Diurnal variation of gas exchange, chlorophyll fluorescence and xanthophyll cycle components of maize hybrids released in different years

Diurnal variation of gas exchange, chlorophyll (Chl) fluorescence, and xanthophyll cycle components of three maize (Zea mays L.) hybrids released in different years, i.e. Baimaya (1950s), Zhongdan2 (1970s), and Nongda108 (1990s), were compared. On cloudless days, the newer hybrids always had higher net photosynthetic rate (P-N), especially at noon, than the older ones. At noon, all the hybrids decreased their maximal yield of photosystem 2 (PS2) photochemistry (F-v/F-m) and actual quantum yield of PS2 (Phi(PS2)), the newer ones always showing higher values. Generally, the newer hybrids displayed higher photochemical quenching of Chl (q(P)) and lower non-photochemical quenching (NPQ). The interhybrid differences in PN may be owing to their differential photochemical efficiency. A midday depression in PN occurred in all hybrids, which might be caused by serious photoinhibition or by decreased stomatal conductance. However, midday depression in PN was more obvious in the older hybrids, especially when leaves were senescent. The higher de-epoxidation state of the xanthophylls was noted in older hybrids, which was confirmed by their larger NPQ. The newer maize hybrids did not need a strong de-epoxidation state since they had a better photosynthetic quantum conversion rate and a lower NPQ.