4.3 Strategies for maintaining high yield under heat stress
Genetic improvement (Frova & Sari-Gorla, 1994; Bohnert, Gong, Li, & Ma, 2006; Muhammad, Muhammad, Akram, Muhammad, & Nisar, 2016) and agronomic management (Ortiz-Monasterio, Dhillon, & Fischer, 1994; Bannayan, Eyshi Rezaei, & Hoogenboom, 2013; Sadras, Vadez, Purushothaman, Lake, & Marrou, 2015) are considered feasible strategies in coping with heat stress, which were both confirmed in the present study. The relative contributions of inbred lines on kernel number, kernel weight, and yield were all smaller than that of sowing date (Figure 10), suggesting that heat tolerance of maize genotypes is limited when subjected to high temperature around flowering and early grain filling. The higher yield reductions in this study were attributed to genotypes and extreme heat waves. Maize inbred lines used were sensitive to high temperature than hybrids (Echarte & Tollenaar, 2006). Heat waves that were over 38℃ frequently occurred during flowering in the experimental region. In this heating condition, kernel number can be significantly reduced even in a single day (Prasad, Bheemanahalli, & Jagadish, 2017), which can be explained by the close correlation between daily maximum temperature and kernel number per ear (Figure 2). These results also imply that it is difficult to breed of and select for heat tolerant maize genotypes in practice. Hence, selecting and improving heat tolerance of maize germplasms is becoming more urgent with warming climate. Heat tolerant inbred lines selected in the present study can be regarded as germplasm resource. As compared with genotype, sowing date was a more effective strategy for coping with heat stress around flowering (Sadras et al., 2015). A proper sowing date that can avoid or reduce coincidence between occurrence of high temperature and sensitive growth period (i.e., flowering and early grain filling) is recommended. However, these two strategies combined together contributed to less than 80% of kernel number and yield (Figure 10), reflecting the limitations of inbred line and sowing date on alleviating high temperature effects. The results also suggested that there were other factors (e.g., precipitation and solar radiation) affecting yield formation for field-grown maize (Figure 2), which increased the difficulty in determining the detailed effects of high temperature and the relevant strategies. Taken together, heat tolerant genotype and proper sowing date should be combined with other alternative strategies to alleviate the negative effects of high temperature around flowering and grain filling in maize.