Pre‐dawn stomatal opening does not substantially enhance early‐morning photosynthesis in H elianthus annuus

Most C 3 plant species have partially open stomata during the night especially in the 3–5 h before dawn. This pre‐dawn stomatal opening has been hypothesized to enhance early‐morning photosynthesis ( A ) by reducing diffusion limitations to CO 2 at dawn. We tested this hypothesis in cultivated H elianthus annuus using whole‐shoot gas exchange, leaf level gas exchange and modelling approaches. One hour pre‐dawn low‐humidity treatments were used to reduce pre‐dawn stomatal conductance ( g ). At the whole‐shoot level, a difference of pre‐dawn g (0.40 versus 0.17 mol m −2  s −1 ) did not significantly affect A during the first hour after dawn. Shorter term effects were investigated with leaf level gas exchange measurements and a difference of pre‐dawn g (0.10 versus 0.04 mol m −2  s −1 ) affected g and A for only 5 min after dawn. The potential effects of a wider range of stomatal apertures were explored with an empirical model of the relationship between A and intercellular CO 2 concentration during the half‐hour after dawn. Modelling results demonstrated that even extremely low pre‐dawn stomatal conductance values have only a minimal effect on early‐morning A for a few minutes after dawn. Thus, we found no evidence that pre‐dawn stomatal opening enhances A .