THE INFLUENCE OF RECOVERY TEMPERATURE ON THE EFFECTS OF A BRIEF HEAT-SHOCK ON WHEAT .1. GRAIN-GROWTH

The responses of wheat yield to moderately high (20-32 degrees C) and very high temperatures (> 32 degrees C) have been studied separately i n the literature, but not in combination, despite the fact that this i s usually how elevated temperatures occur in the field. In this study, controlled environment conditions were used in order to examine the i nteraction of moderately high and very high temperatures during grain filling and their effect on wheat yield. Specifically, we wished to te st the hypothesis that cooler conditions would facilitate greater reco very of grain growth following a brief exposure to very high temperatu re. To this end, wheat was exposed to either 21/16 or 40/16 degrees C (day/night) from 15-19 days after anthesis and subsequently grown unde r one of three moderately high temperature regimes until maturity: 21/ 16, 27/22 or 30/25 degrees C. For all moderately high temperature trea tments, a brief 'heat shock' significantly reduced mature individual k ernel mass by 17%, on average. In the absence of 'heat shock', increas ing moderately high temperature progressively reduced mature individua l kernel mass by ca 2.5% for each 1 degrees C increase in average dail y temperature. After a 'heat shock' event, however, there was not a pr ogressive decline in mature individual kernel mass with increasing mod erately high temperature. A short period of very high temperature appl ied early in grain filling therefore reduced the response of wheat to subsequent moderately high temperatures. We conclude that the reductio n in yield caused by 'heat shock' is not alleviated by cool post-shock conditions.