Effects of heat stress on wheat and possible selection tools for use in breeding for tolerance.

Major physiological mechanisms affected by high temperatures in wheat at the plant and crop levels and possible selection tools for warm environments are examined. Increasing maximum soil temperature (5-cm depth) during the germination to emergence phase (E) significantly decreased crop establishment. Crop emergence to double ridges (GSl) and double ridges to anthesis (GS2) decreased in their real time duration with minor changes in degree days by increasing growing temperature. Growth decreased proportionally to mean temperature. Spike number per unit area was affected, in part, as a reflection of poor crop establishment. Grains per spike and spikes per unit area decreased significantly such that the total number of kernels per unit area decreased at about 4(%) for each degree centigrade increase in mean temperature over the range of 12.2 to 27.5 degree centigrade. High temperature tolerance of physiological processes during GS2 could influence the carbon balance of plants when the spike is growing and affect yield components. Evidence is presented indicating that, through field selection, the photosynthesis per unit leaf area at high temperature may be indirectly improved. Lower kernel weight was observed as a result of heat stress during GS3. Evidence of a relatively constant thermal time for GSl and GS2 across heat environments is presented, supporting the hypothesis that potential growth at high temperatures could be realized as long as radiation, water, and nutrients are acquired by the plants at a sufficient rate to keep up with the temperature induced decreased clock time. This implies optimizing agronomy throughout the life of the crop to approach potential growth and increase grain yield. Based on the physiological responses of the wheat crop to heat stress, traits were identified that may be used in genotype evaluation for increased grain yield in high temperature environments. Screening tools and appropriate screening environments are suggested.