Phytohormones, thermotolerance and thermoinhibition
Phytohormones such as IAA, JA and SA have been demonstrated to interact with ROS during stress tolerance as stress-signaling cues (Clarke, Mur, Wood & Scott 2004; Sharma & Laxmi 2016; Bielach, Hrtyan & Tognetti 2017; Balfagón et al. 2019; Prerostova et al. 2020) and have also been linked to stress thermotolerance (Clarke et al.2009; Shu et al. 2016; Tuan et al. 2019) because increases in the levels of these compounds under high temperature are associated with greater stress-induced dormancy and thermoinhibition (Toh et al. 2012). In our study, we observed drastic increases in IAA, JA, and SA in mature seeds collected from pods exposed to heat stress at the onset of maturation under non-limiting S conditions (Table 3). The 4 late heat peaks induced the greatest increase in IAA and SA, while the CES (which had the longest accumulated days of above-natural temperature) induced the greatest changes in the ABA:GA3 ratio and JA. Therefore, the early mild stress allowed the negative effect of the late heat peaks on IAA and SA concentrations to be alleviated under the non-limiting S supply, thus leading to lower thermoinhibition. In addition, the late 3 heat peaks sequence had less effect than the late 4 heat peaks on IAA and SA, which also suggests that the shorter the period of intense stressing, the lower the impact on the seed concentration.
JA and SA were also reported to confer basal thermotolerance, which is defined as the ability to survive sub- or supra-optimal temperatures (Clarke et al. 2009; Bokszczanin et al. 2013). However, while JA was shown to be involved in acclimation to heat stress (Balfagón et al. 2019), SA was not essential for acquired thermotolerance (Clarke et al. 2004). These results suggest that lower levels of SA and higher JA levels could be expected under priming conditions than under non-primed conditions. Consistently, our results highlighted lower concentrations of SA and higher JA concentrations in the CES than in the late 4 heat peaks, which suggests that an early prior mild stress promoted acquired thermotolerance under the non-limiting S supply.