Forage rape (Brassica napus L) seed quality: Impact of heat stress in the field during seed development

Climate change is predicted to increase the frequency of heat stress events during seed production. In two consecutive seasons, plants within a forage rape (Brassica napus L.) seed crop were covered with plastic sheeting to increase ambient air temperature for the period between seed filling (80% seed moisture content) and seed physiological maturity (50% seed moisture content) = T1, between physiological maturity and harvest (14% seed moisture content) = T2, and between 80% seed moisture content and harvest = T3. This resulted in 47 and 102 h when air temperature exceeded 25 °C for T1, 121 and 173 h for T2, and 145 and 228 h for T3 compared with 9 and 30 h for the uncovered control in each season respectively. Hourly thermal time (T base = 25 °C) was calculated for each treatment. Both T1 and T2 resulted in small (2-10%) reductions in germination in each season, but when season data were meaned and analysed, only T2 and T3 significantly reduced germination. Seed mass (as measured by thousand seed weight) was significantly reduced by T1, but not T2, while seed vigour, as assessed by the accelerated ageing and conductivity tests, was significantly reduced by all three treatments, with T3 > T2 > T1. The number of hours that temperature exceeded 25 °C was negatively correlated with germination and seed vigour, but not seed mass. Approximately 100 h of temperature exceeding 25 °C, or an hourly thermal time of 300 °C h (Tb = 25 °C) were required to reduce the vigour status of the seed lot. Crop management strategies to avoid heat stress during seed development are unlikely to succeed in this environment.