Ecofisiologia dos embriões e plântulas de Mauritia flexuosa (Arecaceae) e o estresse induzido pela seca

Abstract

ABSTRACT The neotropical palm Mauritia flexuosa (buriti or aguaje) is of Amazonian origin; however, its range has expanded to the seasonal climate of the Cerrado, in flooded phytophysiognomies known as veredas. The species produces seeds that exhibit a combination of recalcitrance and dormancy. Despite their intolerance to desiccation, the seeds can form persistent banks (viable for more than two years) in the flooded microenvironments of veredas, and maintain a seedling bank in more drained soils. The buriti palm has great ecological and socioeconomic importance, as well as potential for agroindustrial use, but it is threatened by climate change and anthropogenic actions that disrupt the water stability of its habitats, especially in the Cerrado region. As a result, functional disorders related to seed maintenance in the soil and seedling establishment, the most vulnerable phase of the plant life cycle, may compromise the perpetuation of the species. Since the mechanisms involved in the persistence of recalcitrant seed banks are still not well understood, Chapter 1 of this thesis aimed to evaluate the response of M. flexuosa embryos to water deficit and saturation stresses. Chapter 2 aimed to assess the effect of drought on the dynamics of seed reserves and post-germinative development in the species. Embryos of M. flexuosa with typical water content for dispersal or subjected to hydration were exposed to moderate and severe water potentials (Ψw = 1.5 MPa and Ψw = 2.1 MPa), as well as water saturation (Ψw = 0 MPa). Anatomical, histochemical, and ultrastructural evaluations were conducted on the embryos after 24 hours. Estimates of membrane integrity, endo-β-mannanase activity, and indicators of oxidative stress (H2O2 and MDA contents, and the activities of CAT, SOD, and APX) were also assessed. The structure of the endosperm contributes to the maintenance of the embryo's hydration, while abundant reserves of mucilage enhance resilience to desiccation. Post-dispersal hydration makes the embryos less vulnerable to oxidative stress, which is due to the non-enzymatic antioxidant system. Both moderate water stress and post-dispersal water absorption induce an increase in metabolism and the mobilization of reserves, indicating that hydration/desiccation cycles may favor the overcoming of dormancy. The embryos of M. flexuosa show resilience to water deficit, which is crucial for seed persistence in the soil in seasonal environments. However, successful germination depends on high hydration, which prevents structural and physiological damage. Seedlings had their water potential determined in the aerial part and haustorium (cotyledon blade), and were cultivated under moderate (Ψw = -0.9 MPa) and severe (Ψw = -2.1 MPa) stress for 30 days. For the characterization of the mobilization of seed reserves and oxidative stress, structural and physiological assessments were conducted, similar to those of the embryos. The haustorium plays a central role in the resilience of M. flexuosa seedlings to dehydration, as it adjusts its water potential in order to maintain the absorption of water and nutrients, even under conditions of low water availability. Under drought conditions, the haustorium experiences restricted growth and there is an accumulation of partially degraded reserves in the digestion zone, which favors water retention in the seed. The activity of endo-β-mannanase increases in the endosperm (rich in mannan) adjacent to the embryo under moderate water stress, which favors water retention through the recruitment of osmolytes. Also, under moderate stress, root development is prioritized over the aerial part, contributing to the maintenance of a favorable water status. The seedlings exhibit enzymatic and non-enzymatic antioxidant systems responsive to water stress, both in the aerial part and in the haustorium, which assist in promoting oxidative homeostasis. The resilience of M. flexuosa seedlings to water stress involves a sophisticated adjustment in the dynamics of reserves and the action of an efficient antioxidant system, contributing to their reproductive success and adaptation to seasonal environments.