II.17 Respiratory and photosynthetic metabolism of barley trisomics.
R. G. McDaniel, Department of Agronomy, University of Arizona, Tucson, Arizona 85721, U.S.A.
Presence of an extra chromosome in barley nuclei alters the physical appearance of the plant in most cases. Growth rate, disease resistance and yield parameters may be affected by aneuploidy. The presence of the extra chromosome is generally deleterious to the vigor of the plant, when compared with diploids of the same line. Previous work has correlated the vigor of barley cultivars with mitochondrial activity (McDaniel, Barley Genet. II, 1969, in press). Cultivars with the greatest growth potential had mitochondria with the highest efficiency of energy conservation. When mitochondria from tertiary trisomics 27d, and diploids of R. T. Ramage's experimental line 63-j-18-17, were isolated and compared, no significant differences were found in mitochondrial function. ADP:0 ratios of mitochondria from the diploid averaged 2.08; ADP:0 ratios of mitochondria from the tertiary trisomic were 2.04. Respiratory activity, utilizing alpha-ketoglutarate (method after McDaniel, Crop Sci. 9:823, 1969), was 52.25 and 51.60 mM O2 uptake/min/3 ml reaction for mitochondria of the diploid and trisomic, respectively. These differences were not significant using Duncan's test. Thus, unlike the general case in which growth rate and vigor have been related to mitochondrial activity and efficiency, the presence of an extra, translocated chromosome had no measurable effect on mitochondrial function, and did not appear to be an explanation for reduced vigor of tertiary trisomics.
Photosynthesis is one regulatory system controlling growth rate and productivity of plants. Photosynthetic mutants have been described which have impaired growth rate and other metabolic lesions (Hermann, Planta 90:80, 1970; McIntosh & Baker, Gen. Res. 72:11, 1968). I monitored photosynthesis of tertiary trisomics and diploids in order to test the possible relation of chloroplast function and impaired growth rate of trisomics. Apparent photosynthetic rates of young leaf disks (6.5 mm dia.) of diploids and tertiary trisomics were 22.31 + 1.11 and 16.46 + 0.86 m liters/disk/hr as measured manometrically at 30°, 1.0% CO2, and 2.4 x 105 erg • cm-2 • sec-1 saturating light intensity.
Photosynthetic rates of young leaf disks of diploids and primary trisomics for chromosome 4 in cv. 'Betzes' (Eslick & Ramage, Barley Newsletter 12:17, 1969) were 16.69 + 1.03 and 11.69 + 0.88 m liters/disk/hr, respectively. Both tertiary trisomics for chromosome T27d and primary trisomics for chromosome 4 exhibited photosynthetic rates significantly lower than their respective diploids. These differences (trisomic photosynthetic rates were approximately 70% of the wild type rate) were of the same magnitude whether expressed on the basis of leaf protein, leaf area, or mg chlorophyll. Because of the effect of high CO2 concentrations on photorespiration, comparative experiments were run using a range of CO2 concentrations from 0.02% to 1.0%. Apparent photosynthetic rates of both trisomics and diploids were much lower at physiological CO2 concentrations. The photosynthetic rates of the trisomics still averaged 60-70% of those of the diploids, however. The possible contribution of photorespiration to the differential rates of photosynthesis may thus be eliminated, at least indirectly, by these experiments. These data indicated that the photosynthetic metabolism of the trisomics tested was inferior to that of the normal diploids of respective lines. This would suggest that the presence of an extra chromosome may have caused lesions in the photochemical systems of the trisomics which were manifested phenotypically as impaired photosynthetic rate and ultimately as reduced growth rate compared to diploids. Present experiments are concerned with the biochemical dissection and analysis of the photosynthetic enzyme systems in order to locate the metabolic sites responsible for the phenotypic effects of aneuploidy in these cases.