Mudanças fisiológicas associadas à pré-aclimatação em quatro espécies florestais da Amazônia

Abstract

Information on ecophysiological and silvicultural aspects of most tree species used for reforestation, conservation or reclamation of degraded lands is still limited. Solar radiation is a major factor influencing the survival, growth and development of plants. However, the nature of the morphogenic response may vary considerably among species, according to their acclimatization capability, as well as physiological changes observed in forest species under water stress. The overall objective was to obtain information about how Bertholletia excelsa, Carapa guianensis, Swietenia macrophylla and Hymenaea courbaril respond to changes in the physical environment during the acclimatization process. Gas exchanges, specific leaf area (AFE), chlorophyll fluorescence (the Fv/Fm ratio), survival rate, the monthly increase in height and diameter, leaf nutrient content and dry matter production were assessed in six independent pre-acclimation treatments to full sunlight (TPS) (0, 90, 180, 360, 540 and 720 min) followed by a whole-day exposure to full sunlight during 120 days. Every full sun acclimation treatment (30+120 days) was assessed at two water regimes: seedlings kept at field capacity (hereinafter termed without water stress, SE) and seedlings subjected to mild water restriction (herein termed with moderate water stress, CE). The mean highest maximum photosynthesis (Amax) was found in C. guianensis and S. macrophylla under CE conditions. The maximum carboxylation velocity of Rubisco (Vcmax25) and the maximum electron transport rate (Jmax25) were higher under outdoor conditions than under shading, in all studied species. AFE values were similar in all TPS, as in TH in all species studied, however, in H. courbaril the highest values were found in seedlings under SE conditions. Fv/Fm values decreased after the plants were transferred to full sunlight, particularly in B. excelsa under CE and H. courbaril. The survival rate was maximum (100%) for C. guianensis than in the other species. For H. courbaril, a TPS of 720 min, under CE condition was inadequate since in these treatments the survival rate was 0%. The rates of monthly increase in height and diameter were higher in S. macrophylla than in other species. It was also observed that S. macrophylla had reduced growth when exposed to low light intensity (TPS = 0). The mean leaf nitrogen content across species was higher in the CE treatment than in the SE one. With regard to phosphorus, their values were similar over species for TPS. The same was true for potassium. Calcium and magnesium were higher in S. macrophylla and C. guianensis than in B. excelsa or H. courbaril. Total biomass gain was greater in S. macrophylla and C. guianensis than in the others. For species the largest biomass accumulation occurred in the stem under SE conditions (143.4 g plant-1) and leaf (27.2 g plant-1) respectively. In general, the results obtained in this study indicate that C. guianensis and S. macrophylla have potential to be tested in field experiments on a wide range of environmental conditions because of their high capability to endure stressful conditions caused by sudden exposure to full sunlight and water stress.