Crop responses to ozone: uptake, modes of action, carbon assimilation and partitioning
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.