Pinitol, a Compatible Solute in Mesembryanthemum crystallinum L.?
M. J PAUL
The irrigation of Mesembryanthemum crystallinum L. plants with 400 mol m−3 NaCl to induce crassulacean acid metabolism (CAM) was accompanied by the accumulation of pinitol. Pinitol constituted 71% of the soluble carbohydrate fraction and 9.7% dry weight in the CAM form. Pinitol in the C3 form did not exceed 5% of the soluble carbohydrate fraction. Pinitol appeared metabolically inert: it was not readily degraded during 96 h of darkness in the CAM form or during CAM deinduction. Preparations of CAM M. crystallinum protoplasts, vacuoles and chloroplasts showed pinitol to be chloroplastic at a concentration of about 230 mol m−3 and cytosolic at about 100 mol m−3. No pinitol was detected in vacuoles. CAM leaf extracts possessed a higher myo-inositol phosphate synthesising capacity than C3 extracts, revealing greater activity in the CAM form of glucose-6-phosphate cycloaldolase, an enzyme in the pathway of pinitol synthesis.
Although pinitol accumulation and CAM induction could not be separated and appeared to be specific responses to water stress, there may not be a causal link between them. Pinitol may function as a compatible solute in the cytosol and especially the chloroplasts to counteract the presence of high concentrations of Na+ and Cl− ions in the vacuole. The accumulation of pinitol, though apparently not directly related to CAM may, like CAM, be viewed as an aspect of the adaptation of the plant to a reduction in water availability.