Pre-dawn stomatal opening does not substantially enhance early-morning photosynthesis in Helianthus annuus

Most C-3 plant species have partially open stomata during the night especially in the 3-5h 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 0.17molm(-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 0.04molm(-2)s(-1)) affected g and A for only 5min 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. Most C-3 plant species have partially open stomata during the night especially in the 3-5 hours before dawn, yet the function of predawn stomatal opening has not been established. This work investigates the hypothesis that pre-dawn stomatal opening enhances early morning photosynthesis by reducing diffusion limitations to CO (2) at dawn. We found that, for well-watered Helianthus annuus, predawn opening did not substantially enhance early morning A, indicating that it is unlikely to affect A under well-watered agricultural conditions.