Mesophyll diffusion conductance to CO(2) is a key photosynthetic trait that has been studied intensively in the past years. The intention of the present review is to update knowledge of g(m), and highlight the important unknown and controversial aspects that require future work. The photosynthetic limitation imposed by mesophyll conductance is large, and under certain conditions can be the most significant photosynthetic limitation. New evidence shows that anatomical traits, such as cell wall thickness and chloroplast distribution are amongst the stronger determinants of mesophyll conductance, although rapid variations in response to environmental changes might be regulated by other factors such as aquaporin conductance. Gaps in knowledge that should be research priorities for the near future include: how different is mesophyll conductance among phylogenetically distant groups and how has it evolved? Can mesophyll conductance be uncoupled from regulation of the water path? What are the main drivers of mesophyll conductance? The need for mechanistic and phenomenological models of mesophyll conductance and its incorporation in process-based photosynthesis models is also highlighted.

The stable C isotope composition (δC) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and δC. A significant component of the variation in δC was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km altitude, r =0.49, and in foliage, 1.91‰ km, r =0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO partial pressure into the leaf (P -P) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in P -P of stemwood (r =0.45) and foliage (r=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on δC probably dominated the effect of water availability. We also examined the relationship between plant drought stress (Ψ) and δC within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha, Ψ and δC were less (more negative) than at 250 stems ha indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater δC of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage - attributes that increase rates of photosynthesis, reduce P and increase δC. δC was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the δC-Ψ relationship varied between thinning treatments. Within treatments, the relationship between δC and Ψ was strong (0.38

Internal conductance describes the movement of CO(2) from substomatal cavities to sites of carboxylation. Internal conductance has now been measured in approximately 50 species, and in all of these species it is a large limitation of photosynthesis. It accounts for somewhat less than half of the decrease in CO(2) concentrations from the atmosphere to sites of carboxylation. There have been two major findings in the past decade. First, the limitation due to internal conductance (i.e. C(i)-C(c)) is not fixed but varies among species and functional groups. Second, internal conductance is affected by some environmental variables and can change rapidly, for example, in response to leaf temperature, drought stress or CO(2) concentration. Biochemical factors such as carbonic anhydrase or aquaporins are probably responsible for these rapid changes. The determinants of internal conductance remain elusive, but are probably a combination of leaf anatomy, morphology, and biochemical factors. In most plants, the gas phase component of internal conductance is negligible with the majority of resistance resting in the liquid phase from cell walls to sites of carboxylation. The internal conductance story is far from complete and many exciting challenges remain. Internal conductance ought to be included in models of canopy photosynthesis, but before this is feasible additional data on the variation in internal conductance among and within species are urgently required. Future research should also focus on teasing apart the different steps in the diffusion pathway (intercellular spaces, cell wall, plasmalemma, cytosol, and chloroplast envelope) since it is likely that this will provide clues as to what determines internal conductance.

Limited mesophyll diffusion conductance to CO(2) (g(m)) can significantly constrain plant photosynthesis, but the extent of g(m)-limitation is still imperfectly known. As g(m) scales positively with foliage photosynthetic capacity (A), the CO(2) drawdown from substomatal cavities (C(i)) to chloroplasts (C(C), C(i)-C(C)=A/g(m)) rather than g(m) alone characterizes the mesophyll diffusion limitations of photosynthesis. The dependencies of g(m) on A, foliage structure (leaf dry mass per unit area, M(A)), and the resulting drawdowns across a dataset of 81 species of contrasting foliage structure and photosynthetic potentials measured under non-stressed conditions were analysed to describe the structure-driven potential photosynthetic limitations due to g(m). Further the effects of key environmental stress factors and leaf and plant developmental alterations on g(m) and CO(2) drawdown were evaluated and the implications of varying g(m) on foliage photosynthesis in the field were simulated. The meta-analysis demonstrated that g(m) of non-stressed leaves was negatively correlated with M(A), and despite the positive relationship between g(m) and A, the CO(2) drawdown was larger in leaves with more robust structure. The correlations were stronger with mass-based g(m) and A, probably reflecting the circumstance that mesophyll diffusion is a complex three-dimensional process that scales better with mesophyll volume-weighted than with leaf area-weighted traits. The analysis of key environmental stress effects on g(m) and CO(2) drawdowns demonstrated that the effect of individual stresses on CO(2) drawdowns varies depending on the stress effects on foliage structure and assimilation rates. Leaf diffusion limitations are larger in non-senescent older leaves and also in senescent leaves, again reflecting more robust leaf structure and/or non-co-ordinated alterations in leaf photosynthesis and g(m). According to simulation analyses, in plants with a larger part of the overall diffusion conductance from the ambient atmosphere to the chloroplasts in the mesophyll, photosynthesis is less sensitive to changes in stomatal conductance. Accordingly, in harsher environments that support vegetation with tougher long-living stress-tolerant leaves with lower g(m), reductions in stomatal conductance that are common during stress periods are expected to alter photosynthesis less than in species where a larger part of the total diffusion limitation is determined by stomata. While structural robustness improves plant performance under environmental stress, low g(m) and inherently large CO(2) drawdown in robust leaves limits the photosynthesis of these plants more severely under favourable conditions when stomatal conductance is high. The differences in overall responsiveness to environmental modifications of plants with varying g(m) need consideration in current large-scale ecosystem productivity models.

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 ( ∆ ∆ ∆ ∆ ). 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 CO 2 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 drawdown 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). ∆ ∆ ∆ ∆ 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, ∆ ∆ ∆ ∆ may vary by up to 3‰. This is as large or larger than is commonly reported in many interspecific comparisons of ∆ ∆ ∆ ∆ , and adds to previous warnings about simplistic interpretations of WUE based on ∆ ∆ ∆ ∆ . 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.