Effects of Short-Term High Temperature on Photosynthesis and Photosystem II Performance in Sorghum

Gas exchange and chlorophyll a fluorescence transient were examined in leaves of sorghum at high temperatures. No changes were found in photosynthetic rate (Pn) and photosystem II (PS II) performance index on absorption base (PI(abs)) at 40 degrees C for 1 h. But transpiration rate was enhanced significantly, which served as a self-protection response for dissipating heat. The Pn decreased significantly at 40 degrees C for 3 h, and the decrease became greater at 45 degrees C. Decrease in Pn mainly resulted from stomatal limitation at 40 degrees C for 3 h, whereas it was due to non-stomatal limitation at 45 degrees C. Decline in PS II function indicated by the significant decrease in PI(abs), trapped energy flux and electron transport flux were responsible for the decrease in Pn at 45 degrees C. PS II reaction centre and oxygen-evolving complex in the donor side were not affected at high temperatures, but electron transport in the acceptor side was sensitive to high temperature. The PS II function recovered completely 1 day after high temperature stress even as high as 45 degrees C, which is favourable for sorghum to meet the challenge of global warming. However, Pn did not completely recover possibly due to heat-induced irreversible damage to CO2 fixation process.