Diel trends in stomatal response to ozone and water deficit: a unique relationship of midday values to growth and allometry in Pima cotton?

Plant responses to ozone (O-3) and water deficit (WD) are commonly observed, although less is known about their interaction. Stomatal conductance (g(s)) is both an impact of these stressors and a protective response to them. Stomatal closure reduces inward flux of O-3 and outward flux of water. Stomatal measurements are generally obtained at midday when gas exchange is maximal, but these may not be adequate surrogates for stomatal responses observed at other times of day, nor for non-stomatal responses. Here, we find in Pima cotton that stomatal responses to O-3 observed at midday do not reflect responses at other times. Stomata were more responsive to O-3 and WD near midday, despite being at quasi-steady state, than during periods of active opening or closing in morning or evening. Stomatal responsivity to O-3 was not coincident with maximum gas exchange or with periods of active regulation, but coincident with plant sensitivity to O-3 previously determined in this cultivar. Responses of pigmentation and shoot productivity were more closely related to stomatal responses at midday than to responses at other times of day under well-watered (WW) conditions, reflecting higher stomatal responsivity, sensitivity to O-3, and magnitude of midday gs. Under WD conditions, shoot responses were more closely related to early morning gs. Root responses were more closely related to early morning gs under both WW and WD. Responses of stomata to O-3 at midday were not good surrogates for stomatal responses early or late in the day, and may not adequately predicting O-3 flux under WD or when maximum ambient concentrations do not occur near midday.