Rainfall (water supply), temperature and solar radiation influence rice yield by directly affecting the physiological processes associated with grain production and indirectly through pests and diseases.
In temperate regions, irrigated rice cultivation starts when spring temperatures are between 13°and 20°C and harvested before temperatures drop below 13°C in the autumn. In the tropics, where temperature is favourable throughout the year, rice cultivation starts with the rainy season. In both tropics and temperate regions, the level of solar radiation primarily determines productivity of rice provided irrigation water/rainfall is not limited.
1. Rainfall:
Rainfall is, perhaps, the most important in rainfed rice cultivation. Under irrigated conditions, however, growth and yield are determined largely by temperature and solar radiation. Most of tropical southeast Asia receives abundant annual rainfall above 2000 mm where rice is usually grown during rainy season.
In general, rainfed rice cultivation is limited to areas where the annual rainfall exceeds 1000 mm. In India, rice is grown over a wide range of annual rainfall. In the area adjoining Western Ghats, where the rainfall exceeds 2500 mm, rice is the most important rainy season crop. It is also grown as rainfed crop even when the rainfall is less than 500 mm. At the other extreme, where the rainfall may be around 250 mm, excellent rice crop is cultivated under well-irrigation.
2. Temperature:
Temperature is one of the limiting factors for rice crop in temperate regions. It, generally, influences not only growth duration but also the growth pattern of rice plant. However, temperature is not a serious problem for rice cultivation in tropical countries since its variations are slight in regions between 15°N and S latitudes. Low average yield in tropics is partly due to warm climate.
It is well known that temperature coefficient for photosynthesis is 1.0, while that for respiration is about 2.0. This indicates that for every 10°C rise in temperature, respiration rate increases twice as fast as photosynthetic rate. Therefore, it is logical to assume that the dry matter production would be less in tropics due to less favourable balance between photosynthesis and respiration at high temperature.
Extreme temperatures are destructive to plant growth. The critically low and high temperatures, normally below 20°C and above 30°C vary from one growth stage to another (Table 1.3).
Poor germination, leaf discoloration, stunting, incomplete exsertation of panicle, increased degenerated spikelets, failure in anthesis, increased spikelet sterility, increased grain shattering and delayed heading are some of the adverse effects of low temperature.
Within the critical low and high temperatures, temperature affects grain yield by affecting tillering, spikelet formation and ripening. High temperatures increase the rate of leaf emergence and provide more tiller buds under low light conditions.
Some of the tiller buds may not develop into tillers due to shortage of carbohydrates necessary for growth. Under these conditions, low temperatures may produce more tillers. When light is adequate, however, higher temperatures increase tiller number.
During reproductive stage, spikelet number per plant increases as the temperature drops. Temperature as low as 12°C will not induce sterility if it last for only two days. However, it will induce about 100 per cent sterility if it last for 6 days.
Low temperature induced sterility is normally attributed to low night temperatures. High day temperatures, however, appear to alleviate the effect of low night temperatures. Mean optimum temperature for ripening Japonica rice in Japan is about 20°-22°C.
In the tropics, daily mean temperature as high as 29°C is not detrimental to ripening when solar radiation is high. Yields over 8.0 t ha-1 are usually obtained when rice crop ripen in April and May, two months characterised by high temperatures and high solar radiation in tropics. This suggests that Indica rice varieties are better adapted to high temperatures, while Japonicas require low temperature for better ripening.
The length of ripening is inversely correlated with daily mean temperature. Thus, persistent cloudy weather will be more detrimental to grain filling under high temperature because of a shorter ripening period. Grain development is prolonged at low temperatures so that the grains are fully filled.
Although, the translocation of carbohydrates is slowed down at low temperatures, the prolonged ripening phase result is longer time to fill up the spikelets. Low night temperatures at ripening are favourable since the longevity of leaves is prolonged and consequently dry matter production is increased.
Lower temperatures during ripening are considered effective in reducing the respiratory consumption of carbohydrates. Temperatures lower than critical 20°C, however, may result in imperfect grains, a notched belly development.
High temperature at ripening results in prematurity, mainly due to the inability of spikelet to serve as a sink since the opening of the spikelet is already closed when the plant may still be able to produce carbohydrates.
This prematurity results in partially chalky and milk white kernels. The harvest index is greater at lower temperatures. Food partitioning is quite apparent in the rice plant and has been given as one of the reasons for higher grain yields in the temperate areas.
Until the initiation of panicle primordia, growing points of leaves, tillers and panicles are under water and water temperature affects growth and development. Leaf elongation and plant height, however, are affected by both air and water temperatures. At early growth stages, water temperature affect yield by affecting panicle number per plant, spikelet number per panicle and the percentage of ripened grains.
At later stages, air temperature affect yield by affecting percentages of unfertilised spikelets and percentages of ripened grains. Under most conditions, water temperature is higher than air temperature and increasing the water depth extends the duration during which water temperature controls panicle growth.
Thus, when air temperature goes down below critical level, increasing the water depth to about 15-20 cm at reduction division stage is an efficient method of protecting rice plant against sterility caused by low air temperature.
3. Solar Radiation:
All the varieties grown in rainy season are subjected to low light stress at the vegetative stage irrespective of their duration. Shading at vegetative stage has little effect on overall performance of the plant. Early and medium duration groups may be relatively more affected than the late duration group, as late duration groups have sufficient recover time. However, low light at vegetative stage is characterised by high tiller mortality.
Reproductive stage is most sensitive to low light stress. Early and medium duration varieties are highly affected as their reproductive stage coincides with the low availability of light. As spikelet number is determined during this period, low light affects the production of spikelets per unit area and yield. Decrease in spikelet number is attributed to limited source activity and sink capacity.
Low solar radiation also affects the ripening phase of the early and medium cultivars and lag phase of late cultivars, thus inducing high spikelet sterility in the case of former and heavy tiller mortality in the case of later. Total dry matter production, grain number per panicle and grain size was reduced considerably due to low light at this stage.
Solar radiation of 300 cal cm-2 day-1 during reproductive stage makes yield of 5.0 t ha-1 possible. Less solar radiation during ripening is required to achieve the same yield. Thus, the effect of solar radiation is apparent only when grain yield is higher than 5.0 t ha-1. When grain yield is below that, sunlight per se may not have any direct significance under the conditions of good management during the tropical rainy season.
Summarising the factors affecting rice production, Ramiah (1954) points out that the crop is grown in warm temperate regions mainly in summer months when there is a difference of up to four hrs between length of day and night, while in the tropics the maximum difference is only about an hr. Based on their response to photoperiod, rice varieties can be grouped as sensitive and non- sensitive.
Sensitive varieties flower when the day length is decreasing and when it reaches a critical value for induction of flowering phase. Such varieties are frequently of medium or long duration period. Induction of flowering by the short day length influences their ripening period, so that they are date fixed as regards to maturity date, though their growing period can be extended by earlier sowing.
Non-sensitive varieties do not respond to differences in photoperiod, their length of life being independent of day length so that they can be grown in any season. They are period fixed as regards length of maturation and earlier or later sowing has little influence on the length of life.
The practical application is that farmers must select wisely among the cultivars they plant, since photoperiod sensitivity will determine the growth duration, maturity date and potential yield as well as cultigenic adaptation to double cropping.
In tropical areas, photoperiod sensitive cultivars have traditionally been selected because they could be planted when the monsoon rains begin and are harvested at a fixed time after rain cease and flood waters recede. Such cultivars utilise the high solar radiation at the late growth stages, which have beneficial effects on yield.