Ecophysiological and floristic implications of sex expression in the dioecious neotropical CAM tree Clusia hilariana Schltdl.

Clusia hilariana Schlechtendal is a dioecious tree performing crassulacean acid metabolism (CAM). It grows in the sandy coastal restinga of Brazil. As pioneers and 'nurse plants' individual trees become surrounded by other ve getation forming isolated vegetation islands on the sand plain. The female plants produce large and heavy fleshy fruits suggesting a particular demand for resources. Frugivores may introduce nutrients (faeces) and diaspores i nto the stands of female plants. Thus, the hypothesis was advanced that sex expression in C. hitariana has important general consequences for the phys iology of the trees themselves and also determines aspects of the vegetatio n around them. Field work in the restinga of Macae (Rio de Janeiro State, B razil) showed that morphological leaf characteristics, the expression of CA M and the species diversity and species composition of their respective Veg etation islands were similar for both sexes. The female plants had somewhat higher chlorophyll levels. Differences in effective quantum yield of photo system Il (PS II) and non-photochemical quenching of chlorophyll a fluoresc ence between female and male plants were very small. Female plants were a l ittle less subject to acute photoinhibition at noon. Maximum fluorescence ( Fm), variable fluorescence (Fv) and minimum fluorescence (F-0) were signifi cantly lower in female plants. However, maximum quantum yield ratio of male to female plants as well as fluorescence yield ratio and minimum fluoresce nce ratio of male to female plants related to chlorophyll content were all close to unity. Therefore, although there may be some fine-tuned difference s in the light reactions of photosynthesis, PS II of male and female plants are not different. Hence, the hypothesis of possible important consequence s of sex determination for expression of morphological and physiological tr aits of the plants and for the composition of the 'nursed' vegetation under neath them must be rejected.