Influence of arbuscular mycorrhizae and phosphorus fertilization on growth, nodulation and N2 fixation (15N) in Medicago sativa at four salinity levels

The rose of an isolate of the arbuscular mycorrhizal (AM) fungusGlomus mosseae in the protection ofMedicago sativa (+Rhizobium meliloti) against salt stress induced by the addition of increasing levels of soluble salts was studied. The interactions between soluble P in soil (four levels), mycorrhizal inoculum and degree of salinity in relation to plant growth, nutrition and infective parameters were evaluated. Salt stress was induced by sequential irrigation with saline water having four concentrations of three salts (NaCl, CaCl2, and MgCl2).15N-labelled ammonium sulphate was added to provide a quantitative estimate of N2 fixation under moderate to high salinity levels. N and P concentration and nodule formation increased with the amount of plant-available P or mycorrhizal inoculum in the soil and generally declined as the salinity in the solution culture increased from a moderate to a high level. The mycorrhizal inoculation protected the plants from salt stress more efficiently than any amount of plant-available P in soil, particularly at the highest salinity level applied (43.5 dS m−1). Mycorrhizal inoculation matched the effect on dry matter and nutrition of the addition in the soil of 150 mg P kg−1. Nevertheless the highest saline solution assayed (43.5 dS m−1) affected more severely plants supplemented with phosphorus than those with the addition of mycorrhizal inoculum. Such a saline-depressing effect was 1.5 (biomass), 1.4 (N) and 1.5 (P) times higher in plants supplied with soluble phosphate than with AM inoculum. Mechanisms beyond those mediated by P must be involved in the AM-protectioe effect against salinity. The15N methodology used allowed the determination of N2 fixation as influenced by different P applications compared to mycorrhizal inoculation. A lack of correlation between nodule formation and function (N2 fixation) was evidenced in mycorrhizal-inoculated plants. In spite of the reduced activity per nodule in mycorrhizal-inoculated In spite of the reduced activity per nodule in mycorrhizal-inoculated plants, the N contents determined indicated the highest acquisition of N occurred in plants with the symbiotic status. Moreover, N and P uptake increased while Ca and Mg decreased in AM-inoculated plants. Thus P/Ca ratios and cation/anion balance in general were altered in mycorrhizal treatments. This study therefore confirms previous findings that AM-colonized plants have optional and alternative mechanisms available to satisfy their nutritive requirements and to maintain their physiological status in stress situations and in disturbed ecosystems.The work was supported by CICYT Spain (Project AGR 91-0605-C02/01).Peer reviewe

Influence of mycorrhiza vs. soluble phosphate on growth, nodulation, and N2 fixation (15N) in alfalfa under different levels of water potential

The legume Medicago sativa (+Rhizobium melilott) was grown under controlled conditions to study the interactions between soluble P in soil (four levels), or a mycorrhizal inoculum, and the degree of water potential (four levels) in relation to plant development and N2 fixation. 15N-labelled ammonium sulphate was added to each pot for a qualitative estimate of N2 fixation, in order to rank the effects of the different treatments. Dry-matter yield, nutrient content and nodulation increased with the amount of plant-available P in the soil, and decreased as the water stress increased, for each P-level. The mycorrhizal effect on dry matter, N yield, and on nodulation was little affected by the water potential. Since P uptake was affected by the water content in mycorrhizal plants, additional mechanisms, other than those mediated by P, must be involved in the mycorrhizal activity. There was a positive correlation between N yield and nodulation for the different P levels and the mycorrhizal treatment at all water levels. A high correlation between plant unlabelled N content and atom% 15N excess was also found for all levels of P. In mycorrhizal plants, however, the correlation between unlabelled N yield and 15N was lower. This suggests that mycorrhiza supply plants with other N sources in addition to those derived from the improvement on N2 fixation.The work was included under the CAICYT, Project on Biotechnology.Peer reviewe

Mycorrhiza and phosphate interactions as affecting plant development, N2-fixation, N-transfer and n-uptake from soil in legume-grass mixtures by using a 15N dilution technique

The legume Medicago sativa (alfalfa)(+Rhizobium meliloti) and Lolium perenne (ryegrass) were grown, in a greenhouse, either alone or together in a soil and supplied with increasing amounts of soluble phosphate (P) with or without a vesicular-arbuscular inoculum (VAM). A small amount of 15N-labelled ammonium sulphate was added to each pot to distinguish the sources of N in the plants. The more mycotrophic legume enhance VAM formation by the grass in the mixture at all rates of P additions. Regardless of the cropping system and the P concentration in soil VAM improved, in most cases, dry matter production and the competitive ability of the legume. In spite of that competition from ryegrass reduced alfalfa development with increasing P concentration in the soil. In general VAM increased nodulation and the concentrations of N and P in alfalfa. The 15N enrichment of plant shoots indicated that VAM improved N2-fixation in alfalfa at all rates of P. In mixed cropping, alfalfa derived almost all its N from fixation, but the total amount fixed was decreased by competition from ryegrass in the same pot. The apparent soil N pool size (A-value) for the grass growing alone was significantly higher in mycorrhizal pots and VAM actually increased the total amount of N that the grass derived from soil, supporting a role of VAM in N-uptake. In mixed cropping the various interactions acting on N nutrition of the grass probably mask observations of the actual mechanisms involved, but there was clear isotopic evidence of N-transfer from the legume to the grass in non-mycorrhizal, P-supplemented plants. Apparently VAM enhanced N-transfer in one out of four cases.The work is under the CICYT. Project on Biotechnoloy. One of us (F.E.A.) is on leave from the Department of botany. Science Faculty, University of Damascus (Syria), is indebted to C.S.I.C. (Spain) for facilities allowing her to stay in the Estacion Experimental de! Zaidin.Peer reviewe