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.

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Well watered wheat (Triticum aestivum L, cv. Gabo) plants grown at 20°C were subjected to heat stress (30°C for 3 days), water stress (leaf water potential -2.54 MPa) or exogenous application of abscisic acid (ABA, 3 X

Wheat plants (cv. Gabo) otherwise grown at 20°C were subjected to a temperature of 30°C for 3 days at the onset of meiosis in the anthers. Control plants were maintained at 20°C throughout development. Serial sections through the heat-stressed ovaries just prior to anthesis showed that a third contained abnormal embryo sacs. Abnormalities ranged from the complete absence of an embryo sac accompanied by reduced nucellus development, to small embryo sacs that contained the full complement of cells. No abnormalities were observed in control ovaries.Following pollination with fertile pollen, heat-stressed stigmas had similar numbers of germinated pollen grains to non-stressed controls but there were fewer tubes reaching the ovary. In 7% of the stressed pistils, no pollen tube reached the ovary. Callose was deposited in some of the inhibited pollen grains and tubes that showed abnormal growth.It is concluded that heat stress during meiosis in wheat can reduce yield by causing abnormal ovary development, which results in reduced pollen tube growth and seed set.

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An induced water deficit in lO-day barley seedlings resulted in a decrease in leaf dry weight within a 28-hr period, although the shoots and roots continued to increase in weight. Primordium formation and apex elongation on the main shoot were also inhibited but floral development at the apex continued.

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