Problem Statement

Rationale.  Tropospheric ozone concentrations can inhibit plant growth and yield in many agricultural regions. One management strategy is to utilize ozone-tolerant varieties that perform well in high ozone environments, yet maintain yield and quality in years when ozone impact is minimal. To implement such a strategy, information is needed on the range of ozone sensitivity within cultivars of major crops. For certain crops, available cultivars may not express sufficient ozone tolerance to maintain high levels of productivity under current or future ozone levels. Therefore, new varieties are needed with enhanced ozone tolerance.

What is known.  Plants are known to exhibit a range of ozone sensitivity in terms of visible injury and yield reduction, including significant variation among genotypes or clones of the same species. Presumably, genetic variation represents differences in capacity to express one or more tolerance mechanisms. Studies to date have identified three aspects of plant physiology that determine the impact of ozone stress on plant growth. First, ozone entry into the plant is primarily via leaf stomata, so stomatal processes that limit ozone uptake could ameliorate the effect of ozone stress. However, reduced stomatal conductance also could negatively affect plant growth through reduction of carbon dioxide and water vapor gas exchange, although tolerance to drought might be improved. Second, ozone injury can be minimized or prevented by metabolic pathways that detoxify ozone and the reactive oxygen species formed from ozone. Finally, once ozone injury has been initiated, metabolic responses are induced that replace damaged cellular constituents (e.g., lipids and proteins) or alter whole plant development patterns (e.g., accelerated senescence). There also is evidence that plant responses to ozone share common features with responses to other environmental stresses (e.g., pathogen infection, chilling, drought stress), so that new knowledge about plant tolerance to other stress factors may provide insights into ozone tolerance and vice versa.

Gaps.  Although current knowledge can suggest future research efforts, no definitive ozone tolerance mechanisms have been characterized. Specific mechanisms must be identified, and an understanding is needed of how multiple mechanisms combine to produce the ozone tolerance associated with a particular genotype. The distinctions between injury prevention versus repair processes need to be clarified. Initiation of ozone injury induces a localized or general response that may permanently alter the growth and yield potential of the plant, so mechanisms that prevent injury may have greater impact than mechanisms related to cellular repair.

Evidence suggests that a range of ozone sensitivity exists within each species, but information on ozone-sensitive and tolerant cultivars is not available for each major crop. Knowledge and methodology must be developed to allow rapid selection of ozone-tolerant varieties from available germplasm.

Goals

• Identify specific aspects of physiology and metabolism that distinguish ozone-sensitive and ozone-tolerant plants;
• Formulate strategies based on genetic manipulation of key metabolic pathways that will increase yield potential under elevated ozone environments; and
• Identify ozone-sensitive and -tolerant cultivars within existing germplasm of major crop species.

Approach

Controlled environment facilities and open-top chambers will be used to conduct mechanistic studies and to screen available germplasm under a range of ozone concentrations. Ozone-sensitive and ozone-tolerant genotypes from the same species will be compared to identify biochemical and physiological characteristics that contribute to ozone tolerance. Mutants of model plants will be utilized as a tool to provide insights into tolerance mechanisms not yet recognized. Plants with known tolerance to pathogens or to stresses such as chilling and drought will be tested for cross-tolerance against ozone as an alternative approach for mechanism identification. Specific enzymes or physiological markers with the potential to affect ozone tolerance will be identified and this information used to produce plants for further testing and to develop new methods for rapidly screening germplasm.

Outcomes

• Fundamental knowledge of ozone tolerance will be developed that can be used to produce plants with enhanced performance under elevated ozone environments.
• The potential impact of ozone on crop production and quality will be demonstrated to growers, extension agents, and others.
• Ranking of modern cultivars for ozone-sensitivity will allow growers in areas of high ozone to select ozone-tolerant varieties that are compatible with current farming practices.

Impact

Crop productivity and quality maintained or improved in agricultural areas subjected to elevated tropospheric ozone.

Linkages to Other ARS National Programs

• Global Change
• Integrated Agricultural Systems
• Plant Biological and Molecular Processes