Day‐night changes of citric‐acid levels in crassulacean acid metabolism: phenomenon and ecophysiological significance

A reappraisal is offered of old and new observations of substantial day‐night changes of citric‐acid levels in crassulacean acid metabolism (CAM). In contrast to malic acid, the biochemical consequences and the ecophysiological significance of nocturnal accumulation of citric acid in CAM have not received due attention. Considerations show that citric‐acid accumulation does not provide a means for nocturnal storage of CO 2 . It may, however, serve carbon retention by internal recycling and sustain the water budget affording a vacuolar osmoticum. Since citric‐acid accumulation energetically is considerably more favourable than malic‐acid accumulation, this may have important ecophysiological implications. The questions raised by these reflections can and need to be tackled experimentally.