In C3 plants, CO2 assimilation is limited by RuBP regeneration rate at high CO2. RuBP regeneration rate in turn is determined by either the chloroplast electron transport capacity to generate NADPH and ATP or the activity of Calvin cycle enzymes involved in regeneration of RuBP. Here, transgenic tobacco (Nicotianna tabacum L. cv. W38) expressing an antisense gene directed at the transcript of either the Rieske FeS protein of the cytochrome b6/f complex or the δ subunit of chloroplast ATP synthase have been used to investigate the effect of a reduction of these complexes on chloroplast electron transport rate. Reductions in δ subunit of ATP synthase content reduced electron transport rates. Plants with low ATP synthase content achieved higher electron transport rates per ATP synthase than wild type. In comparison, there was no difference in the electron transport rate per cytochrome b6/f complex in plants with reduced b6/f content and wild type. The electron transport rates decreased more drastically with reductions in cytochrome b6/f complex than ATP synthase content. This suggests that chloroplast electron transport rate is more limited by cytochrome b6/f than ATP synthase content and is a potential target for enhancing photosynthetic capacity in crops.