Changes of water-relation characteristics and levels of organic cytoplasmic solutes during salinity induced transition of Mesembryanthemum crystallinum from C3-photosynthesis to crassulacean acid metabolism

Abstract

NaCl was added to the nutrient solution of 4-6-week old Mesembryanthemum crystallinum plants so that the concentration rose by 50 mM per day. Ten to fifteen days after a concentration of 400 mM was reached, pronounced diurnal oscillations of malate levels indicated that plants had changed from C₃-photosynthesis to crassulacean acid metabolism (CAM). Due to the NaCl-treatment the solute potential (Ψ_s) decreased from about -6 bar to -25 bar, and the water potential (Ψ) changed from about -5 bar to -23 bar on average. Ψ showed small diurnal oscillations both in controls and NaCl-treated plants, with an amplitude of 1 to 3 bar, the value at the end of the dark phase being less negative than that at the end of the light phase. Changes of ion levels due to the NaCl-treatment were average increases in Na⁺ and Cl^- from 10-20 to 370-470 mmol kg^-1 FW and from below 10 to 280-325 mmol kg^-1 FW, respectively, and a decrease in K⁺ from 70-80 to 25 mmol kg^-1 FW. These changes of ion levels corresponded very closely to an increase of dry weight in per cent of fresh weight observed during the NaCl-treatment (e.g. a change of 2% in one experiment), and osmotically they matched the measured change in Ψ_s (e.g. about 18-20 bar in one experiment).Most of the organic solutes analysed did not show any significant changes as a result of the NaCl-treatment. The following compounds were identified within the respective ranges of concentrations: mannitol (0.2 to 0.5 mmol kg^-1 FW), sum of quaternary ammonium compounds (60 to 140 mg kg^-1 FW), choline (0.1 to 0.4 mmol kg^-1 FW), betaine (0.3 to 0.7 mmol kg^-1 FW), hexoses (2-9 mmol kg^-1 FW), pentoses (1-5 mmol kg^-1 FW) and sucrose (2-4 mmol kg^-1 FW). The levels of proline and of total amino acids minus proline rose during the NaCl-treatment from 0.1-1 mmol kg^-1 FW to 2.5-5 mmol kg^-1 FW and from 2.5-4 mmol kg^-1 FW to 6-8 mmol kg^-1 FW, respectively.The changes of Ψ_s and Ψ, and of Na⁺- and Cl^--levels were complete, and new steady levels were attained by the time 400 mM NaCl was reached in the nutrient solution, i.e. many days before pronounced diurnal malate oscillations indicated that the change from C₃-photosynthesis to CAM had occurred. The attainment of new steady levels of proline and K⁺, however, was much slower and coincided with the onset of CAM.