Limitations of Photosynthesis in Pinus taeda L. (Loblolly Pine) at Low Soil Temperatures

The relative importance of stomatal and nonstomatal limitations to net photosynthesis (A) and possible signals responsible for stomatal limitations were investigated in unhardened Pinus taeda seedlings at low soil temperatures. After 2 days at soil temperatures between 13 and 7°C, A was reduced by 20 to 50%, respectively. The reduction in A at these moderate root-chilling conditions appeared to be the result of stomatal limitations, based on the decrease in intercellular CO₂ concentrations (c_i). This conclusion was supported by A versus c_i analysis and measurements of O₂ evolution at saturating CO₂, which suggested increases in stomatal but not biochemical limitations at these soil temperatures. Nonuniform stomatal apertures, which were demonstrated with abscisic acid, were not apparent 2 days after root chilling, and results of our A versus c_i analysis appear valid. Bulk shoot water potential (ψ) declined as soil temperature dropped below 16°C. When half the root system of seedlings was chilled, shoot ψ and gas-exchange rates did not decline. Thus, nonhydraulic root-shoot signals were not implicated in stomatal limitations. The initial decrease in leaf conductance to water vapor after root chilling appeared to precede any detectable decrease in bulk fascicle ψ, but may be in response to a decrease in turgor of epidermal cells. These reductions in leaf conductance to water vapor, which occurred within 30 minutes of root chilling, could be delayed and temporarily reversed by reducing the leaf-to-air vapor-pressure deficit, suggesting that hydraulic signals may be involved in initiating stomatal closure. By independently manipulating the leaf-to-air vapor-pressure deficit of individual fascicles, we could induce uptake of water vapor through stomata, suggesting that nonsaturated conditions occur in the intercellular airspaces. There was an anomaly in our results on seedlings maintained for 2 days at soil temperatures below 7°C. Lower A appeared primarily the result of nonstomatal limitations, based on large increases in calculated c_i and A versus c_i analysis. In contrast, measurements of O₂ evolution at saturating CO₂ concentrations implied nonstomatal limitations per se did not increase at these temperatures. One explanation for this paradox is that calculations of c_i are unreliable at very low gas-exchange rates because of inadequate measurement resolution, and limitations of A are predominantly stomatal. An alternative interpretation is that increases in c_i are real and the results from O₂-evolution measurements are in error. The high CO₂ concentration used in O₂-evolution measurements (15%) may have overcome nonstomatal limitations by enzymes that were down-regulated by a feedback mechanism. In this scenario, carbohydrate feedback limitations may be responsible for nonstomatal reductions in A after 2 days at soil temperatures below 7°C.