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The theme of this year’s ninth National Rice Day is ” Rice: Boon for Food Security “. There is global and national importance of rice. Rice is the staple food for more than 50% of the world’s population and more than 90% rice is produced as well as consumed in Asia. So, rice is a peace making commodity.
Solar radiation is radiant energy from the sun, measured as a total amount (direct beam solar radiation plus sky radiation) expressed in calories per square centimeter per day. Only, the visible part (380-720 mm) of the total solar energy is important for photosynthesis. Plants make foods by photosynthetic process. Solar radiation is not just a form of heat; its spectral bands are also important. The amount (duration and intensity) of sunlight is radiation receipt, is a critical factor affecting rice production. In monsoon climate, light intensity is quite often low during the rainy months.
This sharply limits yield and response to inputs like fertilizers, pesticides and modern varieties. To make the best out of the available sunlight timings of plantings as well as various methods of cropping should be tried. Solar radiation is one of the most important factors in predicting rice yields over a wide variety of locations. Bright sunny weather during flowering is absolutely necessary as most of the grain yield in rice, comes from post flowering photosynthesis. The radiation energy/sunshine hours accumulated during the one or two months preceding harvest, greatly influence final grain yield. With the advent of modern photoperiod-non sensitive rice varieties, photoperiod has become less important in rice culture.
Cloudiness in the rainy season reduces the sunlight available for photosynthesis. There are also more insect pests largely because of favorable conditions, more weeds because water is everywhere, and greater disease incidence due to high humidity. Crop protection is more expensive because there are more alternate hosts and sprayed chemicals are often washed off by rain. Grain drying is more expensive in the rainy season because of low sunshine and high humidity.
The importance of solar energy in tropical agriculture was recognized only after world war II. The average daily solar radiation available during the monsoon season in tropics is one-and-a-half times lower than that available in the temperate rice-growing regions like in Po Valley, Italy, Suicca, Spain, New South Wales, Australia, or Davis, California, America. But because of farmers’ dependence on rainfall, the farmers of rainfed rice in the tropics must grow rice when there is low sunlight intensity.
On the other hand, where irrigation water is available, rice can be grown in the dry season and the grain yield will be higher than in the wet season because of the higher intensity of solar radiation. In the tropics, the correlation between solar radiation for 45-day period before rice harvest (from panicle initiation to crop maturity) and grain yield was highly significant. Earlier experiments indicated a strong correlation between grain yield and solar radiation during the last 30 days of rice growth. Subsequent, Philippines-based International Rice Research Institute (IRRI) research indicated that the increase in dry matter between panicle initiation and harvest was highly correlated with grain yield.
These results indicate that the amount of solar energy received from as early as panicle initiation until crop maturation is important for the accumulation of dry matter during that period. The accumulation of starch in the leaves and culms begins about 10 days before heading. Starch accumulates markedly in the grain during the 30-day period of grain production. With irrigation, the dry-season rice yields in the tropics (11 tons per hectare reported at IRRI) should be similar to those reported for the temperate region (12.5 tons per hectare). From the photosynthetic point of view, IRRI is trying to convert C3 rice to C4 one. If it is successful then 25-50% more rice can be produced with less water and less fertilizers. The Bill-Melinda Gates Foundation (BMGF) has financially helped to IRRI for this Apollo Project. The project says ” Feeding the World with C4 Rice “.
In Terai and Inner Terai of Nepal two crops of rice can be grown i.e. early (dry) and monsoon (wet) season rice. The average rice productivity experienced from long periods in early (dry) season was 3.5 t/ha in the normal (monsoon) season at the Hardinath Agriculture Farm (now National Rice Research Program) located in the Dhanusha, Terai region of Nepal. This farm receives 600 calories per square centimeter per day of solar radiation after flowering in early (dry) season rice but the unit of solar radiation is 500 calories per square per day only after flowering in main/monsoon/normal/wet season rice. Because of the higher intensity of solar radiation, the productivity of early rice is 20-25% higher than the monsoon/wet season rice.
The requirements of solar radiation with rice crop differs from one growth stage to another. Shading during the vegetative stage only slightly effects yield and yield components. Shading during the reproductive stage, however, manifests a pronounced effect on grain number. The shading during ripening reduces grain yield considerably because of a decrease in the percentage of filled grains. Solar radiation at the reproductive stage has the greatest effect on grain yield; that at the ripening stage, the next highest effect; and that at the vegetative stage, an extremely small overall effect.
It is already a well-known fact that ample solar radiation in the reproductive as well as in maturing period is indispensable to get high yields of rice. That is why, Australia, Italy and the United States of America used to get higher rice productivity due to the very strong solar radiation in the maturing period of these countries. Rice is grown under more diverse environmental conditions than any other food crop in the world.
In the tropics, solar radiation is higher in the dry season than in the wet season. Consequently, the productivity per hectare per day of rice during dry season is usually higher. The excessively cloudy weather during the wet season is often considered a serious limiting factor to rice production in monsoon Asia.
However, rice varieties with high-yielding potential can manifest their full capacity when enough size of producer (source: Leaf Area Index) and receiver (sink: grains) of photosynthetic products are formulated and then allowed to function. Environmental factors including cultivation practice affect varietal performances through their effect on the source and sink. The varieties with high yielding potential tend to have a higher sink/source ratio than the traditional varieties.
Therefore, climate- smart farmers must have to know the distribution pattern of solar radiation in the different months of the year, so as to increase rice grain yields by coinciding the reproductive as well as ripening stages during higher intensity of solar radiation. This is really no-cost technology and thus the farmers will be greatly benefited by following this technique without spending extra money. Solar radiation is the free-gift of nature and certainly helps to end hunger.