Volatile Organic Compounds Emitted by C₃ or CAM-Induced Mesembryanthemum crystallinum Plants

Crassulacean acid metabolism (CAM) is an adaptation of certain plants, to arid and water-stressed environments. The expression of the CAM cycle may be strongly modulated by developmental and environmental factors. Mesembryanthemum crystallinum is a well-known facultative halophyte, that can shift its photosynthetic carbon fixation pathway from C₃ to CAM under salinity and other abiotic stress factors. However, until now there has been no study about the volatile organic compounds (VOCs) that are emitted by M. crystallinum in its various life cycles, C₃, and CAM. Plants emit a part of the photosynthetically assimilated carbon into the atmosphere in the form of VOCs. Under normal conditions, isoprenoids (isoprene and monoterpenes) are the most abundant VOCs though methanol, acetaldehyde and C-6 compounds are also emitted in great quantities. Under stress conditions, the emission of these compounds generally is altered. The study of how emissions change depending of stress conditions has become a useful “in vivo” indicator of plant vitality and of the plant response to abiotic stresses. Within this work, we aimed to analyse the VOCs emitted from C₃ or CAM-induced M. crystallinum in order to evaluate the possible role that VOCs may have in the C₃/CAM transition and consequently in the adaptation of this plant to salinity. Results showed that M. crystallinum emits different kind of VOCs: aldehydes, hydrocarbons, ketones, alcohols and terpenoids. VOC emissions were generally higher in plants representing C₃, with only few exceptions as butanone, octanal and ethyl-hexanol that were similar in the III phase of CAM and C₃ plants. Regarding the emission of terpenoids, we could observe that whereas plants in the C₃ mode of photosynthesis emitted three types of monoterpenes: a-pinene, carene and limonene, plants in CAM state did not emit any terpenoid compound.