GAS-EXCHANGE, CARBON ISOTOPE DISCRIMINATION, AND PRODUCTIVITY IN WINTER-WHEAT

Leaf gas exchange theory indicates that carbon isotope discrimination (DELTA) is negatively associated with CER/g(s) (leaf CO2 exchange rate/conductance). Discovery of genetic variation for DELTA in wheat (Triticum aestivum L.) and other species has generated interest in using carbon isotope discrimination in plant breeding. However, the hypothesized negative association between CER/g(s) and DELTA has sometimes not been observed in field-grown crops. In the present study, CER/g(s), DELTA of peduncle tissue, plant productivity and yield were examined in eight winter wheat genotypes under both irrigated and nonirrigated field conditions. Water stress- and genotype-induced variations in CER/g(s) were more consistently correlated with variations in g(s) than with CER. However, low CER in 'Bezostaya' wheat resulted in a consistently low genotypic ranking for CER/g(s). In 1988, values of CER/g(s) increased with water stress, while DELTA values dropped from 18.17 to 17.48 parts per thousand. These results imply a negative relationship between CER/g. and DELTA, consistent with leaf gas exchange theory; however, DELTA was both positively (1988; r = 0.83** [P less-than-or-equal-to 0.011) and negatively (1989; r = -0.79* [P less-than-or-equal-to 0.051) associated with CER/g(s) in irrigated plots. Positive correlations were obtained between A and both biomass productivity (1988, r = 0.54*; 1989, r = 0.45 [P = 0.081) and grain yield (1988, r = 0.66*; 1989, r = 0.55*) when data were averaged across genotypes and irrigation treatments. Consistently low DELTA values were obtained for 'Sturdy, an early-released, drought-susceptible cultivar with low productivity. These results suggest that genotypic rankings for peduncle DELTA must be carefully interpreted, as low DELTA can be associated with low yield and stress susceptibility in wheat.