Pn-PAR response curves, gas exchange, dry mass partitioning and grain yield of four different genotypes of wheat were investigated in a randomized complete block design under different watering regimes (85, 55 and 25% FC). The results indicated that genotype differences existed in the adaptation response to water deficit which included change to growth strategies coupled with photo-physiological strategies. Water treatment was the dominating factor which limited plant growth, productivity and yield. As water deficit increased, stomatal conductance (gs), saturation irradiance (SI) and maximum net photosynthetic rate (Pmax) decreased, while irrigation water use efficiency (IWUE), compensation irradiance (CI) and dark respiration (R(D)) increased in all genotypes, with a more rapid decline in wet climate cultivars. Differences in photosynthetic responses existed in different climate genotypes at 55% FC. Reduction of Pn for the wet climate cultivar was mainly due to gs, while for the dry climate cultivar, Pn coupled with photosynthetic pigment increased. At 25% FC, water deficit decreased Pn attributed to non-stomatal limitations in four genotypes. Dry mass was reduced in droughted plants and the percentage increased in dry mass allocated to roots. Furthermore, the wet climate genotypes had the greatest decrease in Chla/b ratio. These findings demonstrated that the superior drought resistance in dry climate cultivars could be attributed to higher photosynthetic capacity, dry matter allocation and then grain yield.
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