Examining Tolerance to Ozone Levels in Crops

Tolerance to ozone levels in crops is a topic that, considering the differences in photosynthesis among various crops, can make them more or less susceptible to damage caused by ground-level ozone pollution.
For instance, crops like corn and sorghum tolerate ozone levels better than others such as rice or beans. These findings pave the way for improved models to predict crop responses to the effects of global climate change.
Each crop is classified as either C3 or C4 based on whether the carbon dioxide it absorbs from the air is initially converted into a 3-carbon or 4-carbon compound.
Lisa Ainsworth, a research molecular biologist managing global ARS, noted that the overall ability of C4 crops to withstand increased ground-level (or tropospheric) ozone is better than that of C3 crops.
A comprehensive analysis of published and unpublished data was conducted, consisting of an initial set of 46 journal articles and a second set of 46 articles. These 20-year field experiments were conducted in the United States, India, and China.
Specifically, their analysis focused on the responses of five C3 crops (chickpeas, rice, beans, soybeans, and wheat) and four C4 crops (sorghum, corn, giant miscanthus, and switchgrass) to both ambient ozone levels and increased gas concentrations.
Factors such as changes in photosynthetic capacity, chlorophyll content, fluorescence (a form of pigment measurement), leaf antioxidant activity, biomass, and seed yield were among the points of interest.
C3 and C4 crops differ in how they absorb carbon dioxide from the air as a key component of photosynthesis through their leaves. This process allows plants to use sunlight to convert carbon dioxide into glucose, a sugar that aids in their growth, repair, and development while sustaining other forms of life on Earth, including humans.
While both C3 and C4 crops utilize an enzyme called rubisco to convert carbon dioxide into sugar, C4 crops separate rubisco into specialized cells that have a very high concentration of carbon dioxide.
This leads to higher rates of photosynthesis and greater water-use efficiency. Consequently, C4 plants have lower stomatal conductance, resulting in reduced carbon dioxide and ozone release from their leaves.
According to Ainsworth, free-air experiments—correctly referred to as “free-air concentration enrichment”—provide a fundamental truth about crop sensitivity to ozone and carbon dioxide that closed-environment studies cannot offer.
Overall, the team reported in their PNAS paper that exposure to increased ozone levels resulted in a greater reduction in chlorophyll content, fluorescence, and seed yield in C3 crops compared to the C4 group.
However, there were also differences within these two categories of crops; for instance, beans, rice, wheat, chickpeas, soybeans, corn, giant miscanthus, sorghum, and switchgrass exhibited varying degrees of sensitivity to ozone from highest to lowest.
These findings contrast with previous results showing that soybeans were the most sensitive while rice was the least sensitive. Another significant finding highlighted in the PNAS paper was that increased ozone levels reduced seed yield in hybrid lines of corn and rice compared to their parental lines.
Researchers note that current studies benefit from comparing crops side by side under free-air conditions. The protective role of phenolics and other antioxidants in C4 crop leaves also requires further investigation.
According to researchers, ozone pollution has reached levels comparable to other environmental stressors such as pest and disease pressure as well as drought and soil health decline.
Nonetheless, there is hope for better crop resilience. For example, genetic diversity among them could be key to unlocking traits for greater tolerance or photosynthetic efficiency.
Additionally, management decisions such as early planting in the season or using late-maturing varieties could further enhance crop tolerance.