Influence of growth stage on the association between some tropical legumes and two variant species ofGlomusin an andosol

Abstract

Cowpea (Vigna unguiculata L. cv. Kuromame), pigeonpea (Cajanus cajan L. (Millsp.) cv. ICPL 86009), and groundnut (Arachis hypogaea L. cv, Nakate-yutaka) were grown in a gamma ray-sterilized topsoil (26.14 mg P kg^-1 by Bray II; pH 6.6) of an Andosol (Melanudand) and inoculated with two different species of Glomus (Glomus etunicatum-like yellow spore type (G. et.) and E ₃). Plants were raised in the greenhouse to reach different growth stages. Whereas root colonization of pigeonpea and groundnut roots by G. et. reached its highest level at the flowering stage and remained steady until maturity, in cowpea a steady high colonization was observed from the pod-setting stage. Root colonization by E ₃ was very poor except in cowpea (52%) and pigeonpea (34%) at maturity. Plant growth stimulation coincided with the onset of significant root colonization by G. et. in all plants whereas E ₃ was not generally effective on the growth of these legumes except on stem growth in pigeonpea at the maturity stage. G. et. clearly increased the harvest indices (H.I.) of cowpea at the pod-setting and maturity stages and of pigeonpea at the maturity stage whereas for groundnut the inoculations did not exert any significant effect at the respective stages. In all crops (especially in cowpea and pigeonpea), shoot nitrogen was diluted drastically by inoculations with G. et. and even though E ₃ did not produce any significant effects on these legumes, the concentration of shoot N was higher with E ₃ inoculations compared with G. et., presumably due to the small size of the E ₃-plants leading to a concentration effect. Inoculations with G. et. increased the phosphorus concentration in stems of cowpea during the reproductive stage whilst in pigeonpea this enhancement was observed as early as the vegetative phase and persisted until the flowering stage. For groundnut, in addition to producing a sustained high leaf P concentration from flowering till maturity, colonization with G. et. also resulted in a significantly high stem P level at the flowering stage. Colonization by G. et. induced in cowpea the highest dinitrogen fixation activity (acetylene reduction activity) at the flowering stage while in pigeonpea and groundnut at the pod-setting stage. E ₃, however, did not produce any differences in nodule activities except in pigeonpea at maturity when a significantly higher activity was stimulated.