CIRCADIAN RHYTHMIC FLOWERING RESPONSES IN CHENOPODIUM RUBRUM: EFFECTS OF GLUCOSE AND SUCROSE

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Abstract
An endogenous circadian rhythm of flower induction in Chenopodium rubrum that depends on the length of darkness interrupting continuous light has a smaller amplitude and damps out earlier when the light preceding and (or) following darkness is limited in intensity. Glucose or sucrose, fed to plants throughout darkness, can enhance and sustain the rhythm over a considerably longer period. Some degree of specificity of glucose in the rhythmic process was indicated because, when this energy source was applied during a skotophile phase of darkness (particularly the first 9 h) there were inhibitory effects on the rhythmic flowering response, but the same concentration of glucose was stimulatory when applied during the photophile phase(s) of darkness. No similar time dependence resulted from sucrose treatments. To account for the effects of glucose in particular, it is postulated that endogenous sugar formation in the light period is involved in providing substrate for phytochrome-Pfr action or mediation during darkness. Damping out of the oscillation in extended dark periods is a consequence of depletion of sugar reserves during darkness. These and other results support the concept that Pfr acts as a pacemaker or valve and exerts some control over sugar availability and utilization. Comparisons involving the application of glucose to different ecotypes suggest that, for optimal induction, some definite balance is required between the endogenous availability of glucose, or some other sugar derivative(s), and the level of phytochrome Pfr; this balance apparently differs according to genotype.Differences between ecotypes in their rhythmic flowering responses suggest that the endogenous free-running clock is an integral part of the time-measuring system in daily photoperiodic cycles and that it does have a role in measuring time under natural conditions.