Influence of two legume species on hyphal production and activity of two arbuscular mycorrhizal fungi

 Two arbuscular mycorrhizal (AM) fungi (Glomus mosseae and G. intraradices) were compared for abundance of intraradical and soil-borne hyphae in association with Astragalus sinicum, a small-seeded, and Glycine max, a large-seeded legume. A. sinicum was more responsive than G. max to mycorrhizal formation, especially at early growth stages. Biomass allocation was greater in roots than shoots for mycorrhizal A. sinicum, while the opposite was true for G. max. Hyphal development in root and soil compartments was estimated by trypan blue staining and after staining for succinate dehydrogenase (SDH) or alkaline phosphatase (ALP) activity. Total fungal abundance increased steadily in roots and soil with time to a maximum 8 weeks after planting. SDH- and ALP-active AM hyphae increased in roots during plant growth but decreased in soil at later harvests. Mycorrhizal root mass in A. sinicum and G. max increased about 14-fold and 2.5-fold, respectively, but total length of soil hyphae produced per plant differed little, so that the pattern of AM soil to root abundance of the two fungi varied considerably with the host plant. Accepted: 23 July 1997     

红椎和西南桦营养元素的含量与储量特征

营养元素在植物不同器官及不同营养元素在同一器官中的含量与储量特征,不仅反映植物自身的生物学特性,而且还反映了植物在一定的生境下从土壤中吸收和蓄积矿质养分的能力。为了阐明我国南亚热带不同珍优乡土阔叶造林树种营养元素含量与储量特征,该文通过植物样品采集、实验室测试及统计分析的方法,对位于广西凭祥的中国林业科学研究热带林业实验中心的红椎和西南桦的5种营养元素(N、P、K、Ca和Mg)含量与储量特征进行了研究。结果表明:红椎与西南桦不同器官中营养元素总含量排序均为树叶细根树枝中根粗根干皮根蔸干材,林木各器官营养元素含量均以N和K最高,Ca和Mg其次,P最低;2树种叶片的N、P含量均显著(P0.05)高于其他器官,且叶片N∶P表征2树种均是P制约型植物。除根蔸之外,2树种各器官中营养元素总储量的分配为干材树枝干皮树叶粗根细根中根。红椎的营养元素利用效率略高于西南桦。该研究揭示了红椎和西南桦的营养需求,为调节和改善林木生长环境,以及科学制定林地养分管理策略提供了理论依据。     

Nutrient transfer supports a beneficial relationship between the canopy ant,Azteca trigona,and its host tree

1. Energy fluxes between ants and plants have been a focal point for documenting mutualistic behaviour. Plants can provide resources to ants through the production of extrafloral nectaries. In exchange, ants can fertilise plants through their nutrient‐ and microbe‐rich refuse. 2. Here, we test a potential facultative mutualism between the carton‐nesting canopy ant, Azteca trigona, and their host trees. Through observational and experimental approaches, this study documents how nutrient transfer provides a basis for this beneficial ant–plant relationship. 3. In a greenhouse experiment, fertilisation with sterilised refuse (i.e. nutrients only) increased seedling growth three‐fold, while the refuse with its natural microbial community increased growth 11‐fold. 4. Total root density was doubled in refuse piles compared with the surrounding area in situ. On average, refuse provides host trees and the surrounding plant community with access to a > 800% increase in N, P and K relative to leaf litter. 5. Azteca trigona preferentially nests in trees with extrafloral nectaries and on large, longer‐lived tree species. 6. Given the nutrient‐poor nature of the Neotropics, host trees probably experience significant benefits from refuse fertilisation. Conversely, A. trigona benefit from long‐term stable structural support for nests and access to nutrient‐rich extrafloral nectaries. Without clear costs to either A. trigona or host trees, it is proposed that these positive interactions are preliminary evidence of a facultative mutualism.     

Comparison of phosphatase localization in the intraradical hyphae of arbuscular mycorrhizal fungi,Glomus spp. and Gigaspora spp.

The localization of acid and alkaline phosphatases in the intraradical hyphae of the arbuscular mycorrhizal fungi, Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe (Gm), Gl. etunicatum Becker and Gerd. (Ge) and Gigaspora rosea Nicol. and Schenck (Gir) were compared. Marigold (Tagetes patula L.) and leek (Allium porrum L.) were inoculated with each of the three fungi. The mycorrhizal roots were harvested at 3, 4, 5 and 6 weeks after sowing (WAS), treated with a digestion solution containing cellulase and pectinase, and then stained for phosphatase activities at pH 5.0 and pH 8.5. The development of fungal structures in the host root was also examined. Gm formed fine-branched (mature) arbuscules only at the early phase of infection (3 to 4 WAS). Mature arbuscules of Ge and Gir were observed from the early phase (4 WAS) up to the end of experiment. At pH 5.0, the localization of the phosphatase activities of the three fungi were similar irrespective to host plant species. The activity appeared in mature arbuscules and intercellular hyphae, whereas the collapsed arbuscules were inactive. Ten millimolar NaF, an acid phosphatase inhibitor, inhibited the phosphatase activities of Gm and Ge but did not affect that of Gir. At pH 8.5, a difference among the fungal species was found in the localization of phosphatase activity while that between host species was not. The mature arbuscules were also the active sites in all three species. Only Gir showed the activity in the intercellular hyphae while the two Glomus spp. did not. Five millimolar EDTA inhibited the activity of Gir at pH 8.5 while the activities of Ge and Gm were not affected by either 5 mM EDTA or 10 mM KCN (both are alkaline phosphatase inhibitors).     

Extraradical hyphae exhibit more plastic nutrient-acquisition strategies than roots under nitrogen enrichment in ectomycorrhiza-dominated forests

Ectomycorrhizal (ECM) functional traits related to nutrient acquisition are impacted by nitrogen (N) deposition. However, less is known about whether these nutrient-acquisition traits associated with roots and hyphae differentially respond to increased N deposition in ECM-dominated forests with different initial N status. We conducted a chronic N addition experiment (25 kg N ha⁻¹ year⁻¹) in two ECM-dominated forests with contrasting initial N status, that is, a Pinus armandii forest (with relatively low N availability) and a Picea asperata forest (with relatively high N availability), to assess nutrient-mining and nutrient-foraging strategies associated with roots and hyphae under N addition. We show that nutrient-acquisition strategies of roots and hyphae differently respond to increased N addition. Root nutrient-acquisition strategies showed a consistent response to N addition, regardless of initial forest nutrient status, shifting from organic N mining toward inorganic N foraging. In contrast, the hyphal nutrient-acquisition strategy showed diverse responses to N addition depending on initial forest N status. In the Pinus armandii forest, trees increased belowground carbon (C) allocation to ECM fungi thus enhancing hyphal N-mining capacity under increased N availability. By comparison, in the Picea asperata forest, ECM fungi enhanced both capacities of P foraging and P mining in response to N-induced P limitation. In conclusion, our results demonstrate that ECM fungal hyphae exhibit greater plasticity in nutrient-mining and nutrient-foraging strategies than roots do in response to changes of nutrient status induced by N deposition. This study highlights the importance of ECM associations in tree acclimation and forest function stability under changing environments.     

The influence of savanna trees on nutrient,water and light availability and the understorey vegetation

In an East African savanna herbaceous layer productivity and species composition were studied around Acacia tortilis trees of three different age classes, as well as around dead trees and in open grassland patches. The effects of trees on nutrient, light and water availability were measured to obtain an insight into which resources determine changes in productivity and composition of the herbaceous layer. Soil nutrient availability increased with tree age and size and was lowest in open grassland and highest under dead trees. The lower N:P ratios of grasses from open grassland compared to grasses from under trees suggested that productivity in open grassland was limited by nitrogen, while under trees the limiting nutrient was probably P. N:P ratios of grasses growing under bushes and small trees were intermediate between large trees and open grassland indicating that the understorey of Acacia trees seemed to change gradually from a N-limited to a P-limited vegetation. Soil moisture contents were lower under than those outside of canopies of large Acacia trees suggesting that water competition between trees and grasses was important. Species composition of the herbaceous layer under Acacia trees was completely different from the vegetation in open grassland. Also the vegetation under bushes of Acacia tortilis was different from both open grassland and the understorey of large trees. The main factor causing differences in species composition was probably nutrient availability because species compositions were similar for stands of similar soil nutrient concentrations even when light and water availability was different. Changes in species composition did not result in differences in above-ground biomass, which was remarkably similar under different sized trees and in open grassland. The only exception was around dead trees where herbaceous plant production was 60% higher than under living trees. The results suggest that herbaceous layer productivity did not increase under trees by a higher soil nutrient availability, probably because grass production was limited by competition for water. This was consistent with the high plant production around dead trees because when trees die, water competition disappears but the high soil nutrient availability remains. Hence, in addition to tree soil nutrient enrichment, below-ground competition for water appears to be an important process regulating tree-grass interactions in semi-arid savanna.     

Molecular phylogenetics of Hippophae L. (Elaeagnaceae) based on the internal transcribed spacer (ITS) sequences of nrDNA

 The genus Hippophae comprises 7 species and 8 subspecies according to the latest classification, and has shown enormous ecological, nutrient and medicinal values. Here we analyzed the phylogenetic relationships among 15 taxa of the genus by comparing sequences of the internal transcribed spacer (ITS) region of nuclear ribosomal DNA (nrDNA). ITS sequences in Hippophae varied in length from 651 bp to 666 bp. The aligned sequences were 690 bp in length and 269 (39.0%) were variable sites with 150 being parsimony-informative. The amount of polymorphism observed within a taxon was extremely low in most taxa except for two putative hybrid species. The aligned sequences were analyzed by maximum parsimony (MP) and neighbor-joining (NJ) methods. In the strict consensus trees of parsimony analysis, the monophyly of Hippophae was supported by 100% bootstrap value. H. tibetana was at the basal position of the genus, and the remaining taxa formed two clades with high bootstrap support. The first clade included subspecies of H.␣rhamnoides and the other one consisted of remaining species. Parsimony analysis also suggested that the species H. tibetana, H. neurocarpa and H.␣salicifolia were all distinct. Although the sequence divergence among subspecies of H. rhamnoides was also remarkably high, the molecular data supported the monophyly of H. rhamnoides when H. rhamnoides subsp. gyantsensis Rousi was excxluded. The NJ trees showed essentially the same topology. The taxonomical arrangement that divided the genus into two sections was not supported based on the ITS sequences. However, the hybrid origin of H. goniocarpa and H. litangensis proposed previously was supported by the present ITS data. Received January 7, 2002; accepted May 10, 2002 Published online: November 22, 2002 Addresses of the authors: Kun Sun, Xuelin Chen, Ruijun Ma, Qin Wang, Institute of Botany, Northwest Normal University, Lanzhou 730070, China. Changbao Li, Song Ge (e-mail: gesong@ns.ibcas.ac.cn or song_ge@hotmail.com), Laboratory of Systematic and Evolutionary Botany, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China.     

Increased moth herbivory associated with environmental stress of pinyon pine at local and regional levels

 Using 6 years of observational and experimental data, we examined the hypothesis that water and nutrient stress increase the susceptibility of pinyon pine (Pinus edulis) to the stem- and cone-boring moth (Dioryctria albovittella). At two geographic levels, a local scale of 550 km² and a regional scale of 10,000 km², moth herbivory was strongly correlated with an edaphic stress gradient. At a local scale, from the cinder soils of Sunset Crater to nearby sandy-loam soils, nine of ten soil macro- and micronutrients, and soil water content were lowest in cinder-dominated soils. Herbivore damage was six times greater on trees growing in the most water and nutrient deficient site at Sunset Crater compared to sites with well-developed soils. Percentage silt-clay content of soil, which was highly positively correlated with soil nutrient and soil moisture at a local scale, accounted for 56% of the variation in herbivory at a regional scale among 22 sites. Within and across sites, increased stem resin flow was positively associated with reduced moth attack. On the basis of moth distribution across a stress gradient, we predicted that pinyons growing in highly stressful environments would show increased resistance to herbivores if supplemented with water and/or nutrients. We conducted a 6-year experiment at a high-stress site where individual trees received water only, fertilizer only, and water + fertilizer. Relative to control trees, stem growth and resin flow increased in all three treatments, but only significantly in the water + fertilizer treatment. Although there was no significant difference in herbivore damage among these three treatments, there was an overall reduction in herbivore damage on all treatment trees combined, compared to control trees. This experiment suggests that release from stress leads to increased resistance to insect attack and is consistent with our observational data. While other studies have predicted that short-term stress will result in herbivore outbreaks, our studies extend this prediction to chronically stressed host populations. Finally, while flush-feeders are not predicted to respond positively to stressed host plants, we found a positive association between herbivore attack and stressed pinyon populations. Received: 25 December 1995 / Accepted: 15 August 1996     

Ecophysiology of Eucalyptus marginata and Corymbia calophylla in decline in an urban parkland

Eucalypt trees are in decline throughout urban landscapes of south western Australia. This study investigated the cause of decline in Eucalyptus marginata and Corymbia calophylla trees in parkland and compared water and nutrient relations with healthy trees in adjacent bushland in Perth, Western Australia. It was hypothesized that: (i) trees were drought stressed through competition for soil water by the vigorous turf; (ii) excessive uptake of nitrogen, because of fertilizer application to turf, caused toxicity; and/or (iii) micronutrient (Cu, Fe, Mn and/or Zn) deficit was induced by high‐pH irrigation water applied to turf around parkland trees. Leaf water potential showed aseasonal variation in the irrigated parkland trees and foliar δ¹³C indicated that parkland trees generally had low water‐use efficiency and were not drought stressed relative to bushland trees. Foliar N levels were not significantly different between parkland and bushland trees indicating that excess N uptake was not a factor in the decline. Foliar total Fe, ‘metabolically active’ Fe, Cu and Zn concentrations were not significantly different between parkland and bushland trees. Foliar manganese concentrations were indicative of deficiency and significantly lower in parkland trees (5–14 µg g^?1) relative to bushland trees (22–35 µg g^?1). It is concluded that application of alkaline irrigation water to the parkland site reduced the plant‐availability of Mn; however, our study of only one parkland site does not allow us to generalize the results across other parklands.     

Occurrence and expression of acid phosphatase of Hymenoscyphus ericae (Read) Korf & Kernan,in isolation or associated with plant roots

Summary The activity of acid phosphatase produced in pure culture by the endomycorrhizal fungus Hymenoscyphus ericae (Read) Korf & Kernan (H. ericae LPA 2) was inhibited by high phosphorus levels, alkaline pH, fluoride, molybdate and mannosidase, and activated by concanavalin A. Over 80% of the enzyme activity was due to two wall-bound acid phosphatase isozymes with the characteristics of mannose-rich glycoproteins. Antiserum was raised against the major, low-molecular-weight wall isozyme and its activity tested by immunoblotting and ELISA. The antiserum cross reacted 100% with exocellular (excreted) and 28% with cytoplasmic cellular fractions of H. ericae (LPA 2) cultures, and showed high reactivity with other strains of H. ericae but not with fungal isolates from Erica hispidula L. or E. mauritanica L. Ultrastructural localization of acid phosphatase by cytoenzymology and indirect immunogold labelling confirmed its association with the fungal wall in pure culture and showed that the influence of a high phosphorus level, fluoride and molybdate is through inactivation of the enzyme. Intense acid phosphatase activity, sensitive to the latter inhibitors, was also present on external hyphae growing over a host or non-host root but it was weak or absent from intracellular hyphae where these developed within a host root. Indirect immunolabelling confirmed that this acid phosphatase was of fungal origin and that the specific inhibitory effect of host cells is due to inactivation of the enzyme rather than repression of its synthesis. Possible implications of fungal acid phosphatase in ericoid endomycorrhizal infection processes are discussed together with mechanisms that may be regulating the enzyme activity.     

Arbuscular mycorrhizal fungi and organic fertilizer influence photosynthesis,root phosphatase activity,nutrition, and growth of <Emphasis Type="Italic">Ipomoea carnea</Emphasis> ssp. <Emphasis Type="Italic">fistulosa</Emphasis>

The effect of arbuscular mycorrhizal fungi (AMF) inoculation and organic slow release fertilizer (OSRF) on photosynthesis, root phosphatase activity, nutrient acquisition, and growth of Ipomoea carnea N. von Jacquin ssp. fistulosa (K. Von Martinus ex J. Choisy) D. Austin (bush morning glory) was determined in a greenhouse study. The AMF treatments consisted of a commercial isolate of Glomus intraradices and a non-colonized (NonAMF) control. The OSRF was applied at 10, 30, and 100 % of the manufacturer’s recommended rate. AMF plants had a higher net photosynthetic rate (P _N), higher leaf elemental N, P, and K, and generally greater growth than NonAMF plants. Total colonization levels of AMF plants ranged from 27 % (100 % OSRF) to 79 % (30 % OSRF). Root acid phosphatase (ACP) and alkaline phosphatase (ALP) activities were generally higher in AMF than non-AMF plants. When compared to NonAMF at 100 % OSRF, AMF plants at 30 % OSRF had higher or comparable ACP and ALP activity, higher leaf elemental P, N, Fe, Cu, and Zn, and a greater P _N (at the end of the experiment), leading to generally greater growth parameters with the lower fertility in AMF plants. We suggest that AMF increased nutrient acquisition from an organic fertilizer source by enhancing ACP and ALP activity thus facilitating P acquisition, increasing photosynthesis, and improving plant growth.     

Different patterns of biomass allocation of mature and sapling host tree in response to liana competition in the southern temperate rainforest

Lianas can negatively affect their host tree. The evidence comes from studies in tropical forests, where lianas decrease the growth rate and reproduction of their host tree. This is primarily a consequence of water and nutrient competition, two limiting factors in tropical forests. In contrast, for some areas of southern temperate rainforests the competition for these resources could be less severe, because of the high rainfall and fertile soils. But so far, no study has determined the effect of liana competition over their host tree in southern temperate rainforests. The aim of this study was to evaluate, in field conditions, the effect of liana Cissus striata (Ruiz & Pavon) competition over the growth rate of mature Nothofagus obliqua (Bidr Egefam) host tree. In an experimental approach, we determined whether above‐ and/or below‐ground competition is more important in this interaction. We also looked for compensatory strategies that would allow to trees to respond to liana competition. In field conditions, we found that infested trees have a decrease in their relative growth rate of 26% and a reduction of the leaf area index (LAI) of 20% compared with control trees. In the greenhouse experiment, we found that saplings were water stressed and that there was light competition. Neither competition for water nor light had a significant effect on the growth rate of infested saplings. This could be explained because saplings showed compensatory strategies in response to competition. These strategies were based in the biomass distribution between organs (leaf area, slenderness index) and within leaves (LMA). In conclusion, we found that C. striata has a negative effect over the growth of mature and sapling N. obliqua host trees. This was a consequence of above‐ground and below‐ground competition, but we cannot disentangle which type of competition is more important. Trees respond to liana competition, mature host trees change the canopy architecture and saplings allocate resource between and within organs, which allows them to optimize resource capture.     

Effect of Monospecific and Mixed Sea-Buckthorn (Hippophae rhamnoides) Plantations on the Structure and Activity of Soil Microbial Communities

This study aims to evaluate the effect of different afforestation models on soil microbial composition in the Loess Plateau in China. In particular, we determined soil physicochemical properties, enzyme activities, and microbial community structures in the top 0 cm to 10 cm soil underneath a pure Hippophae rhamnoides (SS) stand and three mixed stands, namely, H. rhamnoides and Robinia pseucdoacacia (SC), H. rhamnoides and Pinus tabulaeformis (SY), and H. rhamnoides and Platycladus orientalis (SB). Results showed that total organic carbon (TOC), total nitrogen, and ammonium (NH₄⁺) contents were higher in SY and SB than in SS. The total microbial biomass, bacterial biomass, and Gram⁺ biomass of the three mixed stands were significantly higher than those of the pure stand. However, no significant difference was found in fungal biomass. Correlation analysis suggested that soil microbial communities are significantly and positively correlated with some chemical parameters of soil, such as TOC, total phosphorus, total potassium, available phosphorus, NH₄⁺ content, nitrate content (NH₃⁻), and the enzyme activities of urease, peroxidase, and phosphatase. Principal component analysis showed that the microbial community structures of SB and SS could clearly be discriminated from each other and from the others, whereas SY and SC were similar. In conclusion, tree species indirectly but significantly affect soil microbial communities and enzyme activities through soil physicochemical properties. In addition, mixing P. tabulaeformis or P. orientalis in H. rhamnoides plantations is a suitable afforestation model in the Loess Plateau, because of significant positive effects on soil nutrient conditions, microbial community, and enzyme activities over pure plantations.     

The mycorrhizal fungus Paxillus involutus transports magnesium to Norway spruce seedlings. Evidence from stable isotope labeling

Although it is well established that ectomycorrhizas improve the mineral nutrition of forest trees, there has been little evidence that they mediate uptake of divalent cations such as Mg. We grew nonmycorrhizal seedlings and seedlings mycorrhizal with Paxillus involutus Batsch in a sand culture system with two compartments separated by a 45-μm Nylon mesh. Hyphae, but not roots, can penetrate this net. Labeling the compartment only accessible to hyphae with ²⁵Mg showed that hyphae of the ectomycorrhizal fungus Paxillus involutus transported Mg to their host plant. No label was found in nonmycorrhizal control plants. Our data support the idea that ectomycorrhizas are important for the Mg nutrition of forest trees. This revised version was published online in June 2006 with corrections to the Cover Date.     

鼠李糖乳杆菌碱性磷酸酶的提取条件优化及其降解有机磷农药作用

【背景】我国作为农业大国,对农药的大量使用是不可避免的,但是农药的超范围使用、超标及高检出率对于环境的污染与人体健康的威胁日趋严重。【目的】碱性磷酸酶(alkaline phosphatase,ALP)对有机磷农药具有积极的降解作用,因此,本文对鼠李糖乳杆菌(Lactobacillus rhamnosus) Z23(LGG Z23)所产碱性磷酸酶的提取条件进行优化,并研究其对有机磷农药的降解作用。【方法】使用单因素试验和正交试验优化ALP的提取条件;使用对硝基苯酚法测定酶活力;使用分级沉淀和层析法提纯ALP;使用乙酰胆碱酯酶抑制法测定ALP对有机磷农药的降解率。【结果】LGG Z23所产ALP的最优提取条件为：细胞破碎时间15 min,破碎功率450 W,料液比(质量体积比)1:6,提取液pH 10.0,此条件下酶活力为(4.95±0.26) U/mL,比优化前提高2.11倍;对6种有机磷农药的降解率效果为敌敌畏(95.79%±0.01%)>甲基对硫磷(90.69%±0.03%)>毒死蜱(88.90%±0.02%)>敌百虫(86.07%±0.03%)>马拉硫磷(85.31%±0.02%)>乐果(83.18%±0.03%),其中对敌敌畏和甲基对硫磷的降解效果最好,可达90%以上,并且降解作用差异显著(P<0.05)。【结论】本研究为LGG Z23所产ALP的应用研究提供了理论依据和实验数据。     

Hyphal contribution to water uptake in mycorrhizal plants as affected by the fungal species and water status

Vesicular-arbuscular mycorrhizae may increase resistance of plants to drought by a number of mechanisms, such as increased root hydraulic conductivity, stomatal regulation, hyphal water uptake and osmotic adjustment. However, a substantial contribution of vesicular-arbuscular mycorrhizal (VAM) hyphae to water uptake has not been demonstrated unequivocally. The objective of this investigation was to examine the contribution of hyphae from two VAM fungi to water uptake and transport by the host plant. Lettuce (Lactuca sativa L.) plants were grown in a container divided by a screen into two compartments. One was occupied by roots, the other only by VAM hyphae, which the screen permitted to pass. Roots were colonized by the VAM fungi Glomus deserticola or Glomus fasciculatum, or were left uninoculated but P-supplemented. Water was supplied to the hyphal compartment at a distance of 10 cm from the screen (root). CO₂ exchange rate, water-use efficiency, transpiration, stomatal conductance and photosynthetic phosphorus-use efficiency of VAM or P-amended control plants were evaluated at three levels of water application in the hyphal compartment. Results indicate that much of the water was taken up by the hyphae in VAM plants. VAM plants, which had access to the hyphal compartment, had higher water and nutrient contents. G. deserticola functioned efficiently under water limitation and mycelium from G. fasciculatum-colonized plants was very sensitive to water in the medium. This discrepancy in VAM behaviour reflects the various abilities of each fungus according to soil water levels. Different abilities of specific mycelia were also expressed in terms of nutritional and leaf gas-exchange parameters. G. fasciculatum caused a significant increase in net photosynthesis and rate of water use efficiency compared to G. deserticola and P-fertilized plants. In contrast, the G. deserticola treatment was the most efficient affecting N, P and K nutrition, leaf conductance and transpiration. Since no differences in the intra- and extra-radical hyphal extension of the two endophytes were found, the results demonstrate that mycorrhizal hyphae can take up water and that there are considerable variations in both the behaviour of these two VAM fungi and in the mechanisms involved in their effects on plant water relations.     

Penetration of Hyphae of Sclerotinia sclerotiorum by Coniothyrium minitans Without the Formation of Appressoria

In a study using scanning electron microscopy (SEM), the mode of hyperparasitism of Coniothyrium minitans on its host Sclerotinia sclerotiorum was investigated. The SEM micrographs confirm previous reports, from light microscopic studies, that hyphal tips of C. minitans invade the host hypha by direct penetration, without developing appressoria, and that indentation of the host cell wall at the point of penetration is often evident. There is no functional distinction between amain branch and a side branch hypha of the hyperparasite and tips of either type of hyphae are capable of invading host hyphae by direct penetration.     

Ultrastructure of Ascodichaena rugosa on beech bark

Ascodichaena rugosa Butin is a corkinhabiting fungus, found frequently on the bark of Fagus sylvatica L. The hyphae of the fungus are distributed solely in the phellem cells, stopping their growth in the last-formed cork cell layer. The cell to cell invasion is effected by penetration hyphae, causing no extensive dissolution of the cork wall. Electron microscopical observations revealed fine structural details of the fruit bodies and of the intracellular hyphae. Of special interest were the finger-like hyaline hyphae in the last-formed layer of cork cells, which are interpreted as haustoria on the basis of the fine structure both of hyphae and host cells. This situation is considered as reflecting a parasitic relationship of Ascodichaena to beech bark. The activity of the fungus led also to the increased production of cork cells, perhaps related to the nutrient supply of the fungus.     

Host physiological condition regulates parasitic plant performance: <Emphasis Type="Italic">Arceuthobium vaginatum</Emphasis> subsp. <Emphasis Type="Italic">cryptopodum</Emphasis> on <Emphasis Type="Italic">Pinus ponderosa</Emphasis>

Much research has focused on effects of plant parasites on host-plant physiology and growth, but little is known about effects of host physiological condition on parasite growth. Using the parasitic dwarf mistletoe Arceuthobium vaginatum subsp. cryptopodum (Viscaceae) and its host Pinus ponderosa, we investigated whether changes in host physiological condition influenced mistletoe shoot development in northern Arizona forests. We conducted two studies in two consecutive years and used forest thinning (i.e., competitive release) to manipulate host physiological condition. We removed dwarf mistletoe shoots in April, before the onset of the growing season, and measured the amount of regrowth in the first season after forest thinning (Study I: n=38 trees; Study II: n=35 trees). Thinning increased tree uptake of water and carbon in both studies, but had no effect on leaf N concentration or δ¹³C. Mistletoe shoot growth was greater on trees with high uptake of water and carbon in thinned stands than trees with low uptake in unthinned stands. These findings show that increased resource uptake by host trees increases resources to these heterotrophic dwarf mistletoes, and links mistletoe performance to changes in host physiological condition.     

Beyond the rhizosphere: growth and function of arbuscular mycorrhizal external hyphae in sands of varying pore sizes

Research on nutrient acquisition by symbiotic arbuscular mycorrhizal (AM) fungi has mainly focused on the root–fungus interface and less attention has been given to the growth and functioning of external hyphae in the bulk soil. The growth and function of external hyphae may be affected by unfavourable soil environments, such as compacted soils in which pores may be narrow. The effects of pore size on the growth of two AM fungi (Glomus intraradices and G. mosseae) and their ability to transport ³³P from the bulk soil to the host were investigated. Trifolium subterraneum L. plants were grown individually in `single arm cross-pots' with and without AM fungi. The side arm was separated from the main compartment by nylon mesh to prevent root penetration. It contained three zones: 5 mm of soil:sand mix (HC1); 25 mm of media treatment (HC2); and 20 mm of ³³P-labelled soil (HC3). There were four media treatments; soil and three types of quartz sand with most common continuous pore diameters of 100, 38 and 26 m. AM plants had similar growth and total P uptake in all treatments. However, plants grown with G. intraradices contained almost three times more ³³P than those grown with G. mosseae, indicating G. intraradices obtained a greater proportion of P at a distance from the host roots. Differences in P acquisition were not correlated with production of external hyphae in the four media zones and changes in sand pore size did not affect the ability of the fungi studied to acquire P at a distance from the host roots. Production of external hyphae in HC2 was influenced by fungal species and media treatment. Both fungi produced maximum amounts of external hyphae in the soil medium. Sand pore size affected growth of G. intraradices (but not G. mosseae) and hyphal diameter distributions of both fungi. The results suggest that not only are G. mosseae and G. intraradices functionally complementary in terms of spatial phosphorus acquisition, they are also capable of altering their morphology in response to the soil environment.