Possible involvement of condensed tannins in aluminium tolerance of Lotus pedunculatus

Abstract

We investigated the distribution of aluminium (Al) in the root tips of the Al-tolerant forage legume,Lotus pedunculatus Cav., a species that also accumulates condensed tannin (proanthocyanidin) in the roots and leaves. Clonal cuttings were grown in low ionic-strength nutrient solutions containing Al at levels that were either stimulatory or inhibitory (5–60µM ). The X-ray microanalysis of treated root apices revealed Al deposits at all Al concentrations. In freeze-fractured root samples from high Al concentrations (30 and 60 M ), deposits were found very close to the root tip. These deposits were predominantly composed of Al, phosphorus (P) and silicon (Si). At low Al concentrations (10 µM ), epoxy-embedded root samples were examined and Al deposits were also found near the meristematic areas. At lower concentrations (10 µM ), Al was found associated with P. In all osmium-fixed samples from high and low Al concentrations, Al was generally found in association with osmium-binding vacuoles. Because of the established high affinity of osmium for condensed tannin, the hypothesis is developed that condensed tannins possibly bind and detoxify Al in the root apices of L. pedunculatus.