GRAIN-GROWTH AND MALTING QUALITY OF BARLEY .2. EFFECTS OF TEMPERATUREREGIME BEFORE HEAT-STRESS

Short periods of very high maximum temperature (>35 degrees C) during grain filling appear to reduce grain yield and quality in barley. Tole rance of grain yield and quality to heat stress may be increased when acclimation to high temperature occurs. Two experiments were performed to test the hypothesis that a gradual (or stepped) increase to very h igh temperature reduces the impact of that stress on grain growth and quality of barley, compared with a sudden increase over the same tempe rature range. Plants experiencing either a sudden or a gradual increas e did not exhibit any differences in grain weight or malting quality, but increasing the temperature in 2 steps (so that plants were exposed to 30 or 34 degrees C for 2 h before a 40 degrees C heat stress) appe ared to have produced acclimation, since the reduction in grain weight under the 2-step treatment was about half that of either a sudden or gradual increase in temperature. Heat stress altered grain composition in various ways. The reduction in final grain weight was strongly and linearly related to the reduction in starch content. Grain beta-gluca n was 4.5+/-0.5% across treatments and experiments and was significant ly reduced in the glasshouse but not in the phytotron experiment. Howe ver, beta-glucan degradation was similar between treatments in both ex periments. Grain nitrogen concentration was very high and similar betw een treatments. Consequently, diastatic power was high and there was a trend towards a reduction under heat stress. Free amino nitrogen was higher under heat stress, indicating a higher protein modification tha n in the controls. Malt extract was significantly reduced by heat stre ss in the glasshouse experiment.