Form of aluminium for uptake and translocation in buckwheat (Fagopyrum esculentum Moench)

 The forms of Al for uptake by the roots and translocation from the root to the shoot were investigated in a buckwheat (Fagopyrum esculentum Moench, cv. Jianxi) that accumulates Al in its leaves. The Al concentration in the xylem sap was 15-fold higher in the plants exposed to AlCl₃ than in those exposed to an Al-oxalate (1:3) complex, suggesting that the roots take up Al in the ionic form. The Al concentration in the xylem sap was 4-fold higher than that in the external solution after a 1-h exposure to AlCl₃ solution and 10-fold higher after a 2-h exposure. The Al concentration in the xylem sap increased with increasing Al concentration in the external solution. The Al uptake was not affected by a respiratory inhibitor, hydroxylamine, but significantly inhibited by the addition of La. These results suggest that Al uptake by the root is a passive process, and La³⁺ competes for the binding sites for Al³⁺ on the plasma membrane. The form of Al in the xylem sap was identified by ²⁷Al-nuclear magnetic resonance analysis. The chemical shift of ²⁷Al in the xylem sap was around 10.9 ppm, which is consistent with that of the Al-citrate complex. Furthermore, the dominant organic acid in the xylem sap was citric acid, indicating that Al was translocated in the form of Al-citrate complex. Because Al is present as Al-oxalate (1:3) in the root, the present data show that ligand exchange from oxalate to citrate occurs before Al is released to xylem. Received: 10 December 1999 / Accepted: 3 February 2000     

Chemical forms of aluminum in xylem sap of tea plants (Camellia sinensis L.)

To identify the chemical forms of aluminum (Al) transported from roots to shoots of tea plants (C. sinensis L.), ²⁷Al-nuclear magnetic resonance and ¹⁹F NMR spectroscopy were used to analyze xylem sap.The concentration of Al in collected xylem sap was 0.29 mM, twice as high as that of F. Catechins were not detected in xylem sap. The concentration of malic acid in xylem sap was higher than that of citric acid, whereas the concentration of oxalic acid was negligible.There were two signals in the ²⁷Al NMR spectra of xylem sap, a larger signal at 11 ppm and a smaller one at −1.5 ppm. The former signal was consistent with the peak for an Al-citrate model solution, suggesting that an Al-citrate complex was present in xylem sap. Although the latter signal at −1.5 ppm was thought to indicate the presence of an Al-F complex (at 1.7 ppm) in xylem sap, there was only one signal at −122 ppm in the ¹⁹F NMR spectrum of xylem sap, indicating that the main F complex in xylem sap was F⁻.These results indicate that Al might be translocated as a complex with citrate, while Al-malate, Al-oxalate and Al-F complexes are not major Al complexes in xylem sap of tea plants.     

Effects of litter incorporation and nitrogen fertilization on the contents of extractable aluminium in the rhizosphere soil of tea plant (Camallia sinensis (L.) O. Kuntze)

The effects of litter incorporation and nitrogen application on the properties of rhizosphere and bulk soils of tea plants (Camellia sinensis (L.) O. Kuntze) were examined in a pot experiment. Total of 8 treatments included four levels of tea litter additions at 0, 4.9, 9.8, and 24.5 g kg^–1 in combination with two N levels (154.6 mg kg^–1 and without). After 18 months of growth the rhizosphere soil was collected by removing the soil adhering to plant roots and other soil was referred to as bulk soil. The dry matter productions of tea plants were significantly increased by N fertilization and litter incorporation. The effect of litter was time-depending and significantly decreased the content of exchangeable Al (Al_ex, by 1 mol L^–1 KCl) and Al saturation at 9 months after litter incorporation whereas soil pH was not affected, although the litter contained high Al content. After 18 months, the contents of extractable Al by dilute CaCl₂, CuCl₂ + KCl, NH₄OAC, ammonium oxalate and sodium citrate (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, Al_Oxal, and Al_Cit respectively) and Al_ex, were not affected by litter application, except that of Al_CaCl2 in the rhizosphere soil which was decreased following litter additions. Nitrogen fertilization with NH₄ ⁺ (urea and (NH₄)₂SO₄) significantly reduced soil pH, the contents of exchangeable Ca, K, Mg and base saturation while raised extractable Al levels (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, and Al_ex). In the rhizosphere soils exchangeable K accumulated in all treatments while exchangeable Ca and Mg depleted in treatments without litter application. The depletions of Ca and Mg were no longer observed following litter incorporation. This change of distribution gradients in rhizosphere was possibly due to the increase of nutrient supplies from litter decomposition and/or preferable root growth in soil microsites rich in organic matter. Lower pH and higher extractable Al (Al_CaCl2, Al_ex, and Al_NH4OAC) in the rhizosphere soils, regardless of N and litter treatments, were distinct and consistent in all treatments. Such enrichments of extractable Al in the rhizosphere soil might be of importance for tea plants capable of taking up large amounts of Al.     

Aluminum‐dependent dynamics of ion transport in Arabidopsis: specificity of low pH and aluminum responses

Low‐pH and Al³⁺ stresses are the major causes of poor plant growth in acidic soils. However, there is still a poor understanding of plant responses to low‐pH and Al³⁺ toxicity. Low‐pH or combined low‐pH and Al³⁺ stress was imposed in order to measure rhizosphere pH, ion fluxes, plasma membrane potential and intracellular H⁺ concentration in distal elongation and mature zones (MZs) along the longitudinal axis of Arabidopsis thaliana roots. Low‐pH stress facilitated H⁺ influx into root tissues and caused cytoplasmic acidification; by contrast, combined low‐pH/Al³⁺ treatment either decreased H⁺ influx in the distal elongation zone (DEZ) or induced H⁺ efflux in the MZ, leading to cytoplasmic alkalinization in both zones. Low‐pH stress induced an increase in rhizosphere pH in the DEZ, whereas combined low‐pH/Al³⁺ stress resulted in lower rhizosphere pH in both root zones compared with the low‐pH treatment alone. Low‐pH stress facilitated K⁺ efflux; the presence of Al³⁺ diminished K⁺ efflux or favored K⁺ influx into root tissues. In both zones, low‐pH treatment induced plasma membrane (PM) depolarization, which was significantly diminished (P≤ 0.05) when combined stresses (low‐pH/100 µM Al³⁺) were imposed. After 60 min of exposure, low pH caused PM depolarization, whereas low pH/100 µM Al³⁺ caused PM hyperpolarization. Thus, low pH and Al³⁺ toxicity differentially affect root tissues and, consequently, the rhizosphere, which might underpin the differential mechanisms of plant adaptation to these abiotic stresses.     

Bafilomycin Inhibits Vacuolar pH Regulation in a Fresh Water Charophyte,Chora corallina

Bafilomycin A₁, known as an inhibitor of vacuolar type H⁺-ATPase, was used to study involvement of the vacuolar ATP-dependent H⁺-pump in the vacuolar pH regulation in a fresh water charophyte, Chara corallina. When bafilomycin A₁ (100 nM) was externally given to intact cells, the vacuolar pH (about 5) was not affected. Internodal cells were then pretreated with 100 nM bafilomycin for 1 ? 2 h and the vacuolar sap was replaced with a weakly buffered solution of pH 7.4. The readjustment of the modified vacuolar pH in bafilomycin-treated cells was significantly retarded compared with that in untreated cells. Next, bafilomycin A₁ was directly introduced into the vacuole by vacuolar perfusion with the artificial cell sap of pH 7.4. At 100 nM bafilomycin A₁, the decrease in the vacuolar pH was significantly inhibited. When cell sap was replaced with the artificial cell sap containing no buffer (pH 5.2 ? 5.5), the vacuolar pH increased in the presence of vacuolar bafilomycin, suggesting that the PP₁- dependent H⁺ pumping alone was not sufficient for the pH regulation of Chara vacuoles. Intracellular bafilomycin A₁ had no effect on the plasma membrane potential of tonoplast-free cells, which is evidence that it does not affect the electrogenic H⁺-pump in the plasma membrane. Bafilomycin A₁ inhibited the ATP-dependent H⁺ transport of tonoplast vesicles but not the PP₁-dependent H⁺ transport. The ATPase activity of tonoplast vesicles was also inhibited by bafilomycin A₁.     

Amine Accumulation in Acidic Vacuoles Protects the Halotolerant Alga Dunaliella salina Against Alkaline Stress

Amines at alkaline pH induce in cells of the halotolerant alga Dunaliella a transient stress that is manifested by a drop in ATP and an increase of cytoplasmic pH. As much as 300 millimolar NH₄⁺ are taken up by the cells at pH 9. The uptake is not associated with gross changes in volume and is accompanied by K⁺ efflux. Most of the amine is not metabolized, and can be released by external acidification. Recovery of the cells from the amine-induced stress occurs within 30 to 60 minutes and is accompanied by massive swelling of vacuoles and by release of the fluorescent dye atebrin from these vacuoles, suggesting that amines are compartmentalized into acidic vacuoles. The time course of ammonia uptake into Dunaliella cells is biphasic—a rapid influx, associated with cytoplasmic alkalinization, followed by a temperature-dependent slow uptake phase, which is correlated with recovery of cellular ATP and cytoplasmic pH. The dependence of amine uptake on external pH indicates that it diffuses into the cells in the free amine form. Studies with lysed cell preparations, in which vacuoles become exposed but retain their capacity to accumulate amines, indicate that the permeability of the vacuolar membrane to amines is much higher than that of the plasma membrane. The results can be retionalized by assuming that the initial amine accumulation, which leads to rapid vacuolar alkalinization, activates metabolic reactions that further increase the capacity of the vacuoles to sequester most of the amine from the cytoplasm. The results indicate that acidic vacuoles in Dunaliella serve as a high-capacity buffering system for amines, and as a safeguard against cytoplasmic alkalinization and uncoupling of photosynthesis.     

Buckwheat accumulates aluminum in leaves but not in seeds

Buckwheat (Fagopyrum esculentum Moench. cv Jianxi) is highly resistant to Al stress and is known to be an Al-accumulator. Pot experiments were carried out in a greenhouse to investigate the accumulation of Al in leaves and seeds of buckwheat. Plants were grown for 12 weeks in a strong acid soil amended with or without CaCO₃ at a rate of 1 g kg⁻¹ soil. Old leaves accumulated as much as 10 g kg⁻¹ Al of dry weight when the plants were grown in the acid soil, while the Al concentrations in leaves immediately adjacent to seeds, seed coats, and embryos were, on average, 4516, 41.2 and 7.7 mg kg⁻¹, respectively. The Al concentration significantly decreased in leaves when the plants were grown in the limed soil, and the Al concentrations in leaves immediately adjacent to seeds, seed coats, and embryos were, on average, 1586, 21.3 and 3.1 mg kg⁻¹, respectively. These results show that seeds accumulate much less Al than buckwheat leaves. The underlying mechanisms are discussed. Section Editor: H. Lambers     

Involvement of multiple aluminium exclusion mechanisms in aluminium tolerance in wheat

The goal of this study was to determine if Al-chelators other than malate are released from root apices and are involved in Al-tolerance in different wheat (Triticum aestivum L.) genotypes. Also we wanted to establish if root exudates contribute to increases in rhizosphere pH around the root tip. In seedlings of Al-tolerant Atlas, we have documented a constitutive phosphate exudation from the root apex. Because phosphate can complex Al and bind protons, it could play an important role in Al tolerance, both via complexation of Al³⁺ and by contributing to the alkalinization of rhizosphere pH observed at the apex of Atlas. This study suggests that in wheat, Al-tolerance can be mediated by multiple exclusion mechanisms controlled by different genes.     

DIATOM COMMUNITY DYNAMICS IN STREAMS OF CHRONIC AND EPISODIC ACIDIFICATION: THE ROLES OF ENVIRONMENT AND TIME1

Community dynamics of epiphytic diatoms were studied for 3 years in a chronically and an episodically acidified tributary of Buck Creek, Adirondacks. Both streams experienced pulses of acidity during hydrologic events but these pulses were more pronounced in the episodically acidified stream, where pH decreased over two units (between 4.53 and 6.62) and the acid‐neutralizing capacity (ANC) became negative. In the chronically acidified stream, pH was below 4.9 and the ANC was negative 94% of the time. In this stream, high inorganic acidity following SO₄^2? enrichment from snowmelt or rainstorms alternated with high organic acidity derived from a headwaters wetland during base flow. The fluctuating water chemistry generated shifts in diatom community composition: from exclusive dominance of Eunotia bilunaris (Ehrenberg) Mills during periods of high inorganic acidity to proliferation of several subdominant species during periods of high organic acidity. In the episodically acidified stream, the pulses of acidity were associated with high NO₃^? concentrations and the corresponding high ratios of inorganic monomeric Al (Al_im) to organic monomeric Al (Al_om). Diatom communities there were dominated exclusively by E. exigua (Brébisson) Rabenhorst year round; however, this species peaked during periods of low acidity. Periods of high acidity and Al_im:Al_om ratios were marked by a decline in E. exigua and a concomitant increase in the subdominant species. Variance partitioning into terms of environmental and temporal variance, and their covariance, suggested that diatom communities in the chronically acidic stream were governed primarily by environmental factors while in the episodically acidic stream environmental and temporal factors had equal contributions.     

Involvement of Long-Distance Na<Superscript>+</Superscript> Transport in Maintaining Water Potential Gradient in the Medium-Root-Leaf System of a Halophyte <Emphasis Type="Italic">Suaeda altissima</Emphasis>

The aim of this study was to determine the range of NaCl concentrations in the nutrient solution that allow Suaeda altissima (L.) Pall., a salt-accumulating halophyte, to maintain the upward gradient of water potential in the “medium-root-leaf” system. We evaluated the contribution of Na⁺ ions in the formation of water potential gradient and demonstrated that Na⁺ loading into the xylem is involved in this process. Plants were grown in water culture at NaCl concentrations ranging from zero to 1 M. The water potential of leaf and root cells was measured with the method of isopiestic thermocouple psychrometry. When NaCl concentration in the growth medium was raised in the range of 0–500 mM (the medium water potential was lowered accordingly), the root and leaf cells of S. altissima decreased their water potential, thus promoting the maintenance of the upward water potential gradient in the medium-root-leaf system. Growing S. altissima at NaCl concentrations f 750 mM and 1 M disordered water homeostasis and abolished the upward gradient of water potential between roots and leaves. At NaCl concentrations of 0–250 mM, the detached roots of S. altissima were capable of producing the xylem exudate. The concentration of Na⁺ in the exudate was 1.3 to 1.6 times higher than in the nutrient medium; the exudate pH was acidic and was lowered from 5.5 to 4.5 with the rise in the salt concentration. The results indicate that the long-distance Na⁺ transport and, especially, the mechanism of Na⁺ loading into the xylem play a substantial role in the formation of water potential gradient in S. altissima. The accumulation of Na⁺ in the xylem and acidic pH values of the xylem sap suggest that Na⁺ loading into the xylem is carried out by the Na⁺/H⁺ antiporter of the plasma membrane in parenchymal cells of the root stele.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 549–557.Original Russian Text Copyright © 2005 by Balnokin, Kotov, Myasoedov, Khailova, Kurkova, Lun’kov, Kotova.     

Suitability of Arabidopsis for studies on intracellular pH regulation: correlation between H+ pump activity, cytosolic pH and malate level in wild-type and in a mutant partially insensitive to fusicoccin

Data are presented on the suitability of Arabidopsis thaliana seedlings for studies on intracellular pH regulation. In this material, grown in the dark in liquid medium, the determination of weak acid distribution at equilibrium provides an adequate method for calculating cytosolic pH values, in spite of the failure of benzylamine as a vacuolar pH probe. The stimulation of the H⁺ pump by K⁺ or K⁺ and fusicoccin (FC) is associated with a marked alkalinization of both cytosol and cell sap, and with a strong increase in malate level, whereas its inhibition by erythrosin B (EB) leads to the opposite effects. A good quantitative correlation is evident between the changes in net H⁺ extrusion and those in intracellular pH and malate content, in particular, with FC+K⁺. Cell sap buffer capacity is strongly influenced by the different treatments, its changes being substantially accounted for by changes in malate level. A comparison between the values of intracellular pH and malate level in wt and in the 5-2 mutant shows that in the mutant the cytosolic pH is always more acidic, and the intracellular alkalinization induced by FC+K⁺ and also by K⁺ alone is significatively lower. These results support the view that the partial insensitivity of 5-2 to FC is due to a reduced functionality of the H⁺-extruding system on which FC acts, and that the depression of the H⁺ pump activity in the mutant does not depend on a possible regulation by constitutively higher cytosolic pH values.     

Heterologous expression of vacuolar H+-PPase enhances the electrochemical gradient across the vacuolar membrane and improves tobacco cell salt tolerance

Summary. The vacuolar H⁺-translocating inorganic pyrophosphatase (H⁺-PPase) uses pyrophosphate as substrate to generate the proton electrochemical gradient across the vacuolar membrane to acidify vacuoles in plant cells. The heterologous expression of H⁺-PPase genes (TsVP from Thellungiella halophila and AVP1 from Arabidopsis thaliana) improved the salt tolerance of tobacco plants. Under salt stress, the transgenic seedlings showed much better growth and greater fresh weight than wild-type plants, and their protoplasts had a normal appearance and greater vigor. The cytoplasmic and vacuolar pH in transgenic and wild-type cells were measured with a pH-sensitive fluorescence indicator. The results showed that heterologous expression of H⁺-PPase produced an enhanced proton electrochemical gradient across the vacuolar membrane, which accelerated the sequestration of sodium ions into the vacuole. More Na⁺ accumulated in the vacuoles of transgenic cells under salt (NaCl) stress, revealed by staining with the fluorescent indicator Sodium Green. It was concluded that the tonoplast-resident H⁺-PPase plays important roles in the maintenance of the proton gradient across the vacuolar membrane and the compartmentation of Na⁺ within vacuoles, and heterologous expression of this protein enhanced the electrochemical gradient across the vacuolar membrane, thereby improving the salt tolerance of tobacco cells. Correspondence: J.-R. Zhang, School of Life Science, Shandong University, 27 Shanda South Road, Jinan, People’s Republic of China 250100.     

Microscale heterogeneity of acidity related stress-parameters in the soil solution of a forested cambic podzol

Acid related stress in soils might be caused by high concentrations of H⁺ and Al³⁺ in soil solution. Sampling of soil solution so far integrates over a relatively large soil volume, in the range of dm³. In order to study the microscale heterogeneity of acidity related stress-parameters the soil profile of a podzolic cambisol was covered by a 10×6 matrix of micro suction cups with a grid distance of 2 cm. The soil solution collected at 10 sampling events was analyzed for free cations and anions by capillary electrophoresis and for total metal content by a micro injection technique on ICP-OES. pH and UV absorption were also measured.There was a general trend of increasing pH and decreasing UV absorption with increasing soil depth, however without a clear correlation of concentration isolines to soil horizon borders. The latter was also true for total Al (Al_tot) and Al³⁺, with the exception of the soil horizon border A_he/B_h,which was very well reflected by Al³⁺ and also by the fraction of bound Al. In the A_he horizon less than 30%, in deeper mineral soil less than 50% of Al_tot were present as free Al³⁺. This fact is critical when calculating Ca/Al ratios as a stress parameter, because total metal content measured by ICP clearly overestimates the risk of root damage, even in deeper horizons of acid forest soils, where organic complexation of Al is of minor importance. The heterogeneity of soil solution chemistry and toxicity parameters on the cm-scale was found to be significant, for example with gradients of more than 0,5 pH-units within 2 cm. Because plant roots also experience soil on a microscale, high resolution investigations of soil solution chemistry offer a new approach for looking at the chemical environment relevant for root growth and plant nutrition.     

Response of the plant root to aluminum stress: Analysis of the inhibition of the root elongation and changes in membrane function

Pea root elongation was strongly inhibited in the presence of a low concentration of Al (5 μM). In Al-treated root, the epidermis was markedly injured and characterized by an irregular layer of cells of the root surface. Approximately 30% of total absorbed Al accumulated in the root tip and Al therein was found to cause the inhibition of whole root elongation. Increasing concentrations of Ca²⁺ effectively ameliorated the inhibition of root elongation by Al and 1 mM of CaCl₂ completely repressed the inhibition of root elongation by 50 μM Al. The ameliorating effect of Ca²⁺ was due to the reduction of Al uptake. H⁺-ATPase and H⁺-PPase activity as well as ATP and PPidependent H⁺ transport activity of vacuolar membrane vesicles prepared from barley roots increased to a similar extent by the treatment with 50 μM AlCl₃. The rate of increase of the amount of H⁺-ATPase and H⁺-PPase was proportional to that of protein content measured by immunoblot analysis with antibodies against the catalytic subunit of the vacuolar H⁺-ATPase and H⁺-PPase of mung bean. The increase of both activities was discussed in relation to the physiological tolerance mechanism of barley root against Al stress.     

Stomatal response to drying soil in relation to changes in the xylem sap composition of Helianthus annuus. I. The concentration of cations, anions, amino acids in, and pH of, the xylem sap

Sunflower plants (Helianthus annuus L.) were subjected to soil drying with their shoots either kept fully turgid using a Passioura-type pressure chamber or allowed to decrease in water potential. Whether the shoots were kept turgid or not, leaf conductance decreased below a certain soil water content. During the soil drying, xylem sap samples were taken from individual intact and transpiring plants. Xylem sap concentrations of nitrate and phosphate decreased with soil water content, whereas the concentrations of the other anions (SO₄² and Cl^?) remained unaltered. Calcium concentrations also decreased. Potassium, magnesium, manganese and sodium concentrations stayed constant during soil drying. In contrast, the pH, the buffering capacity at a pH below 5 and the cation/anion ratio increased after soil water content was lowered below a certain threshold. Amino acid concentration of the xylem sap increased with decreasing soil water content. The effect of changes in ion concentrations in the xylem sap on leaf conductance is discussed.     

Influence of aluminum and phosphorus on growth and xylem sap composition in Melastoma malabathricum L.

Melastoma (Melastoma malabathricum L.) is an aluminum-accumulating woody plant that accumulates more than 10 000 mg kg^–1 of aluminum (Al) in mature leaves. The influence of Al and phosphorus (P) applications on plant growth and xylem sap was examined in the present study in order to elucidate the interaction between Al-induced growth enhancement and P nutrition, and to determine the form of Al for translocation from roots to shoots. Although the Al application significantly increased the growth of Melastomaseedlings with the high P pre-treatment, and P concentrations in the leaves and Pi concentrations in the xylem sap regardless of the P pre-treatment, we could not come to the conclusion that a primary cause of the Al-induced growth enhancement in Melastoma is the stimulation of P uptake. The degree of Al-induced growth enhancement corresponded not with the P concentrations but with the Al concentrations in the plant tissue, suggesting that the Al-induced growth enhancement in Melastoma is primarily caused by Al itself in the plant tissue rather than by the stimulation of P uptake. Through the analysis of organic acids and Al in the xylem sap and plant tissue, the form of Al for translocation from roots to shoots was shown to be an Al-citrate complex that was transformed into Al-oxalate complex for Al storage in the leaves. In addition, the xylem sap of Melastoma seedlings grown in the absence of Al contained higher concentrations of malate. In the presence of Al, however, higher concentrations of citrate were found, indicating that Melastoma changes its organic acid metabolism in the presence or absence of Al; more specifically, it increases the synthesis of citrate.     

Differential acidity tolerance of tropical legumes grown for green manure in acid sulfate soils

The growth of four tropical legumes (Cajanus cajan, Sesbania aculeata, S. rostrata, and S. speciosa) used as green manures in the tropics was studied in a glasshouse experiment. Two acid sulfate soils (Typic Sulfaquept, Bang Pakong Series; and Sulfic Tropaquept, Rangsit Series) were adjusted to four pH levels: 3.8 or 4.0 (original soil pH), 4.5, 5.5, and 6.5 (amended with lime). Dry weight was determined 49 days after sowing. Concentrations of N, P, K, Ca, Mg, Fe, Mn, and Al were also determined in aerial plant parts at harvest.The legumes responded differently to soil acidity and liming, but not to soil type. Cajanus cajan had the highest biomass production, followed by S. aculeata, S. rostrata and S. speciosa, in this order. The N concentration closely paralleled biomass production, suggesting that the growth of symbiotic rhizobia and nodulation were perhaps more susceptible to soil acidity than were the host plants. Liming to pH 5.5–6.0 was recommended for the legumes' growth based on the quadratic relationships between dry-matter yield and soil pH. In the unlimed soils, the Ca concentration in C. cajan and S. aculeata (0.32%) was twice as high as that in the two low-yielding legumes (0.15%). Furthermore, plant Ca increased exponentially (or quadratically in case of S. speciosa) as lime additions increased. It was estimated that for adequate growth, the Ca requirement in the shoot dry matter was approximately: C. cajan 1.2% Ca, S. aculeata 0.8%, S. rostrata 0.6%, and S. speciosa 0.4%. In contrast with Ca, the concentration of Fe, and to a lesser extent Mn, was significantly lower in C. cajan and S. aculeata than in S. rostrata and S. speciosa. The ratio of Ca to Al in plant tops was used to characterize plant tolerance to soil acidity, and to quantify the critical Al concentration in the plants. It appears that 90% maximum growth was attained only when Ca/Al was 150 for C. cajan and S. speciosa, 200 for S. rostrata, and 300 for S. aculeata. Cajanus cajan tolerated up to 80 mg Al kg^-1 in the shoot dry matter, whereas significant growth reduction occurred in the Sesbania species at levels > 30 mg Al kg^-1.     

Linking xylem diameter variations with sap flow measurements

Measurements of variation in the diameter of tree stems provide a rapid response, high resolution tool for detecting changes in water tension inside the xylem. Water movement inside the xylem is caused by changes in the water tension and theoretically, the sap flow rate should be directly proportional to the water tension gradient and, therefore, also linearly linked to the xylem diameter variations. The coefficient of proportionality describes the water conductivity and elasticity of the conducting tissue. Xylem diameter variation measurements could thus provide an alternative approach for estimating sap flow rates, but currently we lack means for calibration. On the other hand, xylem diameter variation measurements could also be used as a tool for studying xylem structure and function. If we knew both the water tension in the xylem and the sap flow rate, xylem conductivity and/or elasticity could be calculated from the slope of their relationship. In this study we measured diurnal xylem diameter variation simultaneously with sap flow rates (Granier-type thermal method) in six deciduous species (Acer rubrum L., Alnus glutinosa Miller, Betula lenta L., Fagus Sylvatica L. Quercus rubra L., and Tilia vulgaris L.) for 7–91 day periods during summers 2003, 2005 and 2006 and analyzed the relationship between these two measurements. We found that in all species xylem diameter variations and sap flow rate were linearly related in daily scale (daily average R ² = 0.61–0.87) but there was a significant variation in the daily slopes of the linear regressions. The largest variance in the slopes, however, was found between species, which is encouraging for finding a species specific calibration method for measuring sap flow rates using xylem diameter variations. At a daily timescale, xylem diameter variation and sap flow rate were related to each other via a hysteresis loop. The slopes during the morning and afternoon did not differ statistically significantly from each other, indicating no overall change in the conductivity. Because of the variance in the daily slopes, we tested three different data averaging methods to obtain calibration coefficients. The performance of the averaging methods depended on the source of variance in the data set and none of them performed best for all species. The best estimates of instantaneous sap flow rates were also given by different averaging methods than the best estimates of total daily water use. Using the linear relationship of sap flow rate and xylem diameter variations we calculated the conductance and specific conductivity of the soil–xylem–atmosphere water pathway. The conductance were of the order of magnitude 10⁻⁵ kg s⁻¹ MPa⁻¹ for all species, which compares well with measured water fluxes from broadleaved forests. Interestingly, because of the large sap wood area the conductance of Betula was approximately 10 times larger than in other species.     

Xylem sap calcium concentrations do not explain liming-induced inhibition of legume gas exchange

Background and aims

Liming is considered normal agricultural practise for remediating soil acidity and improving crop productivity; however recommended lime applications can reduce yield. We tested the hypothesis that elevated xylem sap Ca²⁺ limited gas exchange of Phaseolus vulgaris L. and Pisum sativum L. plants that exhibited reduced shoot biomass and leaf area when limed.

Methods

We used Scholander and whole-plant pressure chamber techniques to collect root and leaf xylem sap, a calcium-specific ion-selective electrode to measure xylem sap Ca²⁺, infra-red gas analysis to measure gas exchange of limed and unlimed (control) plants, and a detached leaf transpiration bioassay to determine stomatal sensitivity to Ca²⁺.

Results

Liming reduced shoot biomass, leaf area and leaf gas exchange in both species. Root xylem sap Ca²⁺ concentration was only increased in P. vulgaris and not in P. sativum. Detached leaves of both species required 5 mM Ca²⁺ supplied to via the transpiration stream to induce stomatal closure, however, maximum in vivo xylem sap Ca²⁺ concentrations of limed plants was only 1.7 mM and thus not high enough to influence stomata.

Conclusion

We conclude that an alternative xylem-borne antitranspirant other than Ca²⁺ decreases gas exchange of limed plants.     

Modelling of the dynamics of Al and protons in the rhizosphere of maize cultivated in acid substrate

The object of this study was to analyze the dynamics of Al and protons in the rhizosphere of maize cultivated in a simple acid substrate, so as to allow the use of a dynamic model of the functioning of a rhizosphere consisting of an organic phase (an agarose gel) and a mineral phase (an amorphous aluminium hydroxide). Two cultivars of maize (Zea mays L.), one Al-sensitive and the other Al-tolerant, were cultivated on this substrate in the presence of different proportions of NH ₄ ⁺ and NO ₃ ^- , which served to acidify the rhizosphere to a greater or lesser extent. The state of the agarose gel and of the cell walls of the roots were monitored using an ion exchange model which had previously been calibrated for each substrate. The experiment showed that Al and protons reduce root growth and the Ca and Mg content in the root, while relative growth varies little between pH 4.0 and pH 4.5. The model showed that competition between Al and protons for the binding sites of the cell walls might account for these results. The sensitivity of the model to the rate of Al(OH)₃ dissolution and to the cation exchange capacity of the culture substrate was tested by numerical simulation. When roots release protons and dissolve Al(OH)₃ in the rhizosphere, there is little possibility of Al desorption by protons on the cell walls at pHs compatible with good root growth of maize, plant specie sensitive to Al and H. Furthermore, the phytotoxicity of the different forms of Al hydroxides should be considered only in taking into account the dynamics of the whole system, in particular the solubilisation of Al in the rhizosphere. This revised version was published online in June 2006 with corrections to the Cover Date.