The uptake of 59Fe from FeC13, ferric (Fe 3+) citrate (FeCitr) and Fe3+-EDTA (FeEDTA) was studied in leaf mesophyll of Vigna unguiculata (L.) Walp. Uptake rates decreased in the order FeC13 > FeCitr>> FeEDTA, and uptake depended on an obligatory reduction step of Fe 3+ to Fe 2+, after which the ion could be taken up independently of the chelator, citrate. Uptake was strongly increased by photosynthetically active light (2 > 630 nm), and kinetic analysis revealed saturation kinetics with a K m (FeCitr) of 80-110 laM. In the presence of an external Fe 2+ scavenger, bathophenanthroline disulfonate, the mesophyll also reduced external FeCitr with a K m of approx. 50--60 laM. The reduction rates for FeCitr were fiveto eightfold higher than necessary for uptake. Purified plasma membranes from leaves revealed an NADH-dependent FeCitr-and FeEDTA-reductase activity, which had a pH optimum of 6.5-6.8 and a K m of approx. 20 laM for NADH. Under anaerobic conditions, a K m of 130-170 laM, for ferric chelates was obtained, while in the presence of oxygen a K m (FeCitr) of approx. 100 laM was found. It is concluded that the leaf plasma membrane provides a ferric-chelate-reductase activity, which plays a crucial role in iron uptake of leaf cells. Under in-vivo conditions, however, reactive oxygen species or strong (blue) light may also contribute to the obligatory reduction of Fe 3+ prior to uptake.