The role of plant size and nutrient concentrations in associations betweenMedicago, and Rhizobium and/or Glomus

The aim of this research was to carry out a critical study of the method of obtaining size equivalence between nonsymbiotic alfalfa and alfalfa associ ated with Glomus and/or Rhizobium by applying fixed addition rates of nutri ents to the non-symbiotic controls. The experimental design included three nutrient response curves in which the levels of added phosphorus and/or nit rogen were constant during the whole plant growth process: 1) a phosphorus response curve, in order to compare the growth of double symbiotic plants w ith that of only-Rhizobium inoculated ones; 2) a nitrogen response curve, t hat consisted of a comparison between the growth of double symbiotic alfalf a and four treatments associated only with Glomus; 3) a phosphorus and nitr ogen response curve, to compare the growth of non-inoculated alfalfa with t hat of double symbiotic plants. Although similar size was achieved among so me treatments at harvest, shoot growth over time and nutrient concentration s in tissues differed, indicating that growth equivalence did not mean func tional equivalence. A second experimental design was performed taking into account the establishment of microsymbionts for determining the adequate mo ment to add supplemental phosphorus and/or nitrogen. It included four treat ments: a) double symbiotic plants (MR): 6) plants inoculated with Rhizobium only (R); c) plants inoculated with Glomus only (M), and d) non-inoculated plants (N). Great similarity in terms of plant growth and nutrient content s in tissues were obtained. Moreover, symbiotic plants were able to produce similar dry matter than non-symbiotic ones under P and N limitations.