Total phenolic content and antioxidant capacity in seeds as stress response indicators
Table 3 displays phenolic concentrations and the antioxidant activity (ABST values) measured in mature seeds from podsL≥5cm. Phenolic concentrations ranged from 3.35 to 4.26 mg SA/g seed DM with no significant effect of S supply. By contrast, significant T-modality effects (p<0.05 ) were observed with different T-modality rankings according to S supply. Under HS, extreme seed phenolic concentrations were observed in mod2 (+4.0% compared to T-control) and in mod3 (-13.1% compared to T-control), whereas under LS, extreme values were observed in mod2 (+27.5%) and mod4, which reached the low levels seen in the T-control. These results indicate that under HS, increased seed phenolic concentrations were only observed when the plants were challenged with an early mild stress event at GS72, which contrasted with LS where all the T-modalities except mod4 induced higher seed phenolic concentrations. The antioxidant capacity measured with the ABTS assays revealed both S effects (p<0.05 ) and T-modality effects (p<0.01 ) (Table 3). T-modality rankings were similar to those observed for seed phenolic concentrations with extreme values for mod2 and mod3 seeds under HS, and mod2 and T-control seeds under LS. In addition, differences between HS and LS were observed for T-control (-18.2% in LS compared to HS) and mod4 (-14.9% in LS compared to HS). Overall, under both HS and LS, increased antioxidant capacities were observed in the EMS (mod2) while decreased antioxidant capacities (or levels similar to the T-control) were observed when the stress occurred later, and despite the stress’s greater intensity (mod3 and mod4). In both S conditions the CES induced strong oxidative responses but they were lower than under the EMS alone, and it seemed as though the late heat peaks were beneficial enough to reverse the early mild stress-induced trend. S limitation led to decreased antioxidant capacity in the seeds (but this was not mediated through phenolic production), thus suggesting a crucial role for the S supply in limiting the oxidative damage that was enhanced under the high-temperature sequences. Taking into account the values of all the treatments, antioxidant capacities of the seeds were positively correlated to both phenolic concentration (r=0.93,p<0.001 ) and the [raffinose+stachyose]:sucrose ratio (r=0.64, p<0.05 ) (supplemental data, Figure 1).