The stratospheric ozone depletion and enhanced solar ultraviolet-B (UV-B) irradiance may have adverse impact on living organisms. The impact of UV-B radiation (UV-B, 280~320nm) on growth, biochemical and antioxidant enzyme activity was studied in Indigofera tinctoria (L.) seedling, commonly used as a green manure. The supplementary UV-B radiation significantly decreased the growth, development and changes in UV-B absorbing compounds such as anthocyanin and flavonoids. The antioxidant enzymes were unaffected and showed an enhanced activities of peroxidase, superoxide dismutase, polyphenoloxidase and phenylalanine ammonia-lyase except catalase in UV-B irradiated seedling. Indigofera tinctoria seedling tries to counteract high level of reactive oxygen species produced under UV-B stress through the increased activities of antioxidant enzyme. The results suggest that Indigofera tinctoria is resistant to UV-B radiation damage and the possible negative effect of additional UV-B radiation on the growth of seedling may have been effectively balanced by the UV-B radiation stress through increase in UV-absorbing compound and antioxidant enzymes.

Field studies were conducted to determine the role of defense mechanism in various parameters associated with plant protection subjected to UV-B radiation in Phaseolus trilobus Ait. commonly used as green manure and fodder. Spectrophotometric analysis showed that UV-B radiation decreases the chlorophyll content and increased the amount of UV-B absorbing compounds such as anthocyanins and flavonoids. Other UV-induced alterations included reduction in fresh weight and leaf area. However, wrinkling of leaves and increased leaf thickness was observed in UV-B treated seedlings. The antioxidant enzymes were unaffected and showed enhanced activity, except catalase in UV-B treated seedlings. These results suggest that, in general, UV-B radiation caused oxidative damage. However, plants have evolved complex mechanisms to scavenge the active oxygen species and thereby protect the cellular membrane, pigment and organelle.Key Words: Anthocyanin, catalase, chlorophyll, phenol, superoxide dismutase, UV-B radiation RÉSUMÉDes études sur le terrain ont été conduites pour déterminer le rôle du mécanisme de défense dans divers paramètres liés à la protection des plantes soumises au rayonnement UV-B dans le Phaselus trilobus Ait. communement utilisé comme engrais verts et fourrage. L'analyse spectrophotométrique a prouvé que le rayonnement UV-B diminue le contenu en chlorophylle et augmente la quantité de composés absorbant l'UV-B tels que des anthocyanines et des flavonoïdes. D'autres altérations d'UV-induits ont inclus la réduction de poids à l'état frais ainsi que de la surface foliaire. Cependant, on a observé des feuilles ridées et un accroissement en épaisseur de feuilles des plantules traitées par UV-B. Les enzymes antioxydantes n'étaient pas affectées et ont montré une augmentation de leur activité, à l'exception de la catalase dans les plantules traitées au UV-B. Ces résultats suggèrent que, en général, le rayonnement UV-B cause le dommage oxydative. Cependant, les plantes ont développé des mécanismes complexes pour se débarrasser des espèces d'oxygène actif et ainsi protéger la membrane cellulaire, le pigment et organelle.