The physiology of starch and protein deposition in the endosperm of wheat.

Yield and protein percentage are key issues in the production and marketing of wheat. Yield is a measure of the activity of processes contributing to deposition of starch in the grain, and protein percentage, while not independent of yield, reflects processes in nitrogen metabolism. This paper considers starch and protein deposition in the endosperm of wheat from a physiological point of view and, in particular, explores the extent to which deposition of starch or protein can be manipulated and increased independently of the other product. Rate and duration of both starch and protein deposition in the endosperm of wheat are all independent events, controlled by separate mechanisms. Consideration of this independence can contribute to promoting specific responses within the plant that culminate in starch and protein deposition, whether attempts at improvement be genetic or agronomic in approach. The capacity, or potential, of the grain to accumulate dry matter is established during the grain enlargement phase, that is within the first 15-20 days after anthesis. Genetic, morphological and physiological factors influence development of this capacity, but a major determinant is a substrate effect on mitotic activity in the endosperm. Grain filling commences 10-15 days after anthesis and occupies the last 20-30 days until the grain ripens. Grain filling is the deposition of polymeric product in cells and organelles formed during the grain enlargement phase. Undoubtedly, stress curtails assimilate supply during grain filling but, under adequate growing conditions, both the rate and duration of starch deposition during grain filling are determined mainly by factors that operate within or close to the grain itself. On the other hand, the rate and duration of protein deposition are determined mainly by factors of supply external to the grain. This contrast can be considered in its simplest form as starch deposition lying on an asymptotic region of a rate-versus-supply relationship, while protein deposition lies on a linear region. Strategies for improving starch and protein deposition in wheat are discussed. Starch yield and protein yield should be selected as independent traits in cultivar improvement, and crop management should reflect differences in source-sink relations for starch deposition and protein deposition during the grain filling stage.