A new method of applying a controlled soil water stress, and its effect on the growth of cotton and soybean seedlings at ambient and elevated carbon dioxide

While numerous studies have shown that elevated CO2 can delay soil water depletion by causing partial stomatal closure, few studies have compared responses of plant growth to the same soil water deficits imposed at ambient and elevated CO2. We applied a vacuum to ceramic cups in pots filled with soil to reduce the soil water matric potential to −0.10 MPa. This system resulted in uniform soil water content throughout the pot, and was used to maintain a constant mild stress for seven days. In cotton, the soil water stress treatment reduced stomatal conductance at both 380 and 560 μmol mol−1 CO2, but the reduction was relatively smaller at the higher CO2. No reduction of photosynthesis measured under the daytime growth conditions occurred at elevated CO2 in stressed cotton plants, while photosynthesis was reduced by the stress in the lower CO2 treatment. The soil water stress treatment reduced the leaf area and biomass of cotton at the lower, but not at the higher CO2. In soybean, the soil water stress treatment reduced stomatal conductance, photosynthesis and growth at both CO2 levels, but the effect of water stress was not less at elevated than ambient CO2. In neither species nor CO2 level did the soil water stress treatment cause a detectable change in daytime leaf water potential. In both species, the stomatal closure with the soil water stress may have resulted from the lower soil to leaf hydraulic conductivity. The failure of high CO2 to protect soybean growth from the soil water stress might be related to the lower hydraulic conductivity of stressed soybeans grown at elevated compared with ambient CO2.

► A method was developed to keep soil water potential at −0.1 MPa throughout a pot. ► The soil water potential achieved was independent of transpiration rate. ► This soil water stress decreased the growth of cotton at low but not high CO2. ► This soil water stress decreased the growth of soybean equally at both CO2 levels. ► Stomatal closure resulted from low soil hydraulic conductivity, not leaf water deficits.