EFFECTS OF SUPPLEMENTARY ULTRAVIOLET-B RADIATION ON PHOTOSYNTHESIS IN PISUM-SATIVUM

Pea plants (Pisum sativum L., cv. Greenfeast) were exposed to supplementary UV-B light (up to 8 days) starting on the 17th day after sowing. The effects of this exposure on photosynthesis and the content and activities of some chloroplast components of the mature leaves of these plants were studied. (i) The total chorophyll content of pea leaves was approximately 40% of that in the control leaves on the 8th day of UV-B exposure. Chlorophyll a levels decreased to a greater extent than the content of chlorophyll b. The decrease in carotenoids paralleled the decrease in chlorophyll b. (ii) On a chlorophyll basis, the contents of Photosystem I and cytochrome f were stable, whereas Photosystem II, ATP hydrolysis by the ATP synthase and the maximum ribulose-1,5-bisphosphate carboxylase (Rubisco) activity decreased by 55, 47 and 80%, respectively, when compared with the controls at the end of the 8-day illumination period. (iii) On a leaf-area basis, Photosystem I and cytochrome f content decreased by 58%, Photosystem II by 80%, ATP hydrolysis by 80%, and Rubisco activity by 90%, when compared with the controls. The in vivo activation of Rubisco was markedly increased in UV-B-treated pea leaves. The underlying mechanisms for these results are discussed.