Interactive effects of elevated carbon dioxide concentration and water availability on field-grown soybean (Glycine max)

The concentration of carbon dioxide ([CO2]) in the atmosphere is predicted to reach 730 -- 1020 parts per million (ppm) by 2100, and the risk of drought is predicted to increase. Elevated atmospheric [CO2] directly affects C3 plants by reducing stomatal conductance (gs) and increasing photosynthetic carbon assimilation (A). It is widely hypothesized that reduced gs in elevated [CO2] will decrease plant and canopy water use, conserving soil moisture and ameliorating drought stress. Additionally, stimulation of A by elevated [CO2] often results in increased root biomass, and this is predicted to improve plant access to soil water, enabling avoidance of drought stress. Models of future food supply often assume that these beneficial effects of elevated [CO2] will compensate for the predicted increases in drought stress, but this assumption has not been widely tested in realistic crop production environments. However, elevated [CO2] also increases canopy temperature and leaf area, both of which have the potential to compensate for the effects of reduced gs on canopy water use, and the extent to which this compensation occurs in the field is not well understood. Furthermore, it is not known how stimulation of root growth by elevated [CO2] will alter the distribution of root length relative to soil water resources, or how this may affect whole plant water status, and, in the case of legumes, the symbiosis with nitrogen-fixing bacteria. These knowledge gaps were addressed at the soybean Free-Air CO2 Enrichment (soyFACE) facility, where soybean was grown in the field at ambient [CO2] or elevated [CO 2], as predicted for the middle of this century. I analyzed an eight year field study to test the hypothesis that reduced gs in elevated [CO2] will overwhelm increases in leaf area and canopy temperature to result in conservation of soil moisture. Additionally, I conducted a three-year experiment where a sub-plot of each ambient and elevated [CO2] treatment plot was exposed to reduced precipitation to test the prediction that, by allowing avoidance of drought stress through conservation of soil water and stimulation of root growth, elevated [CO2] will reduce drought sensing and signaling via the plant hormone abscisic acid, and will ameliorate drought-induced reductions in photosynthetic gas exchange. I also tested the predictions that elevated [CO2] will increase the production and size of nitrogen-fixing root nodules, reduced precipitation will inhibit nodule production, and elevated [CO2] will ameliorate the negative effects of reduced precipitation on nodule production. I found that elevated [CO2] did not always conserve soil water, and increased root length in elevated [CO2] occurred in shallow or intermediate soils which tended to be dry, resulting in no improvement in access to soil water and negative effects on plant nitrogen status. Furthermore, I found that elevated [CO2] caused stomata to respond more sensitively to abscisic acid, often resulting in greater drought-induced reductions in photosynthetic gas exchange in elevated [CO2] compared to ambient [CO2]. These results suggest that predicted amelioration of drought stress by elevated [CO2] may not occur in field-grown soybean in the Midwestern U.S.