Effects of pH on NaCl tolerance of American elm (<Emphasis Type="Italic">Ulmus americana</Emphasis>) seedlings inoculated with <Emphasis Type="Italic">Hebeloma crustuliniforme</Emphasis> and <Emphasis Type="Italic">Laccaria bicolor</Emphasis>

In the present study, we investigated the effects of pH treatments on NaCl tolerance in mycorrhizal and non-mycorrhizal American elm. American elm (Ulmus americana) seedlings were inoculated with Hebeloma crustuliniforme, Laccaria bicolor or with both mycorrhizal fungi and subsequently subjected to different pH solutions (pH 3, 6 and 9) containing 0 mM (control) and 60 mM NaCl for 4 weeks. Inoculation with the mycorrhizal fungi did not have a large effect on seedling dry weights when the pH and NaCl treatments were considered independently. However, when the inoculated seedlings were treated with 60 mM NaCl at pH 3 or 6, shoot to root ratios and root hydraulic conductivity were higher compared with non-inoculated plants, likely reflecting changes in seedling water flow properties. At pH 6, transpiration rates were about twofold lower in non-inoculated plants treated with NaCl compared with non-treated controls. For NaCl-treated H. crustuliniforme- and L. bicolor-inoculated plants, the greatest reduction of transpiration rates was at pH 9. Treatment with 60 mM NaCl reduced leaf chlorophyll concentrations more in non-inoculated compared with inoculated plants, with the greatest, twofold, decrease occurring at pH 6. At pH 3, root Na concentrations were higher in inoculated than non-inoculated seedlings; however, there was no effect of inoculation on root Na concentrations at pH 6 and 9. Contrary to the roots, the leaves of inoculated plants had lower Na concentrations at pH 6 and 9, but not at pH 3. The results point to an interaction between ECM fungi and root zone pH for salt tolerance of American elm.     

Responses of ectomycorrhizal American elm (Ulmus americana) seedlings to salinity and soil compaction

American elm (Ulmus americana) seedlings were either non-inoculated or inoculated with Hebeloma crustuliniforme, Laccaria bicolor and a mixture of the two fungi to study the effects of ectomycorrhizal associations on seedling responses to soil compaction and salinity. The seedlings were grown in the greenhouse in pots containing non-compacted (0.4 g cm^?3 bulk density) and compacted (0.6 g cm^?3 bulk density) soil and subjected to 60 mM NaCl or 0 mM NaCl (control) treatments for 3 weeks. All three fungal inocula had similar effects on the responses of elm seedlings to soil compaction and salt treatment. In non-compacted soil, ectomycorrhizal fungi reduced plant dry weights, root hydraulic conductance, but did not affect leaf hydraulic conductance and net photosynthesis. When treated with 60 mM NaCl, ectomycorrhizal seedlings had several-fold lower leaf concentrations of Na⁺ compared with the non-inoculated plants. Soil compaction reduced Na⁺ leaf concentrations in non-ectomycorrhizal plants and decreased dry weights, gas exchange and root hydraulic conductance. However, in ectomycorrhizal plants, soil compaction had little effect on the leaf Na⁺ concentrations and on other measured growth and physiological parameters. Our results demonstrated that ECM associations could be highly beneficial to plants growing in sites with compacted soil such as urban areas.     

Nitrate nutrition enhances zinc hyperaccumulation in Noccaea caerulescens (Prayon)

Nitrate fertilization has been shown to increase Zn hyperaccumulation by Noccaea caerulescens (Prayon) (formerly Thlaspi caerulescens). However, it is unknown whether this increased hyperaccumulation is a direct result of NO₃ ^? nutrition or due to changes in rhizosphere pH as a result of NO₃ ^? uptake. This paper investigated the mechanism of NO₃ ^?-enhanced Zn hyperaccumulation in N. caerulescens by assessing the response of Zn uptake to N form and solution pH. Plants were grown in nutrient solution with 300 μM Zn and supplied with either (NH₄)₂SO₄, NH₄NO₃ or Ca(NO₃)₂. The solutions were buffered at either pH 4.5 or 6.5. The Zn concentration and content were much higher in shoots of NO₃ ^?-fed plants than in NH₄ ⁺-fed plants at pH 4.5 and 6.5. The Zn concentration in the shoots was mainly enhanced by NO₃ ^?, whereas the Zn concentration in the roots was mainly enhanced by pH 6.5. Nitrate increased Zn uptake in the roots at pH 6.5 and increased apoplastic Zn at pH 4.5. Zinc and Ca co-increased and was found co-localized in leaf cells of NO₃ ^?-fed plants. We conclude that NO₃ ^? directly enhanced Zn uptake and translocation from roots to shoots in N. caerulescens.     

Root hydraulic properties and growth of balsam poplar (Populus balsamifera) mycorrhizal with Hebeloma crustuliniforme and Wilcoxina mikolae var. mikolae

The effects of an E-strain fungus (Wilcoxina mikolae var. mikolae) and an ectomycorrhizal fungus (Hebeloma crustuliniforme) on growth and water relations of balsam poplar were examined and compared in the present study. Balsam poplar roots inoculated with W. mikolae var. mikolae (Wm) exhibited structures consistent with ectendomycorrhizal (EEM) associations, including a mantle surrounding the outside of the root and an extensive Hartig net that was located between cortical cells and extended to the vascular cylinder. Roots colonized with H. crustuliniforme (Hc) developed a mantle layer, indicative of an ectomycorrhizal (ECM) association, around the outer part of the root, but no distinct Hartig net was present. Wm-colonized balsam poplar also showed increased shoot growth, stomatal conductance (g _s), and root volumes compared with non-inoculated and Hc-inoculated plants. However, Hc-inoculated plants had higher root hydraulic conductivity (L _pr) compared with non-inoculated plants and Wm-inoculated plants. These results suggest that L _pr was not a growth-limiting factor in balsam poplar and that hyphal penetration of the root cortex in itself may have little influence on root hydraulic properties.     

Growth of mycorrhizal jack pine (Pinus banksiana) and white spruce (Picea glauca) seedlings planted in oil sands reclaimed areas

The effectiveness of ectomycorrhizal inoculation at the tree nursery seedling production stage on growth and survival was examined in jack pine (Pinus banksiana) and white spruce (Picea glauca) planted in oil sands reclamation sites. The seedlings were inoculated with Hebeloma crustuliniforme strain # UAMH 5247, Suillus tomentosus strain # UAMH 6252, and Laccaria bicolor strain # UAMH 8232, as individual pure cultures and in combinations. These treatments were demonstrated to improve salinity resistance and water uptake in conifer seedlings. The field responses of seedlings to ectomycorrhizal inoculation varied between plant species, inoculation treatments, and measured parameters. Seedling inoculation resulted in higher ectomycorrhizal colonization rates compared with non-inoculated control, which had also a relatively small proportion of roots colonized by the nursery contaminant fungi identified as Amphinema byssoides and Thelephora americana. Seedling inoculation had overall a greater effect on relative height growth rates, dry biomass, and stem volumes in jack pine compared with white spruce. However, when examined after two growing seasons, inoculated white spruce seedlings showed up to 75 % higher survival rates than non-inoculated controls. The persistence of inoculated fungi in roots of planted seedlings was examined at the end of the second growing season. Although the inoculation with H. crustuliniforme triggered growth responses, the fungus was not found in the roots of seedlings at the end of the second growing season suggesting a possibility that the observed growth-promoting effect of H. crustuliniforme may be transient. The results suggest that the inoculation of conifer seedlings with ectomycorrhizal fungi could potentially be carried out on a large scale in tree nurseries to benefit postplanting performance in oil sands reclamation sites. However, these practices should take into consideration the differences in responses between the different plant species and fungal strains.     

Responses of ectomycorrhizal Populus tremuloides and Betula papyrifera seedlings to salinity

Roots of trembling aspen (Populus tremuloides Michx.) and paper birch (Betula papyrifera Marsh.) seedlings were inoculated with Hebeloma crustuliniforme or Laccaria bicolor and treated with 25 mM NaCl for 6 weeks. Both tree species appeared to be relatively tolerant of the applied NaCl treatment and did not develop visible leaf symptoms that are characteristic of salt injury. Salt treatment reduced total dry weights in aspen and birch, but did not significantly affect transpiration rates and root hydraulic conductance. Salt-treated ectomycorrhizal aspen maintained higher root hydraulic conductance compared with non-mycorrhizal plants. Na and Cl concentrations increased in shoots and roots of mycorrhizal and non-mycorrhizal aspen and birch in response to NaCl treatment. Roots of NaCl-treated aspen inoculated with H. crustuliniforme had over twofold higher concentrations of Na compared with non-mycorrhizal NaCl-treated plants. Similarly to aspen, Na and Cl concentrations increased in roots and shoots of NaCl-treated birch seedlings. However, in birch, there were no significant differences in Na and Cl concentrations between mycorrhizal and non-mycorrhizal plants. The results suggest that salt exclusion by the ectomycorrhizal associations is host-specific or/and that the processes leading to salt exclusion are activated in ectomycorrhizal plants by a threshold salt level which may vary between plant species.     

Effects of root medium pH on root water transport and apoplastic pH in red‐osier dogwood (Cornus sericea) and paper birch (Betula papyrifera) seedlings

• Soil pH is a major factor affecting plant growth. Plant responses to pH conditions widely vary between different species of plants. However, the exact mechanisms of high pH tolerance of plants are largely unknown. In the present study, we compared the pH responses of paper birch (Betula papyrifera) seedlings, a relatively sensitive species to high soil pH, with red‐osier dogwood (Cornus sericea), reported to be relatively tolerant of high pH conditions. We examined the hypotheses that tolerance of plants to high root zone pH is linked to effective control of root apoplastic pH to facilitate nutrient and water transport processes
• In the study, we exposed paper birch and red‐osier dogwood seedlings for six weeks to pH 5, 7 and 9 under controlled‐environment conditions in hydroponic culture. Then, we measured biomass, gas exchange, root hydraulic conductivity, ferric chelate reductase (FCR) activity, xylem sap pH and the relative abundance of major elements in leaf protoplasts and apoplasts.
• The study sheds new light on the rarely studied high pH tolerance mechanisms in plants. We found that compared with paper birch, red‐osier dogwood showed greater growth, higher gas exchange, and maintained higher root hydraulic conductivity as well as lower xylem sap pH under high pH conditions.
• The results suggest that the relatively high pH tolerance of dogwood is associated with greater water uptake ability and maintenance of low apoplastic pH. These traits may have a significant impact on the uptake of Fe and Mn by leaf cells.

     

Responses of mycorrhizal jack pine (<Emphasis Type="Italic">Pinus banksiana</Emphasis>) seedlings to NaCl and boron

In earlier studies, we established that mycorrhizal associations protect plants against salt stress. However, elevated boron levels are often present in saline soils and little is known about the effects of boron on salt resistance of mycorrhizal plants. In the present study, we inoculated jack pine (Pinus banksiana) seedlings with Hebeloma sp., Suillus tomentosus and Wilcoxina mikolae var. mikolae to study the effects of mycorrhizal associations on seedling responses to boron and salt. Seedlings were grown in the greenhouse and subjected to 60 mM NaCl, 2 mM H₃BO₃ or 60 mM NaCl + 2 mM H₃BO₃ treatments for 4 weeks. Dry weights, shoot:root ratios and chlorophyll concentrations were higher in inoculated seedlings for all treatments compared with the non-inoculated plants. When applied with NaCl, B aggravated needle necrosis while reducing Cl concentrations in shoots of non-inoculated plants. Plants treated with 2 mM H₃BO₃ + 60 mM NaCl had similar concentrations of Na and B to those that were treated separately with 60 mM NaCl and 2 mM H₃BO₃. Plants inoculated with mycorrhizal fungi had lower shoot Na concentrations compared with non-inoculated seedlings, but showed relatively little impact from elevated B concentrations.     

Growth and nodulation in <Emphasis Type="Italic">Alnus sieboldiana</Emphasis> in response to <Emphasis Type="Italic">Frankia</Emphasis> inoculation and nitrogen treatments

Key message

We investigated a Frankia – Alnus sieboldiana symbiosis, including the minimum inoculum dose for constant nodulation, the period of time to nodulation after inoculation, and the effects of N on nodulation.

Abstract

Frankia is a nitrogen-fixing actinomycete that forms root nodules in some dicotyledonous plants, which are called actinorhizal. We studied nodule formation in Alnus sieboldiana, an actinorhizal plant, after inoculation with a Frankia isolate to establish techniques for Frankia inoculation and the cultivation of inoculated plants. Root nodules formed on seedlings of A. sieboldiana by 2 weeks after inoculation, and N₂ fixation measured by acetylene reduction activity started 3 weeks after inoculation. Nodulation was observed after inoculation with a Frankia isolate at 0.001 μL packed cell volume (pcv). The number of nodules formed on the seedlings inoculated with Frankia at more than 0.05 μL pcv was not significantly different. Nodule development and N₂ fixation were reduced when inoculated seedlings were treated weekly with 15 mM NH₄NO₃-N. In contrast, treatment with 3.75 or 0.9375 mM NH₄NO₃-N did not inhibit nodule development or N₂ fixation of inoculated seedlings by 15 weeks of N treatment.

     

Phosphorus deficiency-induced reduction in root hydraulic conductivity in Medicago falcata is associated with ethylene production

Plants grown in phosphorus-deficient solutions often exhibit disruption of water transport due to reduction in root hydraulic conductivity (Lpr) and enhanced ethylene production. To uncover the relationship between the reduction in Lpr and increase in ethylene production, we investigated effect of phosphorus (P) deficiency on ethylene production and Lpr in legume plants of Medicago falcata L. There was an increase in ethylene production and a reduction of Lpr of M. falcata roots when M. falcata seedlings grown in P sufficient solutions (0.5 mM H₂PO₄^2?) were transferred to P-deficient solutions (5 μM H₂PO₄^2?). Antagonists of ethylene biosynthesis, CoCl₂ and aminoethoxyvinylglycine (AVG), abolished the P deficiency-induced ethylene production. Root hydraulic conductivity of M. falcata seedlings grown in P-sufficient solutions was insensitive to CoCl₂ and AVG, while the two chemicals enhanced Lpr for those grown in P-deficient solutions, suggesting that P deficiency-induced decrease in Lpr can be reversed by inhibiting ethylene production. Ethylene precursor 1-amino cyclopropane-1-carboxylic acid (ACC) and ethylene donor ethephon had greater inhibitory effect on Lpr of P-sufficient seedlings than that of P-deficient seedlings. Root hydraulic conductivity of P-sufficient seedlings was more sensitive to HgCl₂ than that of P-deficient seedlings. Taken together, these findings suggest that ethylene induced by P deficiency may play an important role in modulation of root hydraulic conductivity by affecting aquaporins in plants.     

Regulation by fixed nitrogen of host-symbiont recognition in the Rhizobium-clover symbiosis

Either NO₃⁻ (16 millimolar) or NH₄⁺ (1 millimolar) completely inhibited infection and nodulation of white clover seedlings (Trifoliin repens) inoculated with Rhizobium trifolii. The binding of R. trifolii to root hairs and the immunologically detectable levels of the plant lectin, trifoliin, on the root hair surface had parallel declining slopes as the concentration of either NO₃⁻ or NH₄⁺ was increased in the rooting medium. This supports the role of trifoliin in binding R. trifolii to clover root hairs. Agglutination of R. trifolii by trifoliin from seeds was not inhibited by these levels of NO₃⁻ or NH₄⁺. The results suggest that these fixed N ions may play important roles in regulating an early recognition process in the Rhizobium-clover symbiosis, namely the accumulation of high numbers of infective R. trifolii cells on clover root hairs.     

Response of Douglas fir seedlings to nitrate and ammonium nitrogen sources at different levels of pH and iron supply

Douglas fir seedlings were grown for two to three months in sand and soil cultures in a greenhouse to examine their growth response to nitrogen (N) source at different levels of pH and iron (Fe) supply. In the first two experiments nutrient solutions of known pH were automatically applied to the top of the sand cultures and allowed to run to waste from the bottom. Under these conditions seedlings made most growth on nitrate (NO₃–N) under acid (pH4) conditions, but most growth on ammonium (NH₄–N) under neutral (pH7) conditions. Calcium carbonate (CaCO₃) was used to create a range of pH conditions (from 4.0 to 7.2) in a peat and sand artificial soil. Over the pH range 4 to 6 NH₄–N or NO₃+NH₄–N produced larger seedlings than NO₃–N alone, but above pH6 growth on all N sources was depressed. Chemical analysis showed that seedling Ca concentration had increased and Fe concentration had decreased with increase in CaCO₃ application. Both Ca and Fe concentrations were higher in seedlings receiving NO₃–N than in those receiving NH₄ or NO₃+NH₄.In sub-irrigated sand cultures, Doughlas fir seedlings receiving NO₃–N were shown to respond to additions of Fe chelate, but seedlings receiving NH₄–N responded little to Fe chelate. At pH5 seedlings receiving NO₃–N did not grow as big as seedlings receiving NH₄–N in the absence of Fe chelate, but addition of Fe chelate resulted in NO₃-fed seedlings growing larger than NH₄-fed seedlings. The relationship between seedling Fe concentration and N nutrition is discussed.The relatively larger root dry weight and surface area of seedlings grown on NO₃–N, as compared to NH₄–N, in sand culture, was noted.     

Chemical composition of apoplastic transport barriers in relation to radial hydraulic conductivity of corn roots (Zea mays L.)

The hydraulic conductivity of roots (Lp_r) of 6- to 8-d-old maize seedlings has been related to the chemical composition of apoplastic transport barriers in the endodermis and hypodermis (exodermis), and to the hydraulic conductivity of root cortical cells. Roots were cultivated in two different ways. When grown in aeroponic culture, they developed an exodermis (Casparian band in the hypodermal layer), which was missing in roots from hydroponics. The development of Casparian bands and suberin lamellae was observed by staining with berberin-aniline-blue and Sudan-III. The compositions of suberin and lignin were analyzed quantitatively and qualitatively after depolymerization (BF₃/methanol-transesterification, thioacidolysis) using gas chromatography/mass spectrometry. Root Lp_r was measured using the root pressure probe, and the hydraulic conductivity of cortical cells (Lp) using the cell pressure probe. Roots from the two cultivation methods differed significantly in (i) the Lp_r evaluated from hydrostatic relaxations (factor of 1.5), and (ii) the amounts of lignin and aliphatic suberin in the hypodermal layer of the apical root zone. Aliphatic suberin is thought to be the major reason for the hydrophobic properties of apoplastic barriers and for their relatively low permeability to water. No differences were found in the amounts of suberin in the hypodermal layers of basal root zones and in the endodermal layer. In order to verify that changes in root Lp_r were not caused by changes in hydraulic conductivity at the membrane level, cell Lp was measured as well. No differences were found in the Lp values of cells from roots cultivated by the two different methods. It was concluded that changes in the hydraulic conductivity of the apoplastic rather than of the cell-to-cell path were causing the observed changes in root Lp_r. Received: 17 March 1999 / Accepted: 22 June 1999     

Effects of NH4 +, NO3 − and HCO3 − on apoplast pH in the outer cortex of root zones of maize, as measured by the fluorescence ratio of fluorescein boronic acid

A fluorimetric ratio technique was elaborated to measure apoplastic pH in the outer root cortex of maize (Zea mays L.) grown hydroponically. A newly synthesized fluorescent probe, fluorescein boronic acid (pK_a = 5.48), which covalently binds to the cell wall of the outer cell layers, was used. Under conditions of saturating ion concentrations the apoplastic pH was determined along the root axis ranging from 1 to 30 mm behind the root tip. Apoplastic pH was recorded for root segment areas (1 mm²), and pH values of high statistical significance were obtained. With an external solution of pH 5, the apoplastic pH was about pH 5.1 in the division zone, between pH 4.8 and 4.9 in the elongation region and about pH 4.9 in the root hair zone. At an external pH of 8.6, the difference between the external pH and the apoplastic pH was considerably more, with a pH of 5.2–5.3 in all root zones. Addition of 1 mM NH₄ ⁺ caused a small apoplastic pH decrease (0.05 of a pH unit) in all root zones. Apoplastic alkalization upon application of 6 mM NO₃ ⁻ was highest (0.3 of a pH unit) in the zone where root hairs emerge; in the division and early elongation zones, apoplastic pH increased only transiently. In the presence of 10 mM HCO₃ ⁻, NO₃ ⁻ elicited a higher and persistent alkalization (0.06–0.25 of a pH unit) in all root zones. Application of fusicoccin reduced apoplastic pH from 4.85 to 4.75 in the elongation zone, while inhibition of the H⁺-ATPase with vanadate alkalized the apoplast in the root hair zone from pH 5.4 to 5.6. The observed pH differences along the root axis upon differential N supply and application of HCO₃ ⁻ provide evidence that this new pH technique is a useful tool with which to measure apoplastic pH, and in future may permit measurements at microsites at the cell level by use of microscope imaging. Received: 26 August 1998 / Accepted: 4 May 1999     

Ammonium under solution culture alleviates aluminum toxicity in rice and reduces aluminum accumulation in roots compared with nitrate

Al stress and ammonium–nitrogen nutrition often coexist in acidic soils due to their low pH and weak nitrification ability. Rice is the most Al-resistant species among small grain cereal crops and prefers NH₄ ⁺ as its major inorganic nitrogen source. This study investigates the effects of NH₄ ⁺ and NO₃ ^? on Al toxicity and Al accumulation in rice, and thereby associates rice Al resistance with its NH₄ ⁺ preference. Two rice subspecies, indica cv. Yangdao6 and japonica cv. Wuyunjing7, were used in this study. After treatment with or without Al under conditions of varying NH₄ ⁺ and NO₃ ^? supply, rice seedlings were harvested for the determination of root elongation, callose content, biomass, Al concentration and medium pH. The results indicated that Wuyunjing7 was more Al-resistant and NH₄ ⁺-preferring than Yangdao6. NH₄ ⁺ alleviated Al toxicity in two cultivars compared with NO₃ ^?. Both NH₄ ⁺-Al supply and pretreatment with NH₄ ⁺ reduced Al accumulation in roots and root tips compared with NO₃ ^?. NH₄ ⁺ decreased but NO₃ ^? increased the medium pH, and root tips accumulated more Al with a pH increase from 3.5 to 5.5. Increasing the NO₃ ^? concentration enhanced Al accumulation in root tips but increasing the NH₄ ⁺ concentration had the opposite effect. These results show NH₄ ⁺ alleviates Al toxicity for rice and reduces Al accumulation in roots compared with NO₃ ^?, possibly through medium pH changes and ionic competitive effects. Making use of the protective effect of NH₄ ⁺, in which the Al resistance increases, is advised for acidic soils, and the hypothesis that rice Al resistance is associated with the preferred utilization of NH₄ ⁺ is suggested.     

Physiological regulation of plant-atmosphere ammonia exchange

Plants have a compensation point for NH₃ which ranges from 0.1 to 20 nmol mol^-1, and may be several-fold higher or lower than naturally occurring atmospheric NH₃ concentrations. This implies that NH₃ fluxes over vegetated surfaces are bi-directional and that ammonia exchange with the atmosphere in many cases contributes significantly to the nitrogen economy of vegetation. Physiological regulation of plant–atmosphere NH₃ fluxes is mediated via processes involved in nitrogen uptake, transport and metabolism. A rapid turnover of NH₃ ⁺ in plant leaves leads to the establishment of a finite NH₃ ⁺ concentration in the leaf apoplastic solution. This concentration determines, together with that of H⁺, the size of the NH₃ compensation point. Barley and oilseed rape plants with access to NH₃ ⁺ in the root medium have higher apoplastic NH₃ ⁺ concentrations than plants absorbing NO₃ ^-. Furthermore, the apoplastic NH₃ ⁺ concentration increases with the external NH₃ ⁺ concentration. Inhibition of GS leads to a rapid and substantial increase in apoplastic NH₃ ⁺ and barley mutants with reduced GS activity have higher apoplastic NH₃ ⁺ than wild-type plants. Increasing rates of photorespiration do not affect the steady-state NH₃ ⁺ or H⁺ concentration in tissue or apoplast of oilseed rape, indicating that the NH₃ ⁺ produced is assimilated efficiently. Nevertheless, NH₃ emission increases due to a temperature-mediated displacement of the chemical equilibrium between gaseous and aqueous NH₃ in the apoplast. Sugarbeet plants grown with NO₃ ^- seem to be temporarily C-limited in the light due to a repression of respiration. As a consequence, the activity of chloroplastic GS declines during the day causing a major part of NH₃ ⁺ liberated in photorespiration to be assimilated during darkness when 2-oxoglutarate is supplied in high rates by respiration. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ionic balance,proton efflux,nitrate reductase activity and growth ofHippophaë rhamnoides L. ssp.rhamnoides as influenced by combined-N nutrition or N2 fixation

Growth of 2-month-old nonnodulatedHippophaë rhamnoides seedlings supplied with combined N was compared with that of nodulated seedlings grown on zero N. Plant growth was significantly better with combined N than with N₂ fixation and, although not statistically significant for individual harvests, tended to be highest in the presence of NH ₄ ⁺ , a mixture of NH ₄ ⁺ and NO ₃ ^? producing the highest yields. Growth was severely reduced when solely dependent on N₂ fixation and, unlike the combined-N plants, shoot to root ratios had only slightly increased after an initial decrease. An apparently insufficient nodule mass (nodule weight ratio <5 per cent) during the greater part of the experimental period is suggested as the main cause of the growth reduction in N₂-fixing plants. Thein vivo nitrate reductase activity (NRA) of NO ₃ ^? dependent plants was almost entirely located in the roots. However, when grown with a combination of NO ₃ ^? and NH ₄ ⁺ , root NRA was decreased by approximately 85 per cent.H. rhamnoides demonstrated in the mixed supply a strong preference for uptake of N as NH ₄ ⁺ , NO ₃ ^? contributing only for approximately 20 per cent to the total N assimilation. Specific rates of N acquisition and ion uptake were generally highest in NO ₃ ^? +NH ₄ ⁺ plants. The generation of organic anions per unit total plant dry weight was approximately 40 per cent less in the NH ₄ ⁺ plants than in the NO ₃ ^? plants. Measured extrusions of H⁺ or OH^? (HCO ₃ ^? ) were generally in good agreement with calculated values on the basis of plant composition, and the acidity generated with N₂ fixation amounted to 0.45–0.55 meq H⁺. (mmol N_org)^?1. Without acidity control and in the presence of NH ₄ ⁺ , specific rates of ion uptake and carboxylate generation were strongly depressed and growth was reduced by 30–35 per cent. Growth of nonnodulatedH. rhamnoides plants ceased at the lower pH limit of 3.1–3.2 and deterioration set in; in the case of N₂-fixing plants the nutrient solution pH stabilized at a value of 3.8–3.9 without any apparent adverse effects upon plant performance. The chemical composition of experimental and field-growing plants is being compared and some comments are made on the nitrogen supply characteristics of their natural sites.     

Consequences of inoculation with native arbuscular mycorrhizal fungi for root colonization and survival of <Emphasis Type="Italic">Artemisia tridentata</Emphasis> ssp. <Emphasis Type="Italic">wyomingensis</Emphasis> seedlings after transplanting

In arid environments, the propagule density of arbuscular mycorrhizal fungi (AMF) may limit the extent of the plant–AMF symbiosis. Inoculation of seedlings with AMF could alleviate this problem, but the success of this practice largely depends on the ability of the inoculum to multiply and colonize the growing root system after transplanting. These phenomena were investigated in Artemisia tridentata ssp. wyomingensis (Wyoming big sagebrush) seedlings inoculated with native AMF. Seedlings were first grown in a greenhouse in soil without AMF (non-inoculated seedlings) or with AMF (inoculated seedlings). In spring and fall, 3-month-old seedlings were transplanted outdoors to 24-L pots containing soil from a sagebrush habitat (spring and fall mesocosm experiments) or to a recently burned sagebrush habitat (spring and fall field experiments). Five or 8 months after transplanting, colonization was about twofold higher in inoculated than non-inoculated seedlings, except for the spring field experiment. In the mesocosm experiments, inoculation increased survival during the summer by 24 % (p?=?0.011). In the field experiments, increased AMF colonization was associated with increases in survival during cold and dry periods; 1 year after transplanting, survival of inoculated seedlings was 27 % higher than that of non-inoculated ones (p?<?0.001). To investigate possible mechanisms by which AMF increased survival, we analyzed water use efficiency (WUE) based on foliar ¹³C/¹²C isotope ratios (δ ¹³C). A positive correlation between AMF colonization and δ ¹³C values was observed in the spring mesocosm experiment. In contrast, inoculation did not affect the δ ¹³C values of fall transplanted seedlings that were collected the subsequent spring. The effectiveness of AMF inoculation on enhancing colonization and reducing seedling mortality varied among the different experiments, but average effects were estimated by meta-analyses. Several months after transplanting, average AMF colonization was in proportion 84 % higher in inoculated than non-inoculated seedlings (p?=?0.0042), while the average risk of seedling mortality was 42 % lower in inoculated than non-inoculated seedlings (p?=?0.047). These results indicate that inoculation can increase AMF colonization over the background levels occurring in the soil, leading to higher rates of survival.     

Fungal ectomycorrhizal community and drought affect root hydraulic properties and soil adherence to roots ofPinus pinaster seedlings

Pinus pinaster seedlings were grown in a sandy dune soil either inoculated withHebeloma cylindrosporum or let to natural colonisation. Six months later, half of the seedlings of both treatments were subjected to a 3-week moderate drought. Root colonisation analysis showed that root tips were colonised to almost 100% independent of the inoculation. DNA determination of the ectomycorrhizal morphotypes showed that inoculated seedlings were extensively mycorrhized byH. cylindrosporum (more than 75%) whereas non-inoculated seedlings were mycorrhized by the exotic speciesThelephora terrestris (50%) andLaccaria bicolor (30%) and to a lesser extent byH. cylindrosporum (20%). Drought did not affect these frequencies. Total plant biomass was not affected by the mycorrhizal status or by drought but the root/shoot biomass ratio as well as the root/leaf surface area ratio were much lower in seedlings extensively colonised byH. cylindrosporum. Root hydraulic conductivity was higher in plants mainly mycorrhized byH. cylindrosporum, showing that this fungus improved the water uptake capacity of the root system as compared toT. terrestris and/orL. bicolor. This positive effect was also found under drought but to a lesser extent.H. cylindrosporum also increased the amount of root-adhering soil as compared to the other fungal symbionts, illustrating the performance of this association in aggregating sandy soil particles and developing the rhizosheath. The origin of the reduced root hydraulic resistance byH. cylindrosporum mycorrhization is discussed for the whole path including soil, soil-root interface and root cortex.