CIRCADIAN-RHYTHMS HAVE INSIGNIFICANT EFFECTS ON PLANT GAS-EXCHANGE UNDER FIELD CONDITIONS

Circadian rhythms in photosynthesis and stomatal conductance have been widely observed, but their possible adaptive significance is unknown. To determine whether such rhythms have a significant effect on the da ily courses of carbon gain and/or water loss under held conditions, we obtained laboratory data on circadian rhythms in gas exchange of Saur urus cernuus L., a wetland perennial. Using these data we modified a w idely used mathematical model of photosynthesis and stomatal conductan ce by introducing the observed circadian-rhythmic variation into the m aximum rates of electron transport and carboxylation. We measured phot osynthesis and stomatal conductance hourly on the same species growing naturally in the field and compared measured daily courses of photosy nthesis and stomatal conductance with daily courses calculated using t he model as originally formulated and also as modified to include circ adian rhythms. The model fit the field data only slightly better when rhythms were included: the rhythms accounted for only about 1% of the observed daily carbon gain. Thus, these rhythms probably do not affect photosynthesis and stomatal conductance in the held.