Internal leaf CO₂ transfer conductance diffusional limitation and its consequences for modelling photosynthesis in C₃ plant species




Ethier, Gilbert J.

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Virtually all current estimates of the maximum carboxylation rate (V.) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and the maximum electron transport rate (.Imax) for C3 species used to parameterise Land Surface Models (LSMs) implicitly assume an infinite CO2 transfer conductance from intercellular spaces to the sites of carboxylation (gi). And yet, most measurements in perennial plant species or in ageing or stressed leaves show that gi imposes a significant limitation on photosynthesis. In this study, I demonstrate that many current parameterisations of the photosynthesis model of Farquhar, von Caemmerer & Berry (1980) based on the leaf intercellular CO, concentration (Ci) are incorrect for leaves where gi limits photosynthesis. I show how conventional A-C, curve (net CO2 assimilation rate of a leaf - An - as a function of Ci) fitting methods which rely on a rectangular hyperbola model under the assumption of infinite gi can significantly underestimate Vcmax for such leaves. Alternative V., parameterisations of the conventional method based on a single, apparent Michaelis-Menten constant for CO, evaluated at C; and used for all C3 plants are also found to be inaccurate since the relationship between Vcmax and g; is not conserved among species. To address this problem, I present an alternative curve fitting method that accounts for gi through a non-rectangular hyperbola version of the model of Farquhar et al. (1980). Current estimates of a central Farqhuar et al. (1980) model parameter, Kc(1+O/K0) (effective Michaelis-Menten constant for CO2), vary 4-fold, making it very difficult to justify any single value for the parameterisation of large scale, pan-species LSM studies. Following on previous work published over two decades ago, I demonstrate that the current range of Kc(1+O/K0) values chosen for LSMs is partly an artefact of many inaccurate in vitro determinations, and results in widely different estimates of An for given Vcmac values. Once corrected, the average Kc(1 +O/Ko) value determined in vitro for C, plants is essentially identical to the two in vivo values published to date, but considerable variation within the data set remains due to the poor accuracy of the in vitro determinations. The new A-Ci curve fitting method elaborated in this study suggests new ways of obtaining in vivo estimates of Rubisco's kinetic constants, as I demonstrate through a well-documented example. The CO, transfer conductance was originally considered to be a constitutional property of a leaf related to its internal anatomy. This study provides the first estimates of gi in a coniferous species and examines variation in gi through time and space in relation to anatomical and physiological traits. Gas exchange measurements and subsequent novel A-Ci curve analyses, as well as stable carbon isotope, nitrogen (N), protein, and pigment analyses, were made on upper and lower canopy, current- to 4-year-old needles of 50-year-old Pseudotsuga menziesii trees. During the first growing season, needle thickness and leaf mass per area decreased by 30% from the top to bottom of the canopy. These anatomical changes were accompanied by modest variation in area-based estimates of g , but no causal link could be established between anatomical traits and mass-based estimates of gi, whether in current- year or older foliage. Both gi and the stomata] conductance of leaves were closely coupled to Vcmax, Jmax, and An with all variables decreasing with increasing leaf age. The N content of leaves, as well as the amount of Rubisco and other proteins, increased during the first three growing seasons, then stabilised afterwards. Thus, the age-related photosynthetic nitrogen use efficiency decline of leaves was not a consequence of decreased allocation of nitrogen towards Rubisco and other proteins. Rather, loss of photosynthetic capacity was the result of the decreased activation state of Rubisco and proportional down-regulation of electron transport towards the photosynthetic carbon reduction and photorespiratory cycles in response to a reduction of CO, supply to the chloroplasts' stroma.



Douglas fir, photosynthesis, mathematical models