INFLUENCE OF PHOSPHORUS SUPPLY AND LIGHT-INTENSITY ON MYCORRHIZAL RESPONSE IN PISUM-RHIZOBIUM-GLOMUS SYMBIOSIS

The influence of mycorrhizal colonization with Glomus mosseae on param eters of N-2 fixation and plant growth was studied in pot experiments with pea plants (Pisum sativum L.) infected with Rhizobium leguminosar um and supplied with varied levels of phosphorus (P) and nitrogen (N). Reduced light intensities were used to evaluate the dependence of the microsymbionts on assimilate supply. In plants grown with low P suppl y, mycorrhization increased the concentration of P in shoots, and thus N-2 fixation. Reduced light intensity significantly depressed mycorrh izal colonization and nodule growth in low-P plants. When P supply did not limit plant growth and N-2 fixation, however, the percentage of m ycorrhizal colonization was reduced due to the higher P status, and th e microsymbionts were not impaired by low light intensities. To maximi ze carbohydrate supply, another experiment was carried out at high lig ht intensity of 900 mu mol m(-2) s(-1) and with non-limiting P supply. Nitrogen fertilization, given as starter N, enhanced plant growth, bu t delayed nodule formation. Towards flowering, nodulation rapidly incr eased, but less so in Glomus inoculated plants. After 28 days mycorrhi zal plants were lower in shoot dry weight, nodule dry weight and nitro genase activity. The results suggest that under many, but not all, env ironmental conditions the host plant is able to restrict mycorrhizal c olonization and, thus, to prevent impairment of Rhizobium symbiosis.