Between 75% and 85% of the mature wheat grain is starch, so above all else, yield is a measure of the whole-plant processes that culminate in starch deposition in the grain. Protein percentage, on the other hand, is a ratio value, and whilst not independent of yield is obviously an expression of nitrogen metabolism. The rates and durations of both starch and protein deposition in the endosperm of wheat all appear to be independent events controlled by separate mechanisms (Jenner, Ugalde & Aspinall, 1991). It is this independence that gives the opportunity to manipulate specific responses in the plant that culminate in starch and protein deposition, whether the attempts at improvement be genetic or agronomic.
The relationship between substrate supply and dry matter deposition is different for starch and protein, and the responses change during grain development (Jenner et al, 1991). During the grain filling stage (10–15 days after anthesis until the onset of maturity), the rate of starch deposition in healthy plants is mainly influenced by sink-limited factors, that is by factors that operate within or close to the grain itself. By contrast, deposition of protein is influenced to a much greater extent by source-limited factors, that is by factors of supply. Increasing amino acid supply to developing grains leads directly to increases in protein deposition. Within this context, however, the levels of substrate within the endosperm (sucrose and amino acids respectively) appear inconsistent with what may be expected.