Circadian rhythms in photosynthesis

Green beans (Phaseolus vulgaris L.) transferred from a natural photoperiod to constant conditions exhibited persistent rhythms in photosynthesis and stomatal opening. Photosynthesis, measured as the net assimilation of carbon, and stomatal opening, measured as conductance to water vapor, were investigated with a computer controlled gas exchange system that maintained selected environmental and physiological variables at constant values. The photosynthetic and stomatal rhythms were in phase with each other, with maximum values near subjective noon and minimum values near subjective midnight. The periods of the rhythms were approximately 24.5 hours and displayed temperature compensation. The phases of stomatal and photosynthetic rhythms were shifted to the same extent by altered photoperiods.;A rhythm in photosynthesis occurred when either the ambient or the intercellular CO;Plants grown under constant light at constant temperature expressed no rhythms, demonstrating that the rhythms were not spontaneous but induced by an external signal, generally the photoperiod during growth. The rhythm in stomatal opening, but not the rhythm in photosynthesis, could also be entrained by a daily cycle of high and low temperatures.;In either darkness or in light with CO;Phaseolus also expressed a rhythm in leaflet movement with a period close to 27 hours, several hours longer than the rhythms in stomatal conductance and photosynthesis. This difference in free-running period is evidence that anatomically distinct oscillators with different frequencies control different rhythms in Phaseolus.