Global rise in temperature puts the development and yield of many important staple crops at substantial risk, threatening food security. An agronomically important crop, Brassica napus, is the world's second most widely produced vegetable oil crop. We characterised and studied the vegetative and reproductive development of the three spring cultivars DH12075, Topas, and Westar grown at prolonged high temperatures(32°C). At 32°C, B. napus undergoes thermomorphogenesis based on the analysis of the plant's vegetative growth, seed yield, and embryo development. Prolonged growth at elevated temperatures negatively impacts the fertilisation rate, embryo development, seed viability, and yield and accelerates embryo development. During seed maturation, plants developed under 32°C undergo seed coat rupture (SCR) with or without embryo protrusion. The occurrence of SCR in the cultivar Topas was around fifty per cent, severely impacting the yield quality. Further examination of Topas seeds using hormonal, transcriptomic and seed size studies strengthens the assumption that dormancy-related factors do not regulate SCR. Instead, it is controlled by the biophysical aspects of the communication between the seed coat and accelerated embryo development at the seed maturation stage, which is influenced by high temperature. This hypothesis was backed by the Enrichment of demethylesterified pectin subunits in the seed coat cell walls of seeds developed at high temperatures. Such a modification could affect the strength of the developing seed coat. The SCR phenotype in Topas grown at high temperatures is caused by the accelerated embryo development, which, when coupled with the heat stressdependent reduction in the thickness of seed coat layers and untimely cell wall modification, is found to be the cause of the SCR phenotype in Topas grown at high temperatures. Our research provides valuable information for future plant breeders to account for the effects of mechanical properties during seed development, which is misguided by the growth of B. napus at elevated temperatures.