CIRCADIAN-RHYTHMS IN STOMATAL RESPONSIVENESS TO RED AND BLUE-LIGHT

Stomata of many plants have circadian rhythms in responsiveness to env ironmental cues as well as circadian rhythms in aperture. Stomatal res ponses to red light and blue light are mediated by photosynthetic phot oreceptors; responses to blue light are additionally controlled by a s pecific blue-light photoreceptor. This paper describes circadian rhyth mic aspects of stomatal responsiveness to red and blue light in Vicia faba. Plants were exposed to a repeated light:dark regime of 1.5:2.5 h for a total of 48 h, and because the plants could not entrain to this short light:dark cycle, circadian rhythms were able to ''free run' as if in continuous light. The rhythm in the stomatal conductance establ ished during the 1.5-h light periods was caused both by a rhythm in se nsitivity to light and by a rhythm in the stomatal conductance establi shed during the preceding 2.5-h dark periods. Both rhythms peaked duri ng the middle of the subjective day. Although the stomatal response to blue light is greater than the response to red light at all times of day, there was no discernible difference in period, phase, or amplitud e of the rhythm in sensitivity to the two light qualities. We observed no circadian rhythmicity in net carbon assimilation with the 1.5:2.5 h light regime for either red or blue light. In continuous white light , small rhythmic changes in photosynthetic assimilation were observed, but at relatively high light levels, and these appeared to be attribu table largely to changes in internal CO2 availability governed by stom atal conductance.