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.

As temperature rises above 18-22�C, the observed decrease in the duration of deposition of dry matter in the kernel is not accompanied by a compensating increase in the rate of grain filling with the result that grain weight (and yield) is diminished at high temperature. Reduced starch content accounts for most of the reduction in grain dry matter at high temperature. Responses to temperature in the low temperature range, 20-30�C (the LTR), could possibly be ascribed to the temperature response characteristics of the reaction catalysed by soluble starch synthase (SSS), the enzyme synthesising starch. However, the rate of cell enlargement and the rate of accumulation of nitrogen in the grain also do not increase much as temperature rises, so other explanations are conceivable for the temperature responses in the LTR. Variation amongst cultivars of wheat in tolerance of high temperature is evident in the LTR. At temperatures above 30�C (in the high temperature range (HTR) between 30 and 40�C), even for short periods, the rate of starch deposition is slower than that observed at lower temperatures, an effect which is carried over after transfer from high to lower temperatures. This response is attributable to a reduction in the activity, possibly due to thermal denaturation, of SSS. Several forms of SSS are found in cereal endosperm, and some forms may be more tolerant of high temperature than others. Loss of enzyme activity at high temperature is swift, but is partly restored some time after transfer from hot to cool conditions. There appear to be two distinct mechanisms of response to elevated temperature, both resulting in a reduced grain weight through reduced starch deposition, but one of them is important only in the range of temperature above 30�C.

Plants of four cultivars of wheat were exposed during the grain-filling stage in two different experiments to brief episodes (10 or 20 days) of high temperature. About the same number of degree-days above a standard temperature were accumulated in both experiments. While the cultivars differed in detail in their responses, final single grain weight in all cases was reduced by elevating the temperature, and to about the same extent (14%) in both experiments. The numbers of B-type starch granules (but not of A-type) were substantially reduced by warming, but this reduction did not appear to account wholly for the smaller weight of starch per grain resulting from elevated temperature. Nitrogen content per grain was not affected by the warm conditions and, in the experiment with the short duration at the high temperature differential (33/25°C day/night as compared to 21/16°C), percentage nitrogen was significantly increased by warming from 2.61 to 3.05%. It is concluded that starch and nitrogen accumulation in the grain have differing susceptibilities to brief episodes of high temperature during grain-filling.

Abstract.We have investigated the nature and locations of isoforms of starch synthase in the developing endosperm of wheat (Triticum aestivum L.). There are three distinct granule-bound isoforms of 60 kDa (the Waxy gene product), 77 kDa and 100-105 kDa. One of these isoforms, the 77-kDa protein, is also present in the soluble fraction of the endosperm but it contributes only a small proportion of the total soluble activity. Most of the soluble activity is contributed by isoforms which are apparently not also granule-bound. The 60-kDa and 77-kDa isoforms of wheat are antigenically related to isoforms of very similar size in the developing pea embryo, but the other isoforms in the endosperm appear to have no counterparts in the pea embryo. The significance of these results in terms of the diversity of isoforms of starch synthase and their locations is discussed.

Ears of wheat were exposed for up to 10 days during the grain-filling stage to high temperature (35�C) and activities of five enzymes in the sucrose to starch pathway were compared to those in ears maintained at lower temperature (21�C day/16�C night). Two cultivars of wheat known to differ in their post-anthesis tolerance of high temperature were compared. On a per grain basis, the activity of sucrose synthase and of ADPglucose pyrophosphorylase in ears maintained at 21/16�C throughout did not change greatly between days 16 and 32 after anthesis, whereas UDPglucose pyrophosphorylase and soluble starch synthase activities declined with advancing development. Soluble starch synthase activity in grains of heated ears was decreased within 1 day to about one- half of the value in unheated grains, and 3 days' additional heating did not reduce the activity much further. Insoluble starch synthase activity was not significantly reduced by heating. Compared to soluble starch synthase, ADPglucose pyrophosphorylase activity was more slowly affected and decreased to a lesser extent by heat. Sucrose synthase and UDPglucose pyrophosphorylase activities were either not affected or only slightly reduced; part of this reduction could be due to advanced development at the higher temperature. In recovery experiments ears were heated for brief periods and then returned to 21/16�C for a few days. ADPglucose pyrophosphorylase and soluble starch synthase activities recovered in the cooler conditions but the other two enzymes generally only maintained or lost further activity. From a comparison of the activities of these enzymes with the rate of starch deposition, and by taking into account the effects of heating, it is proposed that the influence of heating on final grain dry weight is attributable to the observed reductions of soluble starch synthase activity.