Selection on offspring size varies within and among families in relation to host nutritional quality

Life history theory often assumes a positive relationship between offspring size and fitness, although the strength and form of this relationship is expected to vary with environmental conditions. In arthropods, surprisingly few studies have examined the influence of larval environment on the offspring size–fitness relation. In phytophagous insects, the few studies that have examined variation in larval host plants have found a negative correlation between host plant nutritional quality and the strength of selection favoring larger offspring size, suggesting that this pattern might be general. I present experimental evidence for such a relationship in a population of the moth Rothschildia lebeau feeding on its three primary host plant species. Unlike previous studies, I consider the effect of offspring size on growth and survival at two levels, both among families and among siblings within families. Neonate caterpillar mass had a significant effect on growth and survival. The effect on growth, however, was weak, resulted primarily from variation among families, and did not differ among host plant diets. The effect on survival was stronger and varied among host plant diets, among families, and within families on different host plants in a manner that was generally consistent with the hypothesized negative correlation between host plant nutritional quality and the strength of selection favoring larger offspring size. Overall, results suggest that the consequences of variation in offspring size for survival within and among families are host plant-dependent in this system.     

Ecological correlates of seed mass variation in Phoradendron juniperinum,a xylem-tapping mistletoe

Summary We investigated several ecological correlates of seed mass variation in the hemiparasitic, xylemtapping mistletoe, Phoradendron juniperinum. Mean seed mass varied two-fold among plants between the ages of 4 and 14 years old and was positively correlated with parental plant age. Both the standard deviation and the coefficient of variation in mean seed mass decreased with increasing plant age demonstrating that, on average, younger plants produced seed with more variable mass. Nitrogen concentrations (mg nitrogen per gram of seed) of both the seed and fruit (pericarp) were not correlated with mass or the age of the parent plant from which the seed was taken. However, the nitrogen content per seed (mg nitrogen per seed) was positively correlated with the mean seed dry mass and the age of the seed parent, suggesting that the carbon to nitrogen ratio of individual seeds remained relatively constant as seed mass increased and plants grew older. Seed germination ranged between 20% and 86% and was positively correlated with mass and parental plant age. Heavier seeds (seeds from older plants) also had the highest root radicle growth rates. Furthermore, the final root radicle length after 76 d of growth was positively correlated with seed dry mass. When grown on a medium containing an extract prepared from the host plant foliage, all seeds showed lower germination, grew more slowly and had shorter overall root radicles, but had significantly greater development of the haustorial disks (the holdfast which forms the host-parasite junction in Phoradendron) than seeds grown on a control medium. Our results suggest that, on average, seeds of greater mass produced by older plants have a greater total resource pool per propagule (fruit + seed). This resource pool may be important in conferring a greater potential for dispersal (fruit), survival, colonization, and establishment.     

Phosphorus source alters host plant response to ectomycorrhizal diversity

We examined the influence of phosphorus source and availability on host plant (Pinus rigida) response to ectomycorrhizal diversity under contrasting P conditions. An ectomycorrhizal richness gradient was established with equimolar P supplied as either inorganic phosphate or organic inositol hexaphosphate. We measured growth and N and P uptake of individual P. rigida seedlings inoculated with one, two, or four species of ectomycorrhizal fungi simultaneously and without mycorrhizas in axenic culture. Whereas colonization of P. rigida by individual species of ectomycorrhizal fungi decreased with increasing fungal richness, colonization of all species combined increased. Plant biomass and N content increased across the ectomycorrhizal richness gradient in the organic but not the inorganic P treatment. Plants grown under organic P conditions had higher N concentration than those grown under inorganic P conditions, but there was no effect of richness. Phosphorus content of plants grown in the organic P treatment increased with increasing ectomycorrhizal richness, but there was no response in the inorganic P treatment. Phosphorus concentration was higher in plants grown at the four-species richness level in the organic P treatment, but there was no effect of diversity under inorganic P conditions. Overall, few ectomycorrhizal composition effects were found on plant growth or nutrient status. Phosphatase activities of individual ectomycorrhizal fungi differed under organic P conditions, but there was no difference in total root system phosphatase expression between the inorganic or organic P treatments or across richness levels. Our results provide evidence that plant response to ectomycorrhizal diversity is dependent on the source and availability of P.     

Morphological types of arbuscular mycorrhizal fungi in roots of weeds on vacant land

Morphological types of arbuscular mycorrhizal (AM) fungi in weeds of vacant land were examined in spring and autumn. In total, 33 plant species belonging to 28 genera in 13 families were examined. The number of plant species with Arum-type AM was higher than those with Paris- or intermediate types in both seasons. Thus, Arum-type colonization may be beneficial for fast-growing plant species on vacant land. There was a strong relationship between plant identity and AM morphological type, as the colonization types were mostly distinguished at the plant family level.     

Ectomycorrhizal and arbuscular mycorrhizal colonization of two species of floodplain willows

To understand the relationships between the distribution of Chosenia arbutifolia and Salix sachalinensis and their mycorrhizal colonization, changes in the quality and types of ectomycorrhizas and arbuscular mycorrhizas of the seedlings of two species were studied at five different sites with different soil conditions in the floodplain of the Satsunai River, Hokkaido. High ectomycorrhizal and low arbuscular mycorrhizal colonization were found in roots of both plants. Ectomycorrhizal colonization of S. sachalinensis in wet sandy or muddy soil conditions was at the same level as that in dry gravelly sites. In contrast, ectomycorrhizal colonization of C. arbutifolia seedlings was lower from wet sandy sites than that from dry gravelly sites. In all study sites, the same three morphological types of ectomycorrhizas were dominant.     

Arbuscular mycorrhizal fungal phylogeny-related interactions with a non-host

Arbuscular mycorrhizal fungi (AMF) form symbiotic relationships with most vascular plants including some gymnosperm species. Although species in the gymnosperm family Pinaceae normally develop ectomycorrhizal associations, AMF hyphae and vesicles, typical of members of the Suborder Glomineae, have been reported in the roots of some Pinaceae species. However, it is not known whether AMF belonging to various species and suborders are able to colonize roots of Pinaceae species and to what extent this influences the performance of Pinaceae seedlings. We tested in each of the Glomaceae, Acaulosporaceae and Gigasporineae AMF families two species for their ability to colonize and affect the growth of Pinus strobus (eastern white pine) in the presence or absence of an AMF host plant (Trifolium pretense—red clover). Glomus intraradices was the only AMF that colonized pine roots, predominantly in the presence of clover, forming intracellular hyphae and vesicles but not arbuscules. Colonization, however, did not relate to increased pine biomass and the overall presence of AMF, regardless of colonization abilities, resulted in a biomass reduction. This effect on pine seedling biomass was explained by the AMF family to which the AMF belonged, indicating that the effects of AMF on the non-host pine may be related to phylogeny. Acaulosporaceae species reduced pine biomass the most whereas, Gigasporineae species had the smallest effect on biomass. These preliminary results suggest that AMF may affect the soil microflora differently among AMF families in previously unsuspected ways with potential consequences for non-AMF host growth.     

Arbuscular mycorrhizal morphology in crops and associated weeds in tropical agro-ecosystems

Morphological types of arbuscular mycorrhizae (AM) in crops and associated weeds were examined in agro-ecosystems. In total, 48 plant species (8 crops and 40 weeds) belonging to 43 genera in 18 families were examined. The number of plant species with Arum-type AM was higher than those with Paris-type AM in the examined plants. AM association was absent in 6 weeds, and the average colonization rate was 62.64% in crops and 52.92% in weeds. AM morphology has been reported in 2 crops and 21 weeds for the first time. The influence of plant identity on AM morphology was also analyzed by arranging the examined plants in a current plant phylogenetic scheme. This analysis showed there was a lack of relationship between plant classification and AM morphological type. Actually, the colonization types were not distinguished at the plant family level, but were mostly distinguished at the species level.     

Effects of two types of herbivores on the population dynamics of a perennial herb

The joint effects of multiple herbivores on their shared host plant have received increasing interest recently. The influence of herbivores on population dynamics of their host plants, especially the relative roles of different types of damage, is, however, still poorly understood. Here, we present a modelling approach, including both deterministic and stochastic matrix modelling, to be used in estimating fitness effects of multiple herbivores on perennial plants. We examined the effects and relative roles of two specialist herbivores, a pre-dispersal seed predator, Euphranta connexa, and a leaf-feeding moth, Abrostola asclepiadis, on the population dynamics and long-term fitness of their shared host plant, a long-lived perennial herb Vincetoxicum hirundinaria (Asclepiadaceae). We collected demographic data during 3 years and combined these data with the effects of natural levels of herbivory measured from the same individuals. We found that both seed predation and leaf herbivory reduced population growth of V. hirundinaria, but only very high damage levels changed the growth trend of the vigorously growing study populations from positive to negative. Demographic modelling indicated that seed predation had a greater impact on plant population growth than leaf herbivory. The effect of leaf herbivory was weaker and diminished with increasing level of seed predation. Evaluation of individual fitness components, however, suggested that leaf herbivory contributed more strongly to host plant fitness than seed predation. Our results emphasize that understanding the effects of a particular herbivore on plant population dynamics requires also knowledge on other herbivores present in the system, because the effect of a particular type of herbivory on plant population dynamics is likely to vary according to the intensity of other types of herbivory. Furthermore, evaluating herbivore impact from using individual fitness components does not necessarily reflect the long-term effects on total plant fitness.     

Impact of the belowground herbivore and potential biological control agent, Ceutorhynchus scrobicollis, on Alliaria petiolata performance

We studied the influence of the root-crown weevil Ceutorhynchus scrobicollis on its host plant Alliaria petiolata, a European biennial herb that is currently invading much of temperate North America. Varying timing of attack and herbivore densities in a common garden allowed to assess seasonality of plant response, density-dependence of impact, and the effect of intraspecific competition on C. scrobicollis recruitment (number of F₁ generation adults emerged). Data collected in the common garden were compared with data collected at field sites. C. scrobicollis is a common weevil in Europe, frequently attaining high attack levels on its host plant. In the common garden, weevil attack decreased plant survival by up to 43%, reduced plant height by 54%, increased the number of shoots by up to four–fold and delayed seed ripening, but had no significant negative effect on seed production. Plants infested in spring allocated less biomass to aboveground plant parts, and remained smaller than plants attacked in autumn, indicating that the latter were able to partly compensate for weevil attack. Increasing weevil density rarely had an effect on A. petiolata performance, and did not increase F₁ recruitment, suggesting strong intraspecific competition. At field sites, C. scrobicollis attack is spread over a long time period, which probably alleviates intraspecific competition. In summary, attack by the root-crown feeding weevil, C. scrobicollis, can substantially reduce growth and survival of A. petiolata. If introduced as a biological control agent into North America, C. scrobicollis is likely to decrease the fitness and competitive superiority of A. petiolata.     

Formation of structures resembling ericoid mycorrhizas by the root endophytic fungus Heteroconium chaetospira within roots of Rhododendron obtusum var. kaempferi

A resynthesis study was conducted to clarify the relationship between the root endophyte, Heteroconium chaetospira and the ericaceous plant, Rhododendron obtusum var. kaempferi. The host plant roots were recovered 2 months after inoculation, and the infection process and colonization pattern of the fungus were observed under a microscope. The hyphae of H. chaetospira developed structures resembling ericoid mycorrhizas, such as hyphal coils within the host epidermal cells. These structures were morphologically the same as previously reported ericoid mycorrhizal structures. The frequencies of hyphal coils within the epidermal cells of host roots ranged from 13 to 20%. H. chaetospira did not promote or reduce host plant growth. This is the first reported study that H. chaetospira is able to form structures resembling mycorrhizas within the roots of ericaceous plants.     

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     

荒漠土壤因子和DSE定殖对克隆植物入侵的响应

克隆植物,尤其是游击型克隆植物,具有很强的扩展能力,通过克隆扩展可侵入到不同生境斑块。克隆植物入侵可能会影响入侵地土壤营养状况和微生物群落。为了探明克隆植物入侵对DSE(dark septate endophytes)活动和土壤理化性质的影响,于2013年6月在克隆植物羊柴(Hedysarum laeve)和沙鞭(Psammochloa villosa)群落空地沿根状茎延伸方向设置样方,分别于6月、8月和10月在样方内分0—10、10—20、20—30、30—40、40—50 cm土层采集土样和根样,研究了不同采样时间羊柴和沙鞭群落空地DSE和土壤理化性质时空变化。结果表明,从6月到10月,随时间后延,克隆植物逐渐侵入群落空地,沙鞭入侵群落空地数和分株数高于羊柴。羊柴群落空地根系DSE定殖率随采样时间后延,逐渐降低,最大值在6月;沙鞭群落空地根系DSE定殖率随采样时间后延,逐渐升高,最大值在10月。随着克隆植物入侵,入侵地土壤中可利用的营养物质含量显著提高,羊柴入侵提高了入侵群落空地土壤碱解N、有效P和速效K含量,沙鞭入侵提高了入侵群落空地土壤碱解N和有效P含量。相关性分析表明,羊柴群落空地DSE定殖率与土壤p H值和电导率显著正相关,沙鞭群落空地DSE定殖率与土壤p H值极显著负相关,与电导率、碱解N和有效P极显著正相关。克隆植物入侵使得土壤环境更有利于克隆植物自身生长,为荒漠植被恢复提供了前提。     

Does pre-dispersal seed predation limit reproduction and population growth in the alpine clonal plant Geum reptans?

We studied the impact of the seed damaging gall midge larva Geomyia alpina on its perennial alpine host plant Geum reptans. We analysed the effect of seed predation on reproduction by seeds, i.e. seed number, seed mass, and seed viability and on growth and clonal propagation of non-protected plants in comparison to plants protected from predation by an insecticide. Additionally, we assessed the consequences of seed predation for population growth using matrix projection modelling. Seed predation resulted in a decrease in total seed mass per flower head by 23.8% in non-protected plants (P < 0.05). Individual seed mass decreased with increasing infestation intensity (P < 0.05). Seed number remained unaffected because the sucking feeding behaviour by gall midge larvae does not evoke seed abortion. Percent germination of seeds from non-protected plants was reduced by 97.9% compared to seeds from protected plants. According to reduced seed viability, modelling revealed a decrease in population growth rate from λ = 1.055 to λ = 1.041. Predation did neither influence total plant biomass nor biomass fractions. But stolon dry-weight of non-protected plants increased by 24.1% (P < 0.05), which may indicate a trade-off between sexual reproduction and clonal propagation. Our results demonstrate that despite substantial reduction of viable seeds, predation by gall midge larvae only slightly affected population growth of G. reptans suggesting that in this alpine species, persistence by longevity and clonal propagation can balance potential seed losses by predation, at least for local population growth.     

Resource specialization in a phytophagous insect: no evidence for genetically based performance trade‐offs across hosts in the field or laboratory

We present a field test of the genetically based performance trade‐off hypothesis for resource specialization in a population of the moth Rothschildia lebeau whose larvae primarily feed on three host plant species. Pairwise correlations between growth vs. growth, survival vs. survival and growth vs. survival across the different hosts were calculated, using families (sibships) as the units of analysis. Of 15 pairwise correlations, 14 were positive, 5 significantly so and none were negative. The same pattern was found using complementary growth and survival data from the laboratory. Overall, we found no evidence of negative genetic correlations in cross‐host performance that would be indicative of performance trade‐offs in this population. Rather, variation among families in performance appears to reflect ‘general vigour’ whereby families that perform well on one host perform well across multiple hosts. We discuss the implications of positive genetic correlations in cross‐host performance in terms of the ecology and evolution of host range. We argue that this genetic architecture facilitates colonization of novel hosts and recolonization of historical hosts, therefore contributing to host shifts, host range expansions, biological invasions and introductions, and host ranges that are regionally broad but locally narrow.     

On the biogeography of seed mass in Germany – distribution patterns and environmental correlates

With environmental factors being spatially structured, plant traits that are related to these factors should exhibit a corresponding spatial pattern. We analyzed the distribution pattern of seed mass in Germany. We calculated the median seed mass for 10' longitude by 6' latitude grid cells across Germany using the trait databases BIOLFLOR and the plant distribution database FLORKART. To explain these distribution patterns of median seed mass, we applied multiple regression analyses on twelve selected environmental variables, accounting for spatial autocorrelation. To deal with collinearities of the predictors, we used hierarchical partitioning to analyze the independent and joint explanatory power of the environmental variables. To test whether statistical relationships are due to hidden correlations between seed mass and plant growth form, we conducted seperate analyses for annual, perennial herbs, shrubs and trees. Low median seed mass was found in the lowlands and river valleys whereas high median seed mass was typical for the rich loess regions and the calcareous mountain ranges. Seed mass exhibited a strong positive correlation with soil pH (62% of variance explained) and a negative correlation with soil moisture (25%). Light was less important as a predictor of seed mass. Within the growth forms we observed similar distribution and correlation patterns pointing to a direct link between seed mass and the environmental variables soil pH and moisture. We argue that this striking relationship with soil pH is caused by the high stress from competition on fertile calcareous sites. The negative moisture effect may be due to drought stress. Both relationships are particulary interesting for the prediction of ecosystem responses to climate and land use changes.     

Effect of competitive interactions between ectomycorrhizal and saprotrophic fungi on Castanea sativa performance

In Northeast of Portugal, the macrofungal community associated to chestnut tree (Castanea sativa Mill.) is rich and diversified. Among fungal species, the ectomycorrhizal Pisolithus tinctorius and the saprotroph Hypholoma fasciculare are common in this habitat. The aim of the present work was to assess the effect of the interaction between both fungi on growth, nutritional status, and physiology of C. sativa seedlings. In pot experiments, C. sativa seedlings were inoculated with P. tinctorius and H. fasciculare individually or in combination. Inoculation with P. tinctorius stimulated the plant growth and resulted in increased foliar-N, foliar-P, and photosynthetic pigment contents. These effects were suppressed when H. fasciculare was simultaneously applied with P. tinctorius. This result could be related to the inhibition of ectomycorrhizal fungus root colonization as a result of antagonism or to the competition for nutrient sources. If chestnut seedlings have been previously inoculated with P. tinctorius, the subsequent inoculation of H. fasciculare 30 days later did not affect root colonization, and mycorrhization benefits were observed. This work confirms an antagonistic interaction between ectomycorrhizal and saprotrophic fungi with consequences on the ectomycorrhizal host physiology. Although P. tinctorius is effective in promoting growth of host trees by establishing mycorrhizae, in the presence of other fungi, it may not always be able to interact with host roots due to an inability to compete with certain fungi.     

安徽万佛山自然保护区常见植物种子大小变异

采用样方法收集了安徽万佛山自然保护区常见的206种植物的种子, 通过测量种子千粒重, 研究了种子大小(seed mass, SM)在不同分类水平及生活型间的变异规律。结果表明: 1) SM在植物种间变异较大, 从小到大分为A、B、C、D、E、F、G 7个数量级, SM平均为21.487 g; C级种子(0.1 g ≤ SM < 1.0 g)最多, 共计73种, 占35.4%; D级种子(1.0 g ≤ SM < 10.0g)次之, 共计56种, 占27.2%; 野菰(Aeginetia indica)种子最小, 仅为0.006 g; 木鳖子(Momordica cochinchinensis)种子最大, 重达1775.192 g。2) SM在科间(平均种子大小)存在极显著差异(p < 0.001), 且变异沿系统发育方向呈显著变小趋势(r = -0.079, p = 0.048)。其中, SM较大的5个科分别为葫芦科、柿树科、楝科、安息香科和金缕梅科; SM较小的5个科分别为列当科、兰科、秋海棠科、苦苣苔科和灯心草科。3)该研究中植物的生活型分为8种类型。其中, 多年生草本优势明显, 计83种, 占39.9%, SM变异包括A-E 5个数量级; 一年生草本次之, 计60种, 占28.8%, SM变异包括A-F 6个数量级; 灌木第三, 计34种, 占16.3%, SM变异包括B-F 5个数量级。SM在8种生活型间(平均种子大小)也存在极显著差异(p < 0.001); 从大到小分别为藤本、乔木、灌木、一年生草本、多年生草本、半灌木、一二年生草本、二或多年生草本; 从草本、半灌木、灌木、乔木到藤本, SM与生活型具有极显著的正相关性(r = 0.220, p < 0.001)。4)种类较多的菊科、禾本科、莎草科及蔷薇科, 各科内属间SM也存在极显著差异(p < 0.001)。本研究结果有助于加深对植物生活史对策中的种子大小对策的理解, 提高对森林植被的管理水平, 丰富保育生态学的种子恢复理论。     

Responses of maternal siblings ofPinus densiflora to simulated add rain

Because most research on acid rain does not consider the genetic composition of seeds and the inherent variations in seed mass and germination date, the potentially diverse patterns of plant response are neglected within a population. Therefore, we examined the effects of both family identity and simulated acid rain onPinus densiflora seedlings over a 34-week period, beginning at germination. We were able to evaluate the importance of family and pH treatment independent of seed mass and germination date by considering the latter two variables as covariates. Seeds were collected from six randomly chosen trees that were assumed to represent different genotypes. After germination in a controlled environment, the seedlings were moved outdoors. Each was then subjected twice a week to 40 mL of simulated acid rain, at a pH of either 3.0 or 5.6. We examined variations in germination date, seedling height, needle numbers and lengths, and root and shoot masses. The maternal family effect was significant for all characters over the entire test period. Strong effects of pH treatment were detected only at the later growth stages. Artificial rain with a pH of 3.0 enhanced growth rates, particularly aboveground, compared with seedlings treated with pH 5.6 rain. However, regardless of pH, certain groups of maternal siblings consistently performed better in almost all parameters. The exception was for mean germination date, which differed among families only at a pH of 3.0. Therefore, although we demonstrated that acid rain affected both the date of germination as well as seedling growth rates inP. densiflora, the responses differed among maternal families, and were also associated with seed mass and germination date.     

Variation and covariation among life-history traits in Rumex acetosella from a successional old-field gradient

In this study morphological variation and the potential for competition to affect biomass and seedling selection of the families of five populations of Rumex acetosella L. sampled along a successional old-field gradient have been investigated. Seeds from 25 families were submitted to four competitive regimes: no competition (one plant per pot), medium competition (two plants/ pot taking plants from the same population), high within-population competition (four individuals from the same population in a pot) and high between-population competition (four individuals from two different populations in a pot). Eight traits were analysed after 3 months of growth for variation among families within populations. A significant difference among families within the two older populations was recorded for sexual biomass and related components. High sensitivity of these traits to density was observed in all populations except the youngest, suggesting specialization to particular environmental conditions in late successional populations, and a good adaptive capacity to buffer environmental variation in the pioneer population. Little significant interaction between competitive regimes and families within populations was found, i.e. genotypes within each population showed little variation in their response to environmental variation. Genotypic variance decreased with increasing competitive conditions for the majority of the traits. However, the percentage of variance in sexual reproduction explained by family was stable among treatments. Tradeoffs between vegetative reproduction and sexual reproduction were recorded at the population level along the successional gradient, with increasing competitive conditions. As succession proceeds, we observed a decrease in sexual reproduction and an increase in vegetative reproduction. At the family level, correlation among traits were similar when plants were grown in the absence of competition and at high density, with a significant negative correlation between sexual reproduction and vegetative reproduction. For both sprout number and sexual biomass, the performance of families grown under all the treatments was positively correlated. Together these results indicate allocational constraints on the reproductive biology of R. acetosella that may be favoured by natural selection and have influenced population differentiation along the successional gradient. However, they also revealed that the potential exists for evolutionary specialization through plasticity, in response to variation in environmental conditions.     

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.