The mechanism of rock phosphate solubilization in the rhizosphere

Summary A simple model is described, and experimentally tested, for predicting the rate of dissolution of rock phosphates in soil, including the effect of solubilization by plant roots. The sensitivity of the model to its input parameters is assessed and it is seen that plants may significantly increase their P uptake by acid secretion. The model provides a rational basis for the selection of P solubilizing crops and inter-crops.     

Responses of Pinus halepensis growth, soil microbial catabolic functions and phosphate-solubilizing bacteria after rock phosphate amendment and ectomycorrhizal inoculation

We examined the effects of an ectomycorrhizal (EM) fungus, Pisolithus sp., on of the growth of Pinus halepensis (Allepo pine) seedlings, soil microbial functions and rock phosphate solubilization in a un-disinfected soil amended or not with a Moroccan rock phosphate. Allepo pine seedlings were inoculated with an EM fungus (Pisolithus sp. strain PH4) isolated from a P. halepensis plantation and selected for its high ability to mobilize P from an inorganic form of phosphate. After 4 month’s culture in a disinfected substrate, plants were transferred in 10 l-containers filled with a natural forest soil and amended or not with rock phosphate powder. After 12 month’s culturing, the growth, needle nutrient concentrations of P. halepensis plants were measured. Soil microbial catabolic diversity was assessed by measuring CO₂ production of substrate induced respiration responses. Fluorescent pseudomonads were isolated from each soil treatment and tested in axenic conditions for their ability to solubilize a source of inorganic phosphate. The results clearly showed that (i) P. halepensis growth was greatly promoted by the presence of the ectomycorrhizal fungus Pisolithus strain PH4 in a disinfected soil/vermiculite mixture and in a non disinfected soil, (ii) ectomycorrhizal inoculation induced significant changes in the functions of soil microbial communities and selected microorganisms potentially beneficial to the plant growth (i.e. phosphate-solubilizing fluorescent pseudomonad) and (iii) rock phosphate solubilisation was mainly dependent on EM inoculation and mycorrhizosphere microorganisms. These results were in accordance with previous studies where it was demonstrated that EM symbiosis has a beneficial effect on plant growth through a direct effect on the host plant but also an indirect effect via a selective pressure on soil microbiota that favours microorganisms potentially beneficial to plant growth.     

Influence of inoculation with a Trichoderma bio‐inoculant on ectomycorrhizal colonisation of Pinus radiata seedlings

Trichoderma species, through mechanisms such as mycoparasitism, antibiosis, induced resistance and competition, are able to suppress the growth and activity of soil microbes and therefore have the potential to influence ectomycorrhizal (ECM) fungal colonisation. ECM colonisation potential of Pinus radiata seedlings inoculated with the commercially available Trichoderma species bio‐inoculant, ArborGuard?, was investigated in a commercial containerised nursery setting and in a separate glasshouse experiment. Application of ArborGuard? to P. radiata seedlings in both the containerised nursery and glasshouse experiment had no detrimental effect on the ability of the naturally occurring ECM fungi to colonise the seedlings. Thelephora terrestris was the only ECM species colonising the P. radiata root tips in the containerised system while Tomentella ellisii was the dominant species found colonising the P. radiata root tips in the glasshouse experiment. No seedling growth promotion was observed with the addition of ArborGuard?. The results show that the Trichoderma bio‐inoculant ArborGuard? does not affect ECM colonisation of P. radiata seedlings by Th. terrestris and T. ellisii in a containerised nursery system.     

不同二氧化碳浓度条件下红松和长白赤松幼苗根际土壤微生物数量研究

以连续5年不同CO₂浓度(开顶箱700μmol·mol^-1、500μmol·mol^-1、对照箱和裸地)处理的长白赤松和红松幼苗为研究对象,在2003年7～9月分别对幼苗根际土壤细菌、真菌、放线菌数量进行比较研究.结果表明,高浓度CO₂处理对长白赤松幼苗根际土壤细菌数量起显著的(P≤0.001)促进作用,对根际真菌和放线菌数量的促进作用却不明显;对红松来说,除8月份700μmol·mol^-1CO₂处理和7月份500μmol·mol^-1CO₂处理之外,在各月份中受高浓度CO₂处理的根际土壤细菌数量均较对照箱和裸地显著增多(P≤0.001),而根际土壤真菌数量变化除9月份(P≤0.001)外均不明显,放线菌数量受高浓度CO₂的影响亦不明显.     

Understanding Trichoderma in the root system of Pinus radiata: associations between rhizosphere colonisation and growth promotion for commercially grown seedlings

Two Trichoderma isolates (T. hamatum LU592 and T. atroviride LU132) were tested for their ability to promote the growth and health of commercially grown Pinus radiata seedlings. The colonisation behaviour of the two isolates was investigated to relate rhizosphere competence and root penetration to subsequent effects on plant performance. Trichoderma hamatum LU592 was shown to enhance several plant health and growth parameters. The isolate significantly reduced seedling mortality by up to 29%, and promoted the growth of shoots (e.g. height by up to 16%) and roots (e.g. dry weight by up to 31%). The introduction of LU592 as either seed coat or spray application equally improved seedling health and growth demonstrating the suitability of both application methods for pine nursery situations. However, clear differences in rhizosphere colonisation and root penetration between the two application methods highlighted the need for more research on the impact of inoculum?densities. When spray-applied, LU592 was found to be the predominant Trichoderma strain in the plant root system, including bulk potting mix, rhizosphere and endorhizosphere. In contrast, T. atroviride LU132 was shown to colonise the root system poorly, and no biological impact on P. radiata seedlings was detected. This is the first report to demonstrate rhizosphere competence as a useful indicator for determining Trichoderma bio-inoculants for P. radiata. High indigenous Trichoderma populations with similar population dynamics to the introduced strains revealed the limitations of the dilution plating technique, but this constraint was alleviated to some extent by the use of techniques for morphological and molecular identification of the introduced isolates.     

Relationships between stem structure and bending strength in Pinus radiata seedlings

Summary Seedlings from nine families of Pinus radiata were grown in a glasshouse under conditions of high and low nitrate nitrogen availability to investigate effects on anatomical and strength characteristics of stems. Families were classified into groups dependent upon their previously determined susceptibility to stem deformation prevalent in plantations established on fertile ex-pasture. Nitrogen treatments significantly affected seedling form in terms of both branch production and stem slenderness. The high N treatment resulted in shorter seedlings, a proportion of which were obviously stunted. Stem strength of seedlings, physically supported throughout the experiment, was assessed as stem lean at harvest as well as the bending strength of the fresh stem at 50% stem height. These two variables were found not to be correlated. Stem lean at harvest was greatest in families known to be susceptible to stem deformation. These families produced stems that were also more slender than families of low susceptibility. Increased stem lean was associated mostly with increased stem slenderness while elasticity was more influenced by pith diameter, stem density and wood radius.     

Variations in response to induced stem bending in seedlings of Pinus radiata

Summary The occurrence of stem deformation in Pinus radiata plantations has been examined previously in terms of the factors leading to the onset of bending symptoms. In this study the influence of seedling family and nitrogen availability on the ability of seedlings to recover from deformation was investigated under glasshouse conditions. Seedlings from four families ranging from resistant to susceptible to stem deformation were used. Stem deformation was induced by loading stems with a known weight and comparing bending moments and rates of recovery. Significant differences between families were found with more susceptible families exhibiting greater initial magnitudes of stem lean. Similarly these families also exhibited greater rates of apical elongation following bending despite the larger stem leans. This resulted in more exaggerated stem bends as a consequence of the recovery of the lower stem pushing the upper stem away from the vertical. The strength characteristics of the stems could not explain differences between families. Susceptibility to stem deformation was explained more by stem slenderness expressed as stem height to root collar diameter.     

The ectomycorrhizal flora of primary and secondary stands of Pinus sylvestris in relation to soil conditions and ectomycorrhizal succession

Abstract. Ectomycorrhizal species composition and sporocarp abundance in two 15–20-yr old primary stands of Pinus sylvestris in the central part of the Netherlands was compared with those in two 16- and 27-yr old secondary stands of P. sylvestris in the Northeast of the country. The trees of the primary stands were spontaneously seeded in a drifting sand area. Only thin litter and humus layers were present. The trees of the 16-yr old secondary stand were planted on podzolic sandy soil and those of the 27-yr old stand on non-podzolic sandy soil. In both secondary stands thick litter and humus layers had developed. The litter and humus partly originated from former stands at the same sites and partly from the present stands. In the secondary stands the thick litter and humus layers and herb vegetation were removed (‘sod-cutting’) in order to simulate the thin litter and humus layers in the primary stands. Control treatments were present. Surveys in 1991, 1992 and 1993 showed that sod-cutting enhanced both abundance and diversity of ectomycorrhizal fungi. However, species richness and diversity were higher in the primary stands than in the secondary ones, also even in the sod-cut plots. High species richness and diversity were associated with low concentrations of nitrogen and relatively high pH in the litter and humus layers, and in the mineral soil.     

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     

The effect of ectomycorrhizal fungi on Zn uptake and distribution in seedlings of Pinus sylvestris L

Seedlings of pine, infected with two different ectomycorrhizal fungi, Suillus bovinus (Fr.) O. Kuntze and an unidentified isolate (BP), were exposed to various external Zn concentrations. An additional strain of S. bovinus, cultured on a Zn-enriched medium, was also used. The effect of mycorrhizal associations on Zn uptake and distribution within the plant was determined by AAS.The results show that nonmycorrhizal seedlings have the capability to control the uptake and translocation of Zn to the shoot until the external Zn concentration reaches a threshold level, where no limitation of uptake is possible. Excess Zn is accumulated in the root system to protect the shoot against toxic tissue concentrations. The effect of an ectomycorrhizal infection on Zn uptake and distribution depends on (1) the fungal species (2) the external concentration and (3) the Zn content of the fungal culture medium. Under conditions of low external Zn supply, especially a mycorrhizal infection with S. bovinus led to an increased Zn uptake in root and needles of Pinus sylvestris. Under high external conditions the mycobionts varied considerably in their capability to reduce the transport of Zn to the shoot. Only by an infection with S. bovinus the plant was able to maintain the shoot tissue concentration on a low level. This effect can be enhanced by pretreatment of S. bovinus with high Zn concentrations.     

紫椴根际土壤肥力与外生菌根侵染关系

调查帽儿山老山生态站紫椴(Tilia amurensis Rupr.)外生菌根的侵染情况与根际土壤肥力指标,结合Pearson相关性分析与逐步回归分析的统计方法,分析紫椴外生菌根侵染率与根际土壤肥力指标的相关性。结果表明:紫椴外生菌根侵染率与碱性磷酸酶、碱解氮、有效磷、土壤水分含量呈极显著负相关,与脲酶呈极显著正相关,与速效钾呈显著正相关。逐步回归分析发现,影响紫椴外生菌根侵染率的主要土壤肥力因子是碱性磷酸酶和土壤水分含量。     

Biotite and microcline as potassium sources in ectomycorrhizal and non-mycorrhizal Pinus sylvestris seedlings

 The aim of this study was to investigate the role of plants colonised by two ectomycorrhizal fungi, Paxillus involutus and Suillus variegatus, in mobilising potassium (K) from biotite and microcline, two minerals common in acid to medium-acid bedrock. This was carried out in a 33-week pot study with seedlings of Pinus sylvestris growing in symbiosis with the fungi, where no K was added or where K was added in the form of biotite or microcline. The mineral additions were similar to those found in natural soils. All seedlings, including non-mycorrhizal, were able to access the K in biotite, leading to stimulated growth and K uptake relative to controls. Microcline addition induced growth depression in all seedlings except those colonised by P. involutus, which were stimulated. The soil solution from S. variegatus-colonised seedlings grown with biotite had higher concentrations of citric and oxalic acid. Citric acid concentration was positively correlated to the fungal biomass (ergosterol) in the soil, as well as to the foliar K in S. variegatus-colonised seedlings. Seedlings growing without K addition had low K concentrations in the shoot. Magnesium (Mg) concentrations were enhanced in seedlings with severe K shortage, indicating that Mg can substitute for K, while calcium concentrations did not vary significantly. Accepted: 18 January 1999     

sse of photosynthetic parameters for the diagnosis of copper deficiency in Pinus radiata seedlings

Six-month-old water cultures of Pinus radiataI D. Don seedlings showed optimal growth, and the highest CO₂ assimilation and photosystem I-dependent ascorbate/dichlorophenolindophenol → NADP⁺ electron flow, at 3.0 uM Cu²⁺ (excess) in the hydroponic media. In the nine-month-old water cultures, when the early Cu deprivation has been overcome, the optimum for plant growth and CO₂ fixation shifts to 0.3 u M Cu²⁺ (normal); at that time, the 3.0 uM Cu²⁺ water cultures showed toxic symptoms of foliar chlorosis. Under Cu²⁺ deficient levels (0.03 uM) a clear decrease in the photosystem I-linked electron transport and CO₂ assimilation rates, as well as in the whole plant development, could be observed. Both six- and nine-month-old water cultures showed a close relationship between the Cu²⁺ concentration of the media and the foliar Cu content. However, leaf chlorophyll and the Cu content of thylakoid lamellae showed such a correlation only in the Cu²⁺ deficient and Cu²⁺ normal water cultures. The conclusion from these results is that the electron transport rate ascorbate/dicblorophenolindophenol → NADP⁺, and the Cu content of the photosynthetic membranes, can be used to diagnose a Cu deficiency in Pinus radiata plants.     

The effect of mercury on the establishment of Pinus rigida seedlings and the development of their ectomycorrhizal communities

Metals are toxic to both plants and fungi, and elevated soil metal concentrations have been documented to change the structure of ectomycorrhizal communities. Mercury (Hg) is a highly toxic metal and inhibits the growth of ectomycorrhizal fungi (ECMF) in axenic culture. However, the effects of Hg on the growth of tree seedlings and the development of their ECMF communities have not been explored. In the current study, Pinus rigida seedlings were planted in soil amended with 0–366 μg g^?1 Hg and incubated for 5 months. Survival and growth of P. rigida seedlings was determined, and their ECMF communities were characterized by morphotype analysis. Seedling survival declined with increasing Hg additions, although no reduction in growth was detected among surviving seedlings. The addition of 88 μg g^?1 Hg to soil more than halved the total ectomycorrhizal colonization of root tips and reduced both richness and diversity of root tip morphotypes, while lower Hg additions did not significantly affect ECMF community composition relative to the no Hg control. This suggests that changes in the community of ECMF may occur in contaminated soils before any aboveground effects on surviving seedlings are noticeable, potentially altering the contribution of ECMF to the fitness of established host plants.     

The fate of surface roots of citrus seedlings in dry soil

The top portions of the root system of deeply rooted plantsare frequently in dry soil while deeper roots still have accessto water. We expected that many surface roots would be shedwhen subject to localized soil drying. We further hypothesizedthat the cost of fine root construction per unit root lengthwould be negatively correlated with the rate at which root lengthis shed. Seedlings of four citrus root- stocks that varied widelyin specific root length (cm g^–1 root) were used to testthese hypotheses. Plants were grown for 4 months in a split-potsystem divided into a top and bottom pot. After roots were wellestablished in the bottom pot, water was withheld from the toppots of half of the plants; plants were harvested every 2 weeksthereafter. Sufficient water was supplied to the bottom potto prevent shoots of droughted seedlings from experiencing significantwater stress. All plants were labelled with ¹⁴CO₂ 48 h beforeharvesting, and autoradiographs made of the fine roots harvestedfrom the droughted compartment. Comparisons of the autoradiographswith digitized images of the root system allowed us to assessroot mortality and root sink activity. As expected, the proportionof ¹⁴C-labelled photosyrithate allocated to fine roots in thetop pot declined with soil drying in all four genotypes; however,there was no genotypic effect on this decline. Contrary to ourexpectations, extensive root mortality was not apparent forany genotype, even after 60 d of localized soil drying. Apparently,selection for rapid shedding of roots in response to soil dryinghas not occurred in these Citrus species. Key words: Carbon allocation, drought, root death, split root, root autoradiography     

Hydraulic redistribution of water from Pinus ponderosa trees to seedlings: evidence for an ectomycorrhizal pathway

While there is strong evidence for hydraulic redistribution (HR) of soil water by trees, it is not known if common mycorrhizal networks (CMN) can facilitate HR from mature trees to seedlings under field conditions. Ponderosa pine (Pinus ponderosa) seedlings were planted into root-excluding 61-microm mesh barrier chambers buried in an old-growth pine forest. After 2 yr, several mature trees were cut and water enriched in D(2)O and acid fuchsin dye was applied to the stumps. Fine roots and mycorrhizal root tips of source trees became heavily dyed, indicating reverse sap flow in root xylem transported water from stems throughout root systems to the root hyphal mantle that interfaces with CMN. Within 3 d, D(2)O was found in mesh-chamber seedling foliage > 1 m from source trees; after 3 wk, eight of 10 mesh-chamber seedling stem samples were significantly enriched above background levels. Average mesh-chamber enrichment was 1.8 x greater than that for two seedlings for which the connections to CMN were broken by trenching before D(2)O application. Even small amounts of water provided to mycorrhizas by HR may maintain hyphal viability and facilitate nutrient uptake under drying conditions, which may provide an advantage to seedlings hydraulically linked by CMN to large trees.     

Diversity of ectomycorrhizal fungi naturally established on containerised Pinus seedlings in nursery conditions

The study examined the diversity of ectomycorrhizal fungi, naturally established on roots of containerised Pinus seedlings in a nursery, using PCR-RFLP and sequencing of the nuclear ribosomal internal transcribed spacer. Seventy-two samples, including ectomycorrhizae and fruit bodies, were examined. Molecular typing assigned the fungal symbionts to four ectomycorrhizal Boletales: Rhizopogon rubescens, Suillus bovinus, S. variegatus, and R. luteolus. R. rubescens was abundant (37.5%), while Suillus and R. luteolus species were moderately established (25-26%) and rare (2.8%), respectively. In addition, Rhizopogon species colonised P. nigra ssp. salzmannii seedlings, whereas Suillus species were identified on Pinus nigra ssp. nigra seedlings. The diversity and the ability of these naturally established symbionts under artificial nursery conditions were discussed. The molecular survey investigated here should contribute to successful monitoring of mycorrhizal application under both nursery and plantation conditions.