Responses of Photosynthesis and Photosystem II to Higher Temperature and Salt Stress in Sorghum

Gas exchange and chlorophyll a fluorescence transient were examined in leaves of sorghum under salt stress and high temperature. During salt treatment with 50 and 150 mm NaCl, photosynthetic rate (Pn) decreased, which could be ascribed to stomatal limitation. Salt stress had no effect on photosystem II (PSII) activity. At high temperatures, PSII function was inhibited in leaves of sorghum, indicated by the decrease in PSII performance index on absorption base and PSII maximal photochemistry efficiency (Fv/Fm); however, the decrease was lower in salt-treated sorghum, suggesting that salt adaption enhanced heat tolerance of PSII. The enhanced heat resistance can be expressed on all the components of PSII including reaction centre, donor side and acceptor side. Consistently, a slight decrease in Pn was found in salt-treated sorghum at high temperatures, indicating that salt adaption also enhanced heat tolerance of photosynthesis. Proline plays an important protective role in plant response to environmental stress, and its large accumulation in salt-treated sorghum might be the underlying reason leading to the enhanced heat tolerance. As for this pattern of photosynthetic response, sorghum seems to be a reliable crop species for human beings in the face of global warming and increasing salinity of agricultural land.