Can Stress-Induced CAM Provide for Performing the Developmental Program in Mesembryanthemum crystallinum Plants under Long-Term Salinity?

The development of CAM-type photosynthesis is one of the adaptation mechanisms to severe water deficit. It provides plants with carbon dioxide and permits efficient water spending under extreme environments. In common ice plants, a complete switch from C3 to CAM photosynthesis was observed on the seventh day of salinity (0.5 M NaCl). The indices characterizing this switch were: (1) induction of phosphoenolpyruvate carboxylase; (2) diurnal changes in the organic acid content, which are characteristic of CAM plants, and (3) suppression of transpiration during the daytime. A decrease in the osmotic potential (ψπ) of the leaf sap, which occurred on the second day of salinity, preceded these changes. After long-term salinity stress (four–five weeks), ψπ attained extremely low values (–4.67 MPa), which made possible the water uptake by the root system. The restoration of the ψπ balance between cell compartments resulted from the accumulation of compatible solutes in the cytoplasm, proline primarily, which possesses osmoregulatory and stress-protective properties. This means that a complex of adaptive mechanisms is required for the realization of the common ice developmental program under salinity. These mechanisms maintained plant capacity to uptake water and permitted its efficient utilization. They triggered the development of stress-induced CAM-type photosynthesis, maintained the low osmotic potential in the cell sap, regulated the composition of macromolecules in the cell microenvironment, provided for water storage in tissues, and reduced the time of plant development. A comparison between the time-courses of CAM development and a decrease in the transpiration rate permitted us to suggest that a combination of low ψπ and CO2 in the leaf cells could serve as a signal for the induction of CAM-dependent gene expression in terrestrial plants.