VARIATION AMONG WHEAT CULTIVARS IN THE RESPONSE OF LEAF GAS-EXCHANGE TO LIGHT

This research was done in order to explore genetic variation in carbon exchange rate (CER) of spring wheat (Triticum aestivum) leaves in response to variable photosynthetically active radiation (PAR) and to compare old and new Israeli cultivars in this respect. Leaf gas exchange was measured in detached turgid leaves of 17 cultivars in an open system at 25-degrees-C when PAR was reduced from c. 1200 to 200 mu-mol/m2 per s. Linear regressions of CER, stomatal conductance, transpiration and leaf internal CO2 concentration (Ci) on log PAR were fitted for each leaf (regression r2 was never < 0.79) and the regressions were compared among cultivars by analysis of variance. Genotypes differed significantly for the slope (b) but not the intercept (a) of the regression of CER on log PAR, indicating that genotypic differences for CER increased with increasing PAR. Photosynthetic capacity, as expressed by the ratio of CER/Ci, differed significantly among cultivars only at high PAR. Stomatal conductance and transpiration increased in a linear or a nonlinear fashion with log PAR and differences among cultivars for both were greatest at medium to low PAR. Photosynthetic water-use efficiency (WUE) and its variation among cultivars were greatest at the highest PAR. Genotypic variation in CER at high PAR was confirmed by repeated results for 11 cultivars in two independent experiments. The recently developed high-yielding cultivar V652 had a higher maximum CER, higher photosynthetic capacity and greater WUE at high PAR than older and lower-yielding cultivars. The results suggest an upward genetic shift in photosynthetic capacity and in CER at high PAR when selection for yield was performed under the high-irradiation conditions of Israel.