Physiological and cytological responses to thermal stress in recalcitrant seeds of Mauritia flexuosa (Arecaceae).

Main conclusion: Recalcitrant seeds of Mauritia flexuosa show evidence of thermal stress tolerance, while germination is stimulated by a thermal regime typical of the rainy season. Recalcitrant seeds (sensitive to desiccation) are highly vulnerable to environmental changes, but little is known about their resilience to thermal stress, which will be particularly important in a scenario of climate change. Mauritia flexuosa L. f. is a neotropical palm of ecological and social importance that occurs in flooded environments. The species produces seeds with association of recalcitrance and dormancy (germination blockage) and has the ability to maintain persistent soil seed banks in ecosystems exposed to the markedly seasonal climate of the Cerrado biome. Thermal regimes (30/20, 35/25, 40/30, and 45/35 °C) were employed to investigate the role of temperature on seed physiology (viability, germination, respiratory activity, micropylar biomechanics, oxidative stress, membrane functionality) and cytology (micromorphometry, ultrastructure, and compound dynamics). Germination was stimulated by the 30/20 °C regime (typical of the rainy season), with reduction of the resistance of the tissues adjacent to the embryo, reserve mobilization, and cell expansion. The 40/30 and 35/25 °C regimes contributed to maintaining and increasing dormancy intensity, respectively. The 45/35 °C regime resulted in seed death due to reserve depletion and embryonic cell collapse. Tolerance mechanisms to moderate thermal stress include efficient antioxidant systems, cell homeostasis, and germination restriction. The capacity for differential responses to thermal regimes is important to the establishment of banks of recalcitrant M. flexuosa seeds and constitutes a factor in the adaptation of that species to the Cerrado seasonality. Nonetheless, rising global temperatures due to climate change and increasing local impacts pose risks to the species' reproductive success.