Rhythmical Characteristics at Different Levels of CAM Regulation: Physiological and Adaptive Significance

Abstract

Overt circadian rhythms typical of CAM operations in Kalanchoe blossfeldiana (rhythms in CO₂ exchanges and in malate content) are shown to be in good correlation to rhythms of enzyme capacity (viz. PEP carboxylase) as a function of photoperiodic conditions. The photoperiod-enzyme relationships differ in long days and in short days showing that a central timing mechanism is involved: in physiological long days (i.e. long days or short days + red light flash interruption of the night) the peak of PEP carboxylase capacity is fixed in time of the day; in short days the rhythm of enzyme capacity shifts according to complex transients and shows only relative coordination to the day-night rhythm. The hypothesis that sensitivity of the plant to light-dark signals changes progressively under short days but not under long days is presented and discussed in terms of adaptive significance. Labelling experiments suggest the operation of a regulatory mechanism combining enzyme circadian rhythmicity and feed-back by malate. Experimental and theoretical implications of these findings are discussed.