Plant responses to ozone (O3 ) and water deficit (WD) are commonly observed, although less is known about their interaction. Stomatal conductance (gs ) is both an impact of these stressors and a protective response to them. Stomatal closure reduces inward flux of O3 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 O3 observed at midday do not reflect responses at other times. Stomata were more responsive to O3 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 O3 was not coincident with maximum gas exchange or with periods of active regulation, but coincident with plant sensitivity to O3 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 O3 , 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 O3 at midday were not good surrogates for stomatal responses early or late in the day, and may not adequately predicting O3 flux under WD or when maximum ambient concentrations do not occur near midday.

Palmer amaranth is a highly invasive weed species causing huge economic losses in agricultural cropping systems under a broad range of environmental conditions. Sensitivity of this species to ozone (O3) air pollution and to soil water deficit, relative to native species or competing crops, may affect its competitiveness and invasive potential. In recent years, both high tropospheric O3and soil water deficiency have become common in the San Joaquin Valley of California. Responses to these environmental parameters may help predict the invasiveness of this species and have implications for landscape hydrology. We assessed the impact of O3and soil water deficit on Palmer amaranth. Five- to seven-leaf–stage potted plants were placed in continuous stirred tank reactor chambers and maintained for 30 to 35 d under 12-h mean daylight O3exposures (0700–1900 hours) of 4, 59, or 114 ppb O3. Within the chambers the plants were either well-watered (WW) or exposed to regulated deficit irrigation (RDI) and grown for about 7 wk. Dry weights of the leaves, stems, roots, and leaf area were determined. Day- and nighttime stomatal conductances (gs) were measured at 1.5-h intervals. Nocturnalgswas about 16 to 29% of daytimegs; this suggests that the species could have substantial nighttime water loss, uncoupled from carbon gain in the weed, and could affect water availability for crops and reduce irrigation efficiency. Nocturnalgswas lower in the RDI than in the WW, but daytimegswas not affected by O3or irrigation regime. Neither O3nor irrigation regime affected root or shoot parameters. As O3and drought are two key stressors in the San Joaquin Valley, to which potential competing species have been found to be sensitive, Palmer amaranth may proliferate and become more invasive in the future, potentially altering landscape hydrology and reducing irrigation efficiency.

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