Ohmic analogies and metaphorical circuits: Vascular partitioning of leaf air spaces and stomatal patchiness can create apparent undersaturation and gradient inversion.

Abstract

RationaleAnalyses of leaf gas exchange rely on an Ohmic analogy that arrays single stomatal, internal air space, and mesophyll conductances in series. Such models underlie inferences of mesophyll conductance and the relative humidity of leaf airspaces, reported to fall as low as 80%. An unresolved question is whether such Ohmic models are biased with respect to real leaves, whose internal air spaces are chambered at various scales by vasculature. DescriptionTo test whether undersaturation could emerge from modeling artifacts, we compared Ohmic model estimates with true parameter values for a chambered leaf with varying distributions and magnitudes of leaf surface conductance ("patchiness"). Key ResultsDistributions of surface conductance can create large biases in gas exchange calculations. Both apparent unsaturation and internal CO2 gradient inversion can be produced by the evolution of particular distributions of stomatal apertures consistent with a decrease in surface conductance, as might occur under increasing vapor pressure deficit. Main conclusionIn gas exchange experiments, the behaviors of derived quantities defined by simple Ohmic models are highly sensitive to the true partitioning of flux and stomatal apertures across leaf surfaces. We need new methods to disentangle model artifacts from real biological responses.