Priming effects are dependent on the timing of the stress events
Our initial objective was to pinpoint any beneficial effect from a mild stress event that occurs prior to later repeated intense heat peaks, i.e. whether a beneficial heat stress memory is generated due to induced thermo-sensitization. Our work highlights that the effects of the early stress event combined with late heat peaks are not straightforward and are determined by (i) whether the expected priming event occurs concomitantly with biosynthesis of seed storage compounds, and (ii) whether the temperature is adequate to enhance the targeted process. Figure 2 presented the T-modality effects on yield components of the SESs i.e. early mild, late heat peaks (3 or 4) and the CES according to S supply, and nutritional and physiological quality criteria measured in maturing seeds. Most T-modality effects were S supply specific and for some of them opposite effects were observed (i.e. the ABA:GA3 ratio irrespective of the T-modality, the [raffinose+stachyose]:sucrose ratio in the 3 late heat peaks) thus highlighting the complexity of interactions between stress temperature features and S availability. The CES decreased seed C, FA and PUFA concentrations under HS. These observations suggest that the duration of the heat stress sequence (i.e. the combined sequence having the more number of days above natural temperature over the 17 days of treatment) was the most detrimental feature for maintaining C concentration and hence FA synthesis since the CES induced a greater decrease than the early mild stress or the late heat peaks alone. These results correlate with earlier studies (Aksouhet al. 2001; Aksouh-Harradj et al. 2006) of intense temperature exposure during seed maturation but contrast with others that highlighted increased total FA contents under a long mild stressing event (Brunel-Muguet et al. 2015). Disruption of FA accumulation in OSR has been shown under intense heat stress across a range of temperatures similar to those imposed under our heat-peak conditions, and this was a consequence of photosynthesis inhibition and downregulation of BnWRI1 gene expression, which is a key regulator of FA biosynthesis (Ruuska, Girke, Benning & Ohlrogge 2002; Huang et al. 2019). Our results also revealed higher total FA, SFA, and UFA concentrations under LS, but only in specific high-temperature sequences (Figure 2), which contrasted with prior results (Brunel-Muguet et al. 2015). These differences can be explained in the current work by the lower S supply and the different approaches to seed sampling and analysis (on maturing seeds exposed to the heat sequence). The negative effects of high temperature on total FAs and UFAs also indicated that (i) FA biosynthesis was concomitant with the 17-day high-temperature treatment starting approximately 3 weeks after the onset of flowering (Figure 1b), with FA levels rising during the late storage stage (i.e. 20 after pollination (Niu et al. 2009; Borisjuk et al.2013)) and (ii) the later the heat peaks, the lower the effects. The increased ω6:ω3 ratio under the CES and late heat peaks is also detrimental to oil quality, and is known to result from impaired functioning of the oleic and linoleic desaturases (Aksouh-Harradjet al. 2006).
As usually observed, the accumulations of lipids and proteins (herein linked to seed N concentration) are negatively correlated because they are competitive processes that have spatial and temporal overlaps (Grami & Stefansson 1977; Borisjuk et al. 2013). Therefore, while the CES decreased the total FA concentration, it increased the seed N concentration under both S supplies. However no effects of the late heat peaks were observed under HS, suggesting that the positive increase observed in the CES could not be interpreted as a priming effect that helps to overcome later negative effects. This supports the hypothesis of compensatory effects between the pods that were maturing and the ones that were still developing during the 17-day temperature sequence. Indeed, seed N accumulation was likely to be impaired in filling seeds, and this resulted in N reallocation towards filled and maturing seeds. In contrast, while the SSP concentrations were not increased under high temperature, they increased under LS as previously observed (Brunel-Muguet et al. 2015) except in the early mild stress.
Ultimately, seed physiological characteristics were also highly dependent on the features of the high temperature sequence (duration, intensity), which shapes the dynamics of biosynthesis of storage compounds involved in seed dormancy and stress tolerance. Seed storability and desiccation tolerance can be evaluated by two measured proxies i.e. membrane conductivity and the [raffinose+stachyose]:sucrose ratio. Our results highlighted strong effects of high temperature on seed conductivity, indicating degradation of membrane permeability. However, the [raffinose+stachyose]:sucrose ratio remained unchanged in the CES and in the EMS, which instead suggests acquisition of desiccation tolerance under both S supplies. The beneficial effects of the EMS on desiccation tolerance were maintained throughout the late heat peaks, indicating a more pronounced priming effect under HS. Several phytohormones control seed dormancy, as indicated by the ABA:GA3 ratio, which has been shown to vary according to stresses imposed on the parent plant during seed development (Brunel-Muguet et al. 2015). A high ABA:GA3 ratio indicates higher seed dormancy, which is expected for efficient seed storage before favorable environmental conditions permit the seeds to germinate. The EMS and the late heat peaks modified the ABA:GA3 ratio but to a greater extent in the late 4 heat peaks. These observations also indicate that the timing of the stress exposure is determining i.e. if the stressing event occurs after a given seed compound has been synthesized, and the stressing temperature does not lead to degradation of the compound, no temperature effect will be observed on the synthesized compound. This rule can be extended to an expected priming event, which can occur too early before or too late after the compound’s biosynthesis. Therefore, our results raised questions about what is the optimal temperature for the biosynthesis and maturation processes and how synchronization between these processes and the temperature event occurs so that the expected effects (negative, positive or beneficial priming) can be observed.