Abscisic acid mediated differential growth responses of root and shoot of Vigna radiata (L.) Wilczek seedlings under water stress.

Water stress is a common problem in Indian agriculture and recent global climate change has aggravated this problem further. Plants have an adaptive response to water scarcity as reflected in differential root and shoot growth. In case of Vigna radiata seedlings we have observed earlier promotion of root growth while inhibition of shoot (hypocotyl) growth under mild water stress, which is probably mediated by ABA. However, the exact mechanism of ABA action is far from clear. The present study attempts to elucidate the action of ABA through apoplastic reactive oxygen species (ROS) and its impact upon antioxidant defence system during the growth of root and shoot under water stress. Thus promotion of root growth by application of exogenous ABA (10 μM) and mild water stress (ψ -0.5 MPa) may be correlated with enhanced apoplastic ROS production possibly by activating plasma membrane located NADPH oxidase (NOX) enzyme. On the contrary, in hypocotyls where growth was rather inhibited by application of water stress or ABA, neither NOX activity nor ROS accumulation was significantly detected upon these treatments. Overall activity of antioxidant enzymes [superoxide dismutase (SOD, EC 1.15.1.1), peroxidase (POX, EC 1.11.1.7), ascorbate peroxidase (APX, EC 1.11.1.1) and catalase (CAT, EC 1.11.1.6)] was high and somewhat promoted by ABA and water stress treatment in roots compared to hypocotyls. A possible ROS-mediated role of ABA in promoting growth and antioxidant activity in roots under water stress has been proposed.