Striga seed-germination activity of root exudates and compounds present in stems of Striga host and nonhost (trap crop) plants is reduced due to root colonization by arbuscular mycorrhizal fungi.

Root colonization by arbuscular mycorrhizal (AM) fungi reduces stimulation of seed germination of the plant parasite Striga (Orobanchaceae). This reduction can affect not only host plants for Striga, resulting in a lower parasite incidence, but also false hosts or trap crops, which induce suicidal Striga seed germination, thereby diminishing their effectiveness. In order to better understand these AM-induced effects, we tested the influence of root colonization by different AM fungi on the seed-germination activity of root exudates of the Striga hermonthica nonhost plants cowpea and cotton on S. hermonthica. We also tested the effect of AM fungi on the seed-germination activity of the Striga gesnerioides host plant cowpea on S. gesnerioides. Moreover, we studied whether mycorrhization affects the transport of seed-germination activity to above-ground plant parts. Mycorrhization not only resulted in a lower seed germination of S. gesnerioides in the presence of root exudates of the S. gesnerioides host cowpea but also seed germination of S. hermonthica was also lower in the presence of root exudates of the S. hermonthica nonhosts cowpea and cotton. Downregulation of the Striga seed-germination activity occurs not only in root exudates upon root colonization by different AM fungi but also in the compounds produced by stems. The lowered seed-germination activity does not appear to depend on the presence of seed germination inhibitors in the root exudates of mycorrhizal plants. The implication for Striga control in the field is discusse

Dark septate root endophytic fungi increase growth of Scots pine seedlings under elevated CO2 through enhanced nitrogen use efficiency.

Although increasing concentrations of atmospheric CO2 are predicted to have substantial impacts on plant growth and functioning of ecosystems, there is insufficient understanding of the responses of belowground processes to such increases. We investigated the effects of different dark septate root endophytic (DSE) fungi on growth and nutrient acquisition by Pinus sylvestris seedlings under conditions of N limitation and at ambient and elevated CO2 (350 or 700 µ1 CO2 l-1). Each seedling was inoculated with one of the following species: Phialocephala fortinii (two strains), Cadophora finlandica, Chloridium paucisporum, Scytalidium vaccinii, Meliniomyces variabilis and M. vraolstadiae. The trial lasted 125 days. During the final 27 days, the seedlings were labeled with 14CO2 and 15NH4+. We measured extraradical hyphal length, internal colonization, plant biomass, 14C allocation, and plant N and 15N content. Under elevated CO2, the biomass of seedlings inoculated with DSE fungi was on average 17% higher than in control seedlings. Simultaneously, below-ground respiration doubled or trebled, and as a consequence carbon use efficiency by the DSE fungi significantly decreased. Shoot N concentration decreased on average by 57% under elevated CO2 and was lowest in seedlings inoculated with S. vaccinii. Carbon gain by the seedlings despite reduced shoot N concentration indicates that DSE fungi increase plant nutrient use efficiency and are therefore more beneficial to the plant under elevated CO

Habitat and diversity of ectomycorrhizal fungi in forests of South Cameroon

Information is lacking on habitat and diversity of ectomycorrhizal (ECM) fungi of African humid forests. For three years, mushroom excursions were carried out in four sites with contrasted soil and altitude characteristics of South Cameroon, during wet seasons. Collected fungi were described in fresh state and dried exsiccates examined for microscopic description before morpho-anatomical identification. ECM fungi abundantly fruited exclusively in mixed caesalp, monodominant Gilbertiodendron and Uapaca forest clumps, independently of elevation, rainfall, topography and soil texture. 21 ECM tree species in 11 genera belonging to two families, Ceasalpiniaceae and Phyllanthaceae, dominated ECM forest clumps. More than 100 putative ECM fungal species in 27 genera were identified and assigned to seven families and one super family group. Members of Russulaceae and Amanitaceae recruited the highest number of ECM taxa, followed by species of Boletoids and Cantharellaceae. Paxillus, Clavulina, Coltricia, Scleroderma, Cortinarius and Inocybe genera contained limited species. 12 ECM fungal species were locally edible, including Lactarius gymnocarpus and all chanterelles. Though, species richness was very large in a mountainous area, all four sites shared a substantial large number of ECM mushroom species. Low plant diversity but high fungal richness recommends ECM forest clumps for biodiversity sanctuaries

Molecular diversity of arbuscular mycorrhizal fungi in onion roots from organic and conventional farming systems in the Netherlands

Diversity and colonization levels of naturally occurring arbuscular mycorrhizal fungi (AMF) in onion roots were studied to compare organic and conventional farming systems in the Netherlands. In 2004, 20 onion fields were sampled in a balanced survey between farming systems and between two regions, namely, Zeeland and Flevoland. In 2005, nine conventional and ten organic fields were additionally surveyed in Flevoland. AMF phylotypes were identified by rDNA sequencing. All plants were colonized, with 60% for arbuscular colonization and 84% for hyphal colonization as grand means. In Zeeland, onion roots from organic fields had higher fractional colonization levels than those from conventional fields. Onion yields in conventional farming were positively correlated with colonization level. Overall, 14 AMF phylotypes were identified. The number of phylotypes per field ranged from one to six. Two phylotypes associated with the Glomus mosseae-coronatum and the G. caledonium-geosporum species complexes were the most abundant, whereas other phylotypes were infrequently found. Organic and conventional farming systems had similar number of phylotypes per field and Shannon diversity indices. A few organic and conventional fields had larger number of phylotypes, including phylotypes associated with the genera Glomus-B, Archaeospora, and Paraglomus. This suggests that farming systems as such did not influence AMF diversity, but rather specific environmental conditions or agricultural practice

Rock-eating mycorrhizas: their role in plant nutrition and biogeochemical cycles

A decade ago, tunnels inside mineral grains were found that were likely formed by hyphae of ectomycorrhizal (EcM) fungi. This observation implied that EcM fungi can dissolve mineral grains. The observation raised several questions on the ecology of these ¿rock-eating¿ fungi. This review addresses the roles of these rock-eating EcM associations in plant nutrition, biogeochemical cycles and pedogenesis. Research approaches ranged from molecular to ecosystem level scales. Nutrient deficiencies change EcM seedling exudation patterns of organic anions and thus their potential to mobilise base cations from minerals. This response was fungal species-specific. Some EcM fungi accelerated mineral weathering. While mineral weathering could also increase the concentrations of phytotoxic aluminium in the soil solution, some EcM fungi increase Al tolerance through an enhanced exudation of oxalate. Through their contribution to Al transport, EcM hyphae could be agents in pedogenesis, especially podzolisation. A modelling study indicated that mineral tunnelling is less important than surface weathering by EcM fungi. With both processes taken together, the contribution of EcM fungi to weathering may be significant. In the field vertical niche differentiation of EcM fungi was shown for EcM root tips and extraradical mycelium. In the field EcM fungi and tunnel densities were correlated. Our results support a role of rock-eating EcM fungi in plant nutrition and biogeochemical cycles. EcM fungal species-specific differences indicate the need for further research with regard to this variation in functional traits

Maatregelen om effecten van eutrofiëring en verzuring in bossen met bijzondere natuurwaarden tegen te gaan: synthese

In het kader van OBN-onderzoek in de zogenaamde Voedselarme en Droge Bossen met Bijzondere Natuurwaarden ('Droge Bossen') is in de periode 1992-1998 onderzoek gedaan naar de mogelijkheden om effecten van eutrofiëring en verzuring tegen te gaan. Hiertoe verscheen in 1992 het pre-advies' over de bruikbaarheid van verschillende maatregelen, waarna in een aantal Dennen- en Eikenbossen proefterreinen zijn ingesteld en maatregelen zijn uitgevoerd. Door middel van monitoring zijn de effecten van de maatregelen op de ontwikkeling van deze bossen gevolgd : daarvoor zijn de bosstructuur, bodemvegetatie, mycoflora, bodemgesteldheid en bodemvochttoestand zowel bij aanvang van het project als 5 jaar na dato opgenomen . De resultaten van de heropname zijn recent verschenen', waarmee de eerste fase van de monitoring werd afgesloten

Farmers' agronomic and social evaluation of productivity yield and N2-fixation in different cowpea varieties and their subsequent residual N effects on a succeeding maize crop

Cowpea-maize rotations form an important component of the farming systems of smallholder farmers in the forest/savannah transitional agro-ecological zone of Ghana. We evaluated five cowpea varieties for grain yield, N-2-fixation, biomass production, and contribution to productivity of subsequent maize grown in rotation. We further analyzed the interrelationship between these technical dimensions and the social acceptability of these cowpea varieties for farmers. Cowpea grain yield ranged between 1.1 and 1.4 t ha(-1) with no significant yield differences among the different varieties. Using the N-15 natural abundance technique, the average proportion of N-2 fixed ranged between 61% for Ayiyi and 77% for Legon prolific. This resulted in average amounts of N-2 fixed in above-ground biomass ranging between 32 and 67 kg N ha(-1), respectively. Variation in estimates due to differences in delta N-15 among reference plants were larger than differences between cowpea varieties. The amount of soil-derived N ranged from 15 to 20 kg N ha(-1). The above-ground net N contribution of the cowpea varieties to the soil (after adjusting for N export in grains) was highest for Legon Prolific (31 kg N ha(-1)) due to high N-2-fixation and high leaf biomass production. Maize grain yield after cowpea without application of mineral N fertilizer ranged between 0.4 t ha(-1) with maize after maize to 1.5 t ha(-1) with Legon Prolific. The N fertilizer equivalence values for the cowpea varieties ranged between 18 and 60 kg N ha(-1). IT810D-1010 was ranked by the farmers as the most preferred cowpea variety due to its white seed type, short-duration, ease of harvesting and good market value. Despite the high leaf biomass production and high amount of N-2 fixed by Legon Prolific, it was generally the least preferred variety due to lower market price, late maturity, least potential cash income (due to the red mottled seed type) and difficulty in harvesting. Although farmers recognized the contribution of cowpea to soil fertility and yields of subsequent maize, they did not consider this as an important criterion for varietal selection. Soil fertility improvement must be considered as an additional benefit rather than a direct selection criterion when designing more sustainable smallholder farming systems