Possible involvement of condensed tannins in aluminium tolerance of Lotus pedunculatus

We investigated the distribution of aluminium (Al) in the root tips of the Al-tolerant forage legume, Lotus pedunculatus Cav., a species that also acc umulates condensed tannin (proanthocyanidin) in the roots and leaves. Clona l cuttings were grown in low ionic-strength nutrient solutions containing A l at levels that were either stimulatory or inhibitory (5-60 muM). The X-ra y microanalysis of treated root apices revealed Al deposits at all Al conce ntrations. In freeze-fractured root samples from high Al concentrations (30 and 60 muM), deposits were found very close to the root tip. These deposit s were predominantly composed of Al, phosphorus (P) and silicon (Si). At lo w Al concentrations (10 muM), epoxy-embedded root samples were examined and Al deposits were also found near the meristematic areas. At lower concentr ations (10 muM), Al was found associated with P. In all osmium-fixed sample s from high and low Al concentrations, Al was generally found in associatio n with osmium-binding vacuoles. Because of the established high affinity of osmium for condensed tannin, the hypothesis is developed that condensed ta nnins possibly bind and detoxify Al in the root apices of L. pedunculatus.