Diurnal patterns of chlorophyll a fluorescence and stomatal conductance in species of two types of coastal tree vegetation in southeastern Brazil

Diurnal measurements of chlorophyll fluorescence parameters and stomatal conductance to water vapour were carried out in five woody species and a bromeliad from two coastal vegetation types in Brazil, the rock outcrop of Pao de Acucar and the sand dunes of Macae. The environmental conditions of both study sites are characterized by high temperatures and light levels. The studied species comprised C₃ plants with different degrees of stomatal closure during the day, overall daily stomatal conductance and a plant with a typical CAM pattern. Plants on Pao de Acucar exhibited only a small decline in potential quantum yield throughout the day. The non-photochemical quenching and the approximate photosynthetic electron transport rates were maximal during the peak of irradiance. In Macae, light response curves of fluorescence parameters in the CAM-true Clusia hilariana showed a clear differentiation between phases III and IV of CAM. In phase III, decarboxylation of organic acids probably maintained high internal CO₂ levels and there was only a small decrease in photochemical quenching with saturating light levels. In phase IV, the depletion of the organic acid pool and low stomatal conductances resulted in much lower levels of effective quantum yield and a substantial increase in both 1-qp and non-photochemical quenching with increasing light levels. This behaviour during phase IV in the CAM-tree C. hilariana was comparable to the performance of the C₃-Clusia C. aft parviflora at Pao de Acucar. It is seen that both the C₃ species and the CAM tree occurring in the two coastal communities effectively adjust their photochemical efficiency to environmental conditions and especially to diurnal variations of stress.