The Influence of Rhizobium and Arbuscular Mycorrhizal Fungi on Nitrogen and Phosphorus Accumulation by Vicia faba

The association between nitrogen-fixing Rhizobium and leguminous plants is a very well-known symbiotic relationship. However, in the 'real world' of the soil, other symbiotic relationships also occur, such as those between plant roots and mycorrhizal fungi. This raises a whole series of questions. Can two rather different micro-organisms form mutualistic relationships with the same leguminous plant? Do they interact with each other as well as with the plant? What is the overall effect on plant nutrition and growth? These questions have been addressed in a very thorough study by Yinsou Jia and colleagues (Witwatersrand, South Africa, pp. 251�258). They have looked at a range of interactions (of which we focus here on a selection) between Vicia faba (broad bean), an arbuscular mycorrhizal fungus (AMF) and Rhizobium leguminosarum. Plants infected with the AMF showed a greater uptake of P (as has been shown for other AMF�plant symbioses) which in turn led to enhanced N uptake, increased photosynthesis and greater dry matter accumulation. One of the most obvious effects of the establishment of the Rhizobium�plant symbiosis was an increase in plant N; this was correlated with increased P-uptake, which seemed in this instance to be a result of, rather than a cause of, the increase in plant N. As with the AMF infection, photosynthetic efficiency and plant biomass were also increased. In relation to the interaction between the Rhizobium and the AMF, prior infection with Rhizobium reduced the infectivity of the AMF but, interestingly, the reverse was not true. The lower AMF infection rate in the presence of Rhizobium may have been related to the higher N-status of the plant since, in general, AMF colonization was greater in low-N conditions. Finally, productivity was greatest in plants infected with both AMF and Rhizobium and it is possible that the two symbionts act synergistically rather than just additively. Professor J. A. Bryant, University of Exeter, UKj.a.bryant{at}exeter.ac.uk