The effect of water deficits on growth and yield of rice depend on the stage of crop growth at which the water deficits occur.
i. Vegetative Growth Stage:
Immediately after transplanting, adequate land submergence (5 cm) is necessary to prevent damage to establishing seedlings from high winds and for root development. Following the early rooting stage, a shallow depth of land submergence (2 cm) facilitates tiller production and firm root anchorage in the soil.
Excessive water depth at this stage hampers rooting and decreases tiller production. By and large, water deficit during this stage may reduce plant height, tiller number and leaf area but the yield is least affected if adequate water is provided to permit recovery of the crop before panicle primordial initiation.
ii. Reproductive Stage:
Rice crop is most sensitive to water deficit from panicle primordia development to heading. Major part of its water requirement is consumed during this period. Three days of moisture stress at 11 days and at three days before heading results in maximum yield loss due to high percentage of sterility.
Reduction in grain yield due to water deficit during this stage (PI to heading) is more related to the degree and duration of water deficits than to the stage of crop growth at which the soil moisture stress occurs. Continuous land submergence of 5 cm depth has to be mainlined at reproductive stage for optimum grain yield.
iii. Maturity Stage:
This stage (milk to grain maturity) is least sensitive to soil moisture stress. After yellowish ripening stage, there is no necessity for standing water. Water may be drained from the field about 7-10 days before harvest to facilitate harvesting.
iv. Excess Water and Crop Performance:
Response of rice crop to excess water conditions is different for different varieties. Response also varies with growth stages and season. Turbid water greatly damages the crop than clear water. Flowering to maturity is more sensitive to excess water than the stage from maximum tillering to flowering. Seedling establishment to maximum tillering stage is least sensitive.