Palmer amaranth is a highly invasive weed species causing huge economiclosses in agricultural cropping systems under a broad range of environmentalconditions. Sensitivity of this species to ozone (O3) airpollution and to soil water deficit, relative to native species or competingcrops, may affect its competitiveness and invasive potential. In recentyears, both high tropospheric O3 and soil water deficiency havebecome common in the San Joaquin Valley of California. Responses to theseenvironmental parameters may help predict the invasiveness of this speciesand have implications for landscape hydrology. We assessed the impact of O3 and soil water deficit on Palmer amaranth. Five- toseven-leaf–stage potted plants were placed in continuous stirred tankreactor chambers and maintained for 30 to 35 d under 12-h mean daylight O3 exposures (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 7wk. 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. Nocturnal gs was about 16 to 29% of daytime gs; this suggests that the species could have substantialnighttime water loss, uncoupled from carbon gain in the weed, and couldaffect water availability for crops and reduce irrigation efficiency.Nocturnal gs was lower in the RDI than in the WW, but daytime gs was not affected by O3 or irrigation regime. Neither O3 nor irrigation regime affected root or shoot parameters. As O3 and drought are two key stressors in the San JoaquinValley, to which potential competing species have been found to besensitive, Palmer amaranth may proliferate and become more invasive in thefuture, potentially altering landscape hydrology and reducing irrigationefficiency.