Aims Changes in light and temperature are among the most common and most profound environmental perturbations. The independent effects of light and temperature on photosynthesis and respiration are well studied in single leaves, but are less well studied in whole plants. The short and long term influence of light and temperature on carbon use efficiency is also poorly understood, and is commonly modeled to remain constant over a wide range of conditions. We sought to determine the primary effects of changing light at two growth temperatures on photosynthesis, respiration, and their balance, as defined by carbon use efficiency.Methods We separated respiration into growth and maintenance components using whole-canopy gas-exchange in an elevated CO2 environment in a controlled environment, and supplemented that information with tissue analysis.Important findings Decreases in light level decreased carbon use efficiency through a reduction in the maintenance coefficient, increased the growth coefficient, and reduced partitioning of N in protein. Growth temperature did not significantly affect either maintenance or growth respiration coefficients, suggesting that long-term temperature responses can differ greatly from short-term observations.