Abstract.Various aspects of the biochemistry of photosynthetic carbon assimilation in C3 plants are integrated into a form compatible with studies of gas exchange in leaves. These aspects include the kinetic properties of ribulose bisphosphate carboxylaseoxygenase; the requirements of the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles for reduced pyridine nucleotides; the dependence of electron transport on photon flux and the presence of a temperature dependent upper limit to electron transport. The measurements of gas exchange with which the model outputs may be compared include those of the temperature and partial pressure of CO2(p(CO2) ) dependencies of quantum yield, the variation of compensation point with temperature and partial pressure of O2(p(O2)), the dependence of net CO 2 assimilation rate on p(CO2) and irradiance, and the influence of p(CO2) and irradiance on the temperature dependence of assimilation rate.

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Theory is developed to explain the carbon isotopic composition of plants. It is shown how diffusion of gaseous COz can significantly affect carbon isotopic discrimination. The effects on discrimination by diffusion and carboxylation are integrated, yielding a simple relationship between discrimination and the ratio of the intercellular and atmospheric partial pressures of COZ. The effects of dark respiration and photorespiration are also considered, and it is suggested that they have relatively little effect on discrimination other than cia their effects on intercellular p(COz). It is also suggested that various environmental factors such as light, temperature, salinity and drought will also have effects via changes in intercellular p(C0,). A simple method is suggested for assessing water use efficiencies in the field.

A series of experiments is presented investigating short term and long term changes of the nature of the response of rate of CO2 assimilation to intercellular p(CO2). The relationships between CO2 assimilation rate and biochemical components of leaf photosynthesis, such as ribulose-bisphosphate (RuP2) carboxylase-oxygenase activity and electron transport capacity are examined and related to current theory of CO2 assimilation in leaves of C3 species. It was found that the response of the rate of CO2 assimilation to irradiance, partial pressure of O2, p(O2), and temperature was different at low and high intercellular p(CO2), suggesting that CO2 assimilation rate is governed by different processes at low and high intercellular p(CO2). In longer term changes in CO2 assimilation rate, induced by different growth conditions, the initial slope of the response of CO2 assimilation rate to intercellular p(CO2) could be correlated to in vitro measurements of RuP2 carboxylase activity. Also, CO2 assimilation rate at high p(CO2) could be correlated to in vitro measurements of electron transport rate. These results are consistent with the hypothesis that CO2 assimilation rate is limited by the RuP2 saturated rate of the RuP2 carboxylase-oxygenase at low intercellular p(CO2) and by the rate allowed by RuP2 regeneration capacity at high intercellular p(CO2).

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Variation in carbon-isotope composition among and between wheat genotypes was correlated with variation in water-use efficiency in separate pot experiments conducted in spring-summer and in winter. In the main, winter experiment, the water-use efficiencies ranged from 2.0 to 3.7 mmolC/mol H2O (means of four replicates) while the corresponding isotope effects for leaf material ranged from 1.0225 to 1.0194. 13C was more abundant in grain than in leaves and stems. It is suggested that carbon-isotope analysis may be a useful tool in selection for improved water-use efficiency in breeding programmes for C3 species.

Photosynthetic responses to carbon dioxide concentration can provide data on a number of important parameters related to leaf physiology. Methods for fitting a model to such data are briefly described. The method will fit the following parameters: Vcmax, J, TPU, Rd and gm [maximum carboxylation rate allowed by ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco), rate of photosynthetic electron transport (based on NADPH requirement), triose phosphate use, day respiration and mesophyll conductance, respectively]. The method requires at least five data pairs of net CO2 assimilation (A) and [CO2] in the intercellular airspaces of the leaf (Ci) and requires users to indicate the presumed limiting factor. The output is (1) calculated CO2 partial pressure at the sites of carboxylation, Cc, (2) values for the five parameters at the measurement temperature and (3) values adjusted to 25°C to facilitate comparisons. Fitting this model is a way of exploring leaf level photosynthesis. However, interpreting leaf level photosynthesis in terms of underlying biochemistry and biophysics is subject to assumptions that hold to a greater or lesser degree, a major assumption being that all parts of the leaf are behaving in the same way at each instant.

In big-leaf models of canopy photosynthesis, the Ruhisco activity per unit ground area is taken as the sum of activities per unit leaf area within the canopy, and electron transport capacity is similarly summed. Such models overestimate rates of photosynthesis and require empirical curvature factors in the response to irradiance. We show that, with any distribution of leaf nitrogen within the canopy (including optimal), the required curvature factors are not constant hut vary with canopy leaf area index and leaf nitrogen content. We further show that the underlying reason is the difference between the time-averaged and instantaneous distributions of absorbed irradiance, caused by penetration of sunflecks and the range of leaf angles in canopies.These errors are avoided in models that treat the canopy in terms of a number of layers -the multi-layer models. We present an alternative to the multi-layer model: by separately integrating the sunlit and shaded leaf fractions of the canopy, a single layered sun/shade model is obtained, which is as accurate and simpler. The model is a scaled version of a leaf model as distinct from an integrative approach.