The mechanism of O 3 action on plants remains poorly characterized. Symptoms include visible lesions on the leaf surface, reduced growth and a hypothesized reduction in allocation of carbohydrate to roots. The generality of this latter phenomenon has not been demonstrated. Here, a meta-analysis is performed of all available experimental data, to test the hypotheses that O 3 exposure of the shoot inhibits biomass allocation below ground (the root/shoot allometric coefficient, k ) and inhibits whole-plant growth rate [relative growth rate (RGR)]. Both k and RGR were significantly reduced by O 3 (5.6 and 8.2%, respectively). Variability in k was greater than in RGR, and both exhibited some positive as well as mostly negative responses. The effects on k were distinct from the effects on RGR. In some cases, k was reduced while RGR was unaffected. Slow-growing plants (small RGR) exhibited the largest declines in k . These observations may have mechanistic implications regarding O 3 phytotoxicity. There were no effects of type of exposure chamber on sensitivity to O 3 . The analyses indicate that the O 3 inhibition of allocation to roots is real and general, but variable. Further experiments are needed for under-represented plant groups, to characterize exceptions to this generalization and to evaluate O 3 -environment interactions.

Horseweed has become increasingly difficult to control in the San Joaquin Valley (SJV) of California. Resistance to glyphosate may not fully explain the quasi-invasive behavior of this native species. We contrast glyphosate-resistant (GR) and glyphosate-susceptible (GS) horseweed biotypes for vigor during the vegetative stage and for resistance to ozone (O3). The SJV is impacted by O3 air pollution, which could be a factor in competitiveness of GR vs. GS. Both biotypes were exposed during the seedling and vegetative stages of rosette development to a range of O3 concentrations in greenhouse exposure chambers. Leaf injury was evaluated visually and biomass production and allocation destructively. In O3-free air, the GR biotype exhibited fewer foliar lesions, more vigorous growth, and 40% greater biomass than the GS biotype. The slope of the response to O3 was greater in the GR than in the GS biotype, implying greater relative sensitivity to O3. This was due to greater vigor at low O3, as the biotypes performed similarly at high O3. The competitive advantage of the GR biotype may be reduced in polluted environments. There appeared to be no linkage between the evolution of resistances to O3 and to glyphosate.

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The C4 perennial grasses have many potential virtues as bioenergy crops—high productivity and water use efficiency, multiyear crop cycle that minimizes replanting costs, and wide environmental adaptation. In the productive San Joaquin Valley (SJV) of California, these species also confront high levels of ozone (O3) air pollution. It is often assumed that C4 species will exhibit tolerance to O3, relative to C3 species, based on limited crop yield‐loss data and on responses to a contrasting gaseous air pollutant, sulfur dioxide (SO2). Tolerance to O3 of high‐biomass C4 crops such as sugarcane (Saccharum spp.) has not been demonstrated. Here we tested the hypothesis that a clone of sugarcane currently grown on a small commercial scale in the O3–impacted SJV will exhibit tolerance to O3 The hypothesis was disproven. Biomass production declined by over one‐third and allocation to roots declined by over two‐thirds, over a range of O3 exposures (4, 58, and 114 ppb, 12‐h means). These responses are comparable to responses previously reported for O3–sensitive C3 crop species. Greater O3 tolerance may be required to maximize the potential viability of bioethanol production using tall tropical grasses such as sugarcane (for sucrose) or energy canes (for cellulose) in O3–impacted western valleys. In the case of the locally successful sugarcane clone examined here, the C4 photosynthetic pathway did not confer resistance to O3 exposure.

Plant injury by ozone (O3) occurs in three stages, O3 entrance through stomata, overcoming defences, and attack on bioreceptors. Concentration, deposition, and uptake of O3 are accessible by observation and modelling, while injury can be assessed visually or through remote sensing. However, the relationship between O3 metrics and injury is confounded by variation in sensitivity to O3. Sensitivity weighting parameters have previously been assigned to different plant functional types and growth stages, or by differentially weighting O3 concentrations, but diel and seasonal variability have not been addressed. Here a plant sensitivity parameter (S) is introduced, relating injury to O3 dose (uptake) using three independent injury endpoints in the crop species, Pima cotton (Gossypium barbadense). The diel variability of S was determined by assessment at 2h intervals. Pulses of O3 (15min) were used to assess passive (constitutive) defence mechanisms and dose was used rather than concentration to avoid genetic or environmental effects on stomatal regulation. A clear diel trend in S was apparent, with maximal sensitivity in mid-afternoon, not closely related to gas exchange, whole leaf ascorbate, or total antioxidant capacity. This physiologically based sensitivity parameter provides a novel weighting factor to improve modelled relationships between either flux or exposure to O3, and O3 impacts. This represents a substantial improvement over concentration- or phenology-based weighting factors currently in use. Future research will be required to characterize the variability and metabolic drivers of diel changes in S, and the performance of this parameter in prediction of O3 injury.