Condutância do mesofilo e velocidade máxima de carboxilação da Rubisco em função do horário do dia e da espessura foliar de espécies arbóreas em ambiente de sub-bosque na Amazônia Central

Abstract

In recent decades many studies on the physiological processes of plants have been conducted to predict the effects of climate change on the Amazon rainforest. In the majority of these studies intercellular [CO2] at the intercellular spaces (Ci) have been used. However, climate models could predict with greater accuracy the photosynthetic capacity of the Amazonian rainforest ecosystem should Vcmax and Jmax were calculated on the basis of chloroplast [CO2] concentrations (Cc) instead Ci. This is because of the difference in CO2 concentrations between the intercellular spaces and that at the carboxylation site in the chloroplast. This difference is imposed by mesophyll resistance. Thus, this study aims to determine the maximum carboxylation velocity of Rubisco (Vcmax) and electron transport rate (Jmax) using the values of CO2 concentrations in the chloroplast, and also to investigate whether the mesophyll conductance varies with environmental conditions and leaf characteristics. A total of five tree species in the juvenile stage and growing under understory conditions were used. The species were Myrcia paivae, Minquartia guianensis, Eschweleira bracteosa, Faramea juruana and Psychotria carthagenensis. The photosynthetic parameters were measured using an infrared gas analyzer (IRGA). The mesophyll conductance (gm) was determined using gas exchange measurements in conjunction with fluorescence data obtained using a fluorometer coupled to IRGA head. Furthermore, gm was also determined using IRGA data alone. We also measured leaf thickness, specific leaf area (AFE), leaf nitrogen and phosphorus contents, annual increment in diameter (IAD) and height (IAA), leaf area index (LAI) and the fraction of sky visible. The average gm-IRGA determined with the irga/fluorometer was 0.06 mol m-2 s-1 and 0.1 mol m-2 s-1 when it was determined using IRGA data alone. Vcmax and Jmax determined using Cc values were on average 30% and 12%, respectively, higher than the values obtained using Ci values, which highlights the importance of using Cc for Vcmax and Jmax calculations. The average annual increment in diameter (IAD) and annual increment in height (IAA) were 0.67 mm yr-1 and 75.7 mm yr-1, respectively. The effect of time of day was significant for gm, gs, light saturated photosynthesis (Amax) and electron transport rate obtained with the fluorometer (JF). However, there was no effect of time of day on Vcmax. Leaf thickness and AFE did not affect any of the parameters analyzed. Mean gm calculated using IRGA data alone was almost twice as high the values obtained with the IRGA-fluorometer method. Mesophyll conductance was affected by leaf nitrogen content and the leaf thickness, but sapling height did not affect gm values, which indicates the sensitivity of mesophyll resistance is mainly related to the anatomical characteristics of the leaves.