Effects of Nitrogen on Biochemical Indices of Winter Rice under Low Light Condition

Rice (Oryza sativa L.)is the world’ssingle most important food crop, being the primary food source for more than one-third of the world’spopulation (Shaiful-Islam et al., 2009) as concerned to food security (Jianxin et al., 2023). About 90% of all rice grown in the world is produced and consumed in the Asia region (Viraktamanth, 2007; Bandumula, 2018). In India, it is estimated that rice demand in 2030 will be 142.2 million tons (Goyal and Singh, 2002).

Among the different classes of rice, Kharif rice (Winter rice) accounts for nearly 72% of the total area (Yoshida, 1972; Anonymous, 1987). More than half of the area (55%) under rice cultivation is rain-fed, and 80% of this rain-fed rice area lies in eastern and NE India. Apart from being vulnerable to the vagaries of monsoon along with other biotic and abiotic stresses, occurrence of low light intensity is also a considerable factor. The northeastern region of India covers about 60% of total rice-cropped area with a 48% of the total production only. Numerically about 55% rice cultivation is rain-fed, and 80% of this rain-fed rice area is distributed in eastern and NE India (Adhya et al., 2008; Dutta et al., 2017).

In Assam, rice among the cereals is the single most predominant staple food crop covering about 75% of the gross cropped area. Rice grown in Assam during Kharif season is ensured by sufficient water in the soil by natural precipitation. Rice crop is well adopted to the existing soil water condition, and survives in varying depth of water on the surface of the soil during the peak monsoon season. Kharif rice is characterised by its tropical agro climatic environment of low sunshine hours, high temperature and humidity. It’sgrown indifferent land situations like uplands, medium lands and lowlands with varying conditions of water availability. Due to extreme diversity ingrowing conditions in the state, the situation as a whole is not favourable for higher productivity (Sengupta and Dasgupta, 1978). Rice plant requires about 1500 bright sunshine (BSS) hours for the period from transplanting to maturity. Instead, prevalence of only about 800-900 BSS hours (PPFD<500 µMs-1m-2) during August to December in places like Northeastern region of India not only hampers the physiological efficiencies but also renders nutritional imbalance by retarding nitrogen uptake and ultimately the productivity of winter rice crop (Bharali et al.,1994; Bharali et al., 2020). It’sbecause, solar radiation in tropics is one of the major climatic factors limiting grain yield in rice (Vergara et al.,1976). Low light intensity acts as a stress and determines rice productivity in tropical and subtropical climate. Lower incident radiation is mainly responsible for lower productivity rather than temperature in tropical and subtropical zones (Venkateswarlu and Visperas, 1987; Bormudoi and Bharali, 2016).

Nitrogen (N) at below or above the optimum concentration limits growth and development. Nitrogen is one of the integral constituents of compounds such as amino acids, proteins, RNA, DNA and several phytohormones (Wang and Schjoerring, 2012). In rice, nitrogen is required at early and mid tillering stages to maximize panicle number and to optimize filled spikelets at reproductive stage (Sathiya and Ramesh, 2009). Nitrogen use efficiency (NUE) in cereal crops is generally 33% only (Raun and Johnson, 1999). Rice grain requires N for protein, which is transported directly from the soil or from remobilization during canopy senescence. Under low light intensity, N acquisition is not a limiting factor but its utilization efficiency might be the major reason for reduced productivity. Adequate supply of nitrogen is required to maintain targeted crop yield, but its application costs highly to both the farmers and the environment (Frink et al., 1999). Consequently, there is considerable interest in decreasing fertilizer N inputs by improving plant N use efficiency (Garnett et al., 2009). In general, crops demand higher nitrogen to enrich NUE and carbon gain (Tashiro et al., 1980), and even under abiotic stress situation (Torenpi and Bharali, 2018, Torenpi and Bharali, 2019). Moreover, it is imperative to understand how efficient the nutrient assimilation should be in the presence of external factorse.g. light intensity. However, information on the response of kharif rice genotypes to nitrogen under low light stress condition is scanty, and it deserves further investigation.