Growth stages sensitive to frost
Wheat plants can sense temperature and have the potential to adapt to seasonal cold and freezing environments during the vegetative growth stage. Nevertheless, plants become much more susceptible to low temperatures, especially to frost events, during the reproductive stages. Low temperatures above freezing (0-12°C) can also lead to significant grain yield reduction (Powell et al., 2012). Flowering time has been proven to be a crucial determinant of frost damage. Avoiding flowering window while the probability of frost events is still high has become more difficult with changing climatic patterns, which makes breeding for reproductive frost tolerance a priority for Australian wheat breeding programs (Zheng et al., 2015).
In the current study, most frost QTL overlapped with flowering QTL, and therefore, the phenology genes in the QTL regions responsible for flowering time or anthesis dates were considered as the causal genetic factors for the QTL, i.e., most frost tolerant alleles were associated with late flowering phenotype, which led a frost escape mechanism. In most developed countries, wheat is grown mainly in rainfed marginal land, requiring early flowering to avoid drought (Nazim Ud Dowla et al., 2018). Thus, escape mechanisms are of no utility when breeding for such environments.
In previous drought studies in wheat, the young microspore stage of pollen development, before anthesis, appears to be the most sensitive to mild water stress (Koonjul et al., 2004; Ji et al., 2010). It has been reported that in rice cold-induced pollen sterility at the young microspore stage had effects comparable to those of drought stress (Powell et al., 2012). Cold-tolerant lines at the microspore stage are also tolerant to drought stress. The same mechanism was observed in sorghum (Brooking, 1976, 1979).
The young microspore stage in wheat is the time when the auricle distance (AD) between flag leaf and penultimate leaf is 5-8 cm, around 10 days before anthesis (Saini & Aspinall, 1981; Ji et al., 2010). In our 2019 wheat trials at the Muresk site, the young microspore stage of Suntop was 16 days before anthesis and the AD was 6 cm. In 2018, the frost events at the Williams and Muresk locations occurred 9-18 days before the average anthesis time, which was the most vulnerable growth stage for wheat to endure low temperatures. Although the lowest temperature in Muresk remained above 0°C, plants still suffered from the sudden temperature drop. According to previous studies (Clément et al., 1996; Ji et al., 2011), cold stress induces ABA accumulation in rice anthers, which represses anther cell wall invertase activity. This in turn hampers sugar transport from the tapetum to the pollen and pollen sterility to occur. It is interesting that cold and drought stresses share the same pathways in inducing pollen sterility.