Biossíntese de ácido abscísico (ABA) em folha: a principal fonte da resposta da floresta tropical à seca e às altas temperaturas

Abstract

Tropical forests play a central role in the Earth's climate system by participating in global carbon and water cycles. However, forests are sensitive to extreme drought events, which can cause reductions in net primary productivity (NPP), resulting in reduced biomass gains and increased tree mortality. One of the first responses of trees to drought and high temperature stress is the rapid biosynthesis of the isoprenoid phytohormone abscisic acid (ABA), which stimulates stomatal closure, alters conductance, and reduces water loss through transpiration. The phytohormone ABA helps trees tactically and strategically to conserve water and maintain homeostatic processes. In the current scenario of research involving tropical forests, understanding the functional physiological, biochemical and hydraulic structural responses to environmental factors are extremely important to understand the sensitivity and resistance of trees in the face of future scenarios of climate change. The approach of the experiments aimed to understand the possible variations in the physiological responses, in the biosynthesis of ABA in the leaf, the stomatal sensitivity and the hydraulic architecture in different environmental conditions. Thus, in this thesis, three experiments were conducted at three different sites. The first experiment took place in a seedling nursery. Under controlled conditions, seedlings of native species were subjected to soil water stress in Trancoso-Bahia. Overall, the induced drought was responsible for a significant reduction in physiological processes. It was observed that the species had different defense tactics in the biosynthesis of volatile organic compounds and ABA and in the regulation of leaf temperature. The second experiment took place in an approximately four-year rainfall exclusion experiment in a rainforest in Cairns, Australia. A homeostatic process was observed in physiological and biochemical responses. At the same time, there was a large variation in leaf ABA concentration among the species studied. The third experiment was carried out in a Terra firme tropical forest, in the Reserva Biológica do Rio Cuieiras in central Amazonia. In general, the wood desinity and the edaphic environment (plateau, slope and valley), influence the responses of the functional characteristics of the hydraulic and foliar architecture. As well, it was observed that leaf ABA concentrations varied between predawn and noon, between treatments of leaf water potential and leaf temperature in three genera of species occurring in the three edaphic environments. Finally, we conclude that the leaf ABA constitutes a central component of the interactions of the tree with the environment and acts on the health of tropical forests with global importance for the cycles of water and carbon.