Aluminium avoidance by Mucuna pruriens

The hypothesis was tested that the avoidance of acid subsoil by the velvet bean Mucuna pruriens is based on a mechanism acting on the whole root system rather than on individual roots. In a split-root experiment with circulating nutrient solution the growth of plants with Al-containing (+/+) or Al-free (0/0) solution on both sides of the root system was compared with that of plants which had a choice (0/+). Two levels of Al (110 and 185 μ M ) were tested at two levels of Ca (50 and 1250 μ M ). In the 185 μ M Al treatment the concentration of monomeric Al varied between 53 μ M , directly after refreshing the solution, and 5 μ M at harvest time.
An external Al concentration of 110 μ M had no effect on shoot and root dry weight, while 185 μ M Al applied to both sides of the root system (+/+) increased root dry weight and reduced shoot dry weight and shoot/root ratio, compared with the 0/0 control. Application of 185 μ M Al to half of the roots, ied to a significant shift in root growth in favour of the control side; this response is described here as Al avoidance. On the basis of total root length, root dry weight and root surface area, the ratio of 0/+ roots was 3.1, 2.8 and 2.4, respectively.
Al avoidance at 185 μ M Al was confirmed in another experiment, in which root response was measured to a local P source, supplied in a third compartment containing only KH₂PO₄. A significant increase in root length and dry weight in this compartment was observed, when other roots of the same plant were growing in the presence of Al. This result indicates that Al avoidance by Mucuna roots is related to P preference.