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 daily courses of carbon gain and/or water loss under field conditions, we obtained laboratory data on circadian rhythms in gas exchange of Saururus cernuus L., a wetland perennial. Using these data we modified a widely used mathematical model of photosynthesis and stomatal conductance by introducing the observed circadian‐rhythmic variation into the maximum rates of electron transport and carboxylation. We measured photosynthesis and stomatal conductance hourly on the same species growing naturally in the field and compared measured daily courses of photosynthesis and stomatal conductance with daily courses calculated using the model as originally formulated and also as modified to include circadian 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 field.