ABSTRACT
The Leguminosae constitute the third largest family of flowering plants (Sprent, 2001) with approximately 650 genera and 18 000 species (Polhill et al., 1981). It is the most widely distributed family of flowering plants, occupying habitats ranging from rainforest to arid zones throughout the world (Ravin and Polhill, 1981). The Leguminosae consists of three subfamilies, Papilionoideae, Mimosoideae and Caesalpinioideae. Papilionoideae constitute 65% of the legumes and are represented by trees, shrubs and herbs distributed from the tropics to the arctic. The Mimosoideae are the smallest subfamily, comprising 10% of the legumes. Members of this subfamily are often found in the dry areas of the tropics/subtropics and consist mainly of trees and shrubs. The third subfamily, the Caesalpinioideae, comprising 25% of the Leguminosae, are mainly trees growing in the moist tropics. Many legumes play a major role in both natural ecosystems and in agricultural production systems due to their ability to form symbiotic associations (nodules) with Gram-negative, soil-inhabiting bacteria that fix atmospheric N2. This not only renders the plants independent of soil nitrogen, but also makes them major contributors to soil nitrogen supplies for non-leguminous species. As such they contribute to productivity and sustainability. Approximately 20% of the total legume species have been examined for nodulation, representing all three subfamilies (Sprent, 2001). Nodulation is common within the Papilionoideae and Mimosoideae but only 30% of species within the Caesalpinioideae are nodulated (Allen and Allen, 1981; Sprent, 2001).
