Arbuscular mycorrhizal fungi in the Sub-Saharan Savannas of Benin an their association with yam (Dioscorea spp.) : potential of yam growth promotion and reduction of nematode infestation

摘要

The arbuscular mycorrhiza is the most widely occurring and important microbial symbiosis forudagricultural crops and well known to facilitate plant mineral nutrient uptake, particularly underudconditions of P-limitation - as it is common in tropical soils due to leaching or/and severeudimmobilization - and, moreover, it is understood to improve plant water relations and provideudresistance against pests and pathogens. Yam (Dioscorea spp.) is the most important tuber crop inudterms of coverage area in West Africa, particularly in Benin and Togo. Alarmingly, the annualudyam production per hectare has recently decreased considerably due to a loss of soil fertility andudpest and disease (especially nematode) damage. Under field conditions, yam and arbuscularudmycorrhizal fungi (AMF) are naturally associated with each other. Currently, however, data onudthe ecology of AMF in West Africa are lacking with very limited information on the mycorrhizaludstatus of yam. There may be potential to improve growth and to protect plants against nematodesudby AMF but this is possibly dependent on specific AMF-nematode-host combinations. Theudpresent project aimed at studying AMF indigenous to West Africa, with respect to yam growthudpromotion and yam nematode suppression. The specific objectives focused on assessing the (1)uddiversity of AMF, including their distribution, abundance and relation to agronomic practicesudand ecological conditions; (2) the mycorrhizal status of yam; and (3) the specific associationsudbetween (a) yam-AMF, and (b) yam-AMF -nematode in vivo under greenhouse conditions.udIn the first part of our study, we determined the influence of three ecological zones (from wettestudto driest) and of land use intensity on the diversity of AMF in the yam growing area of Benin,udWest Africa. In each zone, four ‘natural’ and four ‘cultivated’ sites were selected. ‘Natural’ sitesudincluded three natural forest savannas (at least 25-30 years old) and a long-term fallow (6-7 yearsudold). ‘Cultivated’ sites comprised yam fields established immediately following forest clearance,udmixed cropping maize (Zea mays) and peanut (Arachis hypogaea) fields, peanut fields, and fieldsudunder cotton (Gossypium hirsutum), which was the most intensively managed crop. Soil samplesudwere collected towards the end of the wet season in each zone. AMF spores were extracted andudidentified morphologically. A total of 60 AMF species was detected, with only seven speciesudsporulating in AMF trap cultures that were set up with various AMF host plants. Higher speciesududrichness was observed in the northern most, driest ecological zone Sudan Savanna (SU) than inudthe adjacent zones to the south with increasing humidity, namely the Northern Guinea Savannaud(NG) and the Southern Guinea Savanna (SG), mainly due to a high proportion of species in theudGigasporaceae, Acaulosporaceae and Glomeraceae. Within each ecological zone, spore densityudand species richness were generally higher in the natural savannas and in association with yamudthan in the other cultivated sites. These parameters were lowest under the intensively managedudcotton, and intermediate in the fallows, indicating that the high richness of the natural savannasudis not necessarily restored during fallowing.udAssuming that yam is an arbuscular mycorrhizal crop, we addressed the question of which AMFudspecies are associated with yam. Our aim was to propagate the AMF communities from threeudnatural forests and three adjacent yam fields of the SG in Benin in trap cultures and to assess theudAMF richness, identifying those associated with yam. Soil samples were collected in the dryudseason (February 2005) and used to identify AMF spores directly and also to establish AMF trapudcultures on yam (tissue culture plantlets of D. rotundata and D. cayenensis) and, for comparison,udon Sorghum bicolor. In the trap cultures, AMF root colonization was particularly high in yamud(70-95%), compared with S. bicolor (11-20%). Based on spore morphotyping, 37 AMF speciesudwere detected in the ‘trap’ rhizosphere of S. bicolor, while 28 and 29 species were identified asudfungal symbionts of D. cayenensis and D. rotundata, respectively. Following eight monthsudcultivation in trap cultures, yam tuber dry weight was generally higher in mycorrhizal than inudnon-mycorrhizal control pots.udWe also hypothesized that indigenous AMF species and strains isolated from yam plantlets inudtrap cultures may be more beneficial for yam plant growth compared to non-indigenous isolates.udWe screened indigenous AMF species and strains that have been isolated from the trap culturesudand compared their effects on micro-propagated white yam plantlets (D. rotundata) (cv. TDr89-ud02461) against exotic AMF isolates in pot experiments over seven months. First, we testedudseveral indigenous and non-indigenous (South America and Asia) G. etunicatum strains withudregard to their effect on yam growth promotion and mineral accumulation in the tissues.udSecondly, three isolates each of nine indigenous AMF species and three additional non-tropicaludAMF species were screened on the same yam cultivars. We found that most tropical AMFududisolates of G. etunicatum increased yam tuber dry weight, while the non-tropical AMF isolatesudhad a lower or no effect, but instead increased tuber P concentrations, when compared to nonmycorrhizaludcontrols. Glomus mosseae, G. hoi, G. etunicatum, Acaulospora scrobiculata and A.udspinosa generally had a positive effect on tuber growth, while isolates of G. sinuosum andudKuklospora kentinensis generally did not.udFinally, we assessed the interaction between yam and AMF in the presence or absence of plantudparasitic nematodes. Yam vplants cultivated in vitro were used, which were inoculated withudcommercial inocula of G. mosseae and G. dussii (Biorize, Dijon France). In the presence ofudnematodes (Scutellonema bradys and Meloidogyne spp.), inoculation of G. mosseae generallyudincreased growth of micropropagated yam plantlets and yam tuber weight production, especiallyudcultivars from D. alata. Tubers were, in general, less infected with S. bradys, but not necessarilyudwith Meloidogyne spp. However, application of G. mosseae and G. dussii to micropropagatedudplantlets resulted in improved quality of yam tubers, when challenged with nematodes, comparedudto nematode inoculation without AMF, indicating a positive effect of AMF on yam productivity.udOur results indicate that the AMF richness is high in the ‘yam belt’ of Benin, but that it isudstrongly influenced by the ecological zone and by the intensity of land cultivation after forestudclearance. Our results also indicate that in controlled pot studies, AMF can suppress nematodeuddamage and additionally lead to improved quality and weight of yam tubers. The present resultsudremain preliminary, however, while results from ongoing studies currently in the field will helpudto determine further their potential in the longer term. These results provide exciting prospectsudfor African crop production, in addition to illuminating the wide and diverse species richness ofudWest African AMF and their potential benefits.