Mecanismos envolvidos na queda da capacidade fotossintética com o avanço da idade foliar em indivíduos arbóreos na Amazônia Central

Abstract

The renewal of the leaf stock with higher photosynthetic capacity during the dry season in Central Amazon (green-up) has been highlighted as one of the factors that determine the seasonality of the photosynthetic capacity (PC) of the ecosystem. The decline of PC of leaves at an advanced stage of development has already been studied for a variety of tropical and temperate species, however there are still many uncertainties as to the physiological mechanisms that trigger the age-dependent reduction of PC. Thus, the main goals of the study were describe the leaf age canopy dynamics and investigate the effects of leaf-ageing on the PC as well as the possible physiological mechanisms involved. For these purposes, monthly leaf demography was carried out in order to obtain chronological age of leaves on selected branches, and leaf gas exchange measurements of photosynthesis and stomatal conductance curves were done by different approaches (one-point photosynthesis, repeated measurements, and chronosequence) by bi-monthly field campaigns in Central Amazon Terra Firme forest plots. In addition, destructive leaf analysis were done to obtain leaf dry mass per unit area, SPAD index, carbon and nitrogen concentrations, isotopes and non-structural carbohydrates. A weak relationship between photosynthetic capacity (Vcmax) and leaf age at community level was found, and for the individual level only two out of nine trees showed a significant negative regression slopes. It was not found a mechanism common to all trees, among all parameters tested, and regardless of the scale of observation, that explain better the PC with leaf-ageing. However, stomatal control and sucrose accumulation were the physiological mechanisms that better explained the decrease of Vcmax with age for the community studied and for both two individual trees, respectively. The interaction between leaf age canopy composition and the photosynthetic capacity changes defined the seasonality of this specific community, in agreement with the estimates found by remote sensing techonologies of flux towers and satellite vegetation indices.