LOCALIZED INCREASE IN NODULE ACTIVITY BUT NO COMPETITIVE INTERACTION OF COWPEA RHIZOBIA DUE TO PRE-ESTABLISHMENT OF VESICULAR-ARBUSCULAR MYCORRHIZA.

Cowpea [Vigna unguiculata (L.) Walp.] plants were grown in a split-root system using a calcined montmorillonite clay (Turface) as the growth medium. At transplanting, either the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus macrocarpum Tul. & Tul. and 100 mg hydroxyapatite (HAP) or 100, 200 or 400 mg HAP kg-1 without the VAM fungus were mixed into side 1 of the split-root system. Side 2 received only 100 mg HAP kg-1. After 30 d, a combined inoculum of four Rhizobium isolates (TAL169, TAL173, TAL658 and IRC256) was applied to the surface of all pots. At harvest (60 d), root systems were separated from shoots, and nodule activity (acetylene reduction), plant dry weight, nitrogen (N) and phosphate (P) content and the percentage of VAM colonization were determined. There was no significant effect of treatment on either the N or P content of leaves or roots. The VAM fungus, however, significantly (P < 0.01) increased shoot dry weight. Root dry weight and nodule activity were significantly increased only on the VAM side of the split-root system. Rhizobia from 30 nodules per half-root system were identified using fluorescent antibody techniques. Strain TAL173 was detected from 95 % of the nodules and neither the VAM fungus nor P application affected competitive interaction by rhizobia for nodulation sites. Our work suggests the existence of a localized, non-systemic, non-P-mediated influence of a VAM fungus on root dry weight and nodule activity in cowpea. [ABSTRACT FROM AUTHOR]