Photoinhibition and Photoprotection in Ginkgo biloba leaves:: Influence of Temperature, CO2 and O2

In midday ginkgo (Ginkgo biloba L.) leaves have to bear photon nux density over 1 400 mu mol.m(-2).s(-1) in combination with high temperatures around 35 degrees C at natural habitat. They show typical midday depression of stomatal conductance and of CO2 assimilation rate. The zeaxanthin changes with light intensity during the day. The influence of the combination of strong light and temperature on photoinhibition was also examined in the laboratory. A low CO2 internal conductance (31 mmol.m(-1).s(-1)) was found in ginkgo leaves, which had been exposed to excessive light at temperature between 15 degrees C and 35 degrees C with reduced CO2 (80 mu L.L-1) or oxygen (2%) for 2 h, causing a low CO2 concentration at the carboxylation site and a high proportion of photorespiration. The ratio of electron transport to CO2 fixation was rather high in ginkgo (16 e(-)/CO2 at 25 degrees C) as compared with other plants. It increased with temperature also in 2% O-2 which could not be explained solely as due to change of photorespiration. The reduction of oxygen in 340 or 80 mu L.L-1 CO2 had no effect on the extent of photoinhibition at all temperatures, which indicated that electron flow caused by photorespiration in excess light was negligible in protective effect in ginkgo leaves. However, a decreased CO2 concentration increased photoinhibition, especially at high temperature. It is concluded that the dissipation of excessive excitation energy in the PS II antennae through the xanthophyll cycle may be the major protective mechanism to preventing from the deteriorated effects of strong light in ginkgo leaves.