AGRICULTURAL practices have faced various adversities that have threatened our food security. Some serious challenges include high temperatures, cold spells or low temperatures, flooding, drought, gusty winds, and hailstorms. These adversities are particularly responsible for unfilled grains in rice fields. Weather has a profound relationship with the different growth stages of a rice plant, and generally, 15鈥�20 per cent of sterility occurs in rice production. However, adverse weather conditions further increase the amount of unfilled grain in rice production, especially during the Boro season, which begins in November (Kartik-Agrahayan) and ends in late April or May (Baishakh-Jaistha) due to the hot weather. Additionally, when the temperature drops below 10掳C in winter, cold spells begin.
The most sensitive stage in the life cycle of a rice plant is from tiller to panicle. However, the pre-flowering period is also affected as the stamens are most sensitive at this time. Among the various potential causes of unfilled rice grain formation, heat shock or high temperature is a major factor. It affects rice plants at different growth stages. At the tillering stage, high temperatures (>37掳C) cause the upper rice leaves to turn white. Chlorotic and whitish bands or blotches appear on the leaves, while the number of tillers and plant height decrease. At the reproductive stage, if the temperature exceeds 35鈥�38掳C during the nine days before panicle emergence or for an hour or two during flowering and pollination, it can lead to white heads, white spikelets, a reduction in the number of spikelets per head, and chalkiness, all of which negatively impact rice yield.
High temperatures (>35掳C) at the ripening stage hinder the grain formation process and increase the number of half-filled grains, which adversely affects both yield and quality. Furthermore, it can impair the growth of the stamen and the development of pollen grains, reducing pollen viability. The normal movement of pollen grains, including shedding and dispersal onto the stigma for pollination, may be disrupted, diminishing the rate of fertilisation and leading to sterility. Water is essential for pollen tube growth after pollen lands on the stigma. Higher temperatures result in water inadequacy, causing severe sterility in rice, reducing pollen protein, and affecting pollen germination competency, which leads to a higher proportion of sterile spikelets.
Cold spells or low temperatures can also lead to unfilled grains. At the seedling stage, rice plants may die in a cold spell. During the panicle stage, plant growth is reduced, and the plant turns yellowish. At this stage, the ear may fail to emerge fully, and the grains at the top of the ear may die or become completely sterile. Excessive cold disrupts the reproductive phase in rice plants, such as pollen respiration. This leads to sugar accumulation, protein breakdown, and improper accumulation of proline. Pollen sterility may occur due to various nutrient deficiencies. The anther, where pollen is produced, experiences reduced activity in the tapetum (the cell wall of the anther), which hampers pollen movement and results in reduced pollination. Fertilisation is impaired due to reduced stigma and pollen viability. Above all, sterility in spikelets reduces crop size. Additionally, if the temperature drops below 18掳C at the panicle stage or 20掳C at the flowering stage, the embryo inside the panicle may be damaged, leading to white, immature panicles appearing immediately after flowering.
Another contributing factor may be drought and reduced rainfall. Drought decreases the number of effective tillers in the rice plant, and panicle branches fail to develop properly, resulting in grain blanking. At maximum tillering, drought stress severely reduces the number of panicles, leading to fewer grains. Drought at the booting stage causes spikelet sterility and increases the proportion of unfilled grains at maturity. Particularly during kernel emergence, drought inhibits development, resulting in more unfilled grains. During the milking stage, drought prevents stored nutrients from reaching the panicle quickly, which also leads to unfilled grains.
Stormy winds and hailstorms are another cause of unfilled grains. Unexpected storms and hailstorms can injure rice plants during the tillering and flowering stages, causing the grains to turn white. Stormy winds increase transpiration in rice plants, resulting in excess water loss that leaves the plants dehydrated. Stormy winds also disrupt pollination, fertilisation, and grain development, leaving rice malnourished. The green husks of rice may turn brown or black and become scaly.
These weather-related factors are significant challenges for rice production. However, specific measures can help mitigate these issues. Farmers should transplant crops at the appropriate time with seedlings of the correct age, particularly during the boro season. Short-duration varieties should be sown in seedbeds between 15th and 30th November and transplanted between 30th December and 10th January. Long-duration varieties should be sown from 5th to 25th November and transplanted between 20th December and 10th January. Seedlings of 40鈥�45 days should be transplanted for boro, 20鈥�25 days for aush, and 25鈥�30 days for aman. Non-photoperiod-sensitive rice varieties should be prioritised. Adequate water supply must be ensured during the critical stages of rice growth. Early sowing in Agrahayan can help rice plants avoid extreme temperatures, preventing wilting during flowering and grain development stages.
Farmers can mitigate heat shock by maintaining 2鈥�3 inches of water in fields during critical periods until the grains solidify. Even at ambient temperatures above 35掳C, water in the fields acts as a cooling system for the plants. Adjusting the flower opening time to cooler periods, such as early morning, can reduce sterility caused by high temperatures. Early morning flowering techniques can be employed by breeders to develop heat-resistant rice varieties. These innovations, along with weather-based rice management systems, can reduce sterility. Lastly, vulnerable regions like the haor and northern areas, which experience heat stress and cold spells, require careful variety selection and targeted management strategies to adapt to adverse conditions.
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Niaz Md Farhat Rahman is the principal scientific officer of the Agricultural Statistics Division, BRRI; Sadia Afrin Shupta is a scientific officer of the Plant Physiology Division.