Tolerance of photosynthesis to photoinhibition, high temperature and drought stress in flag leaves of wheat: A comparison between a hybridization line and its parents grown under field conditions.

Photosynthesis and its tolerance to photoinhibition, high temperature and drought stress of flag leaves were investigated in a wheat (Triticum aestivum L.) hybridization line (1-12) and its parents (Jing-411 and Xiaoyan-54). From the beginning of flowering to the 10th day, light-saturated CO(2) assimilation rate (P(max)) showed no significant decrease and P(max) of 1-12 was comparable to that of its parents. From the 20th day, P(max) decreased significantly and this decrease was much less in 1-12 than in its parents, whereas no decrease in chlorophyll content was observed in 1-12 and its parents on the 20th day, indicating that photo-oxidative damage occurred in 1-12 and its parents but 1-12 is more resistant to photo-oxidative damage than its parents. To further characterize photo-oxidative damage, tolerance to photoinhibition, high temperature and drought stress was compared in 1-12 and its parents. When exposed to high light (1400μmolm(-2)s(-1)), the maximal efficiency of PSII photochemistry (F(v)/F(m)) decreased significantly with increasing exposure time and such a decrease was much less in 1-12 than in its parents. When exposed to higher temperatures (30-45°C) for 15min, F(v)/F(m) started to decrease at 42°C in 1-12 and its parents. The greatest decrease in F(v)/F(m) was observed in Jing-411. 1-12 and Xiaoyan-54 showed a comparable decrease in F(v)/F(m). Similar results were also observed in the actual PSII efficiency (Φ(PSII)), photochemical quenching (q(P)) and non-photochemical quenching (q(N)). During exposure of detached leaves to air under room temperature conditions, relative water content decreased with increasing exposure time. However, such a decrease was greatest in Jing-411 followed by Xiaoyan-54, and 1-12. There were neither changes in F(v)/F(m) nor q(N) during water loss in the line and its parents. However, there was a decrease in Φ(PSII) and q(P) and greatest decrease was observed in Jing-411 followed by Xiaoyan-54, and 1-12. Also, the decrease in P(max) was greatest in Jing-411, followed by Xiaoyan-54, and 1-12 during water loss. In addition, the activities of ribulose-1,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase, pyruvate phosphate dikinase, NADP-malate dehydrogenase and NADP-malate enzyme were significantly higher in 1-12 than in its parents. The results in this study suggest that high resistance to photo-oxidative damage of the flag leaves in 1-12 may be the physiological basis for its high yield when grown in north China. Our results also suggest that parents can be selected for improved biochemical and physiological traits and crossed to high yielding agronomically elite materials which can be selected for higher performance in yield.