The inhibitory effects of tropospheric O 3 on crop photosynthesis, growth, and yield have been documented in numerous studies over the past 35 years. In large part, the results of this research supported governmental regulations designed to limit tropospheric O 3 levels to concentrations that affected crop production at economically acceptable levels. Recent studies have brought into question the efficacy of these concentration-based O 3 standards compared with flux-based approaches that incorporate O 3 uptake along with environmental and biotic factors that influence plant responses. In addition, recent studies provide insight into the biochemical mechanisms of O 3 injury to plants. Current interpretations suggest that upon entry into the leaf intercellular space O 3 rapidly reacts with components of the leaf apoplast to initiate a complex set of responses involving the formation of toxic metabolites and generation of plant defence responses that constitute variably effective countermeasures. Plant species and cultivars exhibit a range of sensitivity to O 3 , evident as heritable characteristics, that must reflect identifiable biochemical and molecular processes that affect sensitivity to O 3 injury, although their exact makeup remains unclear. Ozone clearly impairs photosynthetic processes, which might include the effects on electron transport and guard cell homeostasis as well as the better-documented effects on carbon fixation via decreased Rubisco activity. Translocation of photosynthate could be inhibited by O 3 exposure as well. Further, the influence of tropospheric O 3 needs to be considered when assessing potential effects of rising concentrations of atmospheric CO 2 on crop production. Advances in O 3 flux modelling and improved understanding of biochemical and molecular effects of O 3 on photosynthetic gas exchange and plant defence processes are leading to more complete, integrated assessments of O 3 impacts on crop physiology that continue to support the rationale for maintaining or improving current O 3 air quality standards as well as providing a basis for development of more O 3 -tolerant crop lines.

The productivity, product quality and competitive ability of important agricultural and horticultural plants in many regions of the world may be adversely affected by current and anticipated concentrations of groundlevel ozone (O 3 ). Exposure to elevated O 3 typically results in suppressed photosynthesis, accelerated senescence, decreased growth and lower yields. Various approaches used to evaluate O 3 effects generally concur that current yield losses range from 5% to 15% among sensitive plants. There is, however, considerable genetic variability in plant responses to O 3 . To illustrate this, we show that ambient O 3 concentrations in the eastern United States cause substantially different levels of damage to otherwise similar snap bean cultivars. Largely undesirable effects of O 3 can also occur in seed and fruit chemistry as well as in forage nutritive value, with consequences for animal production. Ozone may alter herbicide efficacy and foster establishment of some invasive species. We conclude that current and projected levels of O 3 in many regions worldwide are toxic to sensitive plants of agricultural and horticultural significance. Plant breeding that incorporates O 3 sensitivity into selection strategies will be increasingly necessary to achieve sustainable production with changing atmospheric composition, while reductions in O 3 precursor emissions will likely benefit world food production and reduce atmospheric concentrations of an important greenhouse gas.