DOES PHOTORESPIRATION PROTECT THE PHOTOSYNTHETIC APPARATUS IN FRENCH BEAN-LEAVES FROM PHOTOINHIBITION DURING DROUGHT STRESS

Ten days after withholding water from bean (Phaseolus vulgar is L. cv. Contender) plants net photosynthetic CO2 uptake by leaves declined an d no net CO2 uptake occurred after 15 d. A similar decline in stomatal conductance also occurred over this 15-d period. Leaf relative water content and tugor pressure remained very similar to that measured on c ontrol plants during most of the experimental period. The decline of n et CO2 uptake by leaves during dehydration is attributed to stomatal c losure since CO2-dependent O-2 evolution measured on the same leaves a t saturating light and CO2 concentration always remained identical to that of control plants. Dehydration of the leaves had no effect on the quantum yield of CO2-dependent O-2 evolution during the experimental period. Leaves from dehydrating plants maintained in normal (21% O-2 350 mu mol . mol(-1) CO2) air exhibited a substantial rate of photosy nthetic activity (about 38% of that measured at saturation light and C O2 concentration) associated with O-2 reduction. Decreasing the oxygen content of the air from 21% to 2% in order to inhibit photorespiratio n produced a decrease in the linear electron transport rate by ca. 65% However, inhibiting photosynthetic O-2 reduction during high-light tr eatment did not increase the extent of photoinhibition of photosystem II photochemistry and did not change the amount of violaxanthin conver ted to zeaxanthin. It is concluded that photorespiration does not prot ect the photosynthetic apparatus against high-light damage during drou ght conditions. Thermal deactivation of energy in the photosystem II a ntennae appears to be the main protective mechanism against deleteriou s effects of high light.