A Method to Quantify Relative Stomatal, Mesophyll, and Biochemical Limitations to C3 Leaf Photosynthesis in a Specific Growth Environment.

Abstract

Determining relative limitations of stomatal, mesophyll, and biochemical components (l_s, l_m, and l_b, respectively) to photosynthetic rate (A) can help understand leaf ecophysiology for specific growth conditions. The differential method has been widely applied to estimate the relative limitations in C₃ plants, first for comparing two contrasting growth conditions, but also increasingly for a single condition. In addition to the common practical issue that estimating mesophyll conductance is sensitive to measurement noise, the method has a theoretical feature when applied to photosynthesis for a single condition, that is, one of the three in-serial underlying parameters differs from the other two in nature. A new method was proposed, in which a coupled CO₂-diffusion and photosynthesis model was fitted to data for A, using intercellular-CO₂ and ambient-air-CO₂ levels as input, respectively. From two parameter estimates from model-fitting, l_s, l_m, and l_b can be calculated analytically, conditional on that l_s + l_m + l_b = 100%. This method was illustrated using data sets for plants grown under various water, nitrogen, and temperature conditions. The theoretical feature caused the differential method to have a 50% higher estimate of l_b and ~25% lower estimate of l_s and l_m than the new method. Such significant differences were interpreted to result from the differential method defining the limitations in terms of increments in A (dA), whereas the new method defines them in terms of A itself. The measurement noise-sensitive estimation of mesophyll conductance caused an average error of another ~8%. Common misuses and misinterpretations of the differential method were analyzed and discussed.