Problem Statement

Rationale. In many crops, current tropospheric ozone levels inhibit photosynthesis, alter plant structure and development, and suppress biomass and yield. Such effects are mediated by numerous metabolic pathways through which plants produce energy and carry on other activities. However, these pathways remain either unknown or only partially described. This is partly due to the current state of plant science research, which is just beginning to integrate plant biology from the molecular to the organismal level. Extension of these research efforts toward defining the mechanisms of ozone toxicity are required to understand the adverse effects of ozone on plants. Biological impairment by ozone needs further clarification if we are to provide the information needed to counteract its adverse effects.

What is known.  The mechanisms of ozone action involve both toxicity and responses that counteract, perpetuate, or even exacerbate effects of ozone exposure. In addition, some plant responses to ozone might have no role at all in counteracting ozone stress.

Plants take up ozone primarily through their leaves, a process largely influenced by physiological and environmental factors (carbon dioxide concentration, humidity, light, temperature, nutrient and water availability). The complex processes of metabolism following ozone uptake are thought to include decreasing the amount of carbon available for plant growth by suppressing photosynthesis and by stimulating the need for carbon in maintenance and repair processes. In addition, translocation of carbohydrates from leaves to shoot and roots is inhibited by ozone injury. In this respect, carbohydrate metabolism and allocation are important links between carbon dioxide fixation and biomass production. Analysis of carbohydrate pools and plant structure can be a valuable tool in identifying the sensitive steps in plant metabolism that reveal the mechanisms of ozone injury.

In concert with plant responses to ozone injury per se, a number of genetic responses are induced by exposure to ozone. The responses described so far involve photosynthesis, antioxidant and secondary metabolism, pathogen-defense responses, and senescence-related processes. Some of these responses correspond to defense reactions to oxidative stress. In addition, ozone induces several genetic pathways associated with senescence processes, and there are indications that ozone provokes programmed cell death.

Gaps.  A comprehensive understanding of ozone impairment of plant growth and development is lacking. This includes physiological and environmental factors controlling ozone uptake. An integrated understanding of direct and indirect effects of ozone on plant mechanisms and processes, including photosynthesis, ion regulation, carbohydrate, lipid and nitrogen metabolism, water relations, phloem loading, and biomass allocation needs to be developed from the molecular to the organismal level. The significance of ozone-induced changes in antioxidant metabolism, pathogen defense responses, and secondary metabolism needs to be assessed. How senescence processes, especially ethylene production, are induced and interact with ozone needs to be investigated. Many recent studies on photosynthesis, carbohydrate metabolism, and molecular biology have been done with woody plants and model plant systems. These findings should be addressed for applicability to herbaceous crop plants.

Goals

• Improve understanding of physiological processes and environmental factors controlling ozone uptake in crop plants;
• Determine effects of ozone on carbohydrate and nitrogen metabolic pathways;
• Expand our knowledge of genes induced by ozone and their adaptive significance;
• Characterize ozone-induced senescence processes and programmed cell death; and
• Link genetic markers to adaptive physiological traits for ozone resistance.

Approach

A multidisciplinary approach will be used to assess direct and indirect effects of ozone on the quality and quantity of seed and forage crops. Treatment facilities include indoor and outdoor controlled environment chambers, greenhouse chambers, and open-top field chambers. Experiments using model plants and crop plants will be conducted. Experts in molecular biology, plant physiology, and biochemistry will cooperate to measure and assess plant responses to ozone.

Outcomes

• Mechanisms of ozone toxicity will be defined for various crop plants and cropping systems and the information used to counteract adverse effects of ozone exposure.
• Biochemical and molecular markers will be identified that detect ozone stress when visible symptoms are absent or inconclusive.

Impact

A sustainable crop production system that minimizes adverse effects of tropospheric ozone pollution.

Linkages to Other ARS National Programs

• Global Change
• Plant Biological and Molecular Processes
• Plant Microbial and Insect Genetic Resources, Genomics, and Genetic Improvement