Effect of salt stress on proline metabolism in two high yielding genotypes of green gram

The effect of salt stress was studied on free proline accumulation, activities of pyrroline-5-carboxylate reductase (P-5-CR), proline oxidase and γ-glutamyl kinase, glycinebetaine levels and chlorophyll contents in two cultivars of green gram (T-44 and SML-32, salt tolerant and salt sensitive, respectively) under the conditions of absence as well as in the presence of various levels of salinity. Salt stress resulted in a significant accumulation of free proline in shoots of both the cultivars of green gram. The magnitude of increase in free proline accumulation was higher in the tolerant cultivar than in the sensitive cultivar. Salt stress in an accumulation of proline by 5.0 folds in T-44 at 200 mM NaCl (maximum salt stress) and 1.1-fold only in SML-32 at 50 mM NaCl (maximum salt stress). Pyrroline-5-carboxylate reductase and γ-glutamyl kinase activities increased in both the cultivars and the magnitude of increase was more in T-44 than in SML-32. In addition, the proline oxidase activity was inhibited under salt stress in both cultivars. Nevertheless, the reduction in the activity was more in T-44 than in SML-32. The results suggested that during salt stress, proline metabolism was significantly altered and the extent of alteration varied between the cultivars T-44 and SML-32, leading to the maintenance of the turgor by accumulating higher levels of free proline in T-44, supporting its salt tolerance. Further, salt tolerance of T-44 was evident from the higher level of glycinebetaine (GB) compared with SML-32 during salt stress. Furthermore, the salt tolerance of T-44 was indicated by lower amounts of chlorophyll degradation. The physiological significance of recorded changes are analyzed in relation to the function of these enzymes in plant metabolism under salt stress.