Impact of S availability on thermotolerance acquisition
Firstly, the level of S limitation in our assay was mild enough to impact only a few nutritional (FA-related, SSPs) and physiological (ABA:GA3 ratio) quality criteria (Figure 2). By contrast, the indicators of the stress response displayed contrasting temperature modality rankings according to the S supply (Table 3). Our results pinpointed increased antioxidant responses under the 3 late heat peaks and under natural thermoperiod in plants exposed to limiting S supply (Table 3). Nevertheless, S nutrition has been reported to improve antioxidant defenses due to S-containing antioxidant compounds such as the redox-active cysteine residues of glutathione and thioredoxin (Mukwevho, Ferreira & Ayeleso 2014) and also specific phytohormones (Xia et al. 2015; Bashir et al. 2015; Hasanuzzaman et al. 2018). Therefore, it can be assumed that phytohormones such as IAA, SA and JA boosted the antioxidant responses and that their biosynthesis and/or signaling roles (reviewed in Hasanuzzaman et al. 2018) . were enhanced under S limitation. Indeed, IAA biosynthesis relies on the effectiveness of S metabolism via adequate levels of glutathione (Kopriva et al. 2016) and is improved when S is limiting (Nikiforova et al. 2003); JA signaling is dependent on S-containing metabolites (Park et al. 2013) and its synthesis is enhanced under S restriction (D’Hooghe, Escamez, Trouverie & Avice 2013); SA is also known to interact with S during SA homeostasis regulation (Baek et al. 2010). Likewise, our results showed that the enhanced antioxidant responses in S-restricting conditions, especially under the CES and the EMS, correlated with higher concentrations of IAA and JA but not SA. Altogether, these observations not only reflect the direct effects of S restriction on phytohormone synthesis but also the interactions between their specific temperature threshold for synthesis and the features of the temperature sequences (i.e. intensity, duration and period), leading to biosynthesis impairment if these factors are not optimal, regardless of the level of S.