Functional relationship between leaf/stem pseudobulb size and photosynthetic pathway in the Orchidaceae

Abstract

Water storage has been commonly cited as an important function of orchid pseudobulbs, and it is reasonable to assume that orchids which utilize crassulacean acid metabolism (CAM) photosynthesis have larger pseudobulbs than those of C3 orchid taxa, because these foliar (or stem) structures may reflect another type of succulent tissue in CAM plants. On the other hand, it is equally plausible that C3 orchids have larger pseudobulbs, as they lack succulent tissue, as well as the water-conservative CAM pathway. The goal of this study was to compare pseudobulb size in over 100 living orchid species growing at the Missouri Botanical Garden by measurement. Pseudobulb volumes of C3 species did not differ from those of species with CAM photosynthesis in a family-wide comparison of all sampled species, as well as in comparisons of taxa with these two photosynthetic pathways among three subtribes and within one genus. The results did not support a functional relationship between pseudobulb volume and photosynthetic pathway in the Orchidaceae. Pseudobulbs are equally important structures in C3 and CAM orchid taxa, and may function similarly in water, carbohydrates, and (or) essential elements storage in the two groups of orchids. This study lays a foundation for further research into pseudobulb evolution in orchids.