PHOSPHATE NUTRITION EFFECTS ON ALUMINUM AVOIDANCE OF MUCUNA-PRURIENS VAR UTILIS

The response of roots of Mucuna pruriens var. utilis to aluminium depends on the duration of exposure and the type of experiment. In a short-term study, the usual reduction of root elongation rate by Al was found. In contrast, a study for a longer period of time, 4 weeks, showed a stimulation of root dry weight and root surface area for 110 or 185 muM Al3+, compared to a no-Al control. In a split-root experiment, the response of root growth to the presence of Al'' in its environment depended on the presence or absence of Al3+ around other parts of the same root system. It was concluded that Al avoidance, rather than Al toxicity or tolerance, explained the root response of Mucuna to acid subsoil conditions in the field. The question was addressed as to whether the Al-avoidance reaction is related to a local response of plants to P stress, leading to increased branch root development close to a P source. Increasing the P supply to the roots indeed resulted in the disappearance of the Al-avoidance reaction in Mucuna. An experiment with the more Al-tolerant Centrosema pubescens demonstrated that Al avoidance is not restricted to Mucuna species. The Al avoidance described here has an important bearing on selection of crop cultivars for a better root development in acid subsoils. To obtain cultivars with a deep root development in acid subsoils, it is not sufficient to screen for Al tolerance of single roots or intact root systems in a homogeneous medium; it is necessary to test for Al avoidance in heterogeneous media