Sunlight is the ultimate source of energy for the. biosphere. Green plants capture solar energy and convert it

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1 CHAPTER ONE INTRODUCTION Sunlight is the ultimate source of energy for the biosphere. Green plants capture solar energy and convert it into chemical energy in the form of energy rich carbon compounds, by the process of photosynthesis. All other living organisms depend on this process for their energy requirement, either directly or indirectly. The radiant energy from sun which is stored in green plants in the form of energy rich carbon compounds, is called as primary productivity. It is largely determined by the rate and efficiency in the given environmental conditions and resources available at their habitat. Plants and plant products act as the major food sources for almost all other living organisms of this earth including the millions of human population. With the increase of population of this world at an alarming rate, per capita food availability is likely to decrease which needs an increasing demand of global food production. Attempts are being made to improve the productivity of different crop plants in order to meet the increasing demand of global food and biomass production. One of such attempts is the improvement of photosynthet.ic productivity by manipulating the plant and canopy architecture and environmental parameters.

2 2 Rice, a leading cereal crop is vital to more than half the world's population and is the staple food of millions of Asian, Africans and Latin Americans living inthe topics and sub-tropics. In these areas population increase is high and likely to remain high in the coming decades. Thus the production of rice relates to the economic, food and employment status of several Asian, African and Latin American countries (Konokhova, 1985). Since the.last three decades attempts have been made for the improvement of rice production by various cultural and technological manipulations. All rice growing countries have shown a definite upward trend in rice production during this period. However, the rates of increase have not been same in'-these countries and there are large variations in yield levels of different rice growing countries due to various environmental and cultural factors (Yoshida, 1981). Ninty per cent of the world's rice is grown in the wet monsoon season when low solar radiation and high temperature is prevalent. In this condition low light intensity acts as a stress for rice production. The production of rice is per cent lower in wet season than that of dry season which is attributed to sub-optional solar radiation(venkateswarlu and Visperas, 1987 ).

3 3 7 A cumulative solar radiation of 14'000 g cal/cm or 200 hours of bright sunshine during 30 days preceeding harvest is optimal for attaining maximum grain yield ( Vamadevan and Murty, 1978). Solar radiation of 300 cal/cm^day during the reproductive stage makes the yield of 5t/ha possible (Yoshida, 1981). But most of the rice growing regions of the world are usually associated with lower cumulative and daily solar radiation during the main cropping season (wet season). Total sunshine hours and light intensity in a day decreases due to rainy, cloudy and over cast days. Under such conditons per hectare yield of rice is highly affected, particulary that of the high yielding modern rice cultivars, due to the detrimental effect of low light stress on various components of yield. Low light intensity weakens and elongates the plants, reduces effective tillering, causes high spikelet sterility and produces more partially filled grains (Venkateswarlu and "Visperas, 1987) Attempts have been made by many laboratories of Australia, China, Japan, Philippines etc. to increase the production of wet season rice crop by improved crop management and selecting varieties suitable for such low light stress conditons. Significant varietal differences to low light stress tolerance have also been identified under natural and simulated shade conditons. But the various physiological parameters associated with such tolerance have not yet clearly

4 - : 4 : - identified. Physiological mechanisms of low light stress tolerance in rice have not yet been understood. Apart from optimum sunlight, the major nutrient, nitrogen is the most important fertilizer element for all types of rice cultivation and is essential for obtaining higher yield. In places where nitrogen responsive high yielding varieties are grown under improved cultural practices, nitrogen fertilization plays a very significant role. But the uptake of applied nitrogen seldom exceeds 65% even under the best agronomic practices and that during wet season normally ranges between 30-=40! (De Datta, 1981). Such poor recovery and response of applied nitrogen in the wet season are due to the impact of low light stress (Tanaka and Vergara, 1967). The relationship between light intensity, nitrogen response and photosynthesis is considered to be an important aspect in the production of rice. There is a need to identify the physiological parameters linked with nitrogen use efficiency under low light stress conditions. In recent year.s new approaches for low light stress tolerance are assuming importance. Breeding efforts for low light tolerance are also being pursued. Development of suitable cultivars with alternate plant types to enhance potential yield under low light stress conditions is undoubtedly a promising approach for the global improvement of foodgrain production.

5 5 was the Keeping the above points carried out under field and following objectives; in view, this investigation pot culture conditions with * to assess the influence of low light stress on photosynthetic rate, yield and yield altributing characters among rice varieties, * to identify various physiological parameters associated with low light stress tolerance, * to assess the influence of low light stress during different phenological stages, * to study the relationship between photosynthetic rate and associated leaf characters to low light tolerance, * to sudy the impact of low light stress on nitrogen response, and * to characterise elite varieties for low light stress tolerance. * * *