The influence of culture conditions on mycorrhiza formation between the ectomycorrhizal fungus Pisolithus sp. and Afzelia africana Sm. seedlings

Interactions between an isolate of the ectomycorrhizal fungus Pisolithus sp. and Afzelia africana Sm. seedlings were studied at the structural and ultrastructural levels. Several different conditions were tested with or without sugar and in a sterile or nonsterile medium. In the growth cabinet, the A. africana/Pisolithus sp. interactions did not produce ectomycorrhizas. A fungal sheath was formed but no Hartig net, and an unusual host epidermal cell wall was observed. Hyphae of Pisolithus sp. induced modifications of epidermal cells of 15-day-old A. africana seedlings indicative of non-mycorrhizal interactions, such as wall thickening, wall ingrowth, papillae formation, degraded host wall material and the presence of intracellular hyphae. Wall ingrowth consisted of depositions of host cell wall materials giving a positive reaction for polysaccharides; however, wall thickenings and papillae showed no homogeneous reactions for polysaccharides. In glasshouse conditions, inocula of Pisolithus sp. in the form of spores or mycelia entrapped in peat-vermiculite added to sterilized soil produced typical ectomycorrhizae only with 6-month-old A. africana seedlings. Under these conditions, no conspicuous cell wall reactions occurred on A. africana roots. The results demonstrate that the establishment of an association between an ectomycorrhizal fungus and a potential host plant is strongly influenced by seedling age and/or environmental conditions. Therefore, in vitro synthesis is not a conclusive demonstration of a symbiotic relationship.     

Root soluble carbohydrates of Afzelia africana Sm. seedlings and modifications of mycorrhiza establishment in response to the excision of cotyledons

Stem length, number of secondary lateral roots, shoot dry weight and reducing sugar concentrations of root were significantly reduced when translocation of reserves from cotyledons to the roots of Afzelia africana seedlings was interrupted by complete or partial cotyledon excision. The sucrose but not the glucose concentration of lateral roots also decreased significantly after complete cotyledon excision. Hartig net development rather than fungal sheath formation was affected after inoculation with the early fungal isolate E1 and by both late-stage fungal isolates L1 and L2 after partial or complete cotyledon excision. However, mycorrhizal colonization by the early fungal isolate E2 was not affected by cotyledonary reserves, suggesting that this fungal isolate has a lower carbohydrate requirement than fungal isolates E1, L1 and L2. The late-stage fungal isolates L1 and L2 induced a hypersensitivity reaction by epidermal cell walls of the host plant after complete cotyledon excision, suggesting they are more dependent than the early fungal isolate E1 on available root carbohydrate substrates for ectomycorrhizal colonization. These results are discussed in the light of the hypothesis that early and late-stage fungi were different carbohydrate requirements, and that the time sequence of colonization was related to the root carbohydrate status, which increased with time.     

Contrasting responses to ectomycorrhizal inoculation in seedlings of six tropical African tree species

Five caesalpinioid legumes, Afzelia africana, Afzelia bella, Anthonotha macrophylla, Cryptosepalum tetraphylum and Paramacrolobium coeruleum, and one Euphorbiaceae species, Uapaca somon, with a considerable range in seed sizes, exhibited different responses to inoculation by four species of ectomycorrhizal (ECM) fungi, Scleroderma dictyosporum, S. verrucosum, Pisolithus sp. and one thelephoroid sp. in greenhouse conditions. Thelephoroid sp. efficiently colonized seedlings of all of the five caesalpinioid legumes except U. somon, but provided no more growth benefit than the other fungi. Thelephoroid sp. and S. dictyosporum colonized seedlings of U. somon poorly, but stimulated plant growth more than the other fungi. The relative mycorrhizal dependency (RMD) values of the caesalpinioid legumes were never higher than 50%, whilst U. somon had RMD values ranging from 84.6 to 88.6%, irrespective of the fungal species. The RMD values were negatively related to seed mass for all plant species. Potassium concentrations in leaves were more closely related than phosphorus to the stimulation of seedling biomass production by the ECM fungi. Our data support the hypothesis that African caesalpinioid legumes and euphorbe tree species with smaller seeds show higher RMD values than those with the larger seeds.     

Dendrometric characteristics as indicators of pressure of Afzelia africana Sm. dynamic changes in trees found in different climatic zones of Benin

Afzelia africana is a forest species used by local inhabitants for various purposes, especially as forage to feed cattle, as medicinal plant and its wood is used to make furniture or for cooking. Its utilisation in its current form constitutes a threat to this species. However, the lack of data on this species is a hindrance towards drawing up an efficient program for its sustainable management. In order to fill in some gaps in the knowledge of A. africana tree populations, dendrometric characteristics of this species were studied within different climatic zones where it occurs in Benin. Data collected on each of them included height and diameter, and with regard to the levels of pressure, five categories were defined namely: null, weak, moderate, severe and very severe. As far as diameter is concerned every size was taken into consideration in all the climatic zones. However, average diameter and height of the A. africana individuals varied significantly according to climatic zones. Anthropogenic pressure increased while moving from the humid zone towards the drier zone. Moreover, there was a noticeable significant change in the level and quality of pressure between trees found in the different climatic zones in the sense that the lower the height of the trees, the more severe the level of pressure exerted. Such a relationship was not significant when one considers tree diameter in accordance with the climatic zones in the country.     

Solarization in a forest nursery: effect on ectomycorrhizal soil infectivity and soil receptiveness to inoculation with Laccaria bicolor

 Field experiments were carried out in a forest nursery during the summer of 1994 to examine the effect of soil solarization on ectomycorrhizal soil infectivity (ESI) and soil receptiveness to inoculation with Laccaria bicolor. Soil samples from solarized, steamed, fumigated and untreated plots were periodically collected and assayed for ESI. Untreated soil exhibited high ESI. Solarization was as effective as steaming or fumigation in reducing ESI in the uppermost layer. Solarization with a double layer of polyethylene film and fumigation were the only treatments which reduced ESI deeper in the soil. During July, the temperature of covered beds reached 50  °C at a soil depth of 5 cm. Ectomycorrhizal fungi were among the soil-borne fungi most sensitive to solar heating. Soil solarization provides an effective disinfection method for controlled mycorrhization in forest nurseries. Accepted: 10 April 1997     

Characteristic responses of three tropical legumes to the inoculation of two species of VAM fungi in Andosol soils with different fertilities

The growth and mineral nutrition responses were evaluated of three tropical legumes, cowpea (Vigna unguiculata L. cv Kuromame), pigeonpea [Cajanus cajan L. (Millsp.) cv ICPL 86009] and groundnut (Arachis hypogaea cv Nakateyutaka) inoculated with two different species of VAM fungi, Glomus sp. (Glomus etunicatum-like species) and Gigaspora margarita, and grown in Andosols with different fertilities [Bray II-P: topsoil (72 ppm), subsoil (<0.1 ppm)]. Percent fungal root colonization was high in cowpea and groundnut but relatively low in pigeonpea in both soil types. Despite the low rate of root infection, significant growth responses were produced, especially in the inoculated pigeonpea plant. In all legumes, shoot dry matter production was favoured by the inoculations. Increases in shoot biomass due to mycorrhizae were greater in the subsoil than in the topsoil. Mycorrhization raised shoot concentrations of P and Ca (in cowpea and groundnut) and P and K (in pigeonpea) in the topsoil. Whereas the P concentration in shoots in the subsoil was not positively affected by VAM fungi, particularly in cowpea and pigeonpea, the concentration of K in such plants was significantly increased by VAM treatment. The results also showed that mycorrhizal enhancement of shoot micronutrient concentrations was very rare in all plants, with negative effects observed in certain cases. Cu concentration, in particular, was not affected by VAM formation in any of the plants, and Mn and Fe in pigeonpea and groundnut, respectively, remained the same whether plants were mycorrhizal or not. In both soils the three legumes responded to Glomus sp. better than to Gigaspora margarita, and the effects of the VAM fungi on each of the crops relative to the controls were greater in the subsoil than in the topsoil. However, shoot growth of groundnut was not affected as much as cowpea and pigeonpea by the type of soil used. In spite of the relatively low infection of its root, pigeonpea was generally the most responsive of the three legume species in terms of mycorrhizal/nonmycorrhizal ratios.     

构树幼苗对接种丛枝菌根真菌的生长响应

利用石灰岩适生植物构树进行菌根真菌摩西球囊霉(GM)、地表球囊霉(GV)和透光球囊霉(GD)的单独接种(SI)、混合接种(CI)和不接种(N)处理试验,测定了构树幼苗生长3个月后的形态及生长指标.结果表明:接种VA真菌促进了宿主构树的生长,单株地上部分、地下部分生物量和全株生物量等生长指标均较对照组显著提高,4种处理下构树幼苗生物量较对照提高了2.49～8.19倍,其中CI处理生物量最大,而单株叶片数目与对照组没有显著差异.不同接种处理的构树幼苗生长响应不同.在SI处理中,GD对构树幼苗生长效应最大,而CI处理较SI处理对构树幼苗的促进效应更加明显.CI处理中地径、苗高和总叶面积分别是对照组的1.5、2.2和6.0倍.各处理中根冠比最大的为CI(0.446).说明宿主植物与菌种存在一定的选择性.     

Growth and survival of seedlings of native plants in an impoverished and highly disturbed soil following inoculation with arbuscular mycorrhizal fungi

We investigated whether arbuscular mycorrhizas influenced growth and survival of seedlings in an extremely impoverished and highly disturbed soil. Seedlings of four plants species native to the site were either inoculated with native sporocarpic arbuscular mycorrhizal (AM) fungi or fertilised prior to transplanting, and followed over 86 weeks at the site. One treatment was also irrigated with N-rich leachate from the site. In a laboratory experiment, seedlings were fertilised with excess P for 6 weeks, and location of the P store determined. Growth and survival of AM and fertilised seedlings were similar at the site. Inoculated mycorrhizal fungi and roots appeared to extend into the surrounding soil together. P concentration in leaves of all plants was extremely low. Irrigation with leachate increased growth of seedlings. In the laboratory experiment, significantly more P was stored in roots than shoots. We suggest that successful revegetation of extremely disturbed and impoverished sites requires selection of mycorrhizal fungi and plants to suit the edaphic conditions and methods of out-planting.     

Coinoculation efficacy of ectomycorrhizal fungi on Pinus patula seedlings in a nursery

 The coinoculation efficacy of the ectomycorrhizal fungi Laccaria laccata and Thelephora terrestris on the growth and mycorrhizal development of Pinus patula seedlings was studied and compared to individual inoculation of these fungi in a nursery. The total number of mycorrhizas was higher in seedlings inoculated with the combined inoculum than with the individual inocula. The colonization by T. terrestris was higher than L. laccata when the seedlings were inoculated with the two fungi simultaneously. Coinoculation significantly increased the height and dry weight of the seedlings compared with individual inoculation, both in steam-sterilized and unsterilized soil. Accepted: 12 May 1997     

Growth response ofTheobroma cacao L. seeldings to inoculation with vesicular-arbuscular mycorrhizal fungi

Summary The effects of vesicular-arbuscular mycorrhiza (VAM) on the growth and phosphorus uptake of cocoa seedlings (Theobroma cacao L.) grown for 100 days in polythene bags, were studied at five levels of phosphorus fertilization in both steamed and unsterile Bungor Series soil (a fine clayey, kaolinitic isohyperthermic Typic Paleudult). The cocoa seedlings responded well to phosphorus fertilization and mycorrhizal treatments. Plants inoculated with VAM fungi (Gigaspora spp.) gave the most vigorous growth and higher phosphorus in the leaf tissues in unsterile soil compared to plants grown in steamed soil. However, the mycorrhizal effect was significantly more pronounced (P<0.01) in plants grown in steamed than in unsterile soil. High levels of phosphorus application depressed mycorrhizal development. Phosphorus fertilizer applied at the rates of 250 and 500 ug g⁻¹ soil gave maximum root colonization and spore counts in both soil types used.     

Tissue density and growth response of ectomycorrhizal fungi to nitrogen source and concentration

 Amanita rubescens Pers., Lactarius affinis Pk., Leccinum aurantiacum (Fr.) S.F. Gray, Tylopilus felleus (Bull. ex Fe.) Karsten, and two isolates of Suillus intermedius (Smith & Thiers) Smith & Thiers collected from an approximately 55-year-old Pinus resinosa Ait. plantation, and Pisolithus tinctorius (Pers.) Coker & Couch obtained from another source, were tested for their abilities to grow with protein as the primary source of nitrogen. Protein plates contained 63 mg l^–1 N as bovine serum albumen and 7 mg l^–1 N as arginine. Control plates contained only 7 mg l^–1 N as arginine. All isolates except Leccinum aurantiacum and one isolate of S. intermedius attained greater dry weight with protein as the primary source of N. Lactarius affinis, Leccinum aurantiacum, P. tinctorius, and both isolates of S. intermedius had higher tissue densities on protein medium. Amanita rubescens had lower tissue density. To determine if increase in tissue density was an effect of total N concentration or an effect of N source (protein versus arginine), we performed a second experiment in which arginine concentration was increased (7 mg l^–1 N versus 70 mg l^–1 N). The second experiment also included Cenococcum geophilum Fr. but excluded T. felleus. Higher tissue densities with increased nutrients were found in C. geophilum, Lactarius affinis, Leccinum aurantiacum, and both isolates of S. intermedius. Only A. rubescens and P. tinctorius did not have increased densities. The results suggest that these ectomycorrhizal fungi alter their growth forms according to N concentration. At low N concentrations, a growth form likely to promote exploitation of a large volume of medium for a given biomass is produced. At high concentrations, a growth form likely to promote exploitation of a rich source of N is produced. Whether ectomycorrhizal fungi growing in association with roots would act in a similar fashion is not known. Accepted: 30 July 1998     

Effect of ectomycorrhizal fungi on survival and growth of micropropagated plants and seedlings of Castanea sativa mill.

 Four ectomycorrhizal fungi (Amanita muscaria, Laccaria laccata, Piloderma croceum and Pisolithus tinctorius) were used to produce mycorrhiza on seedlings and micropropagated plants of Castanea sativa in vitro. Pisolithus tinctorius was most effective in colonizing roots of both micropropagated plants and seedlings. A. muscaria and L. laccata only colonized a few feeder roots of some plants and Piloderma croceum did not form mycorrhizas. Mycorrhization of micropropagated plants increased survival and growth during weaning. Accepted: 27 February 1996     

Inoculation of containerized Pseudotsuga menziesii and Pinus pinaster seedlings with spores of five species of ectomycorrhizal fungi

 Container-grown Pseudotsuga menziesii and Pinus pinaster seedlings were inoculated with water suspensions of spores of five ectomycorrhizal fungi commonly found in northeastern Spain. Pseudotsuga menziesii seedlings were inoculated with basidiospores of Melanogaster ambiguus, or Rhizopogon subareolatus, or with ascospores of Tuber maculatum. Pinus pinaster seedlings were inoculated with basidiospores of Melanogaster ambiguus, Rhizopogon roseolus or Scleroderma citrinum. The spore concentrations were 10²–10⁷ spores per seedling for Melanogaster ambiguus (in Pseu dotsuga menziesii) and Rhizopogon subareolatus, 10³–10⁷ for Melanogaster ambiguus (in Pinus pinaster), Rhizopogon roseolus, and Scleroderma citrinum, and 10²–10⁴ for Tuber maculatum. Melanogaster ambiguus colonized more short roots in a larger proportion of plants at 10⁷ spores per seedling than at any other rate. The highest colonization by Rhizopogon subareolatus was obtained at 10⁴ spores per seedling and higher, and all inoculated plants became infected at 10⁶ spores per seedling and higher. Tuber maculatum colonized a high percentage of short roots at all rates tested; the proportion of infected plants was over 80% at 10³–10⁴ spores per plant, decreasing to 50% at 10² spores per plant. Rhizopogon roseolus colonized the highest number of short roots on nearly all the inoculated plants when applied at 10⁵ spores per seedling and higher. Scleroderma citrinum colonized a high percentage of short roots on all inoculated plants when applied at 10⁵ spores per seedling and higher. The abundance of sporocarps of Melanogaster ambiguus, Rhizopogon subareolatus, R hizopogon roseolus and Scleroderma citrinum and their colonization ability at relatively low rates allows these spores to be used as ectomycorrhizal inocula on a large scale. Accepted: 27 February 1996     

Field performance in northern Spain of Douglas-fir seedlings Inoculated with ectomycorrhizal fungi

 Experimental plantations were established in northern Spain to determine the effects of different ectomycorrhizal fungi on growth and survival of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) under field conditions. Douglas-fir seedlings were inoculated with Laccaria bicolor S238 mycelia in two bareroot nurseries in central France or with spore suspensions of three hypogeous ectomycorrhizal species: Melanogaster ambiguus, Rhizopogon colossus and R. subareolatus, in a Spanish containerised nursery. The effects of ectomycorrhizal inoculation on plant survival after outplanting were limited, being only significant at the Guipuzkoan (Spain) site, when plants inoculated with L. bicolor S238 were compared to non-inoculated plants grown in non-fumigated soil. L. bicolor S238 had a significant effect on plant growth during the phase of bareroot nursery growth and this difference was maintained after field outplanting. Nursery inoculations with M. ambiguus, R. colossus and R. subareolatus improved plant growth during the first 2 and 3 years after field outplanting. The positive effects of the inoculation treatment on seedling height, root collar diameter and stem volume persisted after 5 years of field growth. Inoculation with these ectomycorrhizal fungi may improve the field performance of Douglas-fir seedlings in northern Spain. Accepted: 12 February 1999     

Effects of carbon dioxide enrichment on response of mycorrhizal pitch pine (Pinus rigida) to aluminum: growth and mineral nutrition

 Carbon dioxide enrichment may increase the Al tolerance of trees by increasing root growth, root exudation and/or mycorrhizal colonization. The effect of elevated CO₂ on the response of mycorrhizal pitch pine (Pinus rigida Mill.) seedlings to Al was determined in two experiments with different levels of nutrients, 0.1- or 0.2-strength Clark solution. During each experiment, seedlings inoculated with the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch were grown 13 weeks in sand irrigated with nutrient solution (pH 3.8) containing 0, 6.25, 12.5, or 25 mg/l Al (0, 232, 463, or 927 μM Al) in growth chambers fumigated with 350 (ambient) or 700 (elevated) μl/l CO₂. At ambient CO₂, in the absence of Al, mean total dry weights (DW) of seedlings at the high nutrient level were 164% higher than those at the low level. Total DW at elevated CO₂, in the absence of Al, was significantly greater than that in ambient CO₂ at the low (+34%) and high (+16%) nutrient levels. Root and shoot DW at both nutrient levels decreased with increasing Al concentrations with Al reducing root growth more than shoot growth. Although visible symptoms of Al toxicity in roots and needles were reduced by CO₂ enrichment, there were no significant CO₂ × Al interactions for shoot or root DW. The percentage of seedling roots that became mycorrhizal was negatively related to nutrient level and was greater at elevated than at ambient CO₂ levels. Generally, elevated CO₂ had little effect on concentration of mineral nutrients in roots and needles. Aluminum reduced concentrations of most nutrients by inhibiting uptake. Received: 18 June 1997 / Accepted: 8 December 1997     

A gene-for-gene model to explain interactions between cultivars of strawberry and races of Phytophthora fragariae var. fragariae

 A gene-for-gene model is postulated to explain the observed interactions between cultivars of strawberry and races of Phytophthora fragariae. Five interacting resistance (R1–R5) and avirulence (Avr1–Avr5) factors explain all the available data involving 15 host genotypes, including the USA and Canadian differential series, and 12 pathogen isolates from North America. Interactions between pathogen isolates and UK and German differentials are also explained by the proposed model. The model makes it possible to develop a universally applicable differential series, to present a systematic, unequivocal nomenclature of races, and to increase the efficiency of breeding programs. Received: 26 April 1996 / Accepted: 19 July 1996     

Relationships between fungal uptake of ammonium, fungal growth and nitrogen availability in ectomycorrhizal Pinus sylvestris seedlings

 Nitrogen deposition and intentional forest fertilisation with nitrogen are known to affect the species composition of ectomycorrhizal fungal communities. To learn more about the mechanisms responsible for these effects, the relations between fungal growth, nitrogen uptake and nitrogen availability were studied in ectomycorrhizal fungi in axenic cultures and in symbiosis with pine seedlings. Effects of different levels of inorganic nitrogen (NH₄) on the mycelial growth of four isolates of Paxillus involutus and two isolates of Suillus bovinus were assessed. With pine seedlings, fungal uptake of ¹⁵N-labelled NH₄ was studied in short-term incubation experiments (72 h) in microcosms and in long-term incubation experiments (3 months) in pot cultures. For P. involutus growing in symbiosis with pine seedlings, isolates with higher NH₄ uptake were affected more negatively at high levels of nitrogen availability than isolates with lower uptake. More NH₄ was allocated to shoots of seedlings colonised by a high-uptake isolate, indicating transfer of a larger fraction of assimilated NH₄ to the host than with isolates showing lower NH₄ uptake rates. Thus low rates of N uptake and N transfer to the host may enable EM fungi avoid stress induced by elevated levels of nitrogen. Seedlings colonised by S. bovinus transferred a larger fraction of the ¹⁵N label to the shoots than seedlings colonised by P. involutus. Seedling shoot growth probably constituted a greater carbon sink in pot cultures than in microcosms, since the mycelial growth of P. involutus was more sensitive to high NH₄ in pots. There was no homology in mycelial growth rate between pure culture and growth in symbiosis, but N uptake in pure culture corresponded to that during growth in symbiosis. No relationship was found between deposition of antropogenic nitrogen at the sites of origin of the P. involutus isolates and their mycelial growth or uptake of inorganic nitrogen. Accepted: 18 September 1998     

Plasticity of seed output in response to soil nutrients and density in Abutilon theophrasti: implications for maintenance of genetic variation

 Seed output is determined by two processes: resource acquisition and the allocation of resources to seeds. In order to clarify how the reaction norm of seed output is controlled by the phenotypic expression of its two components, we examined the genetic components of plasticity of seed dry mass, plant size, and reproductive allocation under different conditions of soil nutrient availability and conspecific competition among eight families of Abutilon theophrasti. Without competition, the reaction norm of seed mass of the families crossed between the lowest and other nutrient levels, although neither of its components, plant size and reproductive allocation, showed such a response. The crossing reaction norm (i.e., reversal of relative fitnesses of different genotypes along the environmental gradient) of seed mass resulted from (1) a trade-off between plant size and reproductive allocation, and (2) changes in the relative magnitude of genetic variances in plant size and reproductive allocation with soil nutrient availability. While allocation was more important in determining seed mass under limiting nutrient conditions, plant size became more important under high-nutrient conditions. There were no significant genetic variances in seed mass, plant size, and reproductive allocation in the competition treatment, except at the highest nutrient level. The results show that plant competition mitigated the effects of genetic differences in plant performance among the families. We discuss the results in relation to maintenance of genetic variation within a population. Received: 16 August 1996 / Accepted 26 April 1997