Transient heat stress during gametophyte development in Brassica napus reduces subsequent floret fecundity

Oilseed rape (Brassica napus L.), also known as canola, is particularly sensitive to high temperatures during flowering. However, the impacts of heat stress during male and female gametophyte development, several days before anthesis and fertilisation, remain unclear. In this study we selected two cultivars, AV-Ruby and YM11, which exhibited different responses to heat stress in a previous study. Precise transient heat stress (maximum 32 ℃ day /22 ℃ night) or control (maximum 25 ℃ day /15 ℃ night) treatments were applied to evaluate the impact of heat stress during male and female gametophyte development on subsequent floret fecundity after pollination. We measured floret fecundity by pod set, number of seeds per pod and average seed size. Heat stress during male gametophyte development reduced pollen viability and germination rate, and resulted in fewer pods and seeds per pod on the main stem. However, despite these negative impacts, pollen tubes successfully traversed the style and were visible at a high percentage of ovules in both heat stress and control treatments. When heat stress was applied to the female gametophyte in the first five buds on the main stem, subsequent floret fecundity was reduced in the lower main stem, while florets in the upper main stem exhibited higher fecundity than in the control treatment. Heat stress during sporogenesis and/or gametogenesis in male and female organs across two cultivars had a lasting negative impact on subsequent floret fecundity, observed as a reduction in the number of pods and seeds per pod, but had no impact on average seed size.