Repeated heat stress events during the reproductive phase impact the dynamic development of seeds in Brassica napus L.

Many studies pointed out the deleterious effects of high temperatures events during the crop reproductive phase on seed yield and quality. However, plant responses to repeated stressing events remain poorly understood, while the increased frequency of extreme abiotic constraints, such as spring and summer heat waves, has been proven as one feature of the on-going and future climate change. The responses of oilseed rape plants subjected to three heat stress sequences that differed in the intensity, the timing of application, the duration and the frequency of the high temperature events were investigated throughout the seed development and maturation phases under controlled conditions. Seed yield and components were measured in three different harvest dates. Biochemical and histological analyses of seeds were carried out in order to monitor the evolution of the main storage compounds (fatty acids, proteins, sugars) involved in seed nutritional quality. Although the effects of heat stress were not significant on total yield, differences in seed number and weight highlighted the strong compensation capacity in indeterminate growth species. Heat stress induced significant decreases and increases in seed oil and protein content respectively, to different extent according to the age of the pods. Soluble sugars concentrations were impacted by heat during seed development, but not when the seeds reached physiological maturity, thus indicating compensatory mechanisms that set up after the stress exposure. Our results led to conclude that the effects of repeated heat stresses on seed yield and quality were tightly related to (i) the optimal temperature of a given compound biosynthesis process, and (ii) the synchrony between the temperature event and the period of biosynthesis of the targeted storage compound. These results highlight the complexity to design thermo-sensitizing protocols to maintain or even improve the various seed quality related criteria, especially in species with indeterminate growth.