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.