Agroforestry Systems 50: 95-105, 2000. © 2000 Kluwer Academie Publishers. Printed in the Netherlands.
Functional compatibility of two arbuscular mycorrhizae with thirteen fruit trees in Senegal A. M. BÂI,*, C. PLENCHETTE2 , P. DANTHU 3, 1, R. DUPONNOIS 4 and T. GUISSOU5 1 CNRFlISRA, BP. 2312, Dakar, Sénégal; 2 Station d'Agronomie/INRA, 17 rue Sully, 21034 Dijon Cedex, France; 3 ClRAD-Forêt, BP. 1716, Dakar, Sénégal, 4Bio-PédologielIRD, BP. 1386, Dakar, Sénégal; 5DPF/INERA, BP. 7047, Ouagadougou, Burkina Faso (*Autlwr for correspondence: Laboratoire de Microbiologie des Sols, centre ISRA/IRD, BP, J386, Dakar, Sénégal; E-mail:
[email protected])
Key words: arbuscular mycorrhizal fungi, mineraI nutrition, multipurpose fruit trees, relative mycorrhizal dependency, root colonization Abstract. Functional compatibility between thirteen tropical fruit trees (Afzelia africana Smith., Adansonia digitata L., Aphania senegalensis Radlk., Anacardium occidentale L., Cordyla pinnata (Lepr. ex A. Rich.) Milne-Redhead, Dialium guineensis Wild., Landolphia heudelottii A.DC., Sclerocarya birrea (A.Roch.) Hochst., Saba senegalensis (A. DC.) Pichon and four reference hosts Balanites aegyptiaca (L.) Del., Parkia biglobosa (Jacq.), Tamarindus indica L. and Zizyphus mauritiana Lam.) and two arbuscular mycorrhizal fungi (AMF) (Glomus aggregatum Schenck and Smith emend. Schenck and Glomus intraradices Schenck ancl Smith), was investigated. Marked differences were found between them in terms of mycorrhizal formation, root colonization, relative mycorrhizal dependency (RMD) and phosphorus concentrations in shoot tissues. A. africana, L. heudelottii and S. senegalensis did not form symbiotic associations, and the growth of A. africana decreased following mycorrhizal inoculation, while L. heudelottii and S. senegalensis showed no dependency. In contrast, A. digitata, A. scnegalensis, A. occidentale, B. aegyptiaca and S. birrea were weil colonized with AMF, but did not significantly increase in biomass production. Five fruit trees did, however, show depenclency by a positive interaction with G. aggregatum, the most effective AMF. Z. mauritiana was found to be very highly dependent (RMD > 75%), T. indica was highly dependent (50-75% RMD), and D. guineensis, P. biglobosa and C. pinnata were moderately dependent (25-50% RMD). Phosphorus absorption probably contributed to this dependency more than the absorption of potassium. Thèse results indicate that some tropical fruit trees do derive benefits l'rom AM inoculation, while others do not.
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Introduction Multipurpose fruit trees are widespread throughout the Sahelian and Sudanian zones in West Africa. They provide high quality products (fruits, medicines, fibers, etc.), that assume food security, health, and provide a source of income for the people of the rural areas (Bonkoungou et al., 1998). However, many of these fruit trees are slow-growing and little is known about their cultivation. In agroforestry, fruit tree domestication has become a priority for research (Nair, 1998). Domestication of these tree crops could be achieved through a combination of approaches and could include the selection of species by local
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Materials and methods Soil preparation
The soil used in the experiment was collected from Bambey (Senegal). It was a savanna soil with 67.2% sand, 21.5% silt, 11.3% clay, 0.6% organic matter, 0.3% total C, 0.02% total N, C/N ratio 16, 500.2 ppm total K, 83.8 ppm total P, 6.6 ppm P-Bray l, Ca 2.56, Mg 0.82, K 0.07 meq 100 s' soit, pH (of a soil/water mixture, ratio 1:2) 6.4 and pH (of a soillKCl mixture, ratio 1:2) 4.8. The soil was crushed, passed through a 2 mm sieve, autoclaved for 1 hr at 120 "C to eliminate native AMF, and transferred into 2 liters plastic bags.
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Fungal inocula and inoculation
Two isolates of AMF were used: G. aggregatum (Ga, isolate IR 27) was obtained from Burkina Faso (Bâ et al., 1996) and G. intraradices (Gi, isolate 89-30-14) was provided by Dr V. Furlan (Agriculture Canada, Québec, Canada). Guissou et al. (1998a) differentiated one efficient fungal isolate as G. aggregatum and one no efficient fungal isolate as G. intraradices. Mycorrhizal inoculation of the soil in plastic bags was achieved by placing 20 g portions of a crude inoculum of AMF consisting of sand, spores, fragments of hyphae and infected roots below the seeds during transplanting. The inoculum density was calibrated by the most probable number method for each fungus as 1800 and 1500 infective propagules per 20 g of G. aggregatum and G. intraradices, respectively (Gianinazzi-Pearson et al., 1985). The uninoculated control plants received 20 g of sterilized sand-root mixture. Plant materials
Seeds of one provenance of each fruit tree species were provided by the CNRF/ISRA (Senegal). Seeds of A. africana (Aa, provenance Diatock), A. digitata (Ad, provenance Bandia). A. senegalensis (As, provenance Bel-air), A. occidentale (Ao, provenance Sangalkam), B. aegyptiaca (Ba, provenance Bandia), D. guineensis (Dg, provenance Ziguinchor), P. biglobosa (Pb, provenance Ncma), S. birrea (Sb, provenance Bandia), T. il/dico (Ti, provenance Thienaba) and Z. mauritiana (Zm, provenance Keur Serigne Touba) were surface sterilized by treating them with 95% sulphuric acid for 120, 360, 45, 240, 60, 45, 60, 120, 30 and 3 min, respectively. Seeds of C. pinnata (Cp, provenance Kolda), L. heudelottii (Lh, provenance Ziguinchor) and S. senegalensis (Ss, provenance Ziguinchor) were surface sterilized with 10% calcium hypochloride for 10 min. The seeds were then rinsed several times and two seeds were sown per plastic bag. After emergence, seedlings were thinned to one plant per plastic bag. Plants were grown under natural light (daylength approximately 12 hr, mean temperature approximately 30 "C day) and watered
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Figure 1. Root colonization of 13 fruit trees inoculated by two arbuscular mycorrhizal fungi in Senegal, West Africa. For a given fruit tree, columns with the same letter do not differ significantly (P < 5%). There was no colonization in the plants of the control treatment. The abbreviations refer to species names of fungi and trees as explained in 'Materials and methods'.
effective in increasing biomass production, while A. occidentale, A. senegalensis, S. birrea, A. digitata, B. aegyptiaca, L. heudelottii and S. senegalensis did not show any significant differences in biomass production between treatment. In A. africana inoculating plants with AMF had a negative effect on biomass production. RMD values differed between fruit tree species and were significantly influenced by AMF (Figure 3). Z. tnauritiana had thc highest RMD values, reaching 78%, while B. aegyptiaca had the lowest RMD value of 0%, irrespective of AME RMD values of fruit trees were generally superior with G. aggregatum. With this fungus, the RMD of fruit trees decreased in the following order: Z. mauritiana, T. indica, D. guineensis, P. biglobosa, C. pinnata, A. occidentale, A. senegalensis, S. birrea, A. digitata and B. aegyptiaca, L. heudelottii, S. senegalensis and A. africana had no RMD. However, no significant relationship between RMD and the extent of AM colonization by G. aggregatum (r = 0.64, P < 5%).
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Figure 3. Relative mycorrhizal dependency (RMD) of 13 fruit trees in Senegal, West Africa. Columns with the same letter do not differ significantly CP < 5%). The abbreviations refer to species names of fungi and trees as explained in 'Materials and methods'.
in shoots of plants inoculated by this fungus were less than 1.2-fold those of controls (Figures 4 and 5).
Discussion We tested the RMD of thirteen African woody fruit trees of semi-arid lands with two AMF under conditions of P deficiency (P = 6.6 ppm). RMD values rangcd from 0% to 77% and was most clcarly dcmonstratcd with G. aggregatum. Based on these data, we propose a ranking of fruit trees according to the RMD categories defined by Habte and Manajunath (1991): Z. mauritiana was considered very highly dependent (RMD > 75%), T indica was highly dependent (50-75% RMD), D. guineensis, P. biglobosa and C. pinnata werc moderately dependent (25-50% RMD), A. occidentale, A. senegalensis, S. birrea and A. digitata were marginally dependent (0-25% RMD), and L. heudolottii, S. senegalensis, B. aegyptiaca and A. africana wcre found not to be dependent on mycorrhizas (RMD = 0%). This last category includcs species that are not hosts to AMF and those that did not respond positively to AM colonization. Our findings generally agree with those observed by Guissou et al. (1996, 1998a), who found that Z. mauritiana was very highly dependently, irrespective of plant-fungus combinations, and that T. indica and P.
103 species, He suggests that root systems with only a few and short root hairs are indicative of RMD (Baylis, 1970). This hypothesis was not supported Guissou et al. (l998a) who found, in contrast, that the density and length of root hairs of Z. mauritiana, T. indica and P. biglobosa were positively correlated with RMD. This study examined the relationship between the increased growth of mycorrhizal fruit trees and the extent of AM colonization and P uptake by the plant, hypothesizing that RMD is associated with a higher P content. However, no significant relationship was found between RMD and the extent of AM colonization by G. aggregatum. Nor did, the high levels of AM colonization (> 50%) of A. occidentale, S. birrea and A. digitata by G. aggregatum foster an increased biomass production. Conversely, some fruit trees (C, pinnata and A. senegalensis) that were poorly colonized « 50%) by G. aggregatum responded weIl in terms of biomass production. Thus, biomass production of non-host plants with AMF either did not benefit to AM inoculation or decreases following mycorrhizal inoculation. Perhaps, these growth reductions can be attributed to the carbohydrate drain of the AMF (Thomson et al., 1994). An evaluation of RMD of plant species is usually achieved when host plants are tested at a wide range of P concentrations in soil solution (Habte and Manajunath, 1991; Habte and Byappanahalli, 1994; Azcon and Barea, 1997). This generalization agrees with findings of Guissou et al. (l998b), who found that RMD of Z. mauritiana decreased with increasing available P in soil fertilized with different levels of rock phosphate. This experimental approach was not included in the present study because the level of P used was close to that generally found in natural ecosystems in West Africa. ft is weIl known that the major contribution of AM syrnbiosis is to improvc P content in plants, because of the ability of AMF to colonize roots extensively and to devclop external hyphae taking up P from soil by passing the phosphate depletion zone immediately around the root (Ravnskov and Jakobsen, 1995; Schweiger et al., 1995; Schweiger et al., 1999). In our study, K concentrations in shoots contributed less than P to the stimulation of biomass production of fruit trees. This suggests that P concentration had the most consistent effect on biomass production of these fruit trees, so accords with the data of Guissou et al. (1998a). Nevertheless, some mycorrhizal fruit trccs (A. senegalensis and S. birrea) did not accumulate more biomass consistent with P uptake in the shoot tissues, indicating that not all species conform to this stratcgy. In conclusion, our data indicate that fruit trees differ in their response to AM inoculation during the first phase of growth and that AMF also improve plant growth and nu trient content. It is therefore suggested that considerations of RMD categories should be the first level of screening for fruit trees that respond well to AM inoculation. However, the RMD values are specifie to the conditions of this particular experiment where the soil was sterilized. Native population of AMF should therefore be taken into account when fruit seedIings are to be grown in agroforestry systems.
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