Ecofisiologia de bancos de sementes recalcitrantes de Mauritia flexuosa (Arecaceae)

Abstract

Mauritia flexuosa, buriti, is a neotropical palm associated with humid environments. The species is originally from Amazonian, but occurs in regions with drier and seasonal climates. In the Brazilian Cerrado (neotropical savanna), buriti characterizes marshy ecosystems known as 'veredas', where it plays a fundamental role in the balance of the water system, in the maintenance of the local fauna and flora, and provides incomes for rural communities. The species produces recalcitrant seeds (sensitive to desiccation and low temperatures) and with dormancy, and maintains soil seed banks (SSB). The persistence of this species is threatened by the overexploitation and destruction of its habitat by local human activities and climate change. Little is known about the ecophysiology of recalcitrant seeds, and buriti is an interesting model for studies of reproductive and adaptive strategies among recalcitrante-seeded species, particularly in seasonal tropical environments. Thus, we sought to characterize SSB of M. flexuosa, determine the effect of soil attributes on SSB dynamics and evaluate the influence of permanence in SSB on the physiological quality and dormancy of the seeds. Faced with the prospect of imminent climate change, we evaluated the physiological and cytological responses of seeds associated with heat stress, and the role of temperature in controlling seed germination. Newly dispersed and SSB (established more than two years earlier) seeds were collected from natural populations in the Brazilian Cerrado. Evaluations of seeds, embryos, and young plants defined 20 indicators of SSB dynamics and were correlated with 17 physical and chemical attributes of the soil. Biometric, biomechanical, cytological and physiological evaluations were performed in newly dispersed and SSB seeds. Despite producing recalcitrant seeds, M. flexuosa forms persistent SSB in humid environments (seeds remain viable for more than two years), wich are mainly affected by attack by predator and pathogen, without the direct influence of physical-chemical soil attributes. The permanence in the SSB does not affect seed viability (they do not lose vigor), wich have high capacity for homeostatic regulation, but it contributes to the reduction in the level of morphophysiological dormancy, associated with increased sensitivity to temperature and embryonic growth. Physiological and cytological aspects of buriti seeds were evaluated after being subjected to high temperature regimes, from moderate to extreme. Germination was favored by the typical thermal regime of the rainy season in the Cerrado (30/20°C), which promoted a reduction in the mechanical resistance of the operculum and cell expansion and reserve mobilization in the embryo. Seeds have mechanisms of resistance to thermal stress, involving the control of oxidative stress, and react preventively at high temperatures (35/25, 40/30°C – associated with periods of low water availability), restricting germination. However, the seeds are sensitive to the thermal regime (45/35 ºC) of possible occurrence in future scenarios. Persistent SSB contribute to the reproductive success of this species and should be considered in the management of natural populations, as they contribute to the recovery of degraded areas. The influence of temperature on germination control of buriti suggests the adaptation of the species to environmental seasonality. Rising global temperatures could impact the SSB dynamics and the reproduction of this important neotropical palm tree.