Stomatal sensitivity to photosynthetic and environmental signals in glycine max grown at elevated atmospheric concentrations of CO2 and O3

Rising atmospheric concentrations of carbon dioxide (CO2) and ozone (O3) will stimulate and impair, respectively, yield and productivity of both managed and natural ecosystems in the 21st century. The ease of diffusion of these gases, as well as water vapor, between the atmosphere and interior of leaves is actively controlled by stomatal pores and termed stomatal conductance (gs). gs is highly regulated in response to environmental factors and a key determinant of plant carbon gain, water use and ozone uptake. The Ball, Woodrow and Berry (1987, Progress in Photosynthetic Research, vol IV, 221-224) model of stomatal conductance is a central component of ecosystem and global models of carbon and water cycling used to predict the impacts and feedbacks of vegetation and climate. It is essential that the m parameter of the model, which characterizes the sensitivity of gs to net photosynthetic CO2 assimilation rate (A), fractional relative humidity at the leaf surface (h) and atmospheric CO2 concentration ([CO2]), be appropriate for the conditions in consideration to avoid errors in large-scale predictions of ecosystem function and global biogeochemical cycling. Characterizing any changes in m with elevated [CO2] and [O3] will be necessary for realistic simulations of the future. Variation in maximum carboxylation capacity (Vcmax) as a result of plant growth in elevated [CO2] or elevated [O3] will change the A that a given ci, permitted by a given gs, will support. Such a Vcmax-induced change in the sensitivity of gs to A may contribute to a change in m. Soybean was exposed to a range of [O3] in the field, and to combinations of different [O3] and [CO2] in environmental growth chambers, to examine the effect of these conditions on m. A positive correlation between m and [O3] was observed in the field. This was reproduced in the controlled environment experiment at the lowest growth [CO2] treatment, but the response was ameliorated by elevated [CO2]. This suggests gs becomes more sensitive to A, h and/or [CO2] when plants are grown in higher [O3] conditions and ambient [CO2]. The estimate of gs was 46% lower for the unparameterized relative to the parameterized Ball et al. (1987) model for the highest growth [O3] of 120ppb and ambient [CO2]. Reparameterization of large-scale models will be necessary to account for this change in stomatal function under these conditions. A strong, negative correlation between Vcmax and m was present whenever m was significantly altered. Estimation of m values based on this correlation may present an easy means to improve simulations of large-scale models for different species and growth conditions involving elevated [O3] without additional parameterization measurements