A dynamic model of leaf photosynthesis for C3 plants has been developed for examination of the role of the dynamic properties of the photosynthetic apparatus in regulating CO2 assimilation in variable light regimes. The model is modified from the Farquhar‐von Caemmerer‐Berry model by explicitly including metabolite pools and the effects of light activation and deactivation of Calvin cycle enzymes. It is coupled to a dynamic stomatal conductance model, with the assimilation rate at any time being determined by the joint effects of the dynamic biochemical model and the stomatal conductance model on the intercellular CO2 pressure. When parametrized for each species, the model was shown to exhibit responses to step changes in photon flux density that agreed closely with the observed responses for both the understory plant Alocasia macrorrhiza and the crop plant Glycine max. Comparisons of measured and simulated photosynthesis under simulated light regimes having natural patterns of lightfleck frequencies and durations showed that the simulated total for Alocasia was within ±4% of the measured total assimilation, but that both were 12–50% less than the predictions from a steady–state solution of the model. Agreement was within ±10% for Glycine max, and only small differences were apparent between the dynamic and steady–state predictions. The model may therefore be parametrized for quite different species, and is shown to reflect more accurately the dynamics of photosynthesis than earlier dynamic models.

Bundle sheath resistance to diffusion of CO2 (rc) is a critical component of C4 photosynthesis which allows accumulation of inorganic carbon in bundle sheath cells of C4 plants. Several analyses were made to evaluate the magnitude of rc in C4 plants. Experimental data on the O2 inhibition of photosynthesis (Dai et al. (1993) Plant Physiol 103: 83-90; (1995) Plant Physiol 107: 815-825) and rates of photorespiration (de Veau and Burris (1989) Plant Physiol 90: 500-511) in Z. mays at different stages of development were analyzed using mathematical models of C4 photosynthesis. In young and senescing leaves modeled values of rc and the CO2 partial pressure in bundle sheath cells (Cbs) were lower and fractional leakage of CO2 from bundle sheath cells (fL) was higher than in mature leaves. Diffusive resistance of bundle sheath cells of C4 plants was also evaluated by analyzing the response of photosynthetic rates to varying CO2 in Amaranthus edulis in which the C4 cycle was dysfunctional by chemical mutagenesis (Dever et al. (1995) J Exp Bot 46: 1363-1376) and in Sorghum bicolor, Panicum maximum and Panicum miliaceum in which the C4 cycle was chemically inhibited (Brown and Byrd (1993) Plant Physiol 103: 1183-1188). These analyses indicate that in mature leaves of C4 plants the values of rc are substantially lower (ca. 50-200 m(2) s mol(-1)) than previous suggested (ca. 500-1500 m(2) s mol(-1)) for C4 photosynthesis and that there is considerable leakage of CO2 from bundle sheath cells. Nevertheless, rc and Cbs values are sufficiently high in mature leaves to minimize photorespiration in C4 plants under normal levels of CO2.

The response curves of leaf photosynthesis to varying light, temperature and leaf‐to‐air vapour pressure deficit were measured in the C3 plants Flaveria pringlei and Oryza sativa in normal air with a computerized open infrared gas analysis (IRGA) system, and the photochemical efficiency of photosystem II, described as (1–F,/F′m) after Genty. Briantais & Baker (1989, Biochimica et Biophysica Acta 990, 87–92), was simultaneously measured with a modulated fluorometer. A model was written for rates of CO2 fixation as a function of the true rate of O2 evolution measured by fluorescene analysis (Jo2), mesophyll conductance and intercellular CO2 partial pressure. A second model was developed for rates of CO2 fixation as a function of Jo2, mesophyll conductance and stomatal conductance. In the latter case, leaf stomatal conductance was simulated using the stomatal model proposed by Leuning (1995, Plant, Cell and Environment18, 339–355). The rates of CO2 fixation predicted from the models were similar to rates measured by IRGA. The results indicate that there is potential to measure CO2 fixation in C3 plants by combining the non‐invasive measurement of Jo2 by chlorophyll fluorescence analysis with the stomatal conductance model.