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The temperature dependence of C 3 photosynthesis is known to vary with growth environment and with species. In an attempt to quantify this variability, a commonly used biochemically based photosynthesis model was parameterized from 19 gas exchange studies on tree and crop species. The parameter values obtained described the shape and amplitude of the temperature responses of the maximum rate of Rubisco activity ( V cmax ) and the potential rate of electron transport ( J max ). Original data sets were used for this review, as it is shown that derived values of V cmax and its temperature response depend strongly on assumptions made in derivation. Values of J max and V cmax at 25 °°°° C varied considerably among species but were strongly correlated, with an average J max : V cmax ratio of 1·67. Two species grown in cold climates, however, had lower ratios. In all studies, the J max : V cmax ratio declined strongly with measurement temperature. The relative temperature responses of J max and V cmax were relatively constant among tree species. Activation energies averaged 50 kJ mol − − − − 1 for J max and 65 kJ mol − − − − 1 for V cmax , and for most species temperature optima averaged 33 °°°° C for J max and 40 °°°° C for V cmax . However, the cold climate tree species had low temperature optima for both J max ( 19 °°°° C) and V cmax (29 °°°° C), suggesting acclimation of both processes to growth temperature. Crop species had somewhat different temperature responses, with higher activation energies for both J max and V cmax , implying narrower peaks in the temperature response for these species. The results thus suggest that both growth environment and plant type can influence the photosynthetic response to temperature. Based on these results, several suggestions are made to improve modelling of temperature responses.

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Plants typically convert only 2% to 4% of the available energy in radiation into new plant growth. This low efficiency has provided an impetus for trying to genetically manipulate plants in order to achieve greater efficiencies. But to what extent can increased photosynthesis be expected to increase plant growth? This question is addressed by treating plant responses to elevated CO 2 as an analog to increasing photosynthesis through plant breeding or genetic manipulations. For plants grown under optimal growth conditions and elevated CO 2 , photosynthetic rates can be more than 50% higher than for plants grown under normal CO 2 concentrations. This reduces to 40% higher for plants grown under the average of optimal and suboptimal conditions, and over the course of a full day, average photosynthetic enhancements under elevated CO 2 are estimated to be about 30%. The 30% enhancement in photosynthesis is reported to increase relative growth rate by only about 10%. This discrepancy is probably due to enhanced carbohydrate availability exceeding many plants' ability to fully utilize it due to nutrient or inherent internal growth limitations. Consequently, growth responses to elevated CO 2 increase with a plant's sink capacity and nutrient status.However, even a 10% enhancement in relative growth rate can translate into absolute growth enhancements of up to 50% during the exponential growth phase of plants. When space constraints and self-shading force an end to exponential growth, ongoing growth enhancements are likely to be closer to the enhancement of relative growth rate.The growth response to elevated CO 2 suggests that increases in photosynthesis almost invariably increase growth, but that the growth response is numerically much smaller than the initial photosynthetic enhancement. This lends partial support to the usefulness of breeding plants with greater photosynthetic capacity, but dramatic growth stimulation should not be expected. The usefulness of increasing photosynthetic capacity can be maximized through changes in management practices and manipulation of other genetic traits to optimize the conditions under which increased photosynthesis can lead to maximal growth increases.Photosynthesis is a relatively inefficient process, with only a maximum of 8% to 10% of the energy in sunlight being converted to the chemical energy in reduced sugars (Long et al., 2006;Zhu et al. 2010). Further considering carbon losses from autotrophic respiration and limitations by other factors such as water and nutrient limitations, realized conversion efficiencies are typically just 2% to 4% of the energy received in sunlight (Long et al., 2006;Zhu et al.