title: Pre-dawn stomatal opening does not substantially enhance  early-morning photosynthesis in Helianthus annuus.
abstract: Most 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.
authors.forename: Lisa
authors.initials: L
authors.lastname: Auchincloss
authors.forename: Hsien M
authors.initials: HM
authors.lastname: Easlon
authors.forename: Diedre
authors.initials: D
authors.lastname: Levine
authors.forename: Lisa
authors.initials: L
authors.lastname: Donovan
authors.forename: James H
authors.initials: JH
authors.lastname: Richards
mesh_terms: D002940:Circadian Rhythm; D006368:Helianthus / Q000378:metabolism / Q000502:physiology*; D008954:Models, Biological; D010788:Photosynthesis; D054046:Plant Stomata / Q000502:physiology*