Field responses to cobalt and molybdenum by different legume species, with inferences on the role of cobalt in legume growth

Abstract

Six legume species were sown over two seasons on a sandy lateritic soil of marginal cobalt and molybdenum status, with varying rates and combinations of applied cobalt and molybdenum. The seeds were from plants previously grown on the same soil without cobalt or molybdenum addition.

Species differed in their responses. Lupinus angustifolius responded strongly to cobalt, which increased dry matter yield by nearly 50%, but at most only marginally to molybdenum. Lupinus cosentinii, Vicia atropurpurea and Trifolium subterraneum responded to molybdenum but not to cobalt. Lupinus luteus, and more doubtfully Trifolium hirtum, responded to neither element.

Yield responses to molybdenum were always accompanied by increased nitrogen concentrations in the tops. Cobalt application resulted in either no change or a reduction ir, nitrogen concentration in the tops, even when yield was increased. No interaction was evident between the two elements.

Neither element increased nodule numbers, which were ample in all treatments, but in L. angustfolius cobalt markedly increased both nodule size and to a lesser extent crown nodule incidence and slightly increased leg-haemoglobin concentration.

Possible reasons are discussed for the unexpectedly unchanged or reduced nitrogen concentrations in the tops of L. angustifolius showing yield responses to cobalt. One suggestion is that enhanced rhizobium and nodule growth resulted in greater cytokinin production, with a greater effect on top growth under the conditions of the experiments than that stemming from increased nitrogen fixation.