On the effects of phosphorus on zinc uptake by barley

Summary The response of barley to phosphate application and the effect of applied phosphorus on the uptake of soil zinc by the crop were tested in pot, Neubauer and incubaticn experiments on four soils differing in native phosphate status.There was a response of the yield to phosphorus application in alluvial soils from Clementina (Ecuador) and Bangla Desh. Barley grown on red soil from Bangla Desh and glacial clay from Uppsala did not show any response to P application.Applied P³² was fixed to a great extent in all the soils studied. But in phosphate-deficient soils, a much higher degree of sorption of P³² was recorded than in phosphate-rich soils. Compared to phosphate-rich soils, the utilization of fertilizer phosphorus is higher than of native soil P in the case of phosphate-deficient soils. It was observed that uptake of Zn⁶⁵ by the crop is counteracted by phosphorus application at the three stages of crop growth studied.This work was financially supported by the International Atomic Energy Agency, Vienna, Austria, and by the National Council for Forestry and Agricultural Research, Sweden.     

Salinity tolerance within the grass speciesAgrostis stolonifera L

Summary Eight populations ofAgrostis stolonifera, collected from maritime and inland habitats, were grown in solution culture and in sand culture at various concentrations of sodium chloride from 0 ppm to 5,000 ppm Na. NaCl had less effect upon the dry weight yield of populations from maritime habitats, with soils of high Na content, than upon populations from inland or maritime habitats with soils of low Na content. The correlation between per cent reduction in dry weight yield and the Na content of the native soil of each population was r=−0.73. Populations from inland or maritime habitats, with soils of low Na content, contained more Na in their shoot materials, and higher Na/K ratios, than did populations from high Na soils, at all Na concentrations. The effect of NaCl upon the root elongation of each population was not correlated with the effect upon dry weight yield, nor with the Na content of their native soil. The effect of Na Cl upon root elongation was highly dependent upon the composition of the culture solution used. It is concluded that the effect of NaCl on root elongation is not an adequate measure of salinity tolerance in this species.     

Selection for Nitrogen Responses in Lolium

Lolium populations from a diallel cross were tested for heritabilityof response to nitrogen in sand culture, in pots of soil andfinally in the field. Heritability of response was found insand culture for several growth parameters including leaf areaand shoot dry weight. Response in pots of soil varied significantlybetween populations, but in a small experiment heritabilitywas not shown to be significant. Selection of individual genotypes within the diallel populationswas then carried out simultaneously for nitrate uptake fromsolutions and for growth in pots. The selected plants were finallytested for nitrogen response in simulated swards in the field.L. perenne plants did not differ after selection for high orlow uptake, but L. multiflorum and interspecific hybrids producedlines which showed significant differences in nitrogen response. A second cycle of selection confirmed the lack of variationin the L. perenne lines chosen, and revealed a complex situationin L. multiflorum whereby the effects of selection were apparentlyreversed. In the L. perenne x L. multiflorum hybrid lines, however,the large differences between high and low response selectionswere maintained. High response plants had broad pale leaveswhile low response plants had rather narrow dark leaves.     

Positive and negative biotic interactions and invasive Triadica sebifera tolerance to salinity: a cross‐continent comparative study

Exotic plant species may exhibit abiotic niche expansions that enable them to persist in a greater variety of habitat types in their introduced ranges than in their native ranges. This may reflect variation in limitation by different abiotic niche dimensions (realized niche shift) or phenotypic effects of biotic interactions that vary among ranges (realized niche expansion). Novel abiotic and biotic environments in the introduced range may also lead to genetic changes in exotic plant traits that enhance their abiotic stress tolerance (fundamental niche expansion). Here, we investigated how biotic interactions (aboveground herbivory and soil organisms) affect plant salinity tolerance using the invasive species Triadica sebifera from China (native range) and US (introduced range) populations grown in common gardens in both ranges. Simulated herbivory significantly reduced survival in saline treatments with reductions especially large at low salinity. Soil sterilization had a negative effect on survival at low salinity in China but had a positive effect on survival at low salinity in the US. Triadica survival and biomass were higher for US populations than for China populations, particularly in China but salinity tolerance did not depend on population origin. On average, arbuscular mycorrhizal (AM) colonization was higher for US populations, US soils and low salinity. These factors had a significant, positive, non‐additive interaction so that clipped seedlings from US populations in low saline US soils had high levels of AM colonization. Overall, our results show that phenotypic biotic interactions shape Triadica's salinity tolerance. Positive and negative biotic interactions together affected plant performance at intermediate stress levels. However, only aboveground damage consistently affected salinity tolerance, suggesting an important role for enemy release in expanding stress tolerance.     

不同生态型东南景天对土壤中Cd的生长反应及吸收积累的差异性

 采用盆栽方法研究了两种生态型东南景天(Sedum alfredii)对土壤中不同含量Cd(即对照, 12.5, 25, 50, 100, 200, 300, 400 mg&;#8226;kg^－1)的生 长反应、吸收和积累Cd的差异性。结果表明,土壤添加重金属Cd后,矿山生态型东南景天生长正常,地上部和根系Cd含量随着土壤中Cd含量的 增加而增加,在400 mg&;#8226;kg^－1 Cd处理下含量分别高达2 900和500 mg&;#8226;kg^－1,其地上部显著大于根部;然而,土壤添加Cd后,非矿山生态型东 南景天的生长受到抑制,地上部和根部的生物量显著降低。当土壤Cd含量为50～100 mg&;#8226;kg^－1 时,非矿山生态型东南景天的地上部和根系Cd含 量随着土壤中Cd含量的增加而增加,而且根系Cd含量则大于地上部。当土壤Cd≤50 mg&;#8226;kg^－1时,矿山生态型东南景天根系Cd含量比非矿山生态 型高 ,但当土壤Cd≥100 mg&;#8226;kg^－1,两者之间无显著差异;然而,但在同一Cd处理水平下,矿山生态型东南景天地上部Cd含量总是高于非矿山 生态型。这些结果表明,矿山生态型东南景天有很强的忍耐和吸收土壤Cd的能力,再次证明其为一种Cd超积累植物。     

Inoculation Response of Legumes in Relation to the Number and Effectiveness of Indigenous Rhizobium Populations

The response of legumes to inoculation with rhizobia can be affected by many factors. Little work has been undertaken to examine how indigenous populations or rhizobia affect this response. We conducted a series of inoculation trials in four Hawaiian soils with six legume species (Glycine max, Vigna unguiculata, Phaseolus lunatus, Leucaena leucocephala, Arachis hypogaea, and Phaseolus vulgaris) and characterized the native rhizobial populations for each species in terms of the number and effectiveness of the population for a particular host. Inoculated plants had, on average, 76% of the nodules formed by the inoculum strain, which effectively eliminated competition from native strains as a variable between soils. Rhizobia populations ranged from less than 6 × 10⁰/g of soil to 1 × 10⁴/g of soil. The concentration of nitrogen in shoots of inoculated plants was not higher than that in uninoculated controls when the most probable number MPN counts of rhizobia were at or above 2 × 10¹/g of soil unless the native population was completely ineffective. Tests of random isolates from nodules of uninoculated plants revealed that within most soil populations there was a wide range of effectiveness for N₂ fixation. All populations had isolates that were ineffective in fixing N₂. The inoculum strains generally did not fix more N₂ than the average isolate from the soil population in single-isolate tests. Even when the inoculum strain proved to be a better symbiont than the soil rhizobia, there was no response to inoculation. Enhanced N₂ fixation after inoculation was related to increased nodule dry weights. Although inoculation generally increased nodule number when there were less than 1 × 10² rhizobia per g of soil, there was no corresponding increase in nodule dry weight when native populations were effective. Most species compensated for reduced nodulation in soils with few rhizobia by increasing the size of nodules and therefore maintaining a nodule dry weight similar to that of inoculated plants with more nodules. Even when competition by native soil strains was overcome with a selected inoculum strain, it was not always possible to enhance N₂ fixation when soil populations were above a threshold number and had some effective strains.     

The Effect of Soil Phosphate on Injury to Winter Barley Caused by Mildew Infection (Erysiphe graminis f.sp. Hordei)

Populations of winter barley were sown in autumn in large tubsof soil to half of which additional phosphate was supplied.Half the plants of each phosphate treatment were infected withpowdery mildew (Erysiphe graminis D.C. f. sp. hordei Marchal)and all plants were transferred to the-field. Infection inducedextensive leaf injury during mid-winter in plants at low phosphate,but injury was greatly reduced in the high soil phosphate treatment.The extent of winter injury had a marked effect on the increasein leaf area in spring, and the accumulation of plant d. wtwas positively correlated with the percentage of total leafarea remaining undamaged at the end of winter. This, in turn,was strongly influenced by the interaction between powdery mildewand soil phosphate. High soil phosphate may act as a ‘buffer’to the effects of infection, minimizing the combined effectsof infection and abiotic stresses suffered by plants in winter. Barley, powdery mildew (Erysiphe graminis hordei), overwinter, phosphate, temperature     

Lead tolerance of Betula and Salix in the mining area of Mechernich/Germany

Natural populations of woody perennials on lead-mining sites in the Mechernich area of the Eifel Mountains were investigated with respect to soil factors determining the degree and type of heavy metal tolerance. Salix caprea L. (Goat Willow) grew on soils with up to 17000 mg kg^–1 total lead (ca. 4000 mg kg^–1 ammonium acetate-exchangeable Pb). Betula pendula Roth (Silver Birch) was found on soils containing as much as 29000 mg kg^–1 total lead (7000 mg kg^–1 ammonium acetate-exchangeable Pb). Other woody perennials, with the exception of the dwarf shrub Calluna vulgaris, were not found in the contaminated area even though they did occur in the immediate vicinity. The two lead-tolerant tree species did not form mixed populations.Because of a significantly lower Pb/Ca ratio in Salix soils (2.2) compared with Betula soils (7.4), a calcium-dependent mechanism of lead tolerance is suggested for Salix, but not for Betula.The Betula population could be divided into two groups, each showing a highly significant correlation between root-lead content and exchangeable lead amounts in the soil, but with different levels of lead uptake. The only soil factor distinguishing the two groups was found to be the level of soluble phosphate. A distinctly low level of soil phosphate correlated with a high lead concentration in roots of the one group (30000 mg Pb kg^–1 DW), whereas high phosphate amounts corresponded with a much lower lead concentration in roots of the other (12000 mg Pb kg^–1 DW^–1). Since the correlation between lead in the soil and in plants was similar for the two groups, it is concluded that the type of lead tolerance in Betula is determined by the status of plant phosphate nutrition, rather than by simple phosphate precipitation in the soil.A comparison of growth between different populations of Betula seedlings on homogenized soils from the mining area revealed the Mechaernich population to be a distinct ecotype with respect to lead tolerance. The control population obtained from a non-contaminated area exhibited a lower degree of lead tolerance coupled with a two-step strategy of adaptation to lead.     

The effect of Chinese tallow tree (Sapium sebiferum) ecotype on soil–plant system carbon and nitrogen processes

The EICA hypothesis predicts that shifts in allocation of invasive plants give rise to higher growth rates and lower herbivore defense levels in their introduced range than conspecifics in their native range. These changes in traits of invasive plants may also affect ecosystem processes. We conducted an outdoor pot experiment with Chinese tallow tree (Sapium sebiferum, Euphorbiaceae) seedlings from its native (Jiangsu, China, native ecotype) and introduced ranges (Texas, USA, invasive ecotype) to compare their relative performances in its native range and to examine ecotype effects on soil processes with and without fertilization. Consistent with predictions, plant (shoot and root) mass was significantly greater and leaf defoliation tended to be higher, while the root:shoot ratio was lower for the invasive ecotype relative to the native ecotype. Seasonal amounts of soil–plant system CO₂ and N₂O emissions were higher for the invasive ecotype than for the native ecotype. Soil respiration rates and N₂O emission increases from fertilization were also greater for the invasive ecotype than for the native ecotype, while shoot-specific respiration rates (g CO₂–C g⁻¹ C day⁻¹) did not differ between ecotypes. Further, soil inorganic N (ammonium and nitrate) was higher, but soil total N was lower for soils with the invasive ecotype than soils with the native ecotype. Compared with native ecotypes, therefore, invasive ecotypes may have developed a competition advantage in accelerating soil processes and promoting more nitrogen uptake through soil–plant direct interaction. The results of this study suggest that soil and ecosystem processes accelerated by variation in traits of invasive plants may have implications for their invasiveness.     

Practical aspects of mycorrhizal technology in some tropical crops and pastures

Summary Greenhouse and field experiments were conducted on the effect of VA mycorrhiza (VAM) on the growth of cassava, various tropical grass and legume species, as well as beans, coffee and tea. A large number of VAM fungal species were evaluated for effectivity in increasing cassava growth and P uptake in acid low-P soils. The effectivity of VAM species and isolates was highly variable and dependent on soil pH and fertilizer applications, as well as on soil temperature and humidity. Two species,Glomus manihotis andEntrophospora colombiana were found to be most effective for a range of crops and pastures, at low pH and at a wide range of N, P, and K levels. At very low P levels nearly all crops and pasture species were highly mycorrhizal dependent, but at higher soil P levels cassava and several pasture legumes were more dependent than grass species. Mycorrhizal inoculation significantly increased cassava and bean yields in those soils with low or ineffective indigenous mycorrhizal populations. In these soils cassava root yields increased on the average 20–25% by VAM inoculation, both at the experiment station and in farmers’ fields. VAM inoculation of various pasture legumes and grasses, in combination with rock phosphate applications, increased their early growth and establishment. Agronomic practices such as fertilization, crop rotations, intercropping and pesticide applications were found to affect both the total VAM population as well as its species composition. While there is no doubt about the importance of VA mycorrhiza in enhancing P uptake and growth of many tropical crops and pastures grown on low-P soils, much more research is required to elucidate the complicated soil-plant-VAM interactions and to increase yields through improved mycorrhizal efficiency.     

Soil conditioning effects of Phragmites australis on native wetland plant seedling survival

Interactions between introduced plants and soils they colonize are central to invasive species success in many systems. Belowground biotic and abiotic changes can influence the success of introduced species as well as their native competitors. All plants alter soil properties after colonization but, in the case of many invasive plant species, it is unclear whether the strength and direction of these soil conditioning effects are due to plant traits, plant origin, or local population characteristics and site conditions in the invaded range. Phragmites australis in North America exists as a mix of populations of different evolutionary origin. Populations of endemic native Phragmites australis americanus are declining, while introduced European populations are important wetland invaders. We assessed soil conditioning effects of native and non‐native P. australis populations on early and late seedling survival of native and introduced wetland plants. We further used a soil biocide treatment to assess the role of soil fungi on seedling survival. Survival of seedlings in soils colonized by P. australis was either unaffected or negatively affected; no species showed improved survival in P. australis‐conditioned soils. Population of P. australis was a significant factor explaining the response of seedlings, but origin (native or non‐native) was not a significant factor. Synthesis: Our results highlight the importance of phylogenetic control when assessing impacts of invasive species to avoid conflating general plant traits with mechanisms of invasive success. Both native (noninvasive) and non‐native (invasive) P. australis populations reduced seedling survival of competing plant species. Because soil legacy effects of native and non‐native P. australis are similar, this study suggests that the close phylogenetic relationship between the two populations, and not the invasive status of introduced P. australis, is more relevant to their soil‐mediated impact on other plant species.     

DIFFERENTIAL RESPONSES TO MAGNESIUM AND CALCIUM BY NATIVE POPULATIONS OF AGROPYRON SPICATUM

Plants of Agropyron spicatum (Pursh) Scribn. and Smith. from populations native to serpentine and nonserpentine soils were grown at varying levels of magnesium and calcium in culture solutions. The yields of plants from the two populations were different. At high Mg levels (low Ca) the yield of the serpentine population was significantly higher than that of the nonserpentine population. At low Mg the yield of the serpentine population leveled off at a Mg: Ca ratio of 1:2, while the yield of the nonserpentine population increased up to a Mg:Ca ratio of 1:8 and showed no leveling off. Chemical analyses of tissue showed that the Ca uptake of plants from the serpentine population was significantly higher than that of the nonserpentine population. In addition, the serpentine population maintained a lower Mg concentration in the shoots than the nonserpentine population at comparable Mg substrate levels. The two populations showed differences in Ca and Mg uptake efficiency and Mg/Ca, Ca + Mg/K + Na, and Ca + Mg + K + Na in the shoots. The ecotypic differentiation with respect to Mg and Ca between native populations of serpentine and nonserpentine A. spicatum does not appear to be due to any single mechanism but, rather, a combination of several possible mechanisms, i.e., differences in root morphology, uptake mechanisms, translocation of nutrients, and interactions between cations.     

Soil O2 and CO2 Effects on Apparent Cell Viability for Roots of Desert Succulents

Roots of desert succulents occupy the upper layers of porous,well-aerated soils. However, roots of Agave deserti, Ferocactusacanthodes, and Opuntia ficus-indica all tolerated many daysof soil anoxia; 0% O₂ in the soil gas phase for 30 d reducedthe fraction of cells taking up the vital stain neutral red,an average of only 18% for the cortex and 6% for parenchymacells within the stele of perennial established roots. Ephemeralrain roots, induced by watering as branches on the establishedroots, were more susceptible to 0% O₂ in the soil gas phase;19 d abolished stain uptake for cortical cells and 32 d forstelar parenchyma cells. Soil CO₂ levels above the 0.1% observedin the root zone in the field rapidly reduced uptake of neutralred; the fraction of cortical cells taking up the stain decreased30% in 10 h at 0.5% CO₂ and was abolished in 9 h at 2% and 7h at 10% CO₂ averaged for the three species. Rain roots weresomewhat more susceptible than established roots to elevatedsoil CO₂ levels, and stelar parenchyma cells were much lesssusceptible than were cortical cells. When uptake of the vitalstain was abolished by elevated soil CO₂, no anatomical evidenceof cellular damage was observed. For A. deserti exposed to 2%CO₂, the pH of macerated root tissue decreased about 0.35 pHunit over 10 h; CO₂ apparently entered the cells, lowered theintracellular and/or cell wall pH, and prevented the accumulationof neutral red. Elevated soil CO₂ also inhibits root respirationfor the three desert succulents considered. Hence, the restrictionof such species to porous soils may reflect the relatively rapidinhibiting effects of elevated soil CO₂ levels rather than arequirement for high soil O₂ levels, consistent with the observationthat desert soils tend to have low gas-phase CO₂ levels near0.1% compared with 1% or more in the root zone of non-desertspecies. Key words: Agave deserti, Ferocactus acanthodes, neutral red, Opuntia ficus-indica, pH     

Do Armeria maritima (Mill.) Willd. ecotypes from metalliferous soils and non-metalliferous soils differ in growth response under Zn stress? A comparison by a new artificial soil method

Morphological and biogeographical evidence suggests that theheavy-metal ecotype of Armeria maritima (Mill.) Willd. has evolvedfrom a hybrid group between the subspecies maritima from saltmarshes and the subspecies elongata from sandy soils. As partof a study on the ecotypic differentiation in A. maritima, Znresistance was compared in populations from these three ecotypes.To study the long-term growth response to elevated Zn concentrations,an artificial soil was made from ion-exchange resin embeddedin an inert sand matrix, in which metal ions were buffered byan ion-exchange system as in natural soils. In contrast to hydroponics,this artificial soil systems is suitable for long-term cultivationand it provides more reproducible growth conditions than a soilsystem. The long-term growth response in the artificial soilsystem was compared to the growth response to elevated Zn concentrationsin a sand nutrient-solution system. In short-term tests, populationsfrom non-metalliferous soils were more sensitive to Zn concentrationsof 1.0 mmol ^–1 than the heavy-metal populations. However,in long-term tests, the growth of adult plants from all populationswas not inhibited by Zn concentrations up to 2.8 mmol kg^–1dry soil (equivalent 26% of cation-exchange capacity). The Znresistance of all populations could therefore be sufficientfor their survival on Zn mine soils. The discrepancy betweenlong-term tests and short-term tests is discussed with respectto the hypothesis that ‘sensitive’ populations maydiffer from ‘resistant’ populations in the expressionof Zn-resistance mechanisms. Key words: Armeria maritima, growth tests, heavymetal resistance, synthetic ionexchange resins, Zn     

Effect of native and introduced arbuscular mycorrhizal fungi on growth and nutrient uptake ofLygeum spartum andAnthyllis cytisoides

The interaction between native and introduced fungi and their effect on plant growth and mineral uptake were studied. The host plants wereLygeum spartum andAnthyllis cytisoides, the introduced fungus wasGlomus fasciculatum. The four soils used were selected from disturbed and contaminated by mining activities areas. Inoculated and uninoculated plants were grown in the unsterilized and sterilized soils (with and withouth native microflora, respectively). Plants inoculated withG. fasciculatum were higher and had higher tissue P concentration than uninoculated plants, especially inA. cytisoides. However, this inoculation was not effective in unsterilized substrates, suggesting a competition between introduced and native fungi. Concentration of mineral elements other than P varied depending on the host plant and soil. Decrease in Fe, Cu, Mn, Zn and Pb was observed in mycorrhizalA. cytiosides plants and a slight increase in Zn concentration was noted in mycorrhizalL. spartum plants. The study showed that the type of soil and their populations of native endophytes have a considerable effect on plant response to mycorrhizal symbiosis, especially in disturbed soils.     

Heavy metal tolerance in Festuca ovina L. from contaminated sites in the Eifel Mountains,Germany

Extremely high degrees of lead tolerance, measured by comparing rates of root extension in culture solutions, are reported from populations of Festuca ovina growing at two lead-mining sites (Westschacht and Keldenich-II) near Mechernich in the Eifel Mountains, Germany. Other populations from nearby heavy metal-contaminated areas show a considerably smaller degree of lead tolerance. Samples of Festuca ovina collected in the field at Westschacht and Keldenich-II contain higher levels of lead in their aerial organs than do those from other lead-contaminated sites. The main soil factor determining the high degree of lead tolerance is the high Pb/Ca ratio. Populations from soils with a low Pb/Ca ratio display a very low degree of tolerance. It is therefore concluded that in Westschacht and Keldenich-II plants, a genuine intracellular tolerance mechanism is present, allowing the accumulation of lead in aerial organs.Leaf samples of zinc-tolerant Festuca contain higher levels of zinc than do samples of non-tolerant plants. Lead and zinc amounts in leaves are correlated with the soil ratios of Pb/Ca and Zn/Ca, respectively, rather than with the absolute soil-metal levels.In a slightly lead-tolerant, but highly zinc-tolerant clone of Festuca ovina from a site contaminated with large amounts of lead and zinc (Plombières), lead was found to be the major factor affecting the inhibition of root extension with combined treatments of lead and zinc in culture solutions. In the highly lead-tolerant, zinc-sensitive population from Westschacht, zinc governs the response of root growth to combinations of the two metals. The results are discussed in terms of discriminating distinct types of heavy-metal tolerance.     

Arbuscular mycorrhizal symbiosis on serpentine soils: the effect of native fungal communities on different Knautia arvensis ecotypes

Serpentine soils represent a unique environment that imposes multiple stresses on vegetation (low Ca/Mg ratios, macronutrient deficiencies, elevated heavy metal concentrations and drought). Under these conditions, a substantial role of arbuscular mycorrhizal (AM) symbiosis can be anticipated due to its importance for plant nutrition and stress alleviation. We tested whether serpentine and non-serpentine populations of Knautia arvensis (Dipsacaceae) differ in the benefits derived from native AM fungal communities. Four serpentine and four non-serpentine populations were characterised in terms of mycorrhizal colonisation and soil characteristics. The serpentine populations showed significantly lower mycorrhizal colonisation than their non-serpentine counterparts. The mycorrhizal colonisation positively correlated with soil pH, Ca and K concentrations and Ca/Mg ratio. Seedlings from each population were then grown for 3 months in their sterilised native substrates, either uninoculated or reinoculated with native AM fungi. Two serpentine and two non-serpentine populations responded positively to mycorrhizal inoculation, while no significant change in plant growth was observed in the remaining populations. Contrary to our hypothesis, serpentine populations of K. arvensis did not show higher mycorrhizal growth dependence than non-serpentine populations when grown in their native soils and inoculated with native AM fungi.     

Nitrogen, Phosphorus, and Potassium Nutrition of the Edaphic Ecotypes in Cynodon dactylon (L.) Pers.

A differential response to nitrogen, phosphorus, and potassiumwas observed for the three edaphic ecotypes in Cynodon dactylon(L.) Pers. naturally occurring at Malla, Shiwalik, and Chandigarh.The soils of these sites differ markedly with respect to availablenutrients. The better growth of the Malla population comparedto the Chandigarh population in low potassium and low nitrogenmedia could be related to the low availability of these nutrientsin the calcareous Malla soil. The Shiwalik population tendedto show an intermediate behaviour. Whilst the Malla populationwas susceptible to ‘phosphate toxicity’, the growthyield of the Shiwalik and the Chandigarh populations improvedwith increase in phosphorus in the medium. The three populationsalso differed from each other with respect to the uptake ofvarious nutrients. These results could be correlated with thenatural habitat conditions in which the three ecotypes occur.It was concluded that besides calcium which was shown to bepartly responsible for the restriction of the three ecotypesto their respective habitats, the availability of nitrogen,potassium, and also phosphorus may play an important role.     

Effects of labile soil carbon on nutrient partitioning between an arctic graminoid and microbes

We measured partitioning of N and P uptake between soil microorganisms and potted Festuca vivipara in soil from a subarctic heath in response to factorial addition of three levels of labile carbon (glucose) combined with two levels of inorganic N and P. The glucose was added to either non-sterilized or sterilized (autoclaved) soils in quantities which were within the range of reported, naturally occurring amounts of C released periodically from the plant canopy. The aims were, firstly, to examine whether the glucose stimulated microbial nutrient uptake to the extent of reducing plant nutrient uptake. This is expected in nutrient-deficient soils if microbes and plants compete for the same nutrients. Secondly, we wanted to test our earlier␣interpretation that growth reduction observed in graminoids after addition of leaf extracts could be caused directly by labile carbon addition, rather than by phytotoxins in the extracts. Addition of high amounts of N did not affect the microbial N pool, whereas high amounts of added P significantly increased the microbial P pool, indicating a luxury P uptake in the microbes. Both plant N and in particular P uptake increased strongly in response to soil sterilization and to addition of extra N or P. The increased␣uptake led to enhanced plant growth when both elements were applied in high amounts, but only led to increased tissue concentrations without growth responses when the nutrients were added separately. Glucose had strong and contrasting effects on plant and microbial N and P uptake. Microbial N and P uptake increased, soil inorganic N and P concentrations were reduced and plant N and P uptake declined when glucose was added. The responses were dose-dependent within the range of 0–450 μg C g⁻¹ soil added to the non-sterilized soil. The opposite responses of plants and microbes showed that plant acquisition of limiting nutrients is dependent on release of nutrients from the soil microbes, which is under strong regulation by the availability and microbial uptake of labile C. Hence, we conclude, firstly, that the microbial populations can compete efficiently with plants for nutrients to an extent of affecting plant growth when the microbial access to labile carbon is high in nutrient deficient soils. We also conclude that reduced growth of plants after addition of leaf extracts to soil can be caused by carbon-induced shifts in nutrient partitioning between plants and microbes, and not necessarily by phytotoxins added with the extracts as suggested by some experiments. Received: 15 February 1997 / Accepted: 12 July 1997     

Evolutionary Divergences in Root Exudate Composition among Ecologically-Contrasting Helianthus Species

Plant roots exude numerous metabolites into the soil that influence nutrient availability. Although root exudate composition is hypothesized to be under selection in low fertility soils, few studies have tested this hypothesis in a phylogenetic framework. In this study, we examined root exudates of three pairs of Helianthus species chosen as phylogenetically-independent contrasts with respect to native soil nutrient availability. Under controlled environmental conditions, seedlings were grown to the three-leaf-pair stage, then transferred to either high or low nutrient treatments. After five days of nutrient treatments, we used gas chromatography-mass spectrometry for analysis of root exudates, and detected 37 metabolites across species. When compared in the high nutrient treatment, species native to low nutrient soils exhibited overall higher exudation than their sister species native to high nutrient soils in all three species pairs, providing support for repeated evolutionary shifts in response to native soil fertility. Species native to low nutrient soils and those native to high nutrient soils responded similarly to low nutrient treatments with increased exudation of organic acids (fumaric, citric, malic acids) and glucose, potentially as a mechanism to enhance nutrition acquisition. However, species native to low nutrient soils also responded to low nutrient treatments with a larger decrease in exudation of amino acids than species native to high nutrient soils in all three species pairs. This indicates that species native to low nutrient soils have evolved a unique sensitivity to changes in nutrient availability for some, but not all, root exudates. Overall, these repeated evolutionary divergences between species native to low nutrient soils and those native to high nutrient soils provide evidence for the adaptive value of root exudation, and its plasticity, in contrasting soil environments.