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Drought can be referred to as a meteorological period without significant rainfall and it is one of such major abiotic stresses that contributes to a huge reduction in crop yield throughout the world. Plant shows a broad range of physiological, morphological, and biochemical changes such as reduced photosynthetic accumulation, altered gene expression, etc. Under the drought stress which ultimately causes reduced growth as well as poor grain yield. Drought stressconditions trigger production of ROS, which disrupts the dynamic balance between ROS production and ROS scavenging systems and its accumulation depends on the intensity as well as duration of water stress, and it varies among species. A plant species that has a better inherited genetic response allowing it to rapidly deploy its antioxidant enzymatic and non-enzymatic defense system, can tolerate drought better than a plant species with a poor antioxidant defense system. Furthermore, enzyme and protein encoding drought specific genes have the ability to enhance drought tolerance. These two enzymatic and genetic engineering strategies are unique and vital tools, which can be used to help alleviate the world’s future problems related to energy, food, and environmental stresses, particularly drought. This chapter attempts to discuss developments in understanding effects of drought stress and underlying mechanisms in plants for its alleviation.

Aims: Arsenic (As) contamination in rice is at alarming level as majority of rice growing regions in India are As contaminated. Present investigation is designed to study the better understanding of the physiological and biochemical mechanisms in the amount of chlorophyll change and antioxidative enzymes activity under arsenic stress on rice variety IET-4786 (Shatabdi). Study Design: Completely Randomized Design. Place and Duration of Study: The experiment was carried out in the departmental laboratory of Plant Physiology, Bidhan Chandra Krishi Viswavidyalaya (BCKV), Mohanpur, Nadia,West Bengal during the year 2017-18. Methodology: Two classes of inorganic arsenic- As(V) in the form of Sodium arsenate (Na2HAsO4.7H2O, M. W. = 321.01) and As(III) in the form of Sodium arsenite (NaAsO2, M.W. =129.91) were added to the modified Hoagland nutrient solution@ 2.5, 5.0, 7.5, 10.0, 12.5,15.0 mg L-1concentration. After 20 days of treatment, rice seedlings under arsenate/arsenite treatments were analyzed for change in total chlorophyll content and antioxidative enzymes activity such as superoxide dismutase (SOD: EC 1.15.1.1), catalase (CAT: EC 1.11.1.6), ascorbate peroxidase (APX: EC1.11.1.11) and peroxidase (POD: EC 1.11.1.7). Results: A noticeable decrease in total chlorophyll content was observed with arsenic treatments as compared to control. Various antioxidative enzymes showed significant variable response upon exposure to As(III) and As(V). SOD activities increased at low arsenic exposure up to 7.5 mg L-1 but decreased with further increase in arsenic treatments.  APX and POD activities were increased with increase in arsenic concentrations while CAT activity displayed decreasing trend. Conclusion: The results are suggestive of differential metabolism of As(III) and As(V) in rice and could exert harmfulness in the early development stage of rice at inappropriate concentrations.