Partitioning stomatal and non‐stomatal limitations to photosynthesis

Abstract. Plant scientists concerned both with crop improvement and with understanding the control mechanisms of complex processes such as photosynthesis need to identify those processes that are most important in restricting the overall rate and to quantify the relative importance of different components. The techniques that have been used for quantifying the relative importance of component processes in limiting net assimilation rate are reviewed and related to a fundamental definition based on sensitivity analysis. It is concluded that many methods currently in use, including standard resistance analysis, frequently give very misleading answers. In addition, possible methods for apportioning the contributions of different component processes to observed changes in net photosynthetic rate (for example after stress) are also reviewed and compared against a fundamental approach based on sensitivity analysis. Unfortunately, the detailed time course of changes in mesophyll and stomatal properties that is required for application of the basic sensitivity analysis is seldom likely to be available, so that it is usually necessary to adopt an approximate method. The standard approximation that is recommended for calculating the contributions of different component processes to a change in assimilation rate, involves measurements at the initial and final states only. The various methods discussed in this paper are compared using published photosynthetic data for a range of species.