By measurement of gas exchange and chlorophyll fluorescence, the effects of salt shock on photosynthesis and the mechanisms to protect photosynthetic machinery against photodamage during salt shock were investigated in leaves of Rumex seedlings. Salt shock induced significant decrease in photosynthesis both in 21 and 2 % O 2 . In 21 % O 2 , quantum yield of photosystem 2 (PS2) electron transport (Ф PS2 ) decreased slightly and q P remained constant, suggesting that the excitation pressure on PS2 did not increase during salt shock. In 2 % O 2 , however, both Ф PS2 and q P decreased significantly, suggesting that the excitation pressure on PS2 increased during salt shock. NPQ increased slightly in 21 % O 2 whereas it increased significantly in 2 % O 2 . The data demonstrated that during salt shock a considerable electron flow was allocated to oxygen reduction in the Mehler-peroxidase reaction (MPR). Under high irradiance and in the presence of saturating CO 2 , the susceptibility of PS2 to photoinhibition in salt-shocked leaves was increased when the electron flow to oxygen in MPR was inhibited in 2 % O 2 . Hence, MPR is important in photoprotection of Rumex seedlings during salt shock.

Biao 810S is a chlorina mutant of the thermosensitive genic male sterile (TGMS) rice. We compared photosynthetic characteristics of these two lines. The contents of chlorophylls and carotenoids in Biao 810S were approximately half of those in 810S. However, the net photosynthetic rate (P N ) of Biao 810S was higher than that of 810S under high irradiance or low concentration of carbon dioxide, and the photon quantum efficiency was higher than that of 810S. The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase in Biao 810S was only 69.80 % of that in 810S, but the activities of phosphoenolpyruvate carboxylase and NADP-malic enzyme were 79.50 and 69.06 % higher than those of 810S, respectively, suggesting that the efficiency of photon energy utilization in Biao 810S was enhanced by reduction of thermal dissipation and increase of electron transfer rate to generate sufficient assimilation power for the dark reactions. Consequently, the increased activities of C 4 photosynthetic enzymes lead to more effective fixation of CO 2 and the synergistic effect of light and dark reactions contributed to the higher P N of Biao 810S.Additional key words: C 3 and C 4 plants; NADP-malic enzyme; net photosynthetic rate; phosphoenolpyruvate carboxylase; quantum efficiency; ribulose-1,5-bisphosphate carboxylase/oxygenase.