Effects of Al on nitrogen (NH 4 + and NO 3 − ) uptake,nitrate reductase activity and proton release in two sorghum cultivars differing in Al tolerance

After growth for 17 to 36 days on nutrient solutions with NH₄NO₃ as nitrogen source (pH 4.2) dry matter of sorghum genotype SC0283 was much less affected by Al (1.5 and 3.0 ppm) than that of genotype NB9040. In the absence of Al both cultivars released protons into the nutrient solution as a result of an excess of cationic nutrients taken up. When Al was present, this proton efflux per unit dry weight increased drastically, especially with the sensitive genotype NB9040. Chemical analysis of plant material and continuous analyses of NO ₃ ⁻ and NH ₄ ⁺ in the nutrient solution indicated, that the Al-induced shift in H⁺-balance of both genotypes could almost completely be attributed to a decreased NO ₃ ⁻ /NH ₄ ⁺ uptake ratio. In vivo nitrate reductase activity (NRA) was reduced in the shoot of NB9040 and to a lesser degree in SC0283. Al-induced decrease in NRA was accompanied by similar percentual decreases in NO ₃ ⁻ tissue concentrations. Therefore this decrease is interpreted as being indirect,i.e., the consequence of the reduced NO ₃ ⁻ uptake of the plants. A direct repression of NRA by Al seems also unlikely because nitrate reductase activity of the roots (where cellular Al-concentrations should be higher than in shoots) was not affected in Al-treated plants of either genotype.     

Nitrate reductase activity and nitrogen compounds in xylem exudate of Juglans nigra seedlings: relation to nitrogen source and supply

Nitrogen (N) limits plant productivity and its uptake and assimilation may be regulated by N source, N availability, and nitrate reductase activity (NRA). Knowledge of how these factors interact to affect N uptake and assimilation processes in woody angiosperms is limited. We fertilized 1-year-old, half-sib black walnut (Juglans nigra L.) seedlings with ammonium (NH₄ ⁺) [as (NH₄)₂SO₄], nitrate (NO₃ ⁻) (as NaNO₃), or a mixed N source (NH₄NO₃) at 0, 800, or 1,600 mg N plant⁻¹ season⁻¹. Two months following final fertilization, growth, in vivo NRA, plant N status, and xylem exudate N composition were assessed. Specific leaf NRA was higher in NO₃ ⁻-fed and NH₄NO₃-fed plants compared to observed responses in NH₄ ⁺-fed seedlings. Regardless of N source, N addition increased the proportion of amino acids (AA) in xylem exudate, inferring greater NRA in roots, which suggests higher energy cost to plants. Root total NRA was 37% higher in NO₃ ⁻-fed than in NH₄ ⁺-fed plants. Exogenous NO₃ ⁻ was assimilated in roots or stored, so no difference was observed in NO₃ ⁻ levels transported in xylem. Black walnut seedling growth and physiology were generally favored by the mixed N source over NO₃ ⁻ or NH₄ ⁺ alone, suggesting NH₄NO₃ is required to maximize productivity in black walnut. Our findings indicate that black walnut seedling responses to N source and level contrast markedly with results noted for woody gymnosperms or herbaceous angiosperms.     

Aluminum accumulation and associated effects on 15NO3 − influx in roots of two soybean genotypes differing in Al tolerance

A study was conducted to examine aluminum (Al) exclusion by roots of two differentially tolerant soybean (Glycine max L. Merr.) lines, Pl-416937 (Al-tolerant) and Essex (Al-sensitive). Following exposure to 80μM Al for up to 2 h, roots were rinsed with a 10 mM potassium citrate solution and rapidly dissected to allow estimation of intracellular Al accumulation in morphologically distinct root regions. Using 10 min exposures to 300μM ¹⁵NO₃ ⁻ and dissection, accompanying effects on NO₃ ⁻ uptake were measured. With Al exposures of 20 min or 2 h, there was greater Al accumulation in all root regions of Essex than in those of Pl-416937. The genotypic difference in Al accumulation was particularly apparent at the root apex, both in the tip and in the adjacent root cap and mucilage. Exposure of roots to Al inhibited the uptake of ¹⁵NO₃ ⁻ to a similar extent in all root regions. The results are consistent with Al exclusion from cells in the root apical region being an important mechanism of Al tolerance.     

The effects of low concentrations of aluminium on the growth and uptake of nitrate-N by white clover

Summary The effects of aluminium (Al³⁺) at concentrations of 0, 25, 50 and 100 μM on the growth of white clover, dependent upon N supplied as NO ₃ ⁻ , were examined in flowing solution culture. Plants were established with a normal nutrient supply for 7 weeks and then grown with carefully controlled pH (at 4.5) and P concentrations, and with 0, 25, 50 or 100 μM Al³⁺ for a further three weeks. There were rapid visual effects (i.e. symptoms of P deficiency and reduction in root extension) and the dry weights of shoots and roots were reduced at 50 and 100 μM. Less than 10% of Al absorbed from solution was transported to the shoots. The uptake of P, and its transport between roots and shoots, were reduced in plants grown with Al. The uptake of NO ₃ ⁻ stopped immediately after the introduction of 50 or 100 μM Al, and was significantly reduced at 25 μM after three weeks. During a second phase of the experiment, plants previously grown at 0, 25, 50 and 100 μM Al, were grown for a further 2 weeks either with NO ₃ ⁻ (with and without 50 μM Al³⁺) or without NO ₃ ⁻ but with inoculation by Rhizobia (and with or without 50 μM Al³⁺). The effects of the previous treatments with Al on N uptake were small during the second phase, but uptake by all plants was restricted when Al was present. Inoculation did not result in nodulation in the second phase when Al³⁺ was present in the solution, but Al already in the plant from the first phase did not prevent nodulation in the absence of Al during the second phase.     

Al avoidance and Al tolerance ofMucuna pruriens var.utilis: Effects of a heterogeneous root environment and the nitrogen form in the root environment

InMucuna pruriens var.utilis, grown with nitrate-N in a hydroponic split-root system, an Al avoidance reaction of root growth was observed, which was ascribed to local P stress in the Al containing compartment. The Al avoidance reaction was similar to the avoidance ofMucuna roots of acid subsoil in the field where roots grew preferentially in the topsoil. In the present paper the effect of different N forms (NO₃ ^– and NH₄ ⁺) on the reactions ofMucuna to Al were studied, since in acid soils N is present as a mixture of NO₃ ^– and NH₄ ⁺. No interaction between the N form and Al toxicity was found. A hydroponic split-root experiment with NH₄NO₃ nutrition, which is comparable to the situation in the field, showed that under these conditions Al avoidance did not occur. It is concluded that a relation between the Al avoidance reaction ofMucuna and P stress is still likely.Abbreviations D_r root diameter - L_pr total root length per plant - L_rw specific root length - NRA nitrate reductase activity - S/R shoot: root ratio     

Nitrate reductase activity of two leafy vegetables as affected by nickel and different nitrogen forms

The author studied the effect of different nickel concentrations (0, 0.4, 40 and 80 μM Ni) on the nitrate reductase (NR) activity of New Zealand spinach (Tetragonia expansa Murr.) and lettuce (Lactuca sativa L. cv. Justyna) plants supplied with different nitrogen forms (NO₃ ⁻–N, NH₄ ⁺–N, NH₄NO₃). A low concentration of Ni (0.4 μM) did not cause statistically significant changes of the nitrate reductase activity in lettuce plants supplied with nitrate nitrogen (NO₃ ⁻–N) or mixed (NH₄NO₃) nitrogen form, but in New Zealand spinach leaves the enzyme activity decreased and increased, respectively. The introduction of 0.4 μM Ni in the medium containing ammonium ions as a sole source of nitrogen resulted in significantly increased NR activity in lettuce roots, and did not cause statistically significant changes of the enzyme activity in New Zealand spinach plants. At a high nickel level (Ni 40 or 80 μM), a significant decrease in the NR activity was observed in New Zealand spinach plants treated with nitrate or mixed nitrogen form, but it was much more marked in leaves than in roots. An exception was lack of significant changes of the enzyme activity in spinach leaves when plants were treated with 40 μM Ni and supplied with mixed nitrogen form, which resulted in the stronger reduction of the enzyme activity in roots than in leaves. The statistically significant drop in the NR activity was recorded in the aboveground parts of nickel-stressed lettuce plants supplied with NO₃ ⁻–N or NH₄NO₃. At the same time, there were no statistically significant changes recorded in lettuce roots, except for the drop of the enzyme activity in the roots of NO₃ ⁻-fed plants grown in the nutrient solution containing 80 μM Ni. An addition of high nickel doses to the nutrient solution contained ammonium nitrogen (NH₄ ⁺–N) did not affect the NR activity in New Zealand spinach plants and caused a high increase of this enzyme in lettuce organs, especially in roots. It should be stressed that, independently of nickel dose in New Zealand spinach plants supplied with ammonium form, NR activity in roots was dramatically higher than that in leaves. Moreover, in New Zealand spinach plants treated with NH₄ ⁺–N the enzyme activity in roots was even higher than in those supplied with NO₃ ⁻–N.     

Effects of soil applied herbicides on leaf nitrate reductase and crude protein in the leaf and seed of two cowpea cultivars

A glass-house study was conducted to determine the effects of four commonly used herbicides (pendimethalin, metobromuron, metolachlor and prometryne) applied pre-emergence at rates of 0, 0.125, 0.625 and 1.25 kg ha^–1, on leaf nitrate concentration (NO₃–C), nitrate reductase activity (NRA), leaf crude protein and seed protein in two cowpea cultivars, 60 day (60D) and Ife brown (IB).Control and treated plants of both cultivars showed separate peaks for NO₃–C and NRA, 49 days after planting (DAP) and 35 DAP for 60D and IB respectively. Herbicide treatment generally enhanced NO₃–C but tended to decrease NRA in both cultivars. Howver, metobromuron at 0.625 kg ha^–1 increased NRA throughout the growth period with an optimum increase of 52.5%, over the control, at 35 DAP. Pendimethalin increased NO₃–C NRA and leaf protein but did not influence seed protein appreciably. In contrast metobromuron increased NO₃–C, decreased NRA, but increased seed protein by 29.6% over the control at 0.125 kg ha^–1 in 60D. Metolachlor and prometryne were most inhibitory to seed protein development. In addition, metolachlor reversed the interdependence of NO₃–C and NRA.     

Antioxidative system in maize roots as affected by osmotic stress and different nitrogen sources

The activities of antioxidative enzymes and contents of proline and total phenolics were assayed in roots of two maize (Zea mays L.) genotypes grown in a medium containing nitrate (NO₃ ⁻) or both nitrogen forms, nitrate and ammonium (NH₄ ⁺/NO₃ ⁻). An increase in the activities of class III peroxidases (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), ascorbate oxidase (AO) and proline content, and decrease in phenolic content were observed in NH₄ ⁺/NO₃ ⁻ in comparison with NO₃ ⁻ grown plants. When polyethylene glycol (PEG) was added to both nitrogen treatments, the content of total phenolics and proline was increased, especially in NH₄ ⁺/NO₃ ⁻ treatment. The PEG treatment decreased enzyme activities in NH₄ ⁺/NO₃ ⁻ grown plants, but in NO₃ ⁻ grown plants activities of POD and SOD were increased, opposite to decreased APX and AO. Isoelectric focusing demonstrated increased activities of acidic POD isoforms in PEG treated NO₃ ⁻ grown plants, and lower activities of both, acidic and basic isoforms in NH₄ ⁺/NO₃ ⁻grown plants.     

Alleviation of nickel toxicity by ammonium supply to sunflower plants

Phytotoxicity of nickel (Ni) varies within plant species and cultivars as well as with the concentration of Ni in the rooting medium. Moreover, it is known that several nutrients can modify the plant response to excess Ni. Nitrogen can be absorbed by plants as different N forms and because N metabolism and Ni are closely related, a hydroponic experiment was conducted to study the effect of Ni toxicity on the growth, nutrient status of the different plant parts and leaf chlorophyll concentrations in sunflower plants (Helianthus annuus L.) cv Quipu grown with different forms of N supply. The plants were grown under controlled conditions for 35 days. Depending on the N source supplied, there were significant differences in the sensitivity of sunflower plants to excess Ni. Tolerance was lowest when grown with NO₃ ⁻ alone. A high Ni and NO₃ ⁻ as the only N source resulted in reduced dry weight and significant decreases in nutrient concentration. Plants supplied with a mixture of NO₃ ⁻ and NH₄ ⁺ absorbed in the presence of Ni in solution about three times less Ni than those supplied with NO₃ ⁻ alone. Consequently, there were great differences in Ni concentrations between treatments. With a N nutrition of 100% NO₃ ⁻-N, Ni supply led to severe growth inhibition. Just contrary, simultaneous supply of NO₃ ⁻ and NH₄ ⁺ not only reduced Ni toxicity, but growth was even stimulated by Ni if supplied to plants fed with NO₃ ⁻ and NH₄ ⁺. This indicates the significant role of the N form supplied in the behaviour of Ni toxicity in sunflower plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of aluminium and mineral nutrition on growth and chemical composition of hydroponically grown seedlings of five different forest tree species

Forest die-back and impaired tree vitality have frequently been ascribed to Al-toxicity and Al-induced nutritional disorders due to increased acidification of forest soils. Therefore, in this experiment effects of Al were studied on growth and nutrient uptake with seedlings of five different forest tree species. During growth in culture solutions with and without Al all five species proved to be very Al-tolerant, despite high accumulation of Al in roots. In the coniferous evergreens Douglas-fir and Scots pine shoot as well as root Al concentrations were significantly higher than in the deciduous broad-leaved species oak and birch. Larch showed intermediate Al levels. In none of the five species did Al reduce nutrient concentrations or the Ca/Al ratio to values below the critical level. Besides differences in Al accumulation, coniferous and broad-leaved species also differed with respect to uptake and assimilation of nitrogen. Due to extra NH ₄ ⁺ uptake, oak and birch showed a much higher N uptake and higher NH ₄ ⁺ preference than the coniferous species. Especially with oak this high NH ₄ ⁺ preference in combination with a low specific root surface area resulted in a high root proton efflux density. In comparison to both broad-leaved trees and Scots pine the NO ₃ ⁻ reduction capacity of larch and Douglas-fir was extremely low. This may have important consequences for both species if grown in NO ₃ ⁻ -rich soils.     

Ammonium nutrition increases photosynthesis rate under water stress at early development stage of rice (Oryza sativa L.)

An hydroponic experiment with a simulated water stress induced by PEG (6000) was conducted in a greenhouse to study the effects of nitrate (NO₃ ⁻), ammonium (NH₄ ⁺) and the mixture of NO₃ ⁻ and NH₄ ⁺, on water stress tolerance of rice seedlings. Rice (Shanyou 63) was grown under non- or simulated water stress condition (10% (w/v) PEG, MW6000) with the 3 different N forms during 4 weeks. Under non-stressed condition no difference was observed among the N treatments. Under simulated water stress, seedlings grown on N-NO₃ ⁻ were stunted. Addition of PEG did not affect rice seedling growth in the treatment of only NH₄ ⁺ supply but slightly inhibited the rice seedling growth in the treatment of mixed supply of NO₃ ⁻ and NH₄ ⁺. Simulated water stress, when only N-NH₄ ⁺ was present, did not affect leaf area and photosynthesis rate, however, both parameters decreased significantly in the NO₃ ⁻ containing solutions. Under water stress, Rubisco content in newly expanded leaves significantly increased in the sole NH₄ ⁺ supplied plants as compared to that in plants of the other two N treatments. Under water stress, the ratio of carboxylation efficiency to Rubisco content was, respectively, decreased by 13 and 23% in NH₄ ⁺ and NO₃ ⁻ treatments, respectively. It is concluded that, water stress influenced the Rubisco activity than stomatal limitation, and this effects could be regulated by N forms. Responsible Editor: Herbert Johannes Kronzucker. Shiwei Guo and Gui Chen contributed equally to this paper.     

Nitrate reductase of primary roots of red spruce seedlings : effects of acidity and metal ions

Nitrate reductase activity (NRA) was found in primary roots, but not in foliage of red spruce (Picea rubens Sarg.) seedlings. Nitrate induced NRA:NH₄⁺ did not induce and slightly depressed NRA in older seedlings. Induction required 8 hours and, once induced, NRA decreased slowly in the absence of exogenous NO₃⁻. Seedlings were grown in perlite with a complete nutrient solution containing NH₄⁺ to limit NR induction. Established seedlings were stressed with nutrient solutions at pH 3, 4, or 5 supplemented with Cl⁻ salts of Al, Cd, Pb, or Zn each at two concentrations. NRA in primary root tips was measured at 2, 14, 28, and 42 days. NRA induction was greatest at pH 3, and remained high during the period of study. NRA induction at pH 4 was lower. Metal ions suppressed NRA at pH 3 and 5, but enhanced NRA at pH 4. It is concluded that acidity and soluble metals in the root environment of red spruce are unlikely to be important factors in nitrogen transformations in red spruce roots.     

The effect of the osmotic potential and specific ion concentration of the nutrient solution on the uptake and reduction of nitrate by barley seedlings

Summary Short-term absorption experiments were conducted with intact barley (Hordeum vulgare L.) seedlings to observe the effects of the osmotic potential (Ψ^π) and salt species on nitrate uptake andin vivo nitrate reduction. The experiments consisted of growing barley seedlings for 5 days in complete nutrient solutions salinized to (Ψ^π) levels of −0.6, −1.8, −3.0, −4.2, and −5.4 bars with NaCl, CaCl₂ or Na₂SO₄. After the absorption period, the seedlings were separated into shoots and roots, weighed, then analyzed for NO₃. The nutrient solutions were sampled for NO₃ analysis each day immediately before renewing the solutions. The accumulative loss of NO₃ from the solutions was considered to be uptake whereas NO₃ reduction was the difference between uptake and seedling content. Lowering the (Ψ^π) of the nutrient solutions resulted in decreased concentrations of NO₃ in the plant, little or no effect (except at the lowest (Ψ^π) level) on uptake, and increased nitrate reductase activity. Increased rates of NO₃ reduction were in particular associated with the Cl concentration of the nutrient solution.     

Aluminum inhibition of shoot lateral branches ofGlycine max and reversal by exogenous cytokinin

Aluminum effects on the morphological development of soybean (Glycine max (L.) Merr.) were characterized in greenhouse and growth chamber experiments. An Al-sensitive cultivar, ‘Ransom’, was grown in an acid soil (Aeric Paleudult) adjusted to 3 levels of exchangeable Al. Lateral shoot development at the nodes of the main stem was extensive in the limed soil containing 0.06 cmol(+) Alkg⁻¹. However, lateral shoot length and weight were severely inhibited in the unlimed soil containing 2.19 cmol(+) Alkg⁻¹, and in the unlimed soil amended to 2.63 cmol(+) Alkg⁻¹ with AlCl₃. This inhibition by the high Al/low pH condition was reversed by the exogenous application of a synthetic cytokinin 6-benzylaminopurine (BA). The daily application of 20 μg mL⁻¹ BA applied locally to the lateral meristems of plants grown in the unlimed soil stimulated lateral shoot growth substantially, such that it was either comparable to or greater than that observed in the limed treatment without BA. Accumulation of K, Ca, and Mg in lateral shoot branches was also stimulated by the local application of BA. The inhibitory effects of Al on lateral shoot development were confirmed in solution culture. In addition, differential sensitivity to Al was evident among the primary root, first order lateral roots, and second order lateral roots. The length of the primary root was only slightly decreased by increasing concentrations of Al up to 30 μM. In contrast, the length of basipetally located first order lateral roots was restricted to greater extent; up to 50% by 30 μM Al. Second order lateral lengths were inhibited even more severely; up to 86% by 30 μM Al. Substantial evidence in the literature indicates that the root apex is a major site for the biosynthesis of cytokinin that is supplied to shoots, and cellular function and development in this region of the root are impaired during Al toxic conditions. This suggests that one mode of action by which Al may affect shoot growth is by inhibiting the synthesis and subsequent translocation of cytokinin to the meristematic regions of the shoot. The present observation of a reversal of Al-inhibited lateral shoot development by exogenously applied cytokinin supports this hypothesis. However, the inability of applied cytokinin to counter the restriction imposed by Al on total shoot dry matter production implies the impairment by Al toxicity of other root functions, such as ion and water transport, also played an important role in altering shoot morphology.     

Epidemiology of wheat take-all as influenced by soil pH and temporal changes in inorganic soil N

Summary Soil pH, NH ₄ ⁺ and NO ₃ ⁻ concentrations in soil, and take-all root rot of winter wheat grown in the field were measured concurrently from sowing to anthesis in order to relate disease development to liming and N fertilization practices. Experimental variables included soil pH (5.5 and 6.0) and three N sources (NH₄NO₃, (NH₄)₂SO₄, NH₄Cl) banded with the seed at sowing in factorial combination with the same three N sources topdressed in the spring. Take-all severity was increased by increasing soil pH and by fertilization with NO ₃ ⁻ . Disease severity on crown roots increased exponentially following spring N fertilization and was affected more by soil pH and N-form than was severity on seminal roots. Grain yield ranged from 4.70 Mgha⁻¹ with spring NH₄NO₃ at soil pH 6.0 to 7.65 Mgha⁻¹ with spring NH₄Cl at soil pH 5.5. Sixty-six percent of the variability in grain yield was explained by the number of take-all infected crown roots per tiller at anthesis. Oregon Agric. Exp. Stn. technical paper no. 7707.     

Optimization and elucidation of interactions between ammonium, nitrate and phosphate inCentella asiatica cell culture using response surface methodology

The effects of macronutrients (NO₃ ⁻, NH₄ ⁺ and PO₄ ³⁻) on cell growth and triterpenoids production inCentella asiatica cell suspension cultures were analyzed using the Box-Behnken response surface model experimental design. In screening and optimization experiments, PO₄ ³⁻ as a single factor significantly influenced cell growth where increasing the phosphate level from 0.1 to 2.4 or 2.6 mM, elevated cell growth from 3.9 to 14–16 g/L. The optimum values predicted from the response surface model are 5.05 mM NH₄ ⁺, 15.0 mM NO₃ ⁻ and 2.6 mM PO₄ ³⁻, yielding 16.0 g/L cell dry weight with 99% fitness to the experimental data. While the NH₄ ⁺-NO₃ ⁻ interaction influenced cell growth positively in the optimization experiment, NH₄ ⁺ and NO₃ ⁻ as single factors; and interactions of NO₃ ⁻-PO₄ ³⁻, NH₄ ⁺-PO₄ ³⁻ and NH₄ ⁺-NO₃ ⁻ were all negative in the screening experiment. Cell growth and the final pH level were positively affected by PO₄ ³⁻, but negatively affected by NH₄ ⁺ and NH₄ ⁺-PO₄ ³⁻ interactions. The different effects of factors and their interactions on cell growth and final pH are influenced by a broad or narrow range of macronutrient concentrations. The productions of triterpenoids however were lower than 4 mg/g cell dry weight.     

Chronic Atmospheric NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Deposition Does Not Induce NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Use by <Emphasis Type="Italic">Acer saccharum</Emphasis> Marsh.

The ability of an ecosystem to retain anthropogenic nitrogen (N) deposition is dependent upon plant and soil sinks for N, the strengths of which may be altered by chronic atmospheric N deposition. Sugar maple (Acer saccharum Marsh.), the dominant overstory tree in northern hardwood forests of the Lake States region, has a limited capacity to take up and assimilate NO₃⁻. However, it is uncertain whether long-term exposure to NO₃⁻ deposition might induce NO₃⁻ uptake by this ecologically important overstory tree. Here, we investigate whether 10 years of experimental NO₃⁻ deposition (30 kg N ha⁻¹ y⁻¹) could induce NO₃⁻ uptake and assimilation in overstory sugar maple (approximately 90 years old), which would enable this species to function as a direct sink for atmospheric NO₃⁻ deposition. Kinetic parameters for NH₄⁺ and NO₃⁻ uptake in fine roots, as well as leaf and root NO₃⁻ reductase activity, were measured under conditions of ambient and experimental NO₃⁻ deposition in four sugar maple-dominated stands spanning the geographic distribution of northern hardwood forests in the Upper Lake States. Chronic NO₃⁻ deposition did not alter the V _max or K _m for NO₃⁻ and NH₄⁺ uptake nor did it influence NO₃⁻ reductase activity in leaves and fine roots. Moreover, the mean V _max for NH₄⁺ uptake (5.15 μmol ¹⁵N g⁻¹ h⁻¹) was eight times greater than the V _max for NO₃⁻ uptake (0.63 μmol ¹⁵N g⁻¹ h⁻¹), indicating a much greater physiological capacity for NH₄⁺ uptake in this species. Additionally, NO₃⁻ reductase activity was lower than most values for woody plants previously reported in the literature, further indicating a low physiological potential for NO₃⁻ assimilation in sugar maple. Our results demonstrate that chronic NO₃⁻ deposition has not induced the physiological capacity for NO₃⁻ uptake and assimilation by sugar maple, making this dominant species an unlikely direct sink for anthropogenic NO₃⁻ deposition.     

Salt tolerance of two differently drought-tolerant wheat genotypes during germination and early seedling growth

Summary Osmotic and specific ion effect are the most frequently mentioned mechanisms by which saline substrates reduce plant growth. However, the relative importance of osmotic and specific ion effect on plant growth seems to vary depending on the drought and/or salt tolerance of the plant under study. We studied the effects of several single salts of Na⁺ and Ca²⁺−NaCl, NaNO₃, Na₂SO₄, NaHCO₃, Na₂CO₃, and Ca(NO₃)₂—on the germination and root and coleoptile growth of two wheat (Triticum aestivum L.) cultivars, TAM W-101 and Sturdy, the former being more drought tolerant than the latter. The concentrations used were: 0, 0.02, 0.04, 0.08, 0.16, and 0.32 mol L⁻¹. Significant two- and three-way interactions were observed between cultivar, kind of salt, and salt concentration for germination, growth of coleoptile and root, and root/coleoptile ratio. Salts differed significantly (P<0.001) in their effect on seed germination, coleoptile and root growth of both cultivars. Germination of TAM W-101 seeds was consistently more tolerant than that of Sturdy to NaCl, CaCl₂, Ca(NO₃)₂, and NaHCO₃ salts at concentrations of 0.02, 0.04, 0.08, 0.16 mol L⁻¹. The osmotic potential, at which the germination of wheat seeds was reduced to 50% of that of the control, was different depending on the kind of salt used in the germination medium. NaCl at low concentrations (0.02 and 0.04 mol L⁻¹) stimulated the germination of both wheat cultivars. At concentrations of 0.02 to 0.16 mol L⁻¹, Ca²⁺ salts (CaCl₂ and Ca(NO₃)₂) were consistently more inhibitory than the respective Na⁺ salts (NaCl and NaNO₃) for germination of Sturdy. This did not consistently hold true for TAM W-101. Among the Na⁺ salts, NaCl was the least toxic and NaHCO₃ and Na₂CO₃ were the most toxic for seed germination. Root and coleoptile (in both wheat cultivars) differed in their response to salts. This differential response of coleoptile and root to each salt resulted in seedlings with a wide range of root/coleoptile ratios. For example, the root/coleoptile ratio of cultivar TAM W-101 changed from 2.09 (in the control) to 3.77, 3.19, 2.8, 2.44, 1.31, 0.32, and 0.0 when subjected to 0.08 mol L⁻¹ of Na₂SO₄, NaCl, CaCl₂, NaNO₃, Ca(NO₃)₂, NaHCO₃, and Na₂CO₃, respectively. Na₂CO₃ at 0.08 mol L⁻¹ inhibited root growth to such an extent that germinated wheat seeds contained coleoptile but no roots. The data indicate that, apart from the clear and more toxic effects of NaHCO₃ and Na₂CO₃ and lesser toxic effect of NaCl on germination and seedling growth, any toxicity-ranking of other salts done at a given concentration and for a given tissue growth may not hold true for other salt concentrations, other tissues and/or other cultivars. The more drought-tolerant TAM W-101, when compared to the less drought tolerant Sturdy, showed higher tolerance (at most concentrations) to NaCl, CaCl₂, Ca(NO₃)₂ and NaHCO₃ during its seed germination and to Na₂SO₄ and CaCl₂ for its root growth. This supports other reports that some drought-tolerant wheat cultivars are more tolerant to NaCl. In contrast, the coleoptile growth of drought-sensitive Sturdy was noticeably more tolerant to NaNO₃, Ca(NO₃)₂ and NaHCO₃ than that of drought-tolerant TAM W-101. Based on the above and the different root/coleoptile ratios observed in the presence of various salts, it is concluded that in these wheat cultivars: a) coleoptile and root tissues are differently sensitive to various salts, and b) at the germination stage, tolerance to certain salts is higher in the more drought-tolerant cultivar.     

Factors controlling soil water and stream water aluminum concentrations after a clearcut in a forested watershed with calcium-poor soils

The 24 ha Dry Creek watershed in the Catskill Mountains of southeastern New York State USA was clearcut during the winter of 1996–1997. The interactions among acidity, nitrate (NO₃⁻), aluminum (Al), and calcium (Ca²⁺) in streamwater, soil water, and groundwater were evaluated to determine how they affected the speciation, solubility, and concentrations of Al after the harvest. Watershed soils were characterized by low base saturation, high exchangeable Al concentrations, and low exchangeable base cation concentrations prior to the harvest. Mean streamwater NO₃⁻ concentration was about 20 μmol l⁻¹ for the 3 years before the harvest, increased sharply after the harvest, and peaked at 1,309 μmol l⁻¹ about 5 months after the harvest. Nitrate and inorganic monomeric aluminum (Al_im) export increased by 4−fold during the first year after the harvest. Al_im mobilization is of concern because it is toxic to some fish species and can inhibit the uptake of Ca²⁺ by tree roots. Organic complexation appeared to control Al solubility in the O horizon while ion exchange and possibly equilibrium with imogolite appeared to control Al solubility in the B horizon. Al_im and NO₃⁻ concentrations were strongly correlated in B-horizon soil water after the clearcut (r ² = 0.96), especially at NO₃⁻ concentrations greater than 100 μmol l⁻¹. Groundwater entering the stream from perennial springs contained high concentrations of base cations and low concentrations of NO₃⁻ which mixed with acidic, high Al_im soil water and decreased the concentration of Al_im in streamwater after the harvest. Five years after the harvest soil water NO₃⁻ concentrations had dropped below preharvest levels as the demand for nitrogen by regenerating vegetation increased, but groundwater NO₃⁻ concentrations remained elevated because groundwater has a longer residence time. As a result streamwater NO₃⁻ concentrations had not fallen below preharvest levels, even during the growing season, 5 years after the harvest because of the contribution of groundwater to the stream. Streamwater NO₃⁻ and Al_im concentrations increased more than reported in previous forest harvesting studies and the recovery was slower likely because the watershed has experienced several decades of acid deposition that has depleted initially base-poor soils of exchangeable base cations and caused long-term acidification of the soil.