TOLERANCE AND AVOIDANCE OF AL TOXICITY BY MUCUNA-PRURIENS VAR UTILIS AT DIFFERENT LEVELS OF P SUPPLY

Previous laboratory experiments showed that velvet bean Mucuna prurien s is moderately tolerant to the presence of Al (up to 185 mu M) in the root environment, but that it only develops a shallow root system in acid soils. Field experiments showed that Mucuna can tolerate acid sub soil conditions in a homogeneous root environment, but avoids subsoil if topsoil is present. Subsequent split-root experiments with a recirc ulating nutrient solution showed that this subsoil avoidance may be ba sed on an Al avoidance mechanism in the root system. This Al avoidance mechanism, however, was not evident when phosphorus (P) supply to the whole plant was adequate. We thus hypothesized that surface applicati on of P may help to overcome Al avoidance in the subsoil. In a field e xperiment on an ultisol in Lampung (Indonesia), only a moderate increa se in aboveground biomass production was found for a wide range of P a pplication rates, although the soil was low in available P, and the P adsorption isotherm was very steep. An increased P status of the topso il and an increased P concentration in the aboveground biomass (from 5 0 to 75 mmol kg(-1)) had no effect on root development in the subsoil.