Tolerance of ozone and drought in common waterhemp (Amaranthus tuberculatus).
Biological invasions in established cropping systems impact biodiversity and agricultural sustainability. Under conditions of multi-year drought and elevated tropospheric ozone (O3), several Amaranthus species have recently become problematic weeds in California. We examined responses of common waterhemp (A. tuberculatus) to 12-h mean O3 exposures of 4, 59, or 114 nmol O3 mol air-1, with two levels of irrigation, 33% (deficit irrigation, DI) or 100% (well-watered, WW) of field capacity. Daytime stomatal conductance (gs) was reduced by O3 but not by DI. Nocturnal gs was uncommonly large but unaffected by either treatment. Plant productivity was not affected by O3 or DI. Leaf mass per unit area and root mass per unit leaf area were reduced by O3 but not by DI. Tolerance of this species to O3 and deficit irrigation may increase its competitiveness with crops. High night-time water loss may reduce water availability for competing crops. The competitiveness of common waterhemp may increase, as climate change creates conditions that are adverse to major crops.