Research Journal of Chemistry and Environment______________________________________Vol.18 (2) February (2014) Res. J. Chem. Environ. 63 Role of exogenous proline in improving water and antioxidant status in Hyacinth bean subjected to salt stress D’souza Myrene R. 1 * and Devaraj V. R. 2 1.Dept. of Chemistry, Mount Carmel College, No. 58, Palace Road, Bangalore, INDIA 2. Dept. of Biochemistry, Central College Campus, Bangalore University, Bangalore, INDIA *myrene83@gmail.com Abstract Proline is an important component of salt stress response in plants. In this study the role of proline in facilitating seed germination and in alleviating salt stress in Hyacinth bean (Lablab purpureus) seedlings was examined. The results showed that 2 mM proline stimulated a better germination percentage in NaCl- treated seeds when compared to untreated seeds. Effect of salinity on Hyacinth bean was also evaluated in 10- day old seedlings with 100–500 mM NaCl over 72 h of exposure. Leaf RWC, endogenous proline, H 2 O 2 , reduced glutathione, MDA, ascorbate, total phenols, total soluble sugar contents as well as catalase, guaiacol peroxidase and glutathione reductase were determined after 72 h of salt treatment. The results showed that salt stress significantly decreased leaf RWC and CAT activity and increased MDA, glutathione and ascorbate contents and POX and GR activities particularly at 300 mM NaCl. The exogenous application of proline (5 mM) significantly alleviated the NaCl induced growth inhibition as noted by lower MDA and higher RWC, POX and GR activity. The CAT activity was less inhibited in the presence of exogenous proline. Thus, addition of exogenous proline to the hydroponics enhances salt tolerance of Lablab purpureus by improving water status and CAT activity in the plant. Keywords: Antioxidants, antioxidant enzymes, hyacinth bean, proline, malondialdehyde, salt stress. Introduction The progressive natural and anthropogenic salinization of arable lands at the rate of three hectares per minute worldwide 27 is a major concern for agricultural crop production 55 . The sustainability of agriculture production in many areas of the world including North and South America, Asia, Europe and Australia is at risk due to soil salinization 18,59,22 . Salinization is known to be associated with large scale irrigation practices. In the Indian subcontinent, the construction of large irrigation canals was said to have initiated the process of secondary salinization in several regions. An example of this is the emergence of salinity in the Deccan Plateau with the commissioning of the Nira Irrigation Project in Maharashtra 68 . Plants resort to many adaptive strategies in response to abiotic stresses. These adaptive mechanisms include changes in physiological and biochemical processes. One of the main consequences of NaCl stress is the loss of intracellular water resulting in growth inhibition 12 . It is widely reported that plants accumulate a variety of compatible solutes such as proline and betaine to increase their hyper osmotic tolerance against salinity and drought 6 . These compatible solutes provide an osmotic pressure within cells and in contrast to inorganic solutes, they do not inhibit the normal metabolic reactions of organisms when accumulated at high concentrations in the cytosol 13 . The high concentrations of compatible solutes reside primarily in the cytosol to balance the high concentrations of sodium and chloride ions in the vacuoles 71 . They protect and stabilize 3D structures of proteins and photosynthetic apparatus 54 , stabilization of membrane and subcellular structures, constituents of cell wall structural proteins 46 and detoxify reactive oxygen species (ROS) 6 produced in response to abiotic stress. Several lines of evidence indicate an important role of proline synthesis in potentiating the pentose-phosphate pathway activity 31 since this pathway is an important component of antioxdative defense meachanisms which need NADPH to maintain glutathione and ascorbate in the reduced state 39 . Many studies have suggested that rapid catabolism of proline upon relief from stress might provide reducing equivalents that support mitochondrial oxidative phosphorylation and the generation of ATP for recovery from stress and repair of stress-induced damage 31 . Exogenous application of proline stimulates seed germination 32 , plant growth 57 and crop productivity 41 under stress conditions by osmoprotection, thereby improving tolerance to salt stress. Although much attention has been paid on the role of proline in stress tolerance as a compatible solute, little attention has been given to its role in other biochemical and physiological processes responsible for stress tolerance in plants 42, 53 . Greenhouse and laboratory techniques are usually used for screening at the germination or early vegetative stages of