G.M. Darrow, The Strawberry: History, Breeding and Physiology
IT IS VERY DIFFICULT to assess the value to breeders of the particular knowledge which is available concerning the morphology and physiology of the strawberry. This knowledge ranges from facts about minute states and processes to general descriptions of the larger complexes in which these facts exist. The difficulty lies in making decisions as to which facts are germane, and which facts are not. It would seem that much of this problem persists because choices have to be made concerning for what sort of people the facts are intended and what uses this information could have for them.
The material in this chapter is supplied either to answer, or to point the way to answering questions that strawberry breeders or investigators might find important. This chapter describes the strawberry plant and its fruit, presenting in an orderly way much of the information which intensive studies have supplied. Such an ordering of information provides a general structure for those who pursue specialized investigations, while, at the same time, the data of special study is included for those who wish it.
The Plant
Figure 19-1 shows the plant of a strawberry, a bit of its root, its crown, leaves, and a fruit cluster with eight fruits, plus three flowers that did not set. There is a great deal more to a strawberry plant however than is revealed in this photograph. In all its parts it is one of the most changeable of all crop plants, and for this reason it is one of the most widely adapted and widely raised of all crops. The following pages contain a survey of some of our knowledge of the strawberry's root, crown, leaves, flowers, and fruit as they affect our understanding of what to breed for a variety.
The Root
Different species and varieties have very different-sized root systems, depending largely on whether they make runners freely and express their vigor in number of runner plants, or whether they make few runner plants and express their vigor in making large individual plants. Within limits, each form can be changed into the other-by restricting runners in the one case, and, in the other, by forcing more runners with nitrogen and by other means.
At the Iowa Experiment Station plants of different ages and with different root systems were compared. Succulent young plants with few lateral rootlets were not anchored as well as the older ones with a large root system, and were injured much more by heaving and by winter's cold. Figure 19-2 shows the two types of roots-the large primary ones which originate in the crown can be seen best in the youngest plant at the right, and, on the other plants, the small secondary lateral roots that make up the mass of the root system and which arise from the primary roots. There are usually 20 to 35, but there may be up to 100 or more primary roots and thousands of small rootlets in a good root system. Though the root is elastic, stretching and contracting as much as a centimeter (Kerner and Oliver, 1895), roots are often seriously broken when alternate freezing and thawing occurs. Mulching with pine needles, straw, or hay helps to prevent this sort of damage.
In general, root development is rapid in the fall and spring when there is not too great a demand for water by the leaves. Darrow (1929) found extensive root development as far north as North Carolina during December and January. Lineberry (1944) showed that ample available nitrogen greatly increased root growth during this period.
The primary roots push out rapidly from the crown and may become several inches in length before they branch. Van Tieghman and Douliot (1888) state that the primary roots arise only at the two sides of a median leaf trace, and White (1927) reports that they arise from the younger portion of the crown just outside the vascular cone. Proper depth of planting is the depth at which the plant formerly grew; if too deep, the crown may rot and the leaves fail to push through; if too shallow, the roots may dry out and be cold-injured in winter. After planting, both new primary and new secondary roots appear.
PARTS OF A ROOT. The wide adaptation of the strawberry is due partly to its variable root system, which is perennial, but perennial only in part. Along the beaches of the Pacific Coast, chiloensis grows in sand dunes and sandy beaches, and roots of these plants may live for many years. In eastern states, virginiana has many primary roots, most of which live for but one or two years. The leaves of virginiana die with cold weather and in the spring new leaves appear from whose base new roots can grow. In general, cultivated varieties follow the pattern of virginiana. Nelson and Wilhelm (1957) describe the development of secondary tissues in the fall or early winter in California. First, vascular cambial strands are laid down, next a complete vascular ring develops, and finally a cork cambium. These outer cambium layers develop into a polyderm, the outer part of which is composed of a protective covering of dead cells, and the inner part of living storage tissue.
The macroscopic parts of a root are (1) the root tip, which is the region of very active growth (Fig. 19-3), (2) the white rootlets which take in most of the water and nutrients, often through root hairs, and (3) the corky-covered region which absorbs some water but is mostly a conducting part. Though Nelson and Wilhelm found starch in primary roots, none was found by White (1927) in Maryland, but only in the secondary roots which he considered to be the overwinter storage organ. A very short exposure-less than a minute in sunny, dry air-may kill all roots not partially corked over. A plant with only old roots absorbs water so slowly that if planted in dry, sunny weather most, or all, leaves must be removed. Figure 19-3 shows a root tip and Figs. 19-4 and 5 are of cross sections of young roots. Figure 19-4 shows a branch root arising opposite a vascular bundle in a tetrarch primary root. The vascular bundles become heavily lignified and fill the center of secondary roots. White noted that the pericycle of primary roots is usually several layered and that of secondary roots few layered. The endodermis is usually distinguished only in very young roots. The root hairs mostly arise from fibrous rootlets, but some may arise from the primary roots.
SYMBIOTIC RELATIONS. Many investigators have found a certain type of fungus (Endogne sp.) in the roots of the strawberry and have suggested that it is mycorrhizal in nature (Figs. 19-6, 19-7 19-8), that is, the fungus may furnish nutrients to the host plant. The mycorrhizal fungus is commonly called Rhizophagus or the Phycomycetous mycorrhizal fungus. White (1929) noted the restricted distribution of chiloensis and its association with mycorrhizal plants as an indication of a possible special need for the fungi. The possibility of the mycorrhizal fungi becoming parasitic under some conditions also was noted by him.
LIFE OF ROOT SYSTEM. In general strawberry roots grow one year
and die the next, during the fruiting season. But when all flower
clusters are removed from a muture plant, most of its root system
does not die at fruitng time. Conditions are extremely variable
and some roots may die when a few weeks old while some at least
may become woody and live many years. Plantations of the Ambato
strawberry in Ecuador in a volcanic soil are very old and under
such conditions the individual roots live for many years
USE OF PLASTIC. Roots of the strawberry grow chiefly downward
in well-drained sandy soils and a few roots may be found as deep
as twenty-four inches. In clay soils they spread more horizontally.
Ball and Mann (1927) found 90 percent of the roots in the top
six inches of soil. In late fall, when the water table rises and
the oxygen in the deeper layers becomes low, root growth is shallow.
The oxygen content of the air in the soil where root growth is
active is nearly that of the air above the soil, but where soil
is water-logged, it may be as little as 1/10.000 the normal. Black
plastic in Florida and clear plastic in Japan and southern California
are used to cover the soil over thousands of acres of strawberries.
The sun warms the soil, and the heat does not radiate so rapidly
off the soil under plastic, so that with several degrees warmer
soil for several months, more extensive root and crown development
occurs (Plates 19-1a and b). Root growth continues
much later in the fall than does leaf growth. The plastic serves
also to conserve soil moisture, and prevent soil washing and weed
growth.
WHERE PRIMARY ROOTS ARISE. Under favorable condi ions new primary
roots grow from the crown at the base of each new leaf. About
fix such roots grow from each leaf base, three from each side.
However, if the crown of the plant is above the ground, the new
roots may not start or may dry up before they reach the soil.
If soil is drawn up around the crowns, new primary roots can grow
to supplement or replace old roots If a new runner plant (Fig.19-10)
lies on moist soil, roots quickly push out; if the soil dries
out, the root tips die (Fig.
19-11). Root distribution around the plant occurs in a
characteristic fashion because the leaf arrangement is in a 2/5
spiral and the roots come from the leaf bases (Fig. 19-12).
Top Growth of Plant
ANNUAL CYCLE OF GROWTH. In the North when a plant is set in the
spring, new leaves appear with a bud in the axil of each leaf.
During the summer some of the buds stay dormant, some develop
into runners, and occasionally one develops into a branch crown.
In the fall, depending on variety and conditions, the buds in
leaf axils develop more often into branch crowns and into flower
buds. In Howard 17, development of buds into runners ceases about
the middle of August and bud growth from then until winter is
by development of runners already initiated, by branch crowns,
or by flowerbud initiation. Varieties like Howard 17 develop very
large individual plants with many branch crowns that are especially
productive, but make their runner plants relatively early in the
season only. In northern states no additional flower-bud initiation
takes place in the spring. In Maryland, a little spring flower-bud
initiation occurs but it is of small value, if any. In eastern
North Carolina and southward the days are short enough when spring
comes, so that extensive initiation occurs and good fruit develops
from winter- and spring-formed buds.
DEVELOPMENT OF PLANT AND RUNNERS. The development of a clone of
the Howard 17 is shown in Fig. 19-13. The mother plant was set
April 1, started its first runner May 27, had four runners on
June 10, and by September 15 had a clone with 11 runners and 109
runner plants. The mother plant was not as large on September
15 as on June 10; its energy went to make new plants of the clone.
A mother plant making as large a clone (Fig.
19-13) does not produce as much fruit as one which has had
its runners kept off, or which does not normally produce many
runners, such as plants of Midland or Earlidawn.
RUNNER PLANTS. The swollen end of a runner normally develops into
a runner plant with roots from the under side and leaves and growing
point at the tip. When plants are set in early spring in Maryland,
runners begin to appear by the end of May and are produced throughout
the summer and with most varieties until September and October.
Plant propagators can often make counts of the number of runners
about September 1 and expect to double the number by the end of
the growing season. Shoemaker (1929) in Ohio and Morrow (1937)
in North Carolina obtained the yields of runner plants that were
rooted at different times in the summer. Plants of the Howard
17 rooted in June produced over fifteen times as many berries
as did those rooted in late October and November, in Ohio. In
North Carolina, plants of three varieties, Blakemore, Klondike,
and Missionary, responded very much alike, with the June-rooted
runner plants having three and one-half times as many berries
as the November-rooted ones.
THE STRAWBERRY CROWN. The crown is actually the very shortened
stem of the plant. In the sand dunes of the Pacific Coast the
crown of F. chiloensis may become as much as two feet in
length and the nodes several inches apart. Fig.
19-14b shows an actual crown with all but the woody vascular
tissue dissolved out. Its structure is shown in Fig.
19-14a. This vascular tissue forms a cylinder with strands
running spirally in both directions. The most striking feature
is the leaf-trace connections to the vascular cylinder, so that
if the roots are cut off any side the leaves of that side do not
wilt, but are supplied by free cross-transfer from any point throughout
the circumference. Thus, reduction of any part of the root system
affects the plant as a whole, not just one side. The central part,
the pith, is made up of large cells which are easily injured and
turned brown by the formation of ice crystals in late fall and
winter (Figs.
19-15 and 19-16).
The narrow cambium layer outside the pith does not seem to be
injured quite so readily by freezing as the pith, but if browned,
the water and food conducting tissue of the plant is destroyed
and the plant may die. Freezing injury is easily seen by cutting
the crowns length wise. Uninjured pith at the center is entirely
white. With slight injury to the crown, but not measurable in
its effect on the plant, browning of the lower part of the pith
occurs; with more severe injury, deeper browning, and with real
damage to the plant, browning and blackening of the outer cambium
occurs. Enormous differences in hardiness of strawberries exist,
from the Ambato and the Red Chilean that are extremely susceptible
to freezing, to the Dunlap, Ogallala, and the native strawberries
of the far North, where temperatures of below-40 are withstood,
even without snow cover.
THE STRAWBERRY LEAF. As stated above, and shown in Fig.
19-12, the leaves are arranged in a 2/5 spiral, each 6th
leaf being just above the first, for maximum light exposure. Leaves
of vesca are thin, those of chiloensis thick, and of virginiana
intermediate (Figs.
19-17 and 19-18). The thin leaves of vesca
are characteristic of the humid woodland plant which vesca is,
while those of chiloensis, with thick cuticles and deep-set stomata,
are characteristic of a dryland plant. Measurements of leaf thickness,
at Corvallis, Oregon, were F. chiloensis, 220 m ; F. virginiana, 143 m ; F. vesca, 99 m ; Marshall, 163
m
; Blakemore, 192 m , and F. nilgerrensis, 163 m (unpublished).
The leaves of F. chiloensis are characteristically evergreen
and live through relatively cold winters, those of virginiana
die soon after severe frosts occur in the fall. Leaf characters
of varieties and hybrids range from those like chiloensis
to those like virginiana. Leaves of most varieties of the
eastern United States are nearer those of virginiana. In
the spring the embryonic leaves, enfolded by stipules, push out
with warm weather, both by cell enlargement and to some extent
by cell division, and in two to three weeks of warm weather reach
full size. The individual leaves live for one to three months-those
of Howard 17 in Maryland averaging 54 days with a range of 21
to 77 days. Though frequently killed by fungi, the leaves usually
die in sequence. In Fig.
19-13 the mother plant, Howard 17, had a leaf area of
530 sq. cms. May 27, 1311 sq. cms. July 8, but only 880 sq. cms.
on September 15. If runners had been kept off, the leaf area would
have been many times this. Overwintering leaves may be scarlet,
purple, or green without a trace of purple, or intermediate. Most
leaves are trifoliate, but some varieties have four or five leaflets;
usually the latter are most recently derived from or most closely
related to F. chiloensis.
ANATOMY. The structure of a young leaf of F. chiloensis
is illustrated in Fig.
19-17
(see also Fig.
8-12), showing the stomata, the air spaces, the large
epidermal cells, and the thick cuticle. A view of the lower surface
of a leaf with its many stomata is shown in Fig.
19-18. Their number, their placement, their response to
high rate of water loss the thickness of the cuticle, and possibly
the extent of the air space within the leaf, together determine
the water loss and the drought resistance of the species or variety.
Plants in humid atmosphere have much thinner, much larger leaves
with fewer, but two to three times larger, vessels. In the shade
there may be but one palisade layer with the green chloroplasts
and in the open, two layers with the leaves much deeper green.
Measurements of the cell surfaces, within the strawberry leaf,
that were exposed to air spaces showed from less than two times
to over four times as much inner leaf surface as outer; a Marshall
leaf having 4.4 to 1, chiloensis 4.1 to 1, and nilgerrensis
ranging from 2.2 to 3.1 to 1. The water loss by the leaves, mostly
by the stomata, in one test averaged 7.6 cc. per 100 sq. cms.
and ranged from 5.3 cc. to 10.8 cc. depending on the variety,
species, and weather condidons (Darrow & Sherwood l931). The
strawberry has rnore stomata per square millimeter than many plants,
300 to 400 versus 246 for the apple, by way of example.
The amount of water a plant uses in a day is dependent on its
leaf area, the extent of its root system, the water supply, the
temperature, the intensity and duration of light period, and the
humidity. On sunny days in August, a plant with ten leaves may
use a third of a pint of water, but on cloudy days not over half
as much. With loss of water greater than the intake, the plants
wilt and if it continues for several days the older leaves may
die. When wilting is this severe the smaller roots are in dry
soil and die. Such a plant has a restricted root system for taking
in water and nutrients, and a restricted top for manufacturing
food. Many weeks may be required for such a plant to regain lost
leaves or roots.
The chlorophyll of the leaf manufactures the food just as long
as the sun furnishes energy and as long as water supplies the
nutrients and carbon dioxide to the leaf and carries the food
and waste away. When it is dark, the chlorophyll stops food manufacture.
Also, if the supply of carbon dioxide from the air stops, as when
the stomata close, food manufacture stops; so also when the nutrient
or water supply ceases, or the sugar is not carried off, or the
temperature is too high or low, or poisons injure the chlorophyll.
Carbon dioxide in the air goes into the interior of the leaf,
chiefly through the stomata. The extensive air spaces of the interior
of each leaf make possible the circulation of the carbon dioxide,
which forms about three parts per 10,000 of the air volume, to
much of the plant. The carbon dioxide is dissolved in the water
in the cell walls which makes it available for plant processes.
LEAF NUMBER AND YIELD. The number of leaves per plant in late
fall is used as a measure of leaf area, which in itself is directly
related to the number of fruits borne by the plant the next year.
Many of the buds in leaf axils turn into flower buds, and usually,
under average conditions within a variety, the more leaves, the
more flower clusters. The older runner plants in general have
the most leaves and greatest leaf area and produce the most fruit.
A two-leaf plant in October may have one small fruit cluster of
three to five berries, while a fifty-leaf plant may bear a quart
of berries. Different varieties have different numbers of fruit
clusters per crown and different types of flower clusters.
In the spring the embryonic leaves within the bud of each crown
develop as soon as growth commences. By the time the first willows
and narcissus are in bloom, one or more of the overwintering leaves
have unfolded. Leaf growth and production of new leaves is rapid
from then on.
DEVELOPMENT OF GROWING POINTS. A drawing of a plant as it appears
at the blossoming season is given in Fig.
19-19. This plant developed from a runner tip during the
previous summer. In the fall the growing point at the end of its
short stem was transformed into a flower bud which, in turn, in
the spring developed into the flower cluster shown. Because its
growing point became a flower bud, no further vegetative development
of the plant could take place except as new growing points appeared,
and, except through such new growing points, no leaves in addition
to those already initiated could develop.
In the plant shown, the three leaves P, Q, and R had already been
initiated in the fall before a flower bud formed. In the spring
these quickly unfolded and reached full size at the time the drawing
was made. The oldest leaf labeled P is the lower one, the next
younger Q and the youngest R. It should be noted that the broad
petiole bases of each of these three leaves encircle the entire
stem or crown of the plant. Before unfolding, these petiole bases
together with their stipules cover and protect the growing-point.
In winter this protection is especially important.
The plant illustrated in this figure had suflficient vigor in
the fall to start the bud in the axil of leaf R. which is just
below the terminal. This developed to such an extent that a fourth
leaf S has now expanded. The base of this leaf, however, encircles
only the bud A. Growing-points B and C may be seen in the axils
of leaves Q and R. If growing point A also turned into a flower
bud then growing-point B would continue the growth of the plant.
This has actually occurred, as can be seen in the longitudinal
section of the crown in the upper right-hand corner of the drawing.
Bud A is seen to consist of a rudimentary leaf and the small flower
buds of a second inflorescence. Still other growing-points can
be found under the old leaf bases shown circling the crown below
the leaf P. If the plant should be given exceptionally good growing
conditions, growing-points B and C might both develop into additional
crowns. If growing points develop during the summer they are runner
tips, but in the fall with shortening days and lower temperatures
they become flower buds.
Guttridge (1959) exposed one plant each of pairs of runner plants
joined by runners to long daily light periods. The effect of the
long light period on one plant of each pair was to increase petiole
lengths and leaf size and to delay flower initiation in the other
plant of each pair exposed to a short day, and he concluded that
there was good evidence for the existence of a growth-regulating
substance(s) that promoted vegetative growth and inhibited flower
initiation and that this substance controlled the vegetative-fruiting
responses of the strawberry. By cutting the tops off in August
soon after the previous harvest and so removing the flower inhibition
produced by leaves, yields were increased from 10 percent to about
100 percent.
RUNNERS. Runners are produced all summer from buds in the axils
of new leaves, and in succession as the leaves develop. Guttridge
suggests that the first axillary bud to differentiate in the spring
becomes the first runWer. These runners are two nodes and two
internodes in length. The more rapidly the plant grows the more
runners are produced. Their size and final length depend on growth
conditions and varietal characteristics. Long runners may be advantageous
in placing the runner plants that develop at their tips at a distance
from the mother plant. Leading varieties have runners of medium
length. The two internodes making up the runner are on the average
about equal in length. The bud at the first node is usually dormant,
but may develop into a runner; such branch runners are usually
much smaller than the primary runner; if the runner tip is cut
off, a plant may form instead of a branch runner. In the spring,
plants with no flower buds start leaves and runners before those
with flower buds, and those with few flower buds before those
with many. Plants producing runners as early as the first fruit
ripens, usually produce less than those starting runners after
harvest. The virginiana normally starts runners much earlier
than chiloensis; and the runners of virginiana are
much more slender than those of chiloensis. The runners
of both are variable in length; both have long or short runners,
depending on conditions of growth. Most runners survive until
winter, though in the southern United States many die earlier.
Runners of chiloensis tend to survive winter's cold, and
often do on the Pacific Coast. Its runners may even become part
of the stem of an old clone and live for many years (Fig.
19-9). Also, in mild climates, by forcing them to grow
erect, the runners of ordinary varieties may live for a full year.
(The ridiculous "climbing" everbearing strawberry!)
Because flower buds, runner buds, and branch crowns all arise
as buds from leaf axils, intermediate structures might be expected
and are known (Fig.
19-20). Under moist conditions the inflorescence may root
at one or more of its nodes and even produce plants. Etter reported
a variety having inflorescences that produced runners. Roots are
often produced by inflorescences when the first internode is short
and the first node has contact with soil under the leaves. F.
orientalis in Maryland often had long inflorescences that
rooted.
ANATOMY. The runner has a thick cortical layer surrounding a cylinder
of very large vessels arranged in bundles separated by rays; the
cylinder in turn surrounds a central pith of thin-walled cells.
The whole structure is well adapted for carrying the large amounts
of water and nutrients necessary to establish runner plants. Food
and water may be carried freely in either direction, and the parent
plant may support, or be supported by, a large clone of runner
plants for months.
Gay (1857) first described the development of the runner series.
At the runner tip a new plant is formed, the first leaf of which
is a scale, or bract-like structure, but whose leaf traces arise
in the crown of the new plant. The bud in the axil of the first
leaf is well placed to receive water and nutrients, and is most
likely to become a runner and to continue a runner series. It
is, however, a new runner, not a continuation of the original
runner. In everbearing varieties the bud in the first leaf axil
may be a flower bud. Gay noted that in F. viridis of Europe the
first runner may be two nodes long, but that the next runners
are one node long.
FLOWER-BUD DEVELOPMENT. In Maryland, the first visible change
of a strawberry plant's growing point into a flower bud is a broadening
of the very tip as shown in Fig.l9-21,
in this case occurring September 1. In seven to ten days this
has developed as shown in Fig.
19-22. By October I the parts of flowers can be distinctly
seen (Fig.
19-23) even with the unaided eye, and by November 20 most
of the fall development has taken place (Fig.
19-24). Crowns of Howard 17 showing flower buds natural size
are shown in Fig.19-25.
In the spring the parts of each flower in a cluster enlarge but
most of the differentiation has already taken place the previous
fall. In southern England (Robertson, 1954) early-rooted runners
began to form flower primordia as follows: Auchincruive Climax
in early August, Royal Sovereign early to mid-September, and Sir
Jos. Paxton in late September.
Development of Inflorescence
In Fig.
19-19, a basal-branching flower cluster is shown, the
details of the branching at the base being illustrated in the
longitudinal section in the upper right-hand corner. The method
of development of such a cluster may be understood best by comparing
it with a flower bud as it develops in the fall in the crown of
a plant. Such a flower bud is shown in Fig.
19-22. Here the primary flower has clearly developed from
the terminal stem growing-point while the secondary flowers developed
from lateral buds. As shown in Fig.
19-22, the pedicel of the primary flower L does not often
elongate to equal those of the branches, especially when the branching
is basal.
In the strawberry buds arise only from leaf axils and the branches
of the inflorescence arise only from bract axils, the leaves being
modified into bracts. In Fig.
19-19, branch W arose from the axil of bract E and branch
X from the axil of bract D. On branch W the flower M is terminal,
but two branches had originated-the first in the axil of bract
F and the second in the axil of bract G. Each of these branches
has terminated in a flower N. but each has also branched.
In the inflorescence shown each branch has three internodes-a
relatively long, a very short, and a second relatively long one.
Thus branch W terminates with flower M. At W is a long internode,
between the bases of F and G is a very short internode, and between
flower M and bract G a second relatively long internode. Branch
Y terminates in flower N. It has a relatively long internode at
Y. a very short one between H and I, and another relatively long
one between I and N. Though the inflorescence in the drawing has
a very short peduncle (from V to the base of E in the longitudinal
section) in most varieties of the strawberries a fairly long peduncle
is usually produced. Typically, then, the primary axis and all
of its branches have the long, short and long internodes. Where
the peduncle is very short as in Fig.
19-19, the inflorescence is said to be basal-branching.
The inflorescences of many strawberry varieties, however, are
quite irregular. Instead of two, several branches may start out
at the base or at any point on the peduncle. The primary axis
may then have one or two long internodes and several very short
ones. Occasionally the primary axis may have several long internodes,
but this is not common, at least in most varieties.
Flower Buds, Light, and Temperature. With most varieties,
when in late summer and fall the photoperiod shortens to eleven
to thirteen hours, the bud in a leaf axil instead of becoming
a branch crown or a runner becomes a flower bud. Van den AA (1942)
found that at least six days and up to 14 days of short photoperiods
(six to twelve hours) were necessary for flower bud initiation,
with six to eight weeks needed for the beginning of flowering
in the Deutsch Evern variety. Commercial growers of Deutsch Evern
shortened the days for four to five weeks from the last of May
to July 1 to start flower bud intiation and to initiate enough
flower buds for a crop. Experimentally Moore and Hough (1962)
were able to obtain flower bud initiation in the Sparkle variety
by shortening the day from sixteen to eight hours at 70 F day
and 65 night temperature, after twelve to fifteen daily cycles.
They had previously obtained initiation within twenty-one days.
If large plants are dug September 1 and placed in a warm greenhouse
with supplementary electric light for four to five hours the flower
buds already initiated develop, but no new ones do. Studies by
Waldo (1930) and others have shown that for the latitude of Maryland
the first flower buds of Howard 17 are inidated in the latter
part of August and additional ones are initiated later. All continue
to develop until freezing weather. Actually, flower-bud initiation
results when either the daily light periods are shortened, or
the temperature is lowered. Along the coast of California with
its cool climate, many varieties initiate flower buds all summer
long. Such varieties are especially sensitive to temperature.
However, up north, along the coast of Oregon and Washington where
the temperatures are as cool, the light periods of midsummer are
enough longer so that the same varieties do not produce as well.
Likewise, southward at Irapuato, Mexico, the light periods are
enough shorter in midsummer, and the elevation high enough for
moderate temperatures, so that varieties like Elorida Ninety,
Klondike, Missionary, and others make flower buds and produce
a crop the year through. From Mexico south to the equator and
to about 15 to 20 degrees south of that at elevations above 3500
ft., most strawberry varieties bear the year through. However,
the elevation must be sufficient to make the temperature cool
enough for the particular variety. Missionary grows and fruits
probably best of any variety in the warmest temperatures under
short days. Florida Ninety may do as well
in some tropical areas.
Flower buds initiate in the short days of fall, winter, and early
spring whenever the temperatures are high enough. The map (Fig.
19-26) shows where in southern states initiation continues
all winter, or intermittently all winter. In Florida it continues
all winter, in southern Louisiana nearly all winter, and in eastern
North Carolina it occurs when warm periods occur. In Louisiana
and North Carolina temperatures are high enough for plant growth
to occur often in late February and through March, when days are
short enough for flower-bud initiation. What is called a "crown
crop" is harvested, usually, in May. To increase this crop
growers use plastic covers to warm the soil and increase root
and crown development. Further north in Maryland, flower bud initiation
in March, and probably in cool Aprils, may occur, but never enough
to be worth harvesting; flower clusters from such buds are mostly
sterile and those that do set, produce small berries. Along the
coast of California, with low temperatures that are still high
enough for flower bud initiation, commercial crops are produced
by many varieties all summer.
Darrow and Waldo (1929) after testing about 140 varieties, suggest
that the response to daily light periods, to temperature, and
to a rest period is so characteristic that the regional adaptation
of new originations could be determined by growing them in the
winter in the greenhouse. Later (1934) they state that "the
Blakemore has not grown as well as Missionary in the short days
of low light intensity of winter and this may be interpreted to
indicate that it will not succeed as well in Florida as the Missionary.
In actual tests (and by grower experience) in Florida this seems
to be borne out." Northern varieties do not grow under the
short days of winter even under a high tenperature unless first
given a rest period. Southern varietiey may grow so vigorously
so late in the season in the North that few flower buds refomed,
and they are relatively unproductive.
Downs and Piringer (1955) reported on responses of everbearing
and June-bearing strawberries to photoperiods in summer. When
grown at photoperiods of eleven, thirteen, fifteen, and seventeen
hours, all three everbearers produced flower clusters but produced
more under the longer light periods. Red Rich produced 5.0 clusters
at thirteen hours and 2S.2 at seventeen hours They tended to produce
the most runners at thirteen-hour photoperiods. Three June bearers
produced their most runners at the fifteen-hour photo period.
Climax produced a flower dusters at the eleven- and thirteen-hour
photoperiods, but none at fiffteen- and seventeen-hour ones.
The response to daily light periods and to temperature is so characteristic
that it has been suggested that by growing varieties in the winter
in the greenhouse their regional adaptation would be indicated.
RATE OR FLOWER BUD DEVELOPMEWT. Waldo (1930) found that in Maryland
different varieties inidated flower buds at different times, but
that in general the degree of development at the end of the growing
season was correlated with the time of initiation. A physiological
test of initiation (referred to above) consists of placing plants
in a warm greenhouse at stated intervals such as September 1 and
15, October 1 and 15, and lengthening the daily light period with
artificial lights. Under these conditions only flower buds already
develop flowers. In Maryland, in one comparison, took 15 days
to develop from initiation to a stage showing primary, secondary
and tertiary flower buds, and fifty-five days to fully developed
buds, while for Dunlap the periods were six and thirty-five days.
In Oregon the Marshall begins to initiate flower buds by September
1, but Ettersburg 121 not until about November 1. However, the
latter was more evergreen and continued development at lower temperatures
than Marshall (Waldo and Darrow, 1932).
FLOWER-BUD DEVELOPMENT AND SIZE OF PLANT. Davis (1922) first called
attention to the relation of time of runner-plant formation to
yield. Runnerplants formed in October produced less fruit than
those produced in August. Morrow (1931) extended this study, and
Sproat and others (1935) showed that the yield per plant was related
to the number of leaves as a measure of leaf area; the older runner
plants having the most leaves and the most fruit the following
year. However, if the older plants are crowded, they may have
few leaves and few fruits. Leaf number in the fall has been found
a good measure of possible yield the following spring.
FLOWER BUD DEVELOPMENT AND MOTHER PLANTS. In both Florida and
California plants are set and handled so that large individual
plants having no runner plants are fruited. These produce maximum
crops. In one study (Darrow, 1929) mother plants with runners
kept off produced 132 fruits while mother plants with runners
allowed to root to September 1, produced 43 fruits. This relation
also holds true for northern regions, but so far it has been difficult
in the Northeast to bring to fruiting a full stand of fully developed
plants. Losses of plants from insects and diseases and weakening
of piants from these and unknown causes have so far prevented
this sytem from being generally adopted in northern states, even
though ideal varieties for this system-Earlidawn and Midland-are
available for some areas.
INFLORESCENCE TYPES. The number of crowns, inflorescen es, and
flowers per plant in matted rows produced by 48 varieties at Glenn
Dale, Md., averaged 1.6 crowns, 2.4 inflorescences, and 23 flowers
under one set of conditions (Darrow, 1929). The inflorescence
is really a modified stem and at each node of the inflorescence
a bract replaces the leaf, while the bud in the axil of the bract
develops into a branch of the inflorescence. The bract at the
first node is often as large as a leaflet of a true leaf. Sometimes
it consists of three leaflets. Bracts at the second, third, and
later nodes are progressively smaller. The branching of a typical
inflorescence has one primary, two secondary, four tertiary, and
eight quaternary flowers. However, different varieties have different
types of inflorescences and even any one variety may have many
types depending in part on where it is grown. Each branch has
three internodes: a long, a very short, and a long one. The effect
of the very short internode is to make it appear that there are
opposite branches at the nodes; however, the lower branch has
the larger bract and the flower and berry on
FLOWER STRUCTURE. The flower arrangement of the strawberry is
typically five-parted as shown in Figs.
19-27 and 28. In vigorous plants extra flower parts
are common both in wild and cultivated kinds (Plates 19-2c
to d). Under unfavorable conditions, as in poor light and
at low temperatures, flower parts are suppressed in a regular
pattern, first the stamens, next petals, then sepals, and epicalyx,
and finally the pistils. When growth is slow at flowerbud development,
the calyx and epicalyx may become foliar. How wide petals, sepals,
and epicalyx open is in part genetical, in part environmental.
FLOWER TYPES. Flower types have already been referred to as male
or staminate, perfect-flowered or hermaphrodite, and female or
pistillate; all the higher chromosome numbered (hexaploid and
octoploid) species have these types in the wild. Pistillates tend
to set all flowers. Males set none, but the fertility of flowers
with stamens grades from pure males with no pistils through all
degrees of pistil development and fertility to those setting practically
all flowers as in Rockhill (Fig.
11-7). Longworth (1854) reported that on the average not
one in 10 flowers of hermaphrodites set fruit. Today hermaphrodites
develop far more of their flowers into fruit than that.
Several records for the average set of flowers of perfect and
imperfectflowered varieties have been made: in Minnesota, 67 percent
vs. 72 percent; at Salisbury, Md., 66 percent vs. 88 percent;
at College Park, Md., 64 percent vs. 82 percent; and at Glenn
Dale, Md., 72 percent vs. 94 percent, 64 percent vs. 95 percent,
and 61 percent vs. 90 percent in different years and conditions.
At Glenn Dale, Md., one season, seven pistillate varieties averaged
31.7 fruits per plant and 1.7 flowers not set. Twenty-one perfect-flowered
varieties averaged 10.2 fruits and 8.8 per plant did not set.
If allowance is made for non-setting equal to that of the pistillate,
then perfect-flowered varieties averaged about 70 percent fertile
and 30 percent infertile.
Change of locale affects flower set also. At Glenn Dale, Maryland,
the European variety La Constante set three fruits, and 39.8 flowers
did not set; and the European White Pineapple set 1.2 fruits,
while 18.4 flowers did not set. Also, Ettersburg 121, which succeeds
in Oregon, set 0.2 flowers, while 17.2 did not set. The Howard
17, which replaced many varieties, set 16.4 and 2.7 did not set.
It is not yet known why a variety like Ettersburg 121 that was
so productive on clay soils in Oregon is so unproductive in Maryland
and the same is so for the affected European varieties. Often
the earlier formed flower buds on a plant develop flower clusters
that set more fruit than do the later flower buds on the same
plant.
Under some conditions varieties called pistillate develop a few
stamens with pollen. Duchesne observed this. Meehan
stated that in moist air and under favorable surroundings pistillate
varieties often become hermaphrodite. Hovey and Crescent were
apparently such varieties. Some
times varieties like Glen Mary that seem to have good stamens
under some conditions do not set well by themselves-and set far
better when cross-pollinated. Its flowers are functionally pistillate
part of the time. By growing Dunlap in pure sand Gardner obtained
91 pistillate and only 2 hermaphrodite plants. Fig.
19-29 shows two flower clusters from one plant with pistillate
flowers on one and hermaphrodite flowers on the other.
STAMENS. The stamens in multiples of five, commonly 20 to 35,
are usually arranged in three whorls (Figs.
19-27 and 19-28),
although Shaffner shows but two (Fig.
19-27>). They differ in size and length and are of
a deep golden color when they contain good pollen (Fig.
19-28 and Plate
19-2a). When pollen degenerates at a late stage the anthers
are not full sized and are pale yellow (Plate 19-2f). Poorly
developed stamens-"staminoclia"-and stamens with good
pollen may be found in the same flower (Plate 19-2e).
Pollen is mature before the flower or the anthers open, but usually
the anthers do not crack until after the flower opens and the
anthers dry a little. The anthers open at the sides (Figs.
19-30 and 19-31),
sometimes under tension so that pollen is thrown onto pistils
and petals; the pollen is at first heavy and sticky but later
becomes dry and is carried by air currents. Pollen (Plate 19-1)
remains viable for several days under ordinary conditions but
if dry can be kept in a refrigerator for weeks. Valleau found
abortive pollen in anthers of all 120 varieties he examined, the
range being from less than 1 percent to 100 percent and the average
being a little over one-third. No self-incompatability was found
in cultivated varieties.
STERILITY AND PARTIAL POLLINATION. When the first flowers of perfectflowered
varieties open and set well, but the later flowers only partially
set or do not set at all, natural sterility is the primary cause.
But if the first flowers develop into nubbins, and yet the later
flowers produce good berries, the poor development is probably
due to partial pollination (Fig.
19-32). Petals and calyx may cover some of the pistils and
prevent pollination. Frost and insect injury may also cause nubbins
and both as well as fungi may kill the flowers. Sterile flowers
are soon infected by fungi which makes diagnosis as to the cause
of non-setting often difficult.
Pollen Production and Climate
Those who have worked with the strawberry have learned that plants
of the same variety vary in their pollen production in any one
area, as well as in different areas. The first flowers to open
in the spring of many varieties may develop good anthers with
abundant pollen in some seasons, but almost none in other years.
Pistillate flowers that have abortive stamens may under other
conditions have good stamens. Castle (1904) noted that Crescent
and Stirling Castle were pistillate in the United States, but
perfect-flowered in England. Hovey, Longworth, and others noted
this response to conditions, in the early 1800's.
PISTILS. The pistils are arranged in a regular spiral on the stem
end of the receptacle and the seeds also, as may be noted by examining
well-shaped berries (Fig.
19-34). The general structure of a pistil is figured by
Winston as shown in Fig.
19-33. The stigma is rough and sticky. Strasburger (1939)
pollinated flowers of F. virginiana with pollen of moschata
and found typical fertilization twenty-four to forty-eight hours
later. The pistil base, the achene, commonly called the seed,
contains one ovary. The ovary contains one ovule. The achenes
are attached on the underside to the receptacle by fibro-vascular
strands as well as by the epidermal layer. The style is also on
the underside of the achene near its attachment to the receptacle.
The style commonly persists until the berry is ripe. The achene
is fully developed several days before the berry is mature. Each
achene contains a single seed; as described by Winston (1902),
there is a many-layered outer hard pericarp, next a soft thin
testa, and then a single-layered endosperm enclosing the embryo.
The food is stored almost entirely in the two cotyledons in the
form of protein and fat with no starches.
No true after-ripening period at low temperatures is necessary
for most of the seed and seed may be sown as soon as the berries
are ripe, or held in the refrigerator dry for years. However,
quicker and more uniform, but no greater, germination is obtained
if the seed is stored moist for a month in an ordinary refrigerator
at 32 to 40°F. Seed of the different varieties and species
vary greatly in speed of germination beginning in four days and
continuing for three to four weeks. Henry (1934) reported that
25 C. (= 77 F.) gave the best germination; the amount of germination
of hybrid seed ranging mostly from 58 to 94 percent, with one
cross as low as 22 percent. Scott (1948) reported that treating
with sulfuric acid for fifteen minutes hastened, but slightly
lowered germination, and soaking in chlorine solution for eight
hours also hastened germination (1955). Bringhurst and Voth (1957)
in California obtained maximum germination after two to four months'
storage moist at 32 to 34 degrees F.
THE BERRY, SIZE, POSITION, AND NUMBER OF SEED. After fertilization,
the ovules develop rapidly. A hormone is produced quickly and
the flesh around reach fertile seed starts swelling (Fig.
19-32). Valleau (1918) and Gardner (1923) both pointed
out that the primary berry of a cluster is the largest and that
the secondary, tertiary, and quaternary are progressively smaller
(Figs.
19-1 and 19-34),
in their tests averaging 80 percent, 47 percent, and 32 percent
of the primary. Darrow (1929) reported 48 percent and 33 percent
as compared with the primary for the average of secondary and
tertiary of one variety. However, Darrow (1929) also reported
that for basal branching clusters secondary berries were 91 percent
of the primary by weight. In general, the berries on basal branching
clusters average far larger than on high branching ones (Fig.
19-36). No basal branching was obsened in Klondike in
Maryland, yet it regularly produces basal branching in the deep
South. For some varieties high branching clusters indicate that
they are being grown north of the area where they succeed best.
Vigorous plants of varieties with many basal branching inflorescences
bear more large fruit than similar plants of varieties with chiefly
high branching clusters (Figs.
19-35 and 19-36).
Primary berries are not only largest and ripen first, but have
the most seeds. Valleau (1918) found 382 seeds for the primary
berry, and 224, 151, and 92 successively for the secondary, tertiary,
and quarternary berries of one variety. Gardner (1923) reported
an average for Gandy of 518 pistils for the primary, and 83 for
the last flowers. Not only is it important for the breeder to
cross the first flowers that open because they have far more seed
than the last flowers, but also because the primary flowers are
the most likely to set. As stated above, in nearly all varieties
some of the later flowers and in some varieties most of the later
flowers do not develop into berries. They are functionally males.
BERRY DEVELOPMENT. The primary flower, the first flower of those
varieties that open in the spring, may have little pollen, less
than later flowers of the same variety. However, the stamens are
so placed that as they crack open they readily scatter pollen
onto many of the pistils; and, as pistils remain receptive for
several days, bees usually can get pollen from the later flowers
to complete the fertilization of these primary ones. Pollination
of all pistils of the flower are necessary for maximum berry size.
Although blossoms may remain receptive for even ten days in cool
weather if not pollinated, the reaction to fertilization is quite
rapid, usually resulting in petal fall and the drying up of the
pistils, sometimes within as short a time as twenty-four to forty-eight
hours. However, flowers pollinated after the pistils have been
receptive for one to several days develop into berries quicker
than those pollinated as soon as the flowers open, and, as a result,
they mature nearly at the same time as those pollinated when the
pistils are first receptive.
GROWTH PERIOD-FLOWER TO RIPE FRUIT. At the beginning of the strawberry
season in Maryland, the average period from flower opening to
berry maturity is about 31 days, and at midseason, with longer
days and higher temperatures, five to six days less. In Oregon
for three years (1912,1913, and 1914) the average periods for
many varieties were twenty-nine, thirty-three, and thirty-five
days. Everbearing varieties that mature their fruit in twenty
to twenty-five days in the long days and high temperatures of
mid-summer take 60 days in the autumn. Though Kerner and Oliver
(1895) gave 2671 as the total daily hour-degrees after January
1 to ripe fruit of F. vesca in Germany, for a true index
it would be necessary to establish the relative effect of different
degrees of temperature on plant and fruit development and how
much those effects are modified by other conditions. Records at
College Park, Maryland, indicate the variation in seasons. the
effect of environment and of varieties:
The approximate ripening period in the United States is best shown
by the map (Fig.
19-37) based on the time when shipments are made from
each section. Along the Atlantic Coast for central and south Florida
this period is four months, one and one-half months for eastern
North Carolina, and 1 month for New Jersey. This difference in
ripening period is based on the length of time of flower-bud formation
in the different regions due to temperature and photoperiod differences
and interaction.
BERRY GROWTH. Tschiercke (1886) has given the detailed structure
and development of the berry (see Figs.
19-38 and 19-39).
The flower base, which is the receptacle, develops into the edible
part of the strawberry and consists of a fleshy pith at the center;
next a ring of vascular bundles with branches leading to the acher
es, the seeds; then a fleshy cortex outside the ring and an epidermis
bearing a few hairs and connected to the superficial achenes.
From the flower stage the pith cells increase in size, especially
in length, and the narrow intercellular spaces already present
increase in width, especially toward maturity. The cells of the
cortical layer are similar to those of the pith, but are thinner
walled and increase in size about twice as fast as the pith (Havis,
1943). The meristematic tissue, next to the epidermis, has no
intercellular spaces, but the cells continue dividing after those
of the pith and cortex cease. The reason for differences in shape
and size of different varieties depends not only on their early
development when flower buds are forming, but on the duration
of cell division in each layer and on how much the cells enlarge
and how much they pull apart. In F. vesca cell division ceases
when the receptacle is the size of a pea, while in the garden
strawberry cell division continues slightly longer. The cells
of the cortex become 4 or 5 times as wide in the garden strawberry
as those in vesca. Havis (1943) found little cell division from
just before fertilization until maturity and concluded that only
15 to 20 percent of the growth after fertilization was due to
cell division, and was not significantly different in the four
varieties studied. The size of air space affects berry size also.
The air spaces in vesca are so large that the berry is extremely
light weight compared to large-fruited varieties. Even large-fruited
varieties vary greatly in density, and a few varieties like Fairfax
have so little intercellular air space that some berries may sink
in water. Redheart also has almost exactly the specific gravity
of water. Berries selected for canning have less air space and
less oxygen inside and keep their color and flavor better than
most varieties.
The length and diameter of the developing fruit was measured at
Willard, North Carolina, to establish a measure of the size at
different stages, from the time of pollen shedding to the stage
of full ripeness, as shown in the table below:
FROST DAMACE. Frosts may either kill the flowers outright or injure
them so as to cause nubbins or misshapen berries. When a flower
is injured by cold the pistils, which are the tenderest part,
are killed first. If most pistils are killed, a nubbin with a
wisp of dead pistils may result, or, if killed after fertilization,
the embryos do not develop and a seedy spot on the berry results
AUXIN CONTROL OF GROWTH. Nitsch (1950) removed all achenes from
fruits four days after pollination and growth stopped. Growth
also stopped when the achenes were removed seven, twelve, nineteen,
and twenty-one days after pollination. Although growth was stopped
when removed at twenty-one days, the berries turned red at the
usual time, twenty-six to thirty days after pollination. When
one ovule was fertilized the flesh around it developed (Fig.
19-42); when three ovules were fertilized three areas
developed; when only a ring of ovules was fertilized only the
ring of flesh developed. When seeds were removed and an auxin
like beta-naphthoxyacetic acid (100 p.p.m. in lanolin paste) was
applied nine days after pollination, the berry grew to almost
full size and ripened like normal berries. Nitsch (1949) found
the greatest auxin activity and the greatest amount of auxin in
the achenes twelve days after pollination and separated seven
different growth substances natural to the berry; the most prominent
being indole-3-acetic acid. No auxin was found in the receptacle.
Thompson (1963) found that when berries were eight days old, 0.1
percent of 2-naphthoxyacetic acid was necessary for effective
growth in replacing achene effects; after ten days, 0.03 percent;
and after twelve days, only 0.003 percent was necessary-less than
1/30 of that required only four days earlier. He noted also that
the embryo sac divided slowly at first, but between seven to ten
days, began to grow rapidly. Zielinski and Garren (1952) applied
50 p.p.m. beta-naphthoxyacetic acid to half-grown Marshall and
obtained 32.7 percent increase in size, indicating that under
some conditions additional auxin may be effective.
Berry Shape in Different Climates
BERRY SHAPE DIFFERS WITH CHANGES IN CLIMATE. Klondike is almost
globose in eastern United States, but is long-conic with a neck
in southern California. The shape of many varieties is more uniform
in the Northeast and on the Pacific Coast than in the South. The
Ambato is long-conic at Ambato, Ecuador, but short, globose-conic
in Maryland (Fig.
9-1).
The tip of the berry may be meristematic under some conditions,
especially when growth is checked in periods of cool weather.
Leaves or even a plant may grow from its tip (Fig.
19-43). The development of bracts instead of seeds is
usually due to the aster-yellows virus disease.
BERRY SHAPE. The common shapes of strawberry varieties are shown
in Fig.
19-44. Most berries of Klondike are oblate, of Shasta
globose conic, and of Florida Ninety long conic with a neck. The
general shape of the berry is indicated to some extent by the
shape of the receptacle in the small flower bud the previous fall.
The shape of the berries on the cluster depends on their position
(Figs.
19-45 and 19-34),
the primary berry tending to be irregular and broad wedge-shaped,
and the later berries much more uniform. However, the shape may
be affected by conditions in the fall and during the growth of
the fruit in the spring. The multiple-tipped shapes of Marshall
shown in Fig.
19-46 were probably caused by cold, dry weather in the
autumn affecting the flower buds. Irregular shapes of berries
also result from conditions affecting the fall growth.
FASCIATION. Fasciation (Fig.
19-47) results from favorable growth conditions in late
fall when days become too short for normal development of the
particular variety and is most serious from Eastern Virginia southward.
The flower bud broadens and in severe cases no fleshy fruit develops
in the spring at all and the plants are barren. All gradations
are found from a slightly flattened stem of the primary berry
(Fig.
19-47A) to a coxcomb berry shape as in Figure
19-47B (see also
Fig. 21-15). Fasciation is a varietal characteristic, some
varieties, like Missionary and Blakemore, never fasciating. Many
varieties succeeding in the North cannot be grown in the South
because of this trouble and all Southern varieties must be quite
resistant to fasciation. In the spring, insects, disease, lack
of pollination, clrouth, humidity, and frost all affect the shape
of the berry.
IRRIGATION AND BERRY SIZE. Irrigated berries are not as firm as
non-irrigated ones, though the difference is not usually great.
The average water content of several strawberry varieties in Maryland
ranged from 92.6 percent to 89.7 percent in 1929; 91.3 percent
to 88.1 percent in 1930; and 92.5 percent to 91:8 percent in 1931.
The primary berries normally had the highest water content and
the later berries slightly lower water content. In 1929, for four
successive pickings the average water content of all varieties
was 92.6, 92.2, 90.9, and 89.7 percent (Darrow, 1936). In 1925,
in Maryland, in a period of low humidity and very high temperature
the ripening berries even dried on the plants to resemble raisins,
a condition seldom observed. Because most of the root system is
in the top six inches of soil, and because the leaves develop
rapidly in the spring and transpire water freely, they pull water
from berries in very dry weather and cause smaller than maximum
size. Irrigation to keep berries growing to maximum size may be
needed as often as every three to seven days, if sufficient rain
does not fall.
FIRMNESS OF BERRY. Though breeding has resulted in much firmer
berries, little is known of the structure or chemistry of a firm
berry. The cell walls of firm berries like those of Blakemore,
Albritton, and Dixieland, do not pull apart as much when growing
as do the cell walls of softer berries. As a consequence, such
varieties do not become as hollow and thus have less air spaces
inside. Their epidermis is tougher. Low water-soluble protein
is usually associated with firm berries (Sistrunk, 1962). The
larger berries of a variety are softer than the small ones, for
they have a higher moisture content and probably have a different
skin composition. High rates of application of nitrogen may produce
larger berries. Temperature and humidity both affect firmness
greatly. Sparkle is considered to be a firm berry in the northern
United States, but it is too soft to be a market berry in the
warmer climate of Maryland. In a very warm humid period berries
of some varieties puff and are unmarketable (Fig.
19-48)Rains in cool weather, unless long continued, do not
greatly soften berries, but in warm rainy weather berries may
absorb water through the skin and become soft. Darrow (1932) reported
on tests in North Carolina that where leaf growth is greatest
the berries are softest, presumably because of the shading effect
of the leaves. Berries soften as the temperature rises. Thus,
when the air temperature at 8 A.M. was 74°F., inside a shaded
berry it was 73°F. and inside berries in the sun it was 91°F.
At ten o'clock with an air temperature of 77°F. berries in
the shade were 84°F. and from 100°F. to 108°F. in
the sun. Pressure tests indicate that for each rise of the temperature
by 21.7°F. the toughness of the skin is doubled. Not only
is the skin more resistant to injury at lower temperatures, but
the respiration rate is lower, fungi that cause rots grow more
slowly, and the berries keep much longer. Popenoe (1921) has told
of the remarkable firmness of the Ambato variety (Fig.
9-1) which grows slowly-in the cold arid climate of highland
Ecuador with a day length of just over twelve hours. The berries
could be carried in large 26- to 33-pound boxes on mule back for
many miles with no apparent injury. When grown in humid climates,
the Ambato is no firmer than ordinary sorts. In Chile, with its
cool, dry climate, the White Chilean is as firm as the Ambato.
Various means have been devised to measure firmness but most breeders
still judge firmness by feel-the friction of the thumb rubbing
the surface of the berries in the field until the skin breaks,
and by shipping tests. Firm-fruited wild plants have been found
in both ovalis and chiloen is and relatively firm ones in virginiana.
These have yet to be utilized by breeders. The Wilson of the United
States and La Constante of Europe were relatively firm l9th-century
varieties. Neunan, Hoffman, Missionary, and Klondike, of earlier
varieties, were also relatively firm; Blakemore and Tennessee
Shipper are much firmer and Massey, Albritton, and Dixieland are
among the firmer large-fruited ones. Recently when grown under
short days in the South, Klonmore, Headliner, Dabreak, and Florida
Ninety have proved to be firm.
DESSERT QUALITY. The dessert quality or flavor of strawberries
is dependent both on inherited characteristics and on how they
are affected by conditions. Caldwell and Culpepper (1935) concluded
that best flavor was dependent on the sugar-acid-tannin ratio
combined with volatile esters that make up aroma. As the berry
matures, the acids decrease and sugars increase. When unripe berries
lose chlorophyll and turn whitish, they have maximum water content.
From the white stage on, sugars increase rapidly and continue
until the berries are fully ripe. Acidity declines rapidly and
astringency slowly.
The tests of Went (1957) on Marshall also serve as a guide to
the development of aromas. Different varieties have been originated
that are best in different regions; that is, that develop high
flavor under the average weather conditions of those regions.
Although the development of flavor of some varieties may follow
that of Marshall, some may not. Suwannee has high flavor under
a very wide range of conditions. Fairfax has very high flavor
under a much narrower range of conditions and probably follows
more closely the general development of flavor in Marshall, although
the latter has high flavor under a wider range of conditions.
DESSERT QUALITY OF RIPENING BERRIES. Smith and Heinze (1958) studied
the development of berries from quarter-colored to fully colored.
Berries of three kinds left to mature on the plants increased
their size 23 percent to 57 percent, from quarter color to full
color, depending on variety, and 12 percent to 23 percent from
three-quarters color to full color. Quarter-colored berries of
four varieties were harvested and stored until fully colored and
rated 72 percent as good in flavor as those fully colored when
picked. Also, the quarter-colored of two kinds stored until fully
colored averaged 78 percent as much sugar and 33 percent more
acid than the fully ripened ones.
Austin and others (1960) found that even greenish-white to 10
percent pink berries of the Sparkle developed full color at 85F.
in forty-eight hours, 90 percent of full color at 65F. in ninety-six
hours and did not develop good color at 55F. The flavor was considered
as good as of those ripened on the plant.
VARIETIES FOR PROCESSING. The chief uses for strawberries have
been freezing for later preserving and for dessert use. Varieties
for preserving should be medium to light red to the center, and
for packaging for dessert, varieties somewhat deeper red should
be used. Varieties with white soft flesh and low in acid are unsatisfactory
for processing. They should be firm, sub-acid, and aromatic. The
varieties used are listed under Sources for Superior Quality.
CAPPING. Berries of the wood strawberry, vesca, separate from
the cap or hull when ripe and are always picked without the cap.
Many of the wild Virginian cap easily, although most do not. The
native commercial chiloensis of Chile mostly cap with ease, and
in picking the caps are left on the plant. Yet the cultivated
Chilean of South America usually cap with some difficulty. Jucunda,
an old variety of England, and still grown in Europe, is regularly
picked without caps. Miss Kronenberg has used it successfully
in breeding to obtain Juspa and Gorella, but they do not pick
without caps as easily as Jucunda. In drought periods berries
are more easily picked without caps than in moist weather, and
under irrigation in the Pacific Coast berries of Marshall, Northwest,
and others are harvested without caps because of this response
(Fig.
19-49). The varieties having the best capping qualities are
listed on page 394.
Vitamin C. Varieties differ greatly in their vitamin C content
and, in one study, ranged from 39 to 89 units (mg) per 100 grams
(Ezell, 1947). The average for strawberries has been estimated
at about 60 units with Catskill having a high content, Midland
about average, Blakemore slightly low, and Aberdeen very low,
about half that of Catskill. Breeding for much higher vitamin
content was shown to be possible by Darrow and associates (Ezell,
1947). Berries on the plant ripeuing in the sun have higher vitamin
C content than those ripening in the shade. After picking, berries
injured by bruising
tend to lose vitamin C rapidly. Uninjured berries lose no vitamin
C for at least three days when stored at 40 to 75°F. Uninjured
half-red berries increase in vitamin C but not so much as if they
were ripened on the plant. Capped berries at about 75°F. lost
between 10 and 15 percent of vitamin C in twenty-four hours and
between 85 and 90 percent in forty-eight hours.
FLAVORS For the esters or volatile compounds, Winters (1964),
states that about 35 odorous substances have been isolated so
far, but that it still is impossible to reconstitute a really
fresh flavor. Only the chief and most stable compounds have been
isolated. When berries are crushed, the finest flavor is developed
in one minute; in five minutes a noticeable change has occurred,
and in ten minutes a marked change. By using low-temperature steam
distillation a distillate (12 to 15 percent of fresh weight) is
obtained which by dilution yields natural strawberry flavor.
Teranishi and associates (1963), of the U.S. Department of Agriculture
at Albany, California, have recently reported on studies of volatile
substances from strawberries. A direct chromatographic method
of analysis of vapor from a single strawberry was developed to
study the more stable components. The report lists 24 compounds
of zone A of the chromatogram of strawberry oil which had over
150 components.