REVERSAL OF ULTRAVIOLET-B (280-320 NM) RADIATION-INDUCED CHANGES IN VIGNA-UNGUICULATA L CHLOROPLASTS BY SUPPLEMENTAL WHITE-LIGHT

Varying ''white light'' (WL) irradiances (2 to 200 W m-2) were provide d simultaneously with UV-B radiation to isolated chloroplasts. UV-B tr eatment for 60 min resulted in severe inhibition of whole chain and ph otosystem (PS) 2 electron transport but only marginally affected PS1 a ctivity. Supplementation of WL reversed these changes to a large exten t. 200 W m(-2) WL not only nullified the UV-B inhibition but also prom oted the photosynthetic activities. At low WL irradiances (2 to 20 W m (-2)) the UV-B inhibition was maximum. The determination of photosynth etic unit size and spectral analysis of chloroplasts exposed to variou s radiation treatments offered evidence for the regulatory role of WL irradiance in influencing UV-B induced changes.