Pea plants were either illuminated with visible light supplemented with ultraviolet-B (UV-B) radiation for five days, or transferred back to control light after short exposures (hours) to UV-B. Spectra of NaOH-extracted pigments from UV-B-exposed plants showed a decrease in absorption in the visible region and an increase in the UV region: the former a consequence of the loss of chlorophyll, the latter probably due to induced synthesis of protective pigments. The decrease in chlorophyll absorption was an earlier event than the increase in UV absorption. In extracts from plants which had recovered, the increase in UV absorption was greater than in leaves subjected to three days of UV-B. This stresses the importance of recovery from UV-B for extensive synthesis of protective pigments. Analysis of tetrapyrrolic pigments showed that UV-B treatment caused no allomerization of chlorophylls. Formation of chlorophyllidesaandbwas greatly enhanced during the degradation of chlorophylls; however, the concentrations of chlorophyllides were three orders of magnitude lower than those of the chlorophylls and did not increase greatly as chlorophyll degradation progressed. No protoporphyrin IX, uro- or copro-porphyrins III were detected, which suggests that early steps in chlorophyll synthesis are not affected by UV-B light.
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