Phytohormone changes as indicators of temperature-induced seed dormancy
The ABA:GA3 ratio ranged from 0.28 to 0.95 with significant lower values in HS than in LS (0.47 and 0.78 on average for HS and LS respectively) (Table 3). For each given T-modality, LS values were significantly higher than HS (p<0.001 ) except for mod1. Different rankings among T-modalities according to S supply were observed with extreme values being in T-control (0.28) and in mod1 (0.74) under HS, and in mod2 (0.60) and in mod4 (0.95) under LS. A high ratio indicates increased secondary seed dormancy, which is induced by thermoinhibition. As expected under high temperature, this ratio increased but under HS, the longest stressing period (mod1) was the most negatively impacted (highest value), whereas under LS the late heat-peaks events (mod3 and mod4) induced the highest ratios. In addition, under LS, the early mild stress alleviated the negative effects of late heat peaks by lowering the ratio (lower value in mod1 than in mod3).
Seed concentrations of IAA, JA, and SA were measured to investigate their variation under the different temperature sequences and S supplies (Table 3). Consistent with the ABA:GA3 ratio, high concentrations of IAA were observed under high temperatures. The highest values were observed in mod3 under both S conditions (380.3 ng/g DW and 279.5 ng/ g DW under HS and LS respectively) and the lowest values in mod2 under HS (-68.0% of HS mod3 value) and in T-control under LS (-42.7% of LS mod3 value). By contrast to the ABA:GA3 ratio, S limitation decreased IAA concentrations except under T-control and mod2, which had levels of +19.2% and +73.7%, respectively, in LS compared to HS (Table 3). Under HS conditions, the early mild stress (EMS) alleviated the strong increase in IAA concentration observed in the late 4 heat-peak sequence, thus suggesting a priming effect. SA concentrations also increased under heat stress for both S supplies. T-modality rankings were modified according to S supply, with the highest concentrations observed in mod3 under HS (+232.4% of HS T-control seeds) and in mod1 under LS (+62.2% of LS T-control). As observed for IAA concentrations, SA concentrations decreased under LS (77.5 and 83.7 ng/g DM on average under LS and HS, respectively) except in the T-control (and to a greater extent in mod2). Similar to IAA concentrations, the EMS had an alleviating effect on the strong increase in the SA concentration under the late 4 heat-peak sequence (mod3), but this was under HS alone. In contrast to the IAA and SA concentrations, JA concentrations were not impacted by S supply, although differences between the HS and LS concentrations were observed for a given T- modality (Table 3). Indeed, while the highest and lowest values were mod1 (2.32 ng/g DM) and T-control (0.82 ng/g DM), respectively, under HS, the same was true for T-control (1.66 ng/g DM) and mod2 (1.07 ng/g DM) under LS. No alleviating effect of the early mild stress sequence was observed because mod1 seeds displayed either the highest JA concentration (in HS), or lower concentrations than in T-control (under LS) due to the S requirements for its synthesis and signaling.