Crop leaves are subject to continually changing light levels in the field. Photosynthetic efficiency of a crop canopy and productivity will depend significantly on how quickly a leaf can acclimate to a change. One measure of speed of response is the rate of photosynthesis increase toward its steady state on transition from low to high light. This rate was measured for seven genotypes of soybean [ Glycine max ( L.) Merr.]. After 10 min of illumination, cultivar `UA4805' ( UA) had achieved a leaf photosynthetic rate ( Pn) of 23.2 mu mol center dot m-2 center dot s-1, close to its steady- state rate, while the slowest cultivar `Tachinagaha' ( Tc) had only reached 13.0 mu mol center dot m - 2 center dot s - 1 and was still many minutes from obtaining steady state. This difference was further investigated by examining induction at a range of carbon dioxide concentrations. Applying a biochemical model of limitations to photosynthesis to the responses of Pn to intercellular CO2 concentration ( Ci), it was found that the speed of apparent in vivo activation of ribulose- 1: 5- bisphosphate carbo xylase/ oxygenase ( Rubisco) was responsible for this difference. Sequence analysis of the Rubisco activase gene revealed single nucleotide polymorphisms that could relate to this difference. The results show a potential route for selection of cultivars with increased photosynthetic efficiency in fluctuating light.