Approaches for Enhancing Nitrogen Use Efficiency in some Upland rice (Oryza sativa L.) Genotypes under Water Stress Conditions

Rice is a semi-aquatic (mesophyte) plant which is commonly grown under submerged conditions. Submersed rice occupies about half of the cultivating areas (79 million hectares) in the world. Alternatively, rice is also grown in upland conditions. The yield of upland rice is reduced to some extent by scarcity of water, so called drought. Drought is defined as a period of no rainfall or no irrigation that affects crop growth (Hanson et al., 1990). Rice is the principal food crop for NorthEastern region of India accounting for more than 80 percent of the food grain production. The crop is extensively cultivated (72 percent of the total cultivated area) in upland, lowland and deep water conditions. On an average 3,869 km2 areas are put under shifting cultivation every year. The productivity of upland rice in N.E. India is very poor (0.9 tonnes per hectare) as compared to the national average (i.e. about 1.9 tonnes per hectare) (Singh 2002)

Water stressor drought is one of the most important abiotic constraints in rice. The effect of varying soil water regimes during different growth phases on rice yield. They reported that the soil water stress applied at any of the growth phases reduced rice grain yield, compared to the continuous flooding irrigation. The ripening phase appeared to be most sensitive as compared to the other phases. Soil water stress during the earlier growth phases (vegetative) reduces the production of effective tillers which lessens grain yield ultimately. Water stress during the later growth phases (reproductive) appeared to affect the reproductive physiology by interfering with pollination, fertilization and grain filling. As a result, there is reduction of grain yield in rice crop (Jana et al. 1971).

Nutrient availability might be further reduced by the often alternating soil water regimes and soil chemistry. Low soil fertility and the limited use of fertilizers contribute considerably to the low productivity of rainfed rice based systems (Haefels and Hijmans, 2007; Wade et al., 1999; Pandey, 1998). Increased yield from fertilizer application even underwater limited conditions were reported repeatedly, but it is often assumed that the economic return to applied fertilizer decreases with increasing drought stress (O’Toole and Baldia, 1982). Indigenous rice genotypes grown indifferent water regimes may vary in nutrient use efficiency. Genotypic differences in nutrient use efficiency have been reported when they were mostly grown in well water intensive lowlands (Broadbent et al., 1987; De Datta and Broadbent, 1990). It is, therefore, one of the major considerations to identify the critical steps controlling plant N use efficiency (NUE). Moll et al. (1982) defined NUE as being the yield of grain per unit of available N in the soil (including the residual N present in the soil and the fertilizer). According to Ladha et al. (1998), desirable cultivars with high nitrogen use efficiency (NUE) should produce large yields at low N supply. This seems even more important in upland environment where no nitrogen rates are applied. Several studies have addressed the optimization of fertilization and the improvement of NUE of crops to achieve high yields with reduced N fertilization rates, and limited environmental side effects related toN leaching (Agostini et al., 2010; Burns, 2006; Neeteson and Carton, 2001; Rahn, 2002). Species and cultivars are expected to playa primary role: the genotype affects both theN uptake and the use of absorbed N, because every genotype has its own morphological and functional characteristics for roots, leaves, etc. (Schenk, 2006; Thorup-Kristense and Sørensen, 1999; Thorup-Kristensen and Vander Boogard, 1999). However, the same genotype can show different NUEs when subjected to different levels of N availability. New technologies in nutrient management in rice have been developed to increase nutrient use efficiency in recent years. Site-specific nutrient management (SSNM) such as Real-Time Nitrogen Management (RTNM) and Fixed-Time adjustable-dose Nitrogen Management (FTNM) were developed to increase theN use efficiency of irrigated rice (Peng et al., 1996 and Dobermann et al., 2002). In RTNM, N is applied only when the leaf N content is below a critical level. In this approach, the timing and number of N applications vary across seasons and locations, while the rate of each N application is fixed. The leaf N content is estimated non-destructively with a chlorophyll meter (SPAD: Soil Plant Analytical Development value) or Leaf Color Chart, commonly known as LCC (Tao et al., 1990, Peng et al., 1996, Balasubramanian et al., 1999 and Yang et al., 2003). In FTNM, the timing and number of N applications are fixed, while the rate of each N application varies across season and location. There is paucity of information on the responses of upland indigenous rice genotypes from North Hill zones (i.e. Karbi Anglong) to varying levels of water stress conditions. Moreover, management of Nitrogen in upland rice crop based on SPAD values underwater stress conditions is lacking. The experiment was conducted to evaluate upland rice genotype(s) for higher nitrogen use efficiency (NUE) and yield potential using the Real Time Nutrient Management (RTNM) approaches, nitrogen use efficiency and productivity underwater regimes and nitrogen levels.