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Environmental factors are known to affect the composition of the rice grain (Juliano, 1985b). Protein content tends to increase with wider spacing or in borders and in response to high N fertilizer application, especially at flowering. Short growth duration and cloudy weather during grain development, as occurs in the wet season, may increase protein content. Stresses such as drought, salinity, alkalinity, high or low temperature, diseases or pests may increase the protein content of the rice grain. An increase in protein content is essentially at the expense of a reduction in starch content.
Environmental factors that increase protein content, such as soil type, ambient temperature during ripening and growth duration, also increase the ash content of brown rice but have no effect on its fat content. Mineral nutrition affects the protein content of the rice grain: soil organic matter, total nitrogen, exchangeable calcium, available copper and molybdenum and total chlorine all tend to increase the grain protein content (Huang, 1990).
As growth duration increases, brown rice protein content decreases (IRRI, 1988b). By contrast, yield and brown rice protein were not always significantly negatively correlated.
Upland culture had a variable effect on the protein content of eight varieties of rice grown in Côte d'Ivoire; five showed a lower milled rice protein content and two showed a higher protein content under upland culture (Villareal, Juliano and Sauphanor, 1990).
In Punjab, Pakistan, high soil salinity increased the brown rice protein content in three of four varieties differing in salinity tolerance but had no effect on the protein content of the fourth (Siscar-Lee et al., 1990). Soil sulphur deficiency reduces grain yield without having any adverse effect on the cysteine and methionine contents of the rice protein (Juliano et al., 1 987).
The mineral content of the grain is affected by the mineral content of the soil and of the irrigation water. For instance, pollution of irrigation water with mine tailings has resulted in high cadmium content in some Japanese rices which has proved to be harmful (Kitagishi and Yamane, 1981).