Water stress and day-to-day variation in apparent photosynthetic acclimation of field-grown soybeans to elevated carbon dioxide concentration

Midday measurements of single leaf gas exchange rates of upper canopy leaves of soybeans grown in the field at 350 (AC) and 700 (EC) mu mol(CO2) mol(-1) in open topped chambers sometimes indicated up to 50% higher net photosynthetic rates (PN) measured at EC in plants grown at AC compared to EC. On other days mean PN were nearly identical in the two growth [CO2] treatments. There was no seasonal pattern to the variable photosynthetic responses of soybean to growth [CO2]. Even on days with significantly lower PN in the plants grown at EC, there was no reduction in ribulose1,5-bisphosphate carboxylase/oxygenase, chlorophyll, or soluble protein contents per unit of leaf area. Over three years, gas exchange evidence of acclimation occurred on days when either soil was dry or the water vapor pressure deficit was high (n = 12 d) and did not occur on days after rain or on days with low water vapor pressure deficit (n = 9 d). On days when photosynthetic acclimation was evident, midday leaf water potentials were consistently 0.2 to 0.3 MPa lower for the plants grown at EC than at AC. This suggested that greater susceptibility to water stress in plants grown at EC caused the apparent photosynthetic acclimation. In other experiments, plants were grown in well-watered pots in field chambers and removed to the laboratory early in the morning for gas exchange measurements. ln these experiments, the amount of photosynthetic acclimation evident in the gas exchange measurements increased with the maximum water vapor pressure deficit on the day prior to the measurements, indicating a tag in the recovery of photosynthesis from water stress. The apparent increase in susceptibility to water stress in soybean plants grown at EC is opposite to that observed in some other species, where photosynthetic acclimation was evident under wet but not dry conditions, and may be related to the observation that hydraulic conductance is reduced in soybeans when grown at EC. The day-to-day variation in photosynthetic acclimation observed here may account for some of the conflicting results in the literature concerning the existence of acclimation to EC in field-grown plants.