Short periods of high temperature (> 35�C) are common during the post-anthesis period in Australian wheat crops and have recently been shown to significantly reduce grain yield and quality. In view of this, 75 cultivars of wheat were screened for tolerance to 3 days of high temperature (max. 40�C). Detailed results for grain yield and quality are presented for five wheat cultivars in order to illustrate the wide range of responses to short periods of high temperature. Individual kernel mass decreased by up to 23%, depending on variety, and the gliadin : glutenin ratio altered in the range -9 to +18% in response to high temperature treatment, also depending on variety. Noodle swelling power was significantly affected by heat in two cultivars, but there was no significant change due to heat in the apparent amylose content in any variety. The marked response of several yield and quality components to a heat treatment lasting only ca 5% of the grain-filling period suggests that starch and protein synthesis do not immediately andlor fully recover from short, severe heat stress. In addition, we conclude that wheat shows considerable genetic variability in tolerance to short periods of high temperature for both grain yield and quality.

Short periods (3-5 days) of high maximum temperature (>35�C), often accompanied by drought, commonly occur during grain filling of cereals. Short periods of high temperature have been shown to reduce grain weight and baking quality in wheat, but little is known about their effects on barley. Consequently, we examined the effects of high temperature and drought, alone or combined, on grain growth for two barley cultivars, Schooner and Franklin. Treatments started 15 days after anthesis and consisted of the factorial combination of three temperatures and three water regimes. The high temperature (maximum 40�C for 6 h day-1) and drought treatments were maintained for 5 or 10 days. Drought reduced individual grain weight much more (ca 20%) than high temperature (ca 5%) for both cultivars. Franklin appeared to be more sensitive to heat stress than Schooner. The reduction in individual grain weight was greatest when both stresses were combimed (ca 30%). The reduction in mature grain weight under high temperature was due to a reduction in duration of grain growth for Schooner and to a reduction in both rate and duration of grain growth for Franklin (8-12%). The reduction in duration of grain growth was the most important cause of reduced grain weight at maturity under drought alone (12-25%) or combined with high temperature (25-33%). Nitrogen content per grain was quite high and similar for all treatments, and nitrogen percentage increased when stress was severe enough to reduce starch accumulation, confirming that starch accumulation is more sensitive to post- anthesis stress than nitrogen accumulation. We conclude that drought, particularly when combined with high temperature, is more likely than heat stress to cause large reductions in grain weight of barley under field conditions.

Short periods of very high temperature (> 35�C) are common in many of the world's wheat growing areas and can be a significant factor in reducing yield and quality of wheat. This study was designed to determine the stage at which grain growth is most sensitive to a short period of high temperature and to examine whether varietal differences in heat tolerance are expressed throughout the whole grain-filling period. Two varieties of wheat differing in heat tolerance (cvv. Egret and Oxley) were exposed to a short (5 days) period of very high temperature (40�C max. for 6 h each day) at 5-day intervals throughout grain filling, starting from 15 days after anthesis (DAA) and concluding at 50 DAA. Responses of grain dry matter accumulation and water content to high temperature were monitored throughout grain filling, and the results compared with controls maintained at 21/16�C day/night. Varietal differences in heat tolerance were expressed throughout the grain-filling period. Mature individual kernel mass was most sensitive to heat stress applied early in grain filling and became progressively less sensitive throughout grain filling, for both varieties. Reductions in mature kernel mass resulted primarily from reductions in duration rather than rate of grain filling.

The responses of 75 cultivars of wheat to a short (3 day) period of very high temperature (40°C max.) applied at either 10 or 30 days after anthesis were examined under controlled conditions. The effect of high temperature on a number of yield (grain number, individual kernel mass and N per kernel) and quality components (protein composition, apparent amylose content and noodle swelling power) is described for the sample population and for a number of varieties which were either particularly heat tolerant or sensitive.Genotypic variation of response to high temperature of the order of 20% was recorded for the majority of yield and quality components. The fact that responses of this magnitude were caused by exposure to high temperatures lasting only 5 to 6% of the grain filling period demonstrates the extent to which short periods of very high temperature may affect wheat yield and quality.

Two varieties of wheat differing in high-molecular-weight glutenin subunit composition (Oxley, 2+12, Glu-D1a and Egret, 5+10, Glu-D1d) and dough properties were examined in order to determine the manner in which differences in mature protein composition were the result of differences in accumulation of proteins during grain filling. To this end, grain samples from each cultivar were taken at 5 day intervals from 10 days after anthesis to maturity, and analysed for protein content and composition. Wheat proteins were separated and quantified as albumin/globulin, monomer, SDS-soluble polymer and SDS-insoluble polymer using size-exclusion high-performance liquid chromatography. For both cultivars, the accumulation of each class of protein was found to be highly asynchronous: synthesis of albumin/globulin was followed by that of monomer, SDS-soluble polymer and finally SDS-insoluble polymer, such that the average molecular size of grain protein increased throughout grain filling. Varietal differences in mature protein composition were almost entirely the result of a greater rate of polymer accumulation in the 2+12 than in the 5+10 genotype.