O3 sensitivity in a potential C4 bioenergy crop: Sugarcane in California
The C4 perennial grasses have many potential virtues as bioenergy crops—high productivity and water use efficiency, multiyear crop cycle that minimizes replanting costs, and wide environmental adaptation. In the productive San Joaquin Valley (SJV) of California, these species also confront high levels of ozone (O3) air pollution. It is often assumed that C4 species will exhibit tolerance to O3, relative to C3 species, based on limited crop yield-loss data and on responses to a contrasting gaseous air pollutant, sulfur dioxide (SO2). Tolerance to O3 of high-biomass C4 crops such as sugarcane (Saccharum spp.) has not been demonstrated. Here we tested the hypothesis that a clone of sugarcane currently grown on a small commercial scale in the O3–impacted SJV will exhibit tolerance to O3. The hypothesis was disproven. Biomass production declined by over one-third and allocation to roots declined by over two-thirds, over a range of O3 exposures (4, 58, and 114 ppb, 12-h means). These responses are comparable to responses previously reported for O3–sensitive C3 crop species. Greater O3 tolerance may be required to maximize the potential viability of bioethanol production using tall tropical grasses such as sugarcane (for sucrose) or energy canes (for cellulose) in O3–impacted western valleys. In the case of the locally successful sugarcane clone examined here, the C4 photosynthetic pathway did not confer resistance to O3 exposure.