Seeds of the C4 dicot Flaveria bidentis and the C3 dicot Nicotiana tabacum were germinated on expanded cotton plugs and grown for 28-35 days in a sterile culture system with liquid nutrients supplied to the roots in the presence or absence of 5% (w/v) sucrose. Despite previous reports that supplying sucrose exogenously to plants inhibits expression of photosynthetic genes, sucrose application in this system stimulated maximum photosynthetic rates and these were comparable to those measured in glasshouse-grown material. The activities of many photosynthetic enzymes in the youngest fully expanded leaves of both Flaveria and tobacco plants were increased by sucrose feeding. Sucrose levels were increased more than 2-fold in tobacco and 3-fold in Flaveria leaves (on a leaf area basis) by sucrose application, and both hexose and starch levels also increased. Chlorophyll per unit leaf area in sucrose-fed plants increased by approximately 20% in tobacco and 2-fold in Flaveria. There was no evidence of sucrose repression of photosynthetic competence in this experimental system. We propose that, over the concentration range present in the leaves in these experiments, sucrose acts as a positive signal for photosynthetic gene expression but that, above a threshold level, inhibition of gene expression seen in other studies could still occur.