Photosynthetic properties of an orchid community in central Italy

摘要

Chlorophyllous Mediterranean orchids share a habitat endangered by climate change and land use change. These orchids are characterized by two mechanisms of carbon assimilation, being autotrophic carbon fixation through photosynthesis supplemented by heterotrophic carbon fixation from mycorrhizal fungi. We investigated whether photosynthesis may sustain autotrophy of several species of orchids co-occurring in the same habitat (the understory of a chestnut forest in the Apennines range) along a vegetative season, and how photosynthesis responds to environmental parameters in the different species. Combined analysis of gas-exchange, chlorophyll fluorescence, optical properties, chlorophylls concentration, and Ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco) activity were carried out to characterize the photosynthetic apparatus of the orchid species. Both in vivo and in vitro measurements indicated that in all orchids, in natural conditions and over the entire vegetative season (May to July), a detectable amount of carbon, typical of autotrophic shade leaves, is fixed. It is therefore suggested that these orchids are predominantly autotrophic. As an exception, however, Limodorum abortivum, a co-occurring orchid in the examined habitat, is unable to photosynthesize at rates compatible with autotrophy. At the low light intensity experienced in the understory habitat all orchids exhibited a similar quantum yield, but photosynthesis of Dactylorhiza saccifera and Cephalanthera longifolia was stimulated by light intensities higher than ambient, indicating that these species may better use sunflecks reaching the understory vegetation. Photosynthesis of all orchids, including Limodorum, positively responded to increasing CO2 concentration and temperature. Whether this will lead to a larger photosynthetic carbon fixation because of present and future climate change needs to be assessed with long-term experiments also including the impacts of climate on mychorrizal activity and host plants.