Stress-induced CAM development and the limit of adaptation potential in Mesembryanthemum crystallinum plants under extreme conditions

The role of the stress-induced water-conserving strategy of the CAM type in the survival of Mesembryanthemum crystallinum L. plants was studied under extreme salinity conditions. In soil-grown plants, NaCl induced the switchi ng from the C-3 to the CAM photosynthesis as manifested by the enhanced pho sphoenolpyruvate carboxylase activity, the CAM-specific diurnal cycle of th e organic acid content, and the daytime decline in the transpiration rate. CAM rapidly developed within a NaCl concentration range of 0.5-1 M and was still detectable at 1.5 M NaCl. However, the potential limit for M. crystal linum adaptation did not exceed 0.75 M NaCl. At higher NaCl concentrations, plants perished, with a concomitant dramatic drop of the osmotic potential and the corresponding increase in Na+ and Cl- ion concentrations, while th e relative water content declined below 70%. We conclude that the survival of M. crystallinum plants under heavy salinity was limited by irreversible changes in the water-salt balance rather than by the resistance of CAM deve lopment to salinity.