Cytological aspects of recalcitrance in dormant seeds of Mauritia flexuosa (Arecaceae)

We evaluated the physiological and cytological aspects of the embryos of the palm tree Mauritia flexuosa, whose seeds show a rare association of recalcitrance and dormancy. Seeds were subjected to dehydration, or stored with stabilized water contents for 420 days. Seed viability and germination, as well as the anatomy, cytochemistry and ultrastructure of the embryos were evaluated using standardized methodologies. Under initial conditions (seeds with water contents of 44.6 %), viability was as high as 94 %, although without germination. Seeds dehydrated to water contents of 20 % lost all viability, whereas 87 % of the seeds stored while hydrated remained viable and 25 % germinated. Embryonic cells showed characteristics associated with recalcitrance in other palms species, such as the presence of large vacuoles and the absence of lipidic reserves, but also had abundant protein bodies and terpenoids in their cytoplasm as well as carbohydrate and protein reserves in their vacuoles-conditions found in the embryo cells of palms having orthodox seeds. Dehydration caused invagination of the cell walls, retraction of the plasma membrane, proliferation of the endoplasmic reticulum and autophagic vacuoles, and increased the densities of vacuolar contents-culminating in the collapse of the protoplast. Stored seeds showed preserved cell structures. M. flexuosa seeds are sensitive to dehydration, but will retain viability if kept hydrated, allowing dormancy to be overcome in seed banks in the swampy soils where this species occurs. The accumulations of secondary metabolites, vacuolation and the storage of carbohydrates and proteins in the vacuole all have important roles in the modulation of recalcitrance.