DIURNAL RHYTHM IN CITRIC-ACID CONTENT PRECEDED THE ONSET OF NIGHTTIMEMALIC-ACID ACCUMULATION DURING METABOLIC CHANGES FROM C-3 TO CAM IN SALT-STRESSED PLANTS OF MESEMBRYANTHEMUM-CRYSTALLINUM

Salt stress did not only enhance the diurnal changes in malic acid con tent (Delta-malate) but also led to a pronounced fluctuation in citric content (Delta-citrate) in expanded leaves of Mesembryanthemum crysta llinum (Mesembryanthemaceae). The mean maximum Delta-citrate of 13 mol m(-3) reached almost 50% of the amplitude of malic acid. Both process es were highly dependent upon plant age. Watering the plants with salt solution had no effect on acid fluctuation until a certain stage of d evelopment was reached. However, the younger the plants were at the on set of stress, the longer was the span of time that a significant Delt a-citrate preceded a Delta-malate. Baseline levels of citrate increase d accumulation of citric acid was minor and only detectable in mature leaves. Removal of salt stress did not eliminate a well-established di urnal rhythm of the citric acid.