INFLUENCE OF SUPPLEMENTAL ULTRAVIOLET-B ON INDOLEACETIC-ACID AND CALMODULIN IN THE LEAVES OF RICE (ORYZA-SATIVA L)

IR68 and Dular rice cultivars were grown under ambient, 13.0 (simulati ng 20% ozone depletion) and 19.1 (simulating 40% ozone depletion) kJ m (-2) day(-1) of biologically effective ultraviolet-B (UV-B-BE) for 4 w eeks. Plant height and leaf area were significantly reduced by supplem ental UV-B-BE radiation. Greater reduction in leaf area than of plant height was observed, A decrease in indole-3-acetic acid (IAA) content and increase in peroxidase and IAA oxidase activities of UV-B treated plants in both cultivars were observed compared with ambient control. Calmodulin content also decreased after plants were treated with high supplemental UV-B for two weeks and medium W-B treatment for four week s. The results indicated that peroxidase and IAA oxidase activities in rice leaves were stimulated by supplemental UV-B, resulting in the de struction of IAA which in turn may cause inhibition of rice leaf growt h. Although the mechanism is unclear, calmodulin is most likely involv ed in leaf growth.