Study of high temperature stress induced damage and recovery in photosystem II (PSII) and photosystem I (PSI) in Spinach leaves (Spinacia oleracia)

High temperature is one of the major abiotic stresses that predominantly affect photosynthetic processes and thereby crop yield. This study was done to get an insight into the mechanism by which plants dissipate excess photon energy during high temperature. For this, measurement of photosystem II (PS II) and photosystem II (PS I) mediated electron transfer rates, P700 redox state and their recovery capacity was done. The results revealed that capacity of open PS II reaction centers to capture excitation energy, the dissipation of excitation energy in the form of heat or non-photochemical quenching [Y(NPQ)] and the quantum yield of PS II [Y(II)] were sensitive to the initial heat treatment (HT) time (2, 5 and 10 min) at 40 degrees C. PS I was stable at 40 degrees C till 5 min, but lost stability with increasing HT time. Under increasing HT time low value of Y(NA) indicated suppression of the over reduction of PS I acceptor side caused by increased quantum yield of cyclic electron flow [Y(CEF)] around PS I. The recovery studies in dark and light conditions showed that at 40 degrees C till 5 min, PS II was recovered completely. In 10 min, slight recovery was observed in PS II in dark while it recovered completely in light condition. This recovery in PS II was due to the recovery in the mentioned sensitive parameters. PS I was recovered completely in 10 min in dark and light conditions both. In 20 min slight recovery was seen in PS II and PS I both. It is concluded that PS II was recovered mainly at the donor side. It is also concluded that cyclic electron flow plays an important role in protection for PS I from heat stress through alleviating the over-reduction of PS I acceptor side.