Al avoidance and Al tolerance of Mucuna pruriens var. utilis: effects of a heterogeneous root environment and the nitrogen form in the root environment

In Mucuna pruriens var. utilis, grown with nitrate-N in a hydroponic split-root system, an Al avoidance reaction of root growth was observed, which was ascribed to local P stress in the Al containing compartment. The Al avoidance reaction was similar to the avoidance of Mucuna roots of acid subsoil in the field where roots grew preferentially in the topsoil. In the present paper the effect of different N forms ($NO_3^ - $ and $NH_4^ + $) on the reactions of Mucuna to Al were studied, since in acid soils N is present as a mixture of $NO_3^ - $ and $NH_4^ + $. No interaction between the N form and Al toxicity was found. A hydroponic split-root experiment with NH₄NO₃ nutrition, which is comparable to the situation in the field, showed that under these conditions Al avoidance did not occur. It is concluded that a relation between the Al avoidance reaction of Mucuna and P stress is still likely.