Natural Biofertilizers for Organic Agriculture: Productivity and Nutrient Uptake of Medicago

16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
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Natural biofertilizers for organic agriculture: productivity and nutrient uptake of Medicago sativa inoculated with different arbuscular mycorrhizal fungi

Avio, L.[1], Pellegrino, E.[2], Bonari, E.[3], & Giovannetti, M.[4]

Key words: arbuscular mycorrhizas, Glomus mosseae, Glomus intraradices, phosphorus uptake, nitrogen uptake

Abstract

Arbuscular mycorrhizas are symbiotic associations that play a key role in plant nutrition by absorbing and translocating mineral nutrients from soil to host plants. Arbuscular mycorrhizal fungi, which are considered natural biofertilizers, show diverse levels of performance, depending on the ability of different isolates to promote plant growth and health. Here we investigated the performance of geographically different isolates of two fungal species, Glomus mosseae and G. intraradices, by assessing plant growth responses and P and N uptake in Medicago sativa, in order to select the most efficient fungi for this host plant. The four selected Glomus isolates significantly increased shoot dry weights and shoot N and P content of mycorrhizal plants, but their performances were different. In particular, G. intraradices IMA6 significantly differed from G. mosseae IMA1 in inducing larger growth responses relative to all parameters measured.

Introduction

Arbuscular mycorrhizas are symbiotic associations established between fungi belonging to the Phylum Glomeromycota and the roots of most land plants. They play a key role in plant nutrition, since plants receive mineral nutrients, such as P, N, S, K, Ca, Fe, Cu, and Zn, that are absorbed and translocated by extraradical hyphae of these fungi, which spread from mycorrhizal roots into the surrounding soil. Thus, arbuscular mycorrhizal (AM) fungi are considered natural biofertilizers (Smith & Read, 1997). However, AM fungi show diverse levels of performance, depending on the ability of different isolates to promote plant growth by improving mineral nutrition and by increasing tolerance to biotic and abiotic stresses (Giovannetti & Avio, 2002; van der Heijden et al., 1998; Avio et al., 2006). Therefore, selection of mycorrhizal endophytes based on their physiological characters represents a fundamental step for practical utilization of AM fungi. Here we investigated the symbiotic performance of four geographically different isolates of two globally distributed AM fungal species, Glomus mosseae and G. intraradices, by assessing plant growth responses and P and N uptake in Medicago sativa (lucerne), a mycotrophic plant species highly dependent on mycorrhizal symbiosis, particularly in nutrient-poor soils.

Materials and methods

The AM fungi used were: G. mosseae (Nicol. & Gerd.) Gerdemann & Trappe, isolate IMA1 from UK and isolate AZ225C from USA, and G. intraradices Schenck & Smith, isolate IMA5 from Italy and isolate IMA6 from France. The plant species used was the forage legume Medicago sativa cv. Messe.

Seeds of M. sativa were planted into 600 ml plastic pots containing a mixture (1:1) of a sandy loam soil and Terragreen (a calcinated clay). The mixture was steam-sterilized to kill naturally occurring AM fungi. Pots were inoculated either with 90 ml of crude inoculum (mycorrhizal roots and soil containing spores and extraradical mycelium) of one of the four fungal isolates, or with 90 ml of a sterilized mixture of them (non-mycorrhizal control). All the pots received 120 ml of a filtrate obtained by sieving a mixture of the four inocula and of agricultural soil from a M. sativa field, through a 50- µm diameter pore sieve, to ensure a common microflora for all treatments. After emergence, seeds of M. sativa were thinned to 10 per pot. Plants were grown in the greenhouse, supplied with tap water as needed and with a weekly fertilization of half-strength Hoagland’s solution (10 ml per pot). The experiment was a completely randomized design with 5 inoculum treatments (fungal isolates and the control) and 5 replicates. Three months after emergence, plant shoots were harvested by cutting them 1 cm above the soil level, and M. sativa dry weights determined after drying at 95° C for 48 h. Percentage of AM colonisation and total root length were assessed on half of each root system after root staining, using the gridline intersect method.

P concentrations of shoots were measured after sulphuric/perchloric acid digestion using the photometric method. Tissue N concentrations of shoots were assessed using the Kjeldahl method. The total P and N contents were calculated by multiplying P and N concentration values by dry weights.

Analysis of variance (ANOVA) was performed with SPSS 11.0 software after the necessary transformations, and differences between means were determined by the appropriate test. Tukey’s B procedure was used for comparing means.

Results

The four Glomus isolates successfully established mycorrhizal symbioses with M. sativa, while no colonization was observed in the uninoculated plants. Shoot dry weights (SDW) were significantly higher in mycorrhizal plants, and since nutrient concentrations were also higher in inoculated plants, shoot N and P contents of inoculated plants increased by much more than did SDW (Fig. 1). In fact, the mean increase of SDW in mycorrhizal plants was 105%, while increases of N and P content were 135% and 216%, respectively. Although all AM fungal isolates used in this study produced positive growth responses in M. sativa, they affected the host differently: G. intraradices isolate IMA6, which was the best-performing fungal endophyte, produced increases in shoot dry weight and N and P content that were consistently higher than with G. mosseae isolate IMA1 (Fig. 1).

Figure 1: Shoot response variables of Medicago sativa inoculated with isolates of Glomus mosseae (IMA1 and AZ225) and Glomus intraradices (IMA6 and IMA5) or not inoculated (NM). Different letters above bars indicates significant difference, P < 0.05. Error bars show ± SEM.

Discussion

This work shows that different AM fungal isolates differ in their ability to increase the growth and P and N nutrition of M. sativa plants, thus contributing to enhanced nutritional quality of this forage crop. In particular, a G. intraradices isolate (IMA6) showed a better symbiotic performance than a G. mosseae isolate (IMA1). Interestingly, the latter isolate, in a previous comparison with the same isolate of G. intraradices on a different lucerne variety, was the less-performing endophyte (Vasquez et al., 2001). The isolates of G. intraradices and G. mosseae used in this work have different patterns of extraradical mycelial growth, as measured by hyphal length and density or by the number of anastomoses (Avio et al., 2006). Interestingly, G. intraradices IMA6 produced the highest values for all parameters related to extraradical fungal growth (Avio et al., 2006). These data are in agreement with the suggestion that size and developmental patterns of soil-exploring mycelium are important factors in AM fungi efficiency (Jakobsen et al., 1992), although other fungal traits may play a role, such as spatial distribution of hyphae or uptake efficiency of hyphal Pi transporters (Smith et al. 2000; Munkvold et al., 2004).

References

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[1]Istituto di Biologia e Biotecnologia Agraria, C.N.R., Via del Borghetto 80, 56124 Pisa, Italy

[2]Dipartimento di Biologia delle Piante Agrarie, Università di Pisa, Via del Borghetto 80, 56124 Pisa, Italy

[3]Scuola Superiore Sant' Anna di Studi Universitari e di Perfezionamento, P.za Martiri della Libertà 33, 56127 Pisa, Italy

[4] Dipartimento di Biologia delle Piante Agrarie, Università di Pisa, Via del Borghetto 80, 56124 Pisa, Italy E-Mail , Internet