Solubility of Gases and the Temperature Dependency of Whole Leaf Affinities for Carbon Dioxide and Oxygen

Abstract

An analysis of the kinetics of simultaneous photosynthesis and photorespiration at the end of a diffusion path is applied to observed net photosynthetic rate as a function of 02 and CO2 concentrations. The data of Ku and Edwards (Plant Physiol. 59. 991-999,1977) from wheat ( Triticum aestivum L.) are analyzed in detail. Ku and Edwards, using an analysis that ignored diffusion resistance between the intercellular air space and fixation site, the competitive effect of CO2 on photorespiration, and the actual concentrations of gases at the fixation site, concluded that: (a) the affinity coefficient of the leaf for CO2 was approximately 3.5 to 5 micromolar, (b) this affinity coefficient is independent of temperature between 25 and 35 C; (c) the effect of 02 was independent of temperature over this range; and (d) competition between CO2 and 02 is responsible for the major share of CO2 loss from photosynthesis due to photorespiration. They suggest that using gas concentrations calculated as equilibium values in the liquid phase is very important in reaching these conclusions. By applying a more complete analysis to their data which includes diffusion in the cell, it is concluded that: (a) the affinity coefficient of the leaf for CO2 is 0.1 to 1.1 micromolar, (b) the temperature dependence of this affinity coefficient cannot be determined from existing data, but there is no evidence to refute independent temperature effect on the two functions of ribulose-1,5-bisphosphate carboxylase-oxygenase being important in the regulation of whole leaf net photosynthesis; and (c) the competitive interplay of CO2 and 02 at ribulose-1,5-bisphosphate carboxylase may under certain conditions lead to a stimulation of fixation by the Calvin cycle because of photorespiration. These conclusions are reached whether CO2 and 02 are expressed as dissolved concentrations or as gas concentrations in the intercellular air space. The relative merits of these two expressions of concentration are discussed. Ku and Edwards (12,13) the whole leaf affinity coefficients for CO2 and 02 regulating total photosynthesis and the photorespiratory process according to the Bowes-Ogren hypothesis (2)? The objective of this paper is to reevaluate the conclusions of Ku and Edwards (12, 13) using an alternative analysis. This second interpretation explains the original data but may lead to very different conclusions, which are fundamental to understanding the photosynthetic response of whole plant leaves to environmental factors.In our analysis (31-33), a stepwise procedure is used to elaborate the environmental regulation of net photosynthesis in terms of physiologically meaningful parameters that characterize: (a) the production of photoproducts in the light reactions of photosynthesis; (b) transport of CO2 from the intercellular air space to the site of fixation; (c) enzymic fixation of C02; and (d) reutilization of photosynthate products in the simultaneous processes of photorespiration and dark respiration. Stomatal resistance is treated as a separ...

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“…The 'subatmospheric' conditions used in assays where the CO2 concentration was 60 to 70% of that found in ambient air is consistent with available data for CO2 in the intercellular spaces (24). In (14).…”

A CO2Concentrating System in Leaves of Higher C3-Plants Predicted by a Model Based on RuBP Carboxylase/Oxygenase Kinetics and14CO2/12CO2Exchange

“…Tenhunen, Weber, Yocum, and Gates (1979) applied RuBPCO kinetics and the diffusion resistance concept to data on photosynthetic gas exchange as influenced by CO 2 and O 2 from Ku and Edwards (1977). They calculated Michaelis constants for stromal CO 2 (K c ) of 0-4 mmol m"…”

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