Differential Responses of Crop and Weed to Ozone and Moisture Stress: A Potential Perturbation of Crop-Weed Competition.

Tropospheric ozone (O₃) is a major air pollutant and an important anthropogenic stressor in agricultural cropping systems. Studies have reported that O₃ can have differential effects on crops and weeds and thereby alter crop-weed competition dynamics in some crops. Weeds such as black nightshade (Solanum nigrum), horseweed (Conyza canadensis), and yellow nutsedge (Cyperus esculentus) have been found to be tolerant to O₃ and in some cases more competitive with crops under elevated O₃ conditions.  Inadequate moisture is also another cause of stress in cropping systems. However, the combined effect of O₃ and moisture deficit on crops and weeds is not known.  Therefore, a study was conducted to determine the effect of O₃ and irrigation levels on cotton (Gossypium barbadense) and common waterhemp (Amaranthus rudis) grown separately. Growth and stomatal conductance (day and night) of these species grown in greenhouse exposure chambers were evaluated under three different O₃ exposure regimes (4, 59, and 114 ppb; 12-hour mean) and two irrigation levels (field capacity and 33% of field capacity). The experiment was a split-plot design, with O₃ as main- and irrigation as sub-factor.   Increase in O₃ concentration and decrease in irrigation level both reduced day-time stomatal conductance (P<0.05) in cotton but not in common waterhemp. In contrast, increased O₃ levels increased night-time (2 AM) stomatal conductance in cotton but not in common waterhemp.  There was no effect of irrigation level in night-time stomatal conductance in  either species  Nocturnal conductance was affected by increasing O₃ levels in cotton but not in waterhemp.  Also, on average, the nocturnal conductance of waterhemp was four-fold higher than in cotton. Elevated O₃ reduced the above- and below-ground biomass in cotton but not in common waterhemp but irrigation level had no effect on the biomass of either species. We conclude that changes in stomatal regulation and biomass productivity may shift crop-weed competition in favor of common waterhemp under elevated O₃.