Ozone reduces plant growth, yield of horticultural and agronomic products, and beauty of ornamental vegetation. But the mechanism of ozone damage is not well understood. As a result, methods to protect plants are not available.
The Air Quality Effects Laboratory, in collaboration with other researchers in California, other states, and other countries, is working to understand the mechanism of ozone damage, to develop production methods to protect crops, and to identify targets for genetic improvement of plants.
It is well known that photosynthesis is damaged by ozone. It is less clear that this is the only or even the primary target of ozone action. Loading of newly photosynthesized sugars into the phloem for long distance translocation to roots and fruits is rapidly inhibited. Allocation of biomass among competing plant parts is disrupted by ozone, reducing root development and hydraulic conductance, and indirectly affecting leaf water relations and photosynthetic gas exchange. The Air Quality Effects Laboratory has shown that in Pima cotton (Gossypium barbadense) carbohydrate translocation is inhibited more than photosynthetic carbon assimilation following brief exposures to high concentrations of ozone.
Pima cotton loads sugars into the phloem from the apoplast, which requires uptake of sucrose across a cell membrane. Muskmelon (Cucumis melo) loads an alternative type of sugar, mostly stachyose, from the symplast, which does not require uptake across a membrane. The Air Quality Effects Laboratory has found that the ratio of stachyose to sucrose in sink tissues, such as fine roots, increases with exposure to ozone in both types of plants. These data suggest that O3 inhibits apoplastic phloem loading of sucrose more than symplastic loading of stachyose. Future research must determine if specific transporter proteins are particularly sensitive to ozone and could be a target for genetic improvement.
Root development is inhibited by ozone. As a result the hydraulic capacity to provide the transpiring shoots with water is reduced. The Air Quality Effects Laboratory has used a model to show that this reduction in root capacity could reduce photosynthesis and plant water use. Total root biomass is reduced, the fraction of plant biomass in root tissues declines, and the number and branching patterns of roots is altered by ozone. Changes in internal anatomy of individual roots is currently under investigation.
Crops are sensitive to weed pressure. The Air Quality Effects Laboratory has demonstrated that purple nightshade, a strong competitor to Pima cotton, is an even stronger competitor in the presence of ozone exposure. While total productivity of nightshade plus cotton declined with increasing ozone concentration, the ratio of cotton to nightshade biomass declined dramatically. Yellow nusedge is another important weedy competitor. In trials with tomato (Lycopersicon esculentum), both tomato and nutsedge were sensitive to ozone and the relative competitiveness of nutsedge did not increase. In ongoing trials with cotton, which is more sensitive to ozone than tomato, competitiveness may have increased.