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.
- Improve understanding of physiological processes and environmental factors
controlling ozone uptake in crop plants;
- Determine effects of ozone on carbohydrate and nitrogen metabolic
- Expand our knowledge of genes induced by ozone and their adaptive
- Characterize ozone-induced senescence processes and programmed cell death;
- Link genetic markers to adaptive physiological traits for ozone
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.
- Mechanisms of ozone toxicity will be defined for various crop plants and
cropping systems and the information used to counteract adverse effects of
- Biochemical and molecular markers will be identified that detect ozone
stress when visible symptoms are absent or inconclusive.
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