PROCEEDINGS OF THE 2016 FACULTY OF SCIENCE INTERNATIONAL CONFERENCE 38 Evaluation of the Effect of Drought on the Physiology Activities in Asian Rice (Oryza sativa L.) Modinat A. Adekoya 1 , Omolayo J. Ariyo 2 , Zaochang Liu 3 , Deyan Kong 3 , Liguo Zhou 3 , Lijun Luo 3 1 Department of Plant Science and Biotechnology, Federal University, Oye-Ekiti 2 Department of Plant Breeding and Seed Technology, Federal University of Agriculture, Abeokuta 3 Shanghai Agrobiological Gene Centre, Shanghai *Corresponding author: modinat.adekoya@fuoye.edu.ng Introduction Rice is one of the major crops feeding the world population and is most important ingredient in food composition in Asia and Africa. Rice is not only a rich source of carbohydrate and proteins but also provides vitamins, minerals and fibre. It constitutes one of the most important staple foods of over half of the world’s population. Globally, it ranks third after wheat and maize in terms o f production (Bandyopadhay and Roy, 1992). In 2012/13, about 491.1 million metric tonnes (FAO, 2014) of rice was produced from 158.4 million hectares (Statista, 2014) of land all over the world. Drought is the opposite of flood and it is defined in relation to plant growth or living condition and duration. It is a condition wherein there is continuous dryness or shortage of water to support plant growth and cultivation or living. A drought condition is declared if there is a continuous dryness for more than 15 days. It affects plant cultivation and loss can be up to 90% depending on the extent and duration. Drought stress had become a global concern in food production. It is a limiting factor accounting for most yield loss in the world. Drought is a major threat as weathers are no longer predictable and rainfall lower than normally received due to climate change and global warming (Seck et al., 2012; Tao et al., 2004). In nature, plant experience multiple or combination of stresses, for example, drought and heat or drought and salinity. In response to heat stress, plants open their stomata to maintain a cooler canopy microclimate through transpiration but under combined heat and drought stress, the sensitive stomata are closed to prevent loss of water, which further increases canopy/tissue temperatures (Rizhsky et al., 2002). Plants are known to develop strategies for coping with stresses when it occur. Stress resistance is based on interaction of the plants with the magnitude and timing of stress - where timing is the stage of plant development when stress occur (Blum, 2011). Exposure of plants to certain environmental stresses can lead to the generation of reactive oxygen species (ROS), including superoxide anion radicals (O 2 ), hydroxyl radicals (·OH), hydrogen peroxide (H 2 O 2 ) and singlet oxygen (O 2 1 ). Injury caused by ROS, known as oxidative stress, is one of the major damaging factors in plants exposed to environmental stresses such as drought (Price et al., 1989). ROS are predominantly generated in the chloroplast by direct transfer of excitation energy from chlorophyll to produce singlet oxygen, or by univalent oxygen reduction at photosystem I, in the Mehler reaction (Foyer et al., 1994; Allen, 1995) and to some extent in mitochondria. Chloroplasts are the first targets in plant cells since this is the major site of ROS production. The increased concentration of ROS inhibits the ability to repair damage to photosystem II and inhibits the synthesis of the D1 protein. Stress- enhanced photorespiration and NADPH activity also contributes to the increased H 2 O 2 accumulation, which may inactivate enzymes by oxidizing their thiol groups. These cytotoxic active oxygen species, which are also generated during metabolic processes in the mitochondria and peroxisomes, can destroy normal metabolism through oxidative damage of lipids, proteins, and nucleic acids (McCord, 2000). Lipid peroxidation, induced by free radicals, is also important in membrane deterioration (Halliwell, 1987; McCord, 2000). To scavenge ROS, which are accumulated during osmotic stress, the measurement of which includes several physiological activities such as proline content, soluble sugar content, peroxidase or superoxide dismutase activity, chlorophyll content, malondialdehyde content etc. (Luo, 2010), plants have evolved specific defense tactics involving both enzymatic and non-enzymatic antioxidant mechanisms. The objective of this study was to evaluate and compare the scavenging ability of the two varieties of Asian rice to changes in physiological activities due to drought stress. Materials and Methods Experimental design: The experiment was laid out in a randomized complete block design with three replicates in the screen-house. Two varieties of rice; Hanyou 8 and Hanyou Xiangqing were used in the experiments. The seeds were sown in plastic box of 68x52x39 cm dimension arranged in three rows in 2012, each representing a water treatment and in four rows in 2013 due to inclusion of an additional water regime. The boxes were filled with soil to brim and compacted to remove air spaces. The soil was later wet to saturation before sowing the seeds. The seeds were sown 20x13.6 cm spacing with a border spacing of 16x6.8cm in 2012 and 13x13 cm spacing with a border spacing of 8x6.5 cm in 2013 thus achieving a seeding rate of 20 seeds per box and one seedling per hill. Three water regime treatments were used in this experiment in 2012, (300, 600 and 900 m 3 /mu) while four water regimes were used in 2013 (300, 450, 600 and 900 m 3 /mu). The 900 m 3 /mu is likened to the paddy flooded field. Determination of Chlorophyll content of samples: 95% ethanol Without the veins, 0.05 g leaf sample was accurately weighed in a 50 mL centrifuge tube with cover, placed in a tube rack. 20 mL of 95% ethanol was then added and the rack with the tubes were covered with paper or aluminum foil and kept from light for 24 hours till the leaves turn white. Chlorophyll content was determined at optical density (OD) of 649 nm and 665 nm, using 95% ethanol as control.