Effects of supplemental UV-B radiation on primary photosynthetic carboxylating enzymes and soluble proteins in leaves of C3 and C4 crop plants

Effects of increased UV-B radiation on activities of primary photosynthetic carboxylating enzymes and on contents of soluble proteins were studied in soybean (Glycine max L.] Merr. cv. Bragg), pea (Pisum sativum L. cv. Little Marvel), tomato (Lycopersicon esculentum L. cv. Rutgers), and sweet corn (Zea mays L. cv. Golden Cross Bantam). The purpose was to evaluate the responses of agronomic crops to increases in solar UV-B radiation. Plants were grown and exposed under greenhouse conditions for 6 h daily to supplemental UV-B radiation which was provided by Westinghouse FS-40 fluorescent sun lamps filtered with 0.127-mm film of cellulose acetate (UV-B treated) or Mylar S (Mylar control). Three UV-B levels were tested: 1.09 (treatment T₁), 1.36 (treatment T₂), and 1.83 (treatment T₃) UV-B_seu where 1 UV-B_seu equals 16.0 mW-m² weighted by EXP-(λ-265)/21]². These UV-B levels corresponded to 6%,21%, and 36%, respectively, of decrease in stratospheric ozone content, based on the interpolations of UV-B irradiances at a solar elevation angle of 60°. Leaves of plants of soybean, pea, and tomato exposed to UV-B radiation were generally low in RuBP carboxylase activity. On a fresh weight basis, all three UV-B radiation levels significantly reduced the enzyme activity in soybean and pea, whereas tomato plants showed significant reduction in RuBP carboxylase activity only when exposed to 1.83 and 1.36 UV-B_seu. An apparent decrease in soluble proteins was observed in leaf extracts of soybean and pea plants exposed to 1.36 and 1.83 UV-B_seu whereas higher amounts of proteins were detected in leaves of tomato plants grown under UV-B radiation. Leaves of sweet corn plants grown under Mylar control were low in PEP carboxylase activity and proteins as compared with those of control plants receiving no supplemental UV and UV-B treatment. Activities of PEP carboxylase in crode extracts from leaves of sweet corn were significantly suppressed under 1.36 and 1.83 UV-B_seu as compared with the no UV control. Some stimulation of PEP carboxylase activity was observed in corn plants exposed to 1.09 UV-B_seu.