Pre‐dawn stomatal opening does not substantially enhance early‐morning photosynthesis in <i><scp>H</scp>elianthus annuus</i> | CU Experts

AbstractMost C3 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 CO2 at dawn. We tested this hypothesis in cultivated Helianthus 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 CO2 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.

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