Testing unified theories for ozone response in C-4 species

There is tremendous interspecific variability in O-3 sensitivity among C-3 species, but variation among C-4 species has been less clearly documented. It is also unclear whether stomatal conductance and leaf structure such as leaf mass per area (LMA) determine the variation in sensitivity to O-3 across species. In this study, we investigated leaf morphological, chemical, and photosynthetic responses of 22 genotypes of four C-4 bioenergy species (switchgrass, sorghum, maize, and miscanthus) to elevated O-3 in side-by-side field experiments using free-air O-3 concentration enrichment (FACE). The C-4 species varied largely in leaf morphology, physiology, and nutrient composition. Elevated O-3 did not alter leaf morphology, nutrient content, stomatal conductance, chlorophyll fluorescence, and respiration in most genotypes but reduced net CO2 assimilation in maize and photosynthetic capacity in sorghum and maize. Species with lower LMA and higher stomatal conductance tended to show greater losses in photosynthetic rate and capacity in elevated O-3 compared with species with higher LMA and lower stomatal conductance. Stomatal conductance was the strongest determinant of leaf photosynthetic rate and capacity. The response of both area- and mass-based leaf photosynthetic rate and capacity to elevated O-3 were not affected by LMA directly but negatively influenced by LMA indirectly through stomatal conductance. These results demonstrate that there is significant variation in O-3 sensitivity among C-4 species with maize and sorghum showing greater sensitivity of photosynthesis to O-3 than switchgrass and miscanthus. Interspecific variation in O-3 sensitivity was determined by direct effects of stomatal conductance and indirect effects of LMA. This is the first study to provide a test of unifying theories explaining variation in O-3 sensitivity in C-4 bioenergy grasses. These findings advance understanding of O-3 tolerance in C-4 grasses and could aid in optimal placement of diverse C-4 bioenergy feedstock across a polluted landscape.