Cinnamate toxicity expression on phenylalanine ammonia-lyase activity,germination and growth of cucumber (Cucumis sativis) seedlings

Low pH (5.2) decreased nodule number and acetylene reduction. Aluminium further depressed those parameters in theRhizobium leguminosarum-Pisum sativum associations examined. In the Al-treated plants nodule formation by strains 128C53 and 128C30 was not affected by 3 or 15 and 30 or 60 μM Al, respectively, as compared with the number of nodules on plants grown at pH 5.2 in the absence of Al. However, improved nodulation rates by those strains did not enhance plant dry weight or reduced nitrogen content. No differences in nitrogenase activity were found among strains of nodulating plants grown at the same aluminium level. These results suggest that Al-ions affected specifically nitrogenase activity and that this effect was primarily responsible for the reduction in plant growth.     

Effects of aluminium on growth and nutrient uptake of Betula pendula seedlings

The response to aluminium concentrations was evaluated for birch seedlings ( Betula pendula Roth, formerly Betula verrucosa Ehrh.) by using a growth technique that provides stable internal concentrations of nutrients in plants. Aluminium was added as aluminium nitrate and aluminium chloride and pH was kept at 3.8±0.2 by adding HCl or NaOH. The seedlings were grown in two different series of nutrient treatments, either with near-optimum conditions (relative addition rate 25% day⁻¹) or with constant nutrient stress (relative addition rate 10% day⁻¹) before the aluminium addition. Growth reduction occurred at aluminium concentrations greater than 3 m M , and lethal effects at aluminium concentrations greater than 15 m M . In plants subjected to near-optimum conditions before aluminium addition, the internal nutrient concentrations decreased with increasing aluminium concentration for all macronutrients. The concentration of the macronutrients N, K and P decreased gradually with increasing aluminium concentration, while the concentration of Ca and Mg decreased fairly abruptly when aluminium concentrations exceeded 1 m M . The same tendency was observed in nutrient stressed birch seedlings, but the pattern was more scattered. Relative growth rate of the seedlings was not affected by a low Ca/Al ratio. In all treatments, the molar Ca/Al ratio in/on the roots was below 0.2 at the end of the experiments. As decrease in growth occurs only at high aluminium concentrations, there is no reason to suggest that aluminium in acid soils is growth limiting for natural birch stands.     

Effect of ammonium- and nitrate-nitrogen nutrition on aluminium tolerance of soybean (Glycine max L.)

Ammonium-nitrogen supply increased Al tolerance (parameter root elongation rate) of soybean (Glycine max L.) plants compared to nitrate-nitrogen supply when grown at constant pH in solution culture. This protective effect of ammonium against Al could only partially be attributed to lowered activity of monomeric aluminium species in the ammonium solution. For ammonium and nitrate-grown plants the relationship between Al concentration in the root tips and total length could be described by the same regression equation. The higher Al tolerance of soybean plants grown in the presence of ammonium was due to restricted ad/absorption of Al which resulted from competition with positively charged Al species for binding sites in the apoplast. Induction of higher symplastic Al tolerance is unlikely because preculture with ammonium decreased rather than increased aluminium tolerance of the plants.     

Influences of high acidity and aluminum on the growth of lowland rice

Summary Lowland rice was grown on nutrient solutions in a continuous-flow system, with pH levels varying from 3.5 to 6.0. The chemical compositions of all nutrient solutions were alike, except for small amounts of HCl or KOH, needed for pH adjustments, and for variations in the Al supply.Rice was found not to be affected in its vegetative growth by H-ion concentrations up to that found at pH 3.5. High acidity suppressed the uptake of metallic cations without affecting vegetative growth.In low concentrations (0.05 and 0.2 ppm), Al suppressed the growth of rice seedlings. Concentrations ten times higher (0.5 and 2 ppm) appeared to be lethal to young rice plants. In contrast, 2 ppm Al when added to the nutrient solution past the seedling stage, exerted no adverse effect on the growth of rice. Two ppm Al, in chelated form, had no deleterious effect on rice in the seedling stage.The decreasing sensitivity of lowland rice to Al with advancing age of the plants might be of practical importance in regions where rice is grown on highly acid soils. By allowing the seedlings to develop on a seedbed consisting of soil close to neutrality, the plants may be transplanted into the acid soil once they have lost most of their initial sensitivity to high levels of soluble Al.     

Mechanism for the detoxification of aluminum in roots of tea plant (Camellia sinensis (L.) Kuntze)

To determine the mechanism of aluminum (Al) detoxification in the roots of tea plants (Camellia sinensis (L.) Kuntze), the amounts of Al and Al-chelating compounds (fluoride (F), organic acids and catechins) were measured and the chemical forms of Al in root cell extracts were identified by the application of 27Al-nuclear magnetic resonance (NMR) spectroscopy. Tea plants were cultivated in nutrient solutions containing 0, 4, 1.0 and 4.0 mM of Al at pH 4.2 for approximately 10 weeks. The levels of soluble Al, water-soluble oxalate and citrate, but not F, malate or catechins in young roots increased with an increase in the concentration of Al in the treatment solution. The 27Al NMR spectra of root tips and cell sap extracted from root tips that had been treated with Al were almost identical and had four signals, with two (11 and 16 ppm) apparently corresponding to the known chemical shifts of Al-oxalate complexes. In the spectra of cell sap, the resonances at 11 and 16 ppm increased with an increase in the Al contents. These results suggest that the levels of Al-oxalate complexes increased in response to an increase in the Al level, implying that oxalate is a key Al-chelating compound in the mechanism of Al detoxification in the tea root.     

Soil moisture and potassium affect the performance of symbiotic nitrogen fixation in faba bean and common bean

Potassium (K) is reported to improve plant's resistance against environmental stress. A frequently experienced stress for plants in the tropics is water shortage. It is not known if sufficient K supply would help plants to partially overcome the effects of water stress, especially that of symbiotic nitrogen fixation which is often rather low in the tropics when compared to that of temperate regions. Thus, the impact of three levels of fertilizer potassium (0.1, 0.8 and 3.0 mM K) on symbiotic nitrogen fixation was evaluated with two legumes under high (field capacity to 25% depletion) and low (less than 50% of field capacity) water regimes. Plants were grown in single pots in silica sand under controlled conditions with 1.5 mM N (¹⁵N enriched NH₄NO₃). The species were faba bean (Vicia faba L.), a temperate, amide producing legume and common bean (Phaseolus vulgaris L.), a tropical, ureide producing species. In both species, 0.1 mM K was insufficient for nodulation at both moisture regimes, although plant growth was observed. The supply of 0.8 or 3.0 mM K allowed nodulation and subsequent nitrogen fixation which appeared to be adequate for respective plant growth. High potassium supply had a positive effect on nitrogen fixation, on shoot and root growth and on water potential in both water regimes. Where nodulation occurred, variations caused by either K or water supply had no consequences on the percentage of nitrogen derived from the symbiosis. The present data indicate that K can apparently alleviate water shortage to a certain extent. Moreover it is shown that the symbiotic system in both faba bean and common bean is less tolerant to limiting K supply than plants themselves. However, as long as nodulation occurs, N assimilation from the symbiotic source is not selectively affected by K as opposed to N assimilation from fertilizer.     

The Effects of Aluminium on the Growth of White Clover dependent upon Fixation of Atmospheric Nitrogen

Jarvis, S. C. and Hatch, D. J. 1985. The effects of aluminiumon the growth of white clover dependent upon fixation of atmosphericnitrogen.–J. exp. Bot. 36: 1075–1086. The effects of aluminium (Al^{3 +}) on the growth of white cloverdependent upon symbiotic fixation of atmospheric nitrogen wereexamined at concentrations that may be encountered in solutionsfrom soils of low pH. Well established plants were grown inflowing solution culture with carefully maintained concentrationsof Al and of P and with pH held constant at a value (4.5) atwhich insoluble precipitates are unlikely to form. After 3 weeksof treatment, there were major visual differences between treatmentsin both shoots and roots. Although added Al at 25, 50 and 100mmol m^–3 reduced dry weight, the differences between theplants were not significant. There were, however, some considerabledifferences in P and Ca contents between the treatments. Muchof the Al held by the roots was displaced when plants were transferredto solutions containing either scandium or gallium. As wellas the effects on the plant, Al had a considerable influenceon the fixation of atmospheric nitrogen. At concentrations of50 and 100 mmol m^–3 Al³⁺ nodule initiation was reducedand there was a much reduced nodule nitrogenase activity perunit of root. Even at 25 mmol m^–3 Al³⁺, when nodule numberswere not reduced, nitrogenase activity was adversely affected. Key words: —Aluminium, Trifolium repens, nitrogen fixation     

Effect of Acidity and Aluminium on the Growth of Pinus massoniana Seedlings

Effect of pH and A1 on the growth of masson pine (Pinus massoniana )seedlings was studied with liquid medium culture method. The masson pine seedlings exhibited strong tolerance toward acid conditions, growing normally at the pH range of 3.5—5.5. Growth was promoted in ultra-low concentration of Al. The masson pine seedlings were moderately sensitive to the toxicity of aluminium. The threshold of toxic concentration for aluminium to masson pine was about 4.0 ppm. Calcium ions could ameliorate the inhibition effect of A1 to some extent. The threshold ratio of A1 to Ca was about 0.2 at the level of A1 inhibition to growth of masson pine.     

Solution pH modifies the response of Norway spruce seedlings to aluminium

Norway spruce (Picea abies) was exposed to nutrient solutions containing a range of aluminium (Al) concentrations at several pH levels (3.2, 4 and 5). Root growth was reduced by 100 µM and 400 µM Al at pH 4 and 5, but at pH 3.2 only by 400 µM Al. The Al content of the roots increased with increasing pH. The Al content of the roots was higher at the root tips than at the older root parts at all pH values. Using X-ray microanalysis it could be shown that higher levels of Al at increased pH were mainly due to increased Al contents in root cortex cell walls. In seedlings, mycorrhizal with Pisolithus tinctorius or Lactarius rufus, the Al concentration of cortex cell walls was higher when nitrate (NO₃) rather than ammonium (NH₄) was the nitrogen (N) source.     

Symbiotic dinitrogen fixation and host-plant growth during development of and recovery from phosphorus deficiency

Characterization of nodule growth and function, phosphorus and nitrogen status of plant tissues and host-plant growth of nodulated soybean ( Glycine max L. Merr.) plants developing and recovering from phosphorus deficiency was used to evaluate the role of phosphorus in symbiotic dinitrogen fixation. The sequence of physiological responses during recovery from phosphorus deficiency was; (1) rapid uptake of phosphorus, (2) rapid increases in the phosphorus concentration of leaves and nodules, (3) enhanced growth and function of nodules, (4) increased nitrogen concentrations in all plant organs and (5) enhanced plant growth. The sequence of physiological responses to onset of phosphorus deficiency was; (1) decreased phosphorus uptake, (2) decreased phosphorus concentrations in leaves and nodules, (3) decreased nodule function, (4) decreased nitrogen concentration in plant organs and (5) decreased plant growth. These results, in conjunction with previously published data (Sa and Israel, Plant Physiol. 97: 928–935, 1991), support an interpretation that the total response of symbiotic dinitrogen fixation in soybean plants to altered phosphorus supply is a function of both indirect effects on host-plant growth and more direct effects on the metabolic function of nodules.     

Effects of aluminium on growth and kinetics of K+(86Rb) uptake in two cultivars of wheat (Triticum aestivum) with different sensitivity to aluminium

Two cultivars of wheat (Triticum aestivum L. cvs Kadett and WW 20299) were grown for 9 days with 20% relative increase in nutrient supply per day at pH 4.1. Aluminium at 50 μ M retarded the growth of roots more than that of shoots in both cultivars, thus decreasing the root/shoot ratio. The inhibition was largest in WW 20299. With long term Al treatment (9 days), K_m for K⁺(⁸⁶Rb) influx increased five times in both cultivars and V_max decreased in WW 20299. Efflux of K⁺(⁸⁶Rb) was little affected. When the roots were treated with aluminium for two days, only relative growth rate of roots was retarded, while growth of shoots was unaffected and influx of K⁺(⁸⁶Rb) adjusted to the actual K⁺ demand of the plants. It is concluded that the effects of aluminium on K⁺ uptake in these wheat cultivars are not primary factors contributing to aluminium sensitivity. However, in soil with Al the demand for a comparatively high concentration of K⁺ to maintain an adequate K⁺ uptake rate, in combination with a slow growth rate of the roots, may secondarily lead to K⁺ deficiency in the plants.     

The effect of aluminium in Atlantic salmon (Salmo salar) with special emphasis on alkaline water

Atlantic salmon (Salmo salar) parr were exposed to aluminium under both steady state and non-steady state chemical conditions in alkaline water. Under alkaline (pH 9.5) steady state conditions, approximately 350 microg Al l(-1) (predominantly aluminate, Al(OH)(4)(-)) had no acute toxic effect on the salmon. The fish, however, showed a physiological response after 3 weeks of exposure ( approximately 300% increase in blood glucose concentration, about 30% increase in blood haematocrit, and about 15% decrease in plasma Cl(-) concentration). No increase in toxicity was evident under non-steady state conditions, i.e. lowering Al solubility as pH was lowered from 9.5 to 7.5. The results indicate that the toxicity of the aluminate ion (Al(OH)(4)(-)) is low, and particularly lower than the corresponding toxicity of cationic Al hydroxides. The effects observed in fish exposed to Al-rich water at pH 9.5 were counteracted as Al solubility was decreased by lowering pH to 7.5. This is contrary to previous observations where Al solubility has been lowered by increasing pH from 5.0 to 6.5.     

The Effect of Nitrogen Source on Transport and Metabolism of Nitrogen in Fruiting Plants of Cowpea (Vigna unguiculata (L.) Walp.)

Nitrogen metabolism and transport were studied during reproductivedevelopment of cowpea (Vigna unguiculata (L.) Walp. cv. Vita3) under three contrasting nitrogen regimes: (1) nitrate suppliedcontinuously (plants non-nodulated), (2) symbiotic N₂ fixation(no combined nitrogen), (3) nitrogenstarvation post-anthesisof previously N₂-fixing plants. The last treatment involveddaily flushing of the root systems with 100% oxygen which suppressedpost-anthesis N₂-fixation by 76–79%, thereby making fruitgrowth almost entirely reliant upon mobilization of previouslyaccumulated nitrogen. The bulk of the xylem nitrogen (root bleedingsap or peduncle tracheal sap) of nitrate-fed plants was nitrateand amide, that of symbiotic and O₂-treated plants largely ureide.The composition of fruit cryopuncture phloem sap, however, wasclosely similar in all treatments, with most nitrogen as amidesand amino acids. The evidence suggested intense metabolic transferof root derived nitrate-N or ureide-N to amino acids by vegetativeplant parts prior to translocation to fruits. All tissues offruits showed patterns of development of enzymic activitiesconsistent with release of nitrogen from both ureides and amidesand re-assimilation of ammonia to form amino acids. Althoughthe levels of enzyme activities varied between treatments thedifferences could not be readily associated with individualpatterns of nitrogen transport in the treatments. Nitrogen sufficiencyin the NO₃-fed plants was marked by elevated vegetative biomassand low harvest indices for dry matter and nitrogen, while nitrogendeficiency of the O₂-treated plants was associated with seedabortion, small seed size and low seed nitrogen concentration,and efficient mobilization of nitrogen from vegetative partsto fruits. Key words: Nitrogen, Translocation, Cowpea     

Drought-induced changes in xylem pH, ionic composition, and ABA concentration act as early signals in field-grown maize (Zea mays L.).

Early signals potentially regulating leaf growth and stomatal aperture in field-grown maize (Zea mays L.) subjected to drought were investigated. Plants grown in a field lysimeter on two soil types were subjected to progressive drought during vegetative growth. Leaf ABA content, water status, extension rate, conductance, photosynthesis, nitrogen content, and xylem sap composition were measured daily. Maize responded similarly to progressive drought on both soil types. Effects on loam were less pronounced than on sand. Relative to fully-watered controls, xylem pH increased by about 0.2 units one day after withholding irrigation (DAWI) and conductivity decreased by about 0.25 mS cm(-1) 1-3 DAWI. Xylem nitrate, ammonium, and phosphate concentrations decreased by about 50% at 1-5 DAWI and potassium concentration decreased by about 50% at 7-8 DAWI. Xylem ABA concentration consistently increased by 45-70 pmol ml(-1) at 7 DAWI. Leaf extension rate decreased 5 DAWI, after the changes in xylem chemical composition had occurred. Leaf nitrogen significantly decreased 8-16 DAWI in droughted plants. Midday leaf water potential and photosynthesis were significantly decreased in droughted plants late in the drying period. Xylem nitrate concentration was the only ionic xylem sap component significantly correlated to increasing soil moisture deficit and decreasing leaf nitrogen concentration. Predawn leaf ABA content in droughted plants increased by 100-200 ng g(-1) dry weight at 7 DAWI coinciding with a decrease in stomatal conductance before any significant decrease in midday leaf water potential was observed. Based on the observed sequence, a chain of signal events is suggested eventually leading to stomatal closure and leaf surface reduction through interactive effects of reduced nitrogen supply and plant growth regulators under drought.     

Changes in specific area and energy of root surface of cereal plants in Al-solution cultures. Water vapor adsorption studies

Surface areas and energetic properties of the shooting stage roots of rye (Secale L.), triticale (Triticale), barley (Hordeum L.) and four wheat (Triticum L.) varieties were estimated from experimental water vapor adsorption data. Roots stressed during 10 days at pH 4 with aluminium concentrations ranging from 0 to 40 mg dm^–3 were studied. Roots grown continuously at pH 7 were taken as controls. The surface properties of the roots grown at pH 4 without Al addition were apparently the same as those of the control roots. With the increase of the concentration of the aluminium treatment the surface area of the roots increased for all of the plants, beginning at 5 mg Al dm^–3 for barley, at 10 mg Al dm^–3for wheat and triticale, and at 40 mg Al dm^–3 for rye. The average water vapor adsorption energy of the root surface decreased in general with the increase of Al stress concentration for all plants but triticale, for which this increased. The sensitive cereal varieties seem to have greater amount of high energy adsorption centers (more polar surface) than the resistant ones (lower surface polarity), however more data is needed to justify this hypothesis. For Al-sensitive roots, fraction of high energy adsorption sites decreased and fraction of low energy sites increased under the Al stress. Smaller changes in adsorption energy sites were noted for roots of Al-resistant plants.     

Influence of aluminium on phosphorus and calcium localization in roots of beech (Fagus sylvatica)

Beech plants ( Fagus sylvatica L. provenance Maramures) were grown in nutrient solution at low pH (4.2) and exposed to different concentrations of AlCl₃. Uptake and leakage of Ca²⁺(⁴⁵Ca²⁺) and H²PO₄-(³²P) were studied. A high external aluminium concentration (1.0m M ) reduced the uptake and export to the shoot of both calcium and phosphate, while 0.1 m M Al increased the phosphorus level in the roots. To determine the impact of aluminium on the localization of calcium and phosphate, leakage of the elements from both intact plants and plants frozen prior to the leakage experiment was studied. The leakage of Ca²⁺ from intact plants was not affected by prior exposure to 0.1 m M Al. Freezing of the beech plants before the leakage experiment increased leakage of calcium slightly more from roots of control plants than for roots exposed to 0.1 m M Al, indicating that even low concentrations of alminium may impede the influx of calcium across the plasma membrane in the roots. The patterns of Ca²⁺ leakage from roots previously exposed to 1.0 m M Al indicated that very little Ca²⁺ was located extracellularly. The extracellular fraction of phosphate increased with increasing Al concentration in the nutrient solution. Low Al concentration (0.1 m M ) only reduced the intracellular phosphate concentration to a minor extent, while 1.0 m M Al profoundly decreased it. It is concluded that 0.1 m M AlCl₃ has a limited effect upon the localization of Ca²⁺ and phosphate in the roots. At higher levels of Al, 0.1–1.0 m M , there is a more dramatic change in nutrient localization in the free space and uptake over the plasma membrane.     

Aluminium accumulation in root cell walls coincides with inhibition of root growth but not with inhibition of magnesium uptake in Norway spruce

High levels of aluminium in the soil solution of forest soils cause stress to forest trees. Within the soil profile, pH and aluminium concentration in the soil solution vary considerably with soil depth. pH strongly influences the speciation of A1 in solution, and is a factor when considering toxicity of A1 to roots. Norway spruce ( Picea abies [L.] Karst.) seedlings were grown for 7 weeks in nutrient solutions at pH 3.2, 4.0 or 5.0 containing 0, 100 or 400 µ M A1. At the end of this period, seedling growth, the cation exchange capacity of the roots and the amount of exchangeable Ca and Mg in roots were determined. A1 concentrations in whole roots, root segments, and in needles were measured. Using X‐ray microanalysis, the concentrations of Al, Ca, Mg and P were determined in cortical cell walls. We wanted to test the hypotheses that (1) the amount of Al bound to cation exchange sites can be used as a marker for Al toxicity and (2) the Mg concentration of needles is controlled by the amount of Mg bound to cation exchange sites. Low pH reduced the inhibition of Al on root growth and shoot length. Both low pH and Al lowered the concentration of Ca and Mg in needles. Al concentrations in the roots decreased as the pH decreased. In the roots, Al displaced Mg and Ca from binding sites at the root cortical cell walls. A comparison of the effects of Al at the different pH values on root growth and Mg concentration in the needles, suggests that, at pH 5.0, an Al fraction in the apoplast inhibits root growth, but does not affect Mg uptake. This fraction of Al is not available for transport to the shoots. In contrast, Mg uptake is strongly affected by Al at pH 3.2, although only very low levels of Al were detected in the roots. Thus, Al accumulation in the apoplast is a positive marker for Al effects on root growth, but not Mg uptake. The Mg concentration of needles is not controlled by the amount of Mg bound to cation exchange sites.     

Antisense inhibition of NADH glutamate synthase impairs carbon/nitrogen assimilation in nodules of alfalfa (Medicago sativa L.)

Legumes acquire significant amounts of nitrogen for growth from symbiotic nitrogen fixation. The glutamine synthetase (GS)/NADH-dependent glutamate synthase (NADH-GOGAT) cycle catalyzes initial nitrogen assimilation. This report describes the impact of specifically reducing nodule NADH-GOGAT activity on symbiotic performance of alfalfa (Medicago sativa L.). Four independent transgenic alfalfa lines, designated GA89, GA87, GA88, and GA82 (for GOGATantisense), containing an antisense NADH-GOGAT cDNA fragment under the control of the soybean leghemoglobin (lbc3) promoter were evaluated. The GA plants were fertile and showed normal growth in non-symbiotic conditions. The NADH-GOGAT antisense transgene was heritable and the T1 plants showed phenotypic alterations - similar to primary transformants. Clonally propagated plants were inoculated with Sinorhizobium meliloti after rooting and the symbiotic phenotype was analyzed 21 days post-inoculation. Nodules of each GA line had reduced NADH-GOGAT activity, ranging from 33 to 87% of control plants, that was accompanied by comparable decreases in RNA and protein. Plants from the GA89 line, with the lowest NADH-GOGAT activity (c. 30%), presented a strikingly altered symbiotic phenotype: concomitantly activities of key enzyme for carbon and nitrogen assimilation decreased; nodule amino acids and amides were reduced while sucrose accumulated. Antisense GOGAT plants were chlorotic, reduced in fresh weight, and had a lower N content than control plants. Photosynthesis was also impaired in antisense plants. Specifically, reducing NADH-GOGAT in nodules resulted in plants having impaired nitrogen assimilation and altered carbon/nitrogen metabolic flux.     

Metabolite analysis of the effects of elevated CO2 and nitrogen fertilization on the association between tall fescue (Schedonorus arundinaceus) and its fungal symbiont Neotyphodium coenophialum

Atmospheric CO₂ is expected to increase to between 550 ppm and 1000 ppm in the next century. CO₂‐induced changes in plant physiology can have ecosystem‐wide implications and may alter plant‐plant, plant‐herbivore and plant‐symbiont interactions. We examined the effects of three concentrations of CO₂ (390, 800 and 1000 ppm) and two concentrations of nitrogen fertilizer (0.004 g N/week versus 0.2 g N/week) on the physiological response of Neotyphodium fungal endophyte‐infected and uninfected tall fescue plants. We used quantitative PCR to estimate the concentration of endophyte under altered CO₂ and N conditions. We found that elevated CO₂ increased the concentration of water‐soluble carbohydrates and decreased the concentration of plant total amino acids in plants. Fungal‐derived alkaloids decreased in response to elevated CO₂ and increased in response to nitrogen fertilization. Endophyte concentration (expressed as the number of copies of an endophyte‐specific gene per total genomic DNA) increased under elevated CO₂ and nitrogen fertilization. The correlation between endophyte concentration and alkaloid production observed at ambient conditions was not observed under elevated CO₂. These results suggest that nutrient exchange dynamics important for maintaining the symbiotic relationship between fungal endophytes and their grass hosts may be altered by changes in environmental variables such as CO₂ and nitrogen fertilization.     

Estimating the chemical compositions of soil solutions by obtaining saturation extracts or specific 1:2 by volume extracts

The aluminium tolerance of several tree species was studied in a cloud forest in Northern Venezuela, growing on a very acid soil and rich in soluble Al. The Al-accumulator species (>1000 ppm in leaves) were compared to non-accumulator ones in relation to total Al concentration in xylem sap, pH and Al concentration in vacuoles, and rhizosphere alkalinization capacity. The Al³⁺ concentration in the soil solution and the xylem sap were also measured. The results show that in the Al-accumulator plant Richeria grandis, xylem sap is relatively rich in Al and about 35% of it is present in ionic form. In the non-accumulator plant studied (Guapira olfersiana) there is no Al detectable in xylem sap. The pH of vacuolar sap of several Al-accumulator species studied was very acidic and ranged between 2.6–4.8, but the presence of Al in vacuoles was not correlated with the acidity of the vacuolar sap. Both Al-accumulator and non accumulator plants had the capacity to reduce acidity of the rhizosphere and increased the pH of the nutrient solution by one unit within the first 24 hours. Trees growing in natural, high acidity-high Al³⁺ environment show a series of tolerance mechanisms, such as deposition of Al in vacuoles, Al chelation and rhizosphere alkalinization. These partially ameliorate the toxic effects of this element, but they probably impose a high ecological cost in terms of photosynthate allocation and growth rate.