A Model of the Photosynthesizing Leaf

Abstract

AbstractA model is proposed for the relationship between net photosynthetic rate (N) and light Intensity at a given concentration of CO2 in the air ([CO2]a). The model provides a prediction of the sum of the diffusion resistances (Σr), the capacity (K) of the leaf to fix CO2, the concentration of CO2 at the point of photosynthesis ([CO2]g), and the respiration rate (R). The model fits the available data well and provides a frame work by which future research may be guided.The calculated values of [CO2]g decreased from [CO2]g at the compensation point to a nearly constant value at high tight intensities. [CO2]g high light infensitit‐s range from 32 to 144 μ/l (volume) depending on the species. When these values of [CO2]g, are used in the diffusion equation, the resulting levels of the mesophyll resistance (rm) are lower than those calculated by using the assumptions that [CO2]g equals zero. The plants which had (he higher photosynthetic rates at a given light intensity and [CO2]a had grealer values of [CO2]g than those with lower photo‐synthetic rates.The calculated rates of respiration of wheat leaves were twice as high as those measured in the dark. This suggests that the light respiration rate may be twice as great as the dark respiration rate at the same temperature.The calculated values of K demonstrate variability within and between species. The maximum N was independent of K. A relationship between K and the maximum quantum efficiency, at constant levels of [CO2]g, was demonstrated in several species.The Σr was inversely related to the maximum rate of photosynthesis for the species investigated. The values of rm calculated for cotton were inversely related to [CO2]a suggesting that the transfer of [CO2] in the cell may involve a concentration dependent chemical reaction in addition to or rather than a physical diffusion process.