Internal conductance does not scale with photosynthetic capacity: implications for carbon isotope discrimination and the economics of water and nitrogen use in photosynthesis

Central paradigms of ecophysiology are that there are recognizable and even explicit and predictable patterns among species, genera, and life forms in the economics of water and nitrogen use in photosynthesis and in carbon isotope discrimination (Delta). However most previous examinations have implicitly assumed an infinite internal conductance (g(i)) and/or that internal conductance scales with the biochemical capacity for photosynthesis. Examination of published data for 54 species and a detailed examination for three well-characterized species -Eucalyptus globulus, Pseudotsuga menziesii and Phaseolus vulgaris- show these assumptions to be incorrect. The reduction in concentration of CO2 between the substomatal cavity (C-i) and the site of carbon fixation (C-c) varies greatly among species. Photosynthesis does not scale perfectly with g(i) and there is a general trend for plants with low g(i) to have a larger draw-down from C-i to C-c, further confounding efforts to scale photosynthesis and other attributes with g(i). Variation in the g(i)-photosynthesis relationship contributes to variation in photosynthetic 'use' efficiency of N (PNUE) and water (WUE). Delta is an information-rich signal, but for many species only about two-thirds of this information relates to A/g(s) with the remaining one-third related to A/g(i). Using data for three well-studied species we demonstrate that at common WUE, Delta may vary by up to 3 parts per thousand. This is as large or larger than is commonly reported in many interspecific comparisons of Delta, and adds to previous warnings about simplistic interpretations of WUE based on Delta. A priority for future research should be elucidation of relationships between g(i) and g(s) and how these vary in response to environmental conditions (e.g. soil water, leaf-to-air vapour pressure deficit, temperature) and among species.