Stomatal and non‐stomatal limitations to carbon assimilation: an evaluation of the path‐dependent method

Abstract Environmental stresses can decrease photosynthesis by a direct effect on photosynthetic capacity of the mesophyll or by a CO₂ limitation resulting from stomatal closure. In the present study, a ‘path‐dependent method’ (Jones, 1985) for the partitioning of a stress‐related decline in assimilation rate between non‐stomatal and stomatal factors was evaluated, using light quality as a ‘stress’. Kinetic data on assimilation rate and conductance of Phragmipedium longifolium following a change in light quality from 95 μmol m⁻²s⁻¹ white light to 95 μmol m⁻²s⁻¹ red light failed to generate a smooth response curve for conductance. Partitioning of limitations on assimilation by a path‐dependent method that utilizes the actual trajectories of conductance and assimilation was therefore not feasible. A simplified path‐dependent method (Jones, 1985) which assumes that either mesophyll cells or guard cells respond first to a stress was applied to steady‐state measurements of assimilation and conductance under red and white illumination. Either 5% or 23% of the observed reduction in assimilation rate under white light was attributable to stomatal factors, depending on whether the ‘stomatal first’ or the ‘mesophyll first’ path was assumed. In the absence of additional information indicating the appropriate choice of path, arbitrary choice may therefore lead to widely divergent estimates, and potentially erroneous conclusions. An alternative approach to the evaluation of the importance to carbon assimilation of stomatal and non‐stomatal factors is suggested.