The effects of O-3 fumigation during leaf development on photosynthesis ofwheat and pea: An in vivo analysis

Previous studies have shown that short exposure of plants to high doses of ozone decreases subsequent photosynthesis; initially by reducing carboxylat ion capacity. This study tests the hypothesis that this is also the primary cause of loss of photosynthetic capacity in leaves affected by development under a low level of ozone. Triticum aestivum and Pisum sativum plants wer e exposed from germination to ozone in air (80 nmol mol(-1)) for 7 hours pe r day, for 18 days. Leaves that had completed lamina expansion at this time were free of visible injury and light absorptance was unaffected. However, some significant changes in photosynthetic gas exchange were evident. Phot osynthetic CO2 uptake at light saturation was decreased significantly by 35 % in T. aestivum but was unchanged in P. sativum. The reduction in photosyn thesis of T. aestivum was accompanied by a 31% decline in the maximum veloc ity of carboxylation measured in vivo. Decreased stomatal conductance did n ot contribute to this reduction of photosynthesis because there was no sign ificant change in the stomatal limitation to CO2. Processes directly depend ent upon photochemical reactions; that is, the quantum yield of CO2 uptake and capacity for regeneration of ribulose 1,5-bisphosphate were not affecte d by O-3 fumigation in either species. This suggests that for wheat, the qu antitative cause of decreased photosynthetic rate in vivo is a decrease in the quantity of active ribulose-1,5-bisphosphate carboxylase-oxygenase.