Diurnal Rhythm in Citric Acid Content Preceded the Onset of Nighttime Malic Acid Accumulation during Metabolic Changes from C 3 to CAM in Salt-stressed Plants of Mesembryanthemum crystallinum

Abstract

Salt stress did not only enhance the diurnal changes in malic acid content (Δ-malate) but also led to a pronounced diurnal fluctuation in citric acid content (Δ-citrate) in expanded leaves of Mesembryanthemum crystallinum (Mesembryanthemaceae). The mean maximum Δ-citrate of 13 mol m-3 reached almost 50% of the amplitude of malic acid. Both processes were highly dependent upon plant age. Watering the plants with salt solution had no effect on acid fluctuation until a certain stage of development was reached. However, the younger the plants were at the onset of stress, the longer was the span of time that a significant Δ-citrate preceded a Δ-malate. Baseline levels of citrate increased two-fold during stress, reaching a mean maximum of 10 mol m-3. In non-stressed plants, nighttime accumulation of citric acid was minor and only detectable in mature leaves. Removal of salt stress did not eliminate a well-established diurnal rhythm of the citric acid.