Modern hexaploid wheat differs from diploid and tetraploid ancestors in the importance of stress tolerance versus stress avoidance

In recent decades, global climate has been changing rapidly. It is predicted that the average global temperature would climb sharply in this century, with projected rising air temperatures of 1.1 - 6.4℃. In plant breeding, growing spring cereal crops under greenhouse conditions is often used to speed up generation advance in winter times. In this situation, the crop often receives high temperatures, but suffers from relatively weak radiation. It is of critical importance to understand how high temperature and weak radiation stress affects growth, development and biomass allocation, and what mechanisms in wheat genotypes differing in ploidy levels have acquired to adapt to such adverse environments, during natural and human selection processes. We hypothesized that adaptive strategy may be another major factor influencing yield formation in different wheat genotypes. To test this hypothesis, a study under controlled environmental conditions was designed to: 1) investigate physiological and biochemical responses of three ploidy levels of wheat genotypes in response to a combined high temperature and weak radiation stress, and 2) identify adaptive strategies of various wheat genotypes to cope with adverse environmental conditions. The aim of our study was to identify the link between adaptive mechanisms and yield formation. This should provide researchers and breeders with key insights into the adaptive strategies of crop plants to cope with the anticipated increase in abiotic stresses with future climate change.

In this study, wheat genotypes of different ploidy levels were compared under high temperature and weak radiation. We found that diploid and tetraploid genotypes showed higher avoidance ability against high temperature and weak radiation stress. However, this adaptation strategy resulted in a greater loss in harvestable yield under stress. In contrast, modern hexaploid wheat has acquired better tolerance through effective osmoregulation, antioxidant systems, leaf water conservation and photosynthetic capacity. Our results indicate a shift from stress avoidance to stress tolerance in wheat evolution from diploid, through tetraploid to the modern hexaploid wheat varieties. Avoidance and tolerance strategies influence differently the formation of yield. Reinforcing stress tolerance may result in improved yield potential. This strategy maintains a clear advantage when compared to the stress avoidance strategy in high temperature and weak radiation environments.