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.