Dynamics of concentrations and nutrient fluxes in the xylem of Ricinus communis– diurnal course, impact of nutrient availability and nutrient uptake

Diurnal courses of nutrient transport in the xylem and their response to external availability of nutrients were studied. In soil culture, maximal concentrations in all analysed substances were observed during night‐time. Over experimental periods of up to 20 d, concentrations of some ions increased, most by accumulation in the soil. Stringent nutrient conditions were established in a novel pressure chamber. An aeroponic nutrient delivery system inside allows the sampling of xylem sap from intact plants under full control of the nutrient conditions at the root. Analysis of xylem transport under these highly defined conditions established that (1) diurnal variations in concentrations and fluxes in the xylem are dominated by plant‐internal processes; (2) concentrations of nutrients in the xylem sap are highly but specifically correlated with each other; (3) nitrate uptake and nitrate flux to the shoot are largely uncoupled; and (4) in continuous light, diurnal variations of xylem sap concentrations vanish. Step changes in nitrate concentrations of the nutrient solution established that (5) the concomitant increase in nitrate concentration and flux in the xylem is delayed by 2–3 h and is only transient. Diurnal variations of xylem sap composition and use of the new technique to elucidate xylem‐transport mechanisms are discussed.     

Sequential response of whole plant water relations to prolonged soil drying and the involvement of xylem sap ABA in the regulation of stomatal behaviour of sunflower plants

Sunflower plants ( Helianihus animus cv. Tall Single Yellow} were grown in the greenhouse in drain pipes (100 mm inside diameter and 1 m long) rilled with John Innes No. 2 compost. When the fifth leaf had emerged, half of the plants were left unwatered for 6 days, rewatered for 2 days and then not watered for another 12 days. Measurements of water relations and abaxial stomatal conductance were made at each leaf position at regular intervals during the experimental period. Estimates were also made of soil water potentials along the soil profile and of ABA concentrations in xylem sap and leaves.
Soil drying led to some reduction in stomatal conductance alter only 3 days but leaf turgors were not reduced until day 13 (6 days after rewatering). When the water relations of leaves did change, older leases became substantially dehydrated while high turgors were recorded in younger leaves. Leaf ABA content measured on the third youngest leaf hardly changed over the first 13 days of the experiment, despite substantial soil drying, while xylem ABA concentrations changed very significantly and dynamically as soil water status varied, even when there was no effect of soil drying on leaf water relations. We argue that the highest ABA concentrations in the xylem, found as a result of substantial soil drying, arise from synthesis in both the roots and the older leaves, and act to delay the development of water deficit in younger leases.
In other experiments ABA solutions were watered on to the root systems of sunflower plants to increase ABA concentrations in xylem sap. The stomatal response to applied ABA was quantitatively very similar to that to ABA generated as a result of soil drying. There was a log-linear relationship between the reduction of leaf conductance and the increase of ABA concentration m xylem sap.     

Influence of [S, S]-EDDS on phytoextraction of copper and zinc by Elsholtzia splendens from metal-contaminated soil

Two pot experiments were conducted to investigate the time course effects of the (S, S)-N, N'-ethylenediamine disuccinic acid (EDDS) addition to contaminated soil on the uptake of Cu and Zn by the Cu accumulator Elsholtzia splendens and on plant Cu and Zn concentrations at different growth stages. EDDS increased the amounts of Cu and Zn soluble in the soil, taken up by plants, concentrated in the xylem sap, and translocated from roots to stems and leaves. The increase in soil-soluble metals, especially Cu, resulted in a corresponding increase in metal concentrations in the xylem sap and leaves. The addition of EDDS to the soil increased plant Cu and Zn concentrations, especially in the leaves, and changed the proportions of Cu and Zn taken up by different plant parts. The proportions of Cu and Zn taken up by the roots were higher than by the leaves of control plants, but EDDS-treated plants showed the opposite trend. EDDS exerted greater effects at the end of the vegetative growth stage than at the start of the flowering or reproductive stages.     

Signalling mechanisms involved in the response of two varieties of Humulus lupulus L. to soil drying: II. changes in the concentration of abscisic acid catabolites and stress-induced phytohormones

Abscisic acid (ABA) is one of the most common stress signals that appear in plant organs in response to soil drying. Equilibrium between ABA biosynthesis and catabolism regulates ABA accumulation in plants under water stress. The aim of our work was to explore the dynamics of changes in ABA metabolites as well as other stress-induced phytohormones such as jasmonic acid, indole-3-acetic acid, and their respective metabolites in hop [Humulus lupulus (L.)] plants during drying and to identify among them potential signals involved in drought signalling. We showed that the concentrations of all ABA metabolites (except the concentration of ABA glucosyl ester in leaves) increased in the same manner in leaves and xylem sap approximately at the same level of soil water content when the relative water content of leaves decreased. The predominant metabolites in leaves and xylem sap were phaseic acid and dihydroxyphaseic acid. ABA glucosyl ester was not a source of the increased concentration of ABA in leaves and xylem sap because of its considerably lower concentration compared to ABA. The concentration of jasmonates decreased in leaves of hop plants. Changes in auxin concentration suggest that this hormone is involved in the response of hop plants to soil drying.     

The role of mineral nutrients in the increased growth response of tomato plants in solarized soil

Soil solarization is a non-chemical disinfestation technique that frequently promotes plant growth in the absence of known major pathogens, a phenomenon termed increased growth response (IGR). The effect of solarization on plant nutrients and their role in the IGR was studied with tomato plants grown in solarized or non-solarized (control) sandy soil, under controlled conditions. Solarization considerably increased the soil concentrations of water extractable N, K, Ca, Mg and Na at most sites, whereas Cl and DTPA extractable Mn, Zn, Fe and Cu were decreased by the treatment. Plant growth and specific leaf area were enhanced in solarized as well as in N-supplemented control soil. In tomato plants grown in solarized soil, concentrations of most nutrients in the xylem sap, including N, were increased compared to the control, whereas Cl and SO4 levels decreased. The most significant increase in leaf nutrient concentration caused by soil solarization was recorded for N. Furthermore, leaf N concentration was highly and positively correlated with shoot growth. The concentration of Cu increased in leaves from the solarization vs. the control treatment, whereas that of SO4 and Cl decreased, the latter presumably below the critical toxicity level. The correlation between shoot growth and leaf concentration was positive for Cu and inverse for Cl and SO4. In conclusion, we found that soil solarization significantly affects nutrient composition in tomato plants, and provided strong evidence that N, and eventually also Cl, play a major role in IGR.     

Root and bacterial secretions regulate the interaction between plants and PGPR leading to distinct plant growth promotion effects

Background and aims

Long distance signals in xylem from roots to leaves are important in plant response to drought stress. Abscisic acid (ABA) plays a key role in drought signaling in plants but apoplastic pH may modulate its effect by distributing ABA into various compartments in leaves. We aimed to reveal the dynamics of changes in sap pH and its relationships with the transport of inorganic and organic ions in eight herbaceous plant species under continuously declining soil water content. We tested several hypotheses related to the mechanism of pH changes in xylem.

Methods

We used a pressure chamber to collect xylem sap and to measure of leaf/stem water potential at various stages of soil drying. We measured pH and concentrations of the most abundant inorganic (NO₃ ^?, SO₄ ^2?, PO₄ ^3? and Cl^?) and organic (malate and citrate) anions in xylem sap.

Results

Species differed considerably in the dynamics of pH changes in xylem in drying soil. Changes in xylem sap pH during drying did not relate to the nitrogen assimilation strategy but may be affected by sap flow rate. Simultaneous changes in the concentrations of inorganic and organic anions were highly species-specific.

Conclusions

High variability among species in the observed relationships in response to drought indicates that comparisons among different studies and the generalization of results should be made with caution.

     

Stomatal control by fed or endogenous xylem ABA in sunflower: interpretation of correlations between leaf water potential and stomatal conductance in anisohydric species

The stomatal conductance of several anisohydric plant species, including field-grown sunflower, frequently correlates with leaf water potential (φ₁), suggesting that chemical messages travelling from roots to shoots may not play an important role in stomatal control. We have performed a series of experiments in which evaporative demand, soil water status and ABA origin (endogenous or artificial) were varied in order to analyse stomatal control. Sunflower plants were subjected to a range of soil water potentials under contrasting air vapour pressure deficits (VPD, from 0.5 to 2.5 kPa) in the field, in the glasshouse or in a humid chamber. Sunflower plants were also fed through the xylem with varying concentrations of artificial ABA, in the glasshouse and in the field. Finally, detached leaves were fed directly with varying concentrations of ABA under three contrasting VPDs. A unique relationship between stomatal conductance (g_s) and the concentration of ABA in the xylem sap (xylem [ABA]) was observed in all cases. In contrast, the relationship between φ₁ and g_s varied substantially among experiments. Its slope was positive for droughted plants and negative for ABA-fed whole plants or detached leaves, and also varied appreciably with air VPD. All observed relationships could be modelled on the basis of the assumption that φ₁ had no controlling effect on g_s. We conclude that stomatal control depended only on the concentration of ABA in the xylem sap, and that φ₁ was controlled by water flux through the plant (itself controlled by stomatal conductance). The possibility is also raised that differences in stomatal ‘strategy’ between isohydric plants (such as maize, where daytime φ₁ does not vary appreciably with soil water status) and anisohydric plants (such as sunflower) may be accounted for by the degree of influence of φ₁ on stomatal control, for a given level of xylem [ABA]. We propose that statistical relationships between φ₁ and g_s are only observed when φ₁ has no controlling action on stomatal behaviour.     

Relationships between feeding of Philaenus spumarius (L.) and the amino acid concentration in the xylem sap

Summary. 1. P.spwnarius nymphs selected young, upper leaves of Xanthium strumarium plants, which have a relatively high amino acid concentration in the xylem sap.
2. Nymphs selected or rejected a host leaf as a feeding site after a test ingestion of plant sap.
3. Nymphs fed on detached leaves kept with their petioles in a solution of amino acids, in preference to leaves with petioles in a solution containing no amino acids.
4. Nymphs caged on leaves with relatively high amino acid concentrations in the xylem sap suffered a lower mortality after 7 days than nymphs caged on leaves with low amino acid concentrations in the xylem sap.
5. Mean excretory rate increased with the increase in the amino acid concentration in the xylem sap for third and fourth instar nymphs, and adults, but not in fifth instars.     

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.     

Accumulation of weathered pp'-DDE in xylem sap of grafted watermelon

Movement of weathered p,p'-dichlorodiphenyldichloroethane (p,p'-DDE) from contaminated soil to the rhizosphere pore water to the xylem sap of grafted watermelon was studied under green house conditions. p,p'-DDE concentrations in pore water and xylem sap was compared in intact plants, homografted, and compatible heterografts of Cucurbita pepo spp. pepo and Citrullus lanatus plants. An average p,p'-DDE concentrations in pore water of contaminated soil ranged from 0.36 microg/L to 0.55 microg/L and there were no statistically significant among the cultivars. Conversely, the xylem sap p,p'-DDE concentration of heterografted watermelon having a zucchini rootstock and watermelon scion was 71 microg/L and it was greater than intact watermelon plants (0.49 microg/L) but less than that of intact plants of zucchini (141 microg/L). Homografting showed no effect on xylem sap p,p'-DDE concentrations of the identical cultivars. The bio-concentration factors (BCFs) which is an average p,p'-DDE concentration in xylem sap over average p,p'-DDE in pore water were 344, 325, 197, 1.28, and 0.89 for intact plant of zucchini, homografted zucchini, heterografted watermelon, homografted watermelon, and intact plant of watermelon, respectively. Xylem sap p,p'-DDE concentrations of the heterografted watermelon plants were clearly influenced by plant phylogeny and enhanced by the zucchini rootstock compared to intact watermelon plants.     

Plant Nutrient Partitioning in Coffee Infected with Meloidogyne konaensis

Two experiments were conducted to assess nutrient partitioning in coffee (Coffea arabica cv. Typica land race Guatemala) infected with Meloidogyne konaensis. Nutrient levels were quantified from soil, roots, and leaves. In the first experiment, 500-cm3 aliquants of a Kealakekua Andisol were infested with four initial population densities of M. konaensis ranging from 0 to 1,500 freshly hatched second-stage juveniles. Coffee plants (~3 months old) were transplanted into the soil and grown for 25 weeks. Plants responded to nematode infection with decreases (P < 0.05) in concentrations of Ca, Mg, P, and B and increases (P < 0.05) in concentrations of Mn, Cu, Zn, and Ca/B in the roots. Mn and Cu uptake by roots was decreased (P < 0.05) by nematode infection even though concentrations of Mn and Cu increased (P < 0.05) in the roots. Concentrations of Ca and Mg also decreased (P < 0.05) in the leaves, whereas the concentration of Zn increased (P < 0.05). In the second experiment, the soil was amended with Zn at 0 or 5 mg/kg soil and infested with M. konaensis at 0, 100, 1,000 or 10,000 eggs/1,200 cm³ soil. Three-month-old coffee seedlings of similar height were weighed and transplanted into pots and then placed in a greenhouse and grown under 50% shade for 23 weeks. Concentrations of P, K, Ca, Mg, Mn, B, and Zn increased in roots of nematode-free plants growing in Zn-amended soil. The beneficial effects due to the Zn amendment were not apparent in nematode-infected plants. Mn, B, and Zn uptake by coffee roots and P and B concentrations in coffee leaves responded similarly. Management of M. konaensis is necessary to achieve optimal nutrient management in coffee.     

Water and Nutrient Dynamics in Surface Roots and Soils are not Modified by Short-term Flooding of Phreatophytic Plants in a Hyperarid Desert

Little is known of the mechanisms employed by woody plants to acquire key resources such as water and nutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the water table, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viable shallow roots may be equally necessary for nutrient uptake. We sought to better understand the interaction between water and nutrient uptake from soil horizons differing in the relative abundance of these resources. To this end, we monitored plant water and nutrient status before and after applying flood irrigation to four phreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sap flow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica (Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using the heat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitrate reductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in the leaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on water and nutrient status. Rates of sap flow measured during the day and at night in the absence of transpiration did not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5–25 cm h⁻¹) detected during the day in soils that had a near zero water content at the surface indicated that all species had contact to groundwater. There was no evidence from sap flow data that plants had utilised flood water to increase maximum rates of transpiration under similar climatic conditions, and there was no evidence of a process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. the passive movement of water from moist soil to very dry soil via roots). Measurements of plant water status, xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were also unaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of water and nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newly available nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.     

三角叶滨藜根吸水特点与其抗盐性的关系

采用压力室和冰点渗透压计测定了三角叶滨藜在不同浓度NaCl的根系环境溶液中根木质部的压力势和伤流液的渗透势,并利用原子吸收分光光度计测定了植株和伤流液以及环境溶液中Na 含量。结果表明:随着根环境溶液NaCl浓度的增加,三角叶滨藜植株和木质部伤流液中Na 含量虽呈上升趋势,但根系的过滤系数和体内Na 相对累积量逐渐降低,说明三角叶滨藜根细胞对盐分有很强的过滤作用;木质部伤流液的渗透势随着环境溶液渗透势的降低而降低,但根木质部溶液的水势则逐渐高出根外环境溶液的渗透势;表明三角叶滨藜能够利用较低的木质部负压来抵抗根外溶液的低渗透势而反渗透吸水,并利用根细胞对盐分的过滤作用来避免从环境摄取过量的盐分。     

Omics of root-to-shoot signaling under salt stress and water deficit

Maximizing crop yield depends on the leaves receiving an optimal supply of water, mineral nutrients, small organic molecules, proteins, and hormones from the root system via the xylem. Soil drying and salinization alter these xylem fluxes, and modern omics techniques offer unparalleled opportunities to understand the complexity of these responses. Although absolute xylem concentrations of any constituent depend on the genotype and xylem sap sampling methodology, analysis of the relative changes in concentrations has revealed some conserved behavior. Typically, these stresses increase xylem concentrations of the plant hormone abscisic acid (ABA) that limits crop water loss, but decrease the concentrations of certain cytokinins that stimulate expansive growth and prevent premature leaf senescence. Further understanding of the ionic and biophysical alterations in the rhizosphere environment that cause increased xylem concentrations of the ethylene precursor (ACC) is needed. Interactions of these plant hormones with plant nutrient status and xylem nutrient delivery may be important in tuning plant responses to their environment. Xylem proteomics is an emerging area that will help understand mechanisms of plant stress adaptation. Using omics techniques to underpin rootstock-mediate plant improvement is likely to improve crop yields in dry or saline soil.     

The effects of nutrient supply,predominantly addition of iron,and rhizobial inoculation on the tolerance of Lupinus pilosus genotypes to a calcareous soil

Two glasshouse experiments were conducted to examine the effects of nutrient supply and rhizobial inoculation on the performance of Lupinus pilosus genotypes differing in tolerance to calcareous soils. In experiment 1, plants were grown for 84 days in a calcareous soil (50% CaCO₃; soil water content 90% of field capacity) at four nutrient treatments (no-added nutrients, added nutrients without Fe, added nutrients with soil applied FeEDDHA, added nutrients with foliar applied FeSO₄). In experiment 2, plants were grown for 28 days with supply of NH₄NO₃ without inoculation or inoculated with Bradyrhizobium sp. (Lupinus). Chlorosis in the youngest leaves was a good indicator of the relative tolerance of the genotypes to the calcareous soil in both experiments, except the treatment with FeEDDHA at 5 mg kg^–1 soil which was toxic to all genotypes. Chlorosis scores correlated with chlorophyll meter readings and chlorophyll concentrations. The foliar application of FeSO₄ did not fully alleviate chlorotic symptoms despite concentrations of active or total Fe in the youngest leaves being increased. Adding nutrients and chemical nitrogen did not change the severity of chlorosis or improve the growth of the plant. The nutrient supply did not alter the ranking of tolerance of genotypes to the calcareous soil. The results suggest that nutrient deficiency or poor nodulation was not a major cause of poor plant growth on calcareous soils and that bicarbonate may exert a direct effect on chlorophyll synthesis. The mechanism for tolerance is likely to be related to an ability to exclude bicarbonate or prevent its transport to the leaves.     

The chemical form of trivalent chromium in xylem sap of maize (Zea mays L.)

Abstract

The toxicity, mobility and bioavailability of Cr, a versatile industrial metal and a contaminant, depends on its chemical form, viz: Cr(lll) and Cr(VI). It may enter humans through plants grown on contaminated soil or irrigated by contaminated water. The phytoavailability and transfer through agricultural food chains requires an understanding of mechanisms of Cr uptake and translocation by plants. Xylem sap transports both nutrient and non-nutrient ions after absorption by roots to aerial parts of the plant. lt transports cations by complexation with organic ligands. Trivalent chromium, though prone to hydrolysis, also complexes O donor ligands. The chemical form in which Cr(lll) is transported by xylem sap was investigated. ln vitro studies were performed by mixing the xylem sap of maize plants at three stages of plant growth with radiotagged Cr(III). The speciation change was investigated after 10 days and 30 days by anion and cation exchange elution chromatography. The elution curves were compared with those of pure Cr(III) and Cr(III) complexes of different synthetic acids. Complexation of Cr(III) with ligands of xylem sap especially with carboxylates was evident. Cationic Cr(III) was vitally being transported as anionic organic complex species. The major species seemed to be that of Cr(III)-citrate. Citric acid was the major complexing acid of xylem sap as determined by HPLC. These mobile and soluble complexes may get immobilized and stored in leaves and other edible plant parts. This may also be a mechanism used by plants for detoxification of toxic Cr(VI) which may become reduced and then complexed.     

Xylem-solute technique to measure N2 fixation by Phaseolus vulgaris L.: Calibration and sources of error

Utilising the ¹⁵N dilution technique the relationship between the proportion of N derived from N₂ fixation and relative abundance of ureides in xylem sap was evaluated for Phaseolus vulgaris L. cv. Mokcham during vegetative and reproductive development. In order to establish calibration curves for time integrated estimates of N₂ fixation, plants were raised in sand culture during the dry season in northern Thailand and continuously supplied with a N-free nutrient solution or the same solution amended with 0, 3, 6 or 9 mol m^–3 nitrate. Large changes in plant dependence on N₂ fixation were concomitantly reflected by corresponding alterations in N solutes in xylem sap. Regression analyses of the data suggested high correlations between relative ureide content and N₂ fixation, but different slopes and line intercepts indicated the requirement for the use of calibration curves established for different phases of the development of the plant. Largest age related differences were noted between vegetative and reproductive development. Judging from 95% confidence limits, utilisation of appropriate calibrations can reduce errors of the technique to close to ±5%.A second experiment, involving similarly cultivated plants exposed to different sources of mineral N, indicated an effect of ammonium on xylem sap composition. This implies that calibrations, in which N₂ fixation is regulated only by applications of various concentrations of nitrate, may lead to errors in situations where a major proportion of the plant available soil nitrogen fraction is present in the form of ammonium.     

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.     

Interference from xylem sap in an enzyme-linked immunosorbent assay for zeatin riboside

The extent of interference from xylem sap in an enzyme-linked immunosorbent assay was determined for a woody perennial [ Populus trichocarpa Torr. & Gray x P deltoides Bart, ex Marsh (Hybrid 1l–ll)] and a herbaceous annual ( Phasesolus vulgaris L. cv. Contender). Crude xylem sap collected from excised roots from both species interfered with the assay for zeatin riboside. Assays for zeatin riboside in xylem sap collected from Popidus overestimated endogenous levels, and added standards could not be accurately measured from a range of sap dilutions. When Phaseolus plants were grown under various nutrient regimens, interference in the assay was dependent on nutrient availability. Of xylem sap components (inorganic minerals, amino acids and sucrose) which may vary with environmental conditions or among species, only sucrose interfered at the concentrations tested. Since the pH of xylem sap varies it was necessary to buffer samples prior to analysis. Partial purification using anion exchange columns and Sep-Paks cffectively eliminated interference. These results demonstrate that estimates of plant growth regulators in xylem sap by the ELISA (enzyme-linked immunosorbant assay) method can be influenced by species and environmental conditions such as plant nutritional status.     

Transpiration inhibition by stored xylem sap from well-watered maize plants

There is increasing evidence that a chemical signal exists in xylem sap of plants subjected to water deficits which influences physiological responses in plant shoots. An important method of studying this signal is the transpiration response of excised leaves exposed to xylem sap collected from plants. However, Munns et al [Plant, Cell & Environment 16, 867–877] cautioned that transpiration inhibition is observed when xylem sap collected from wheat and barley is stored before determining physiological activity. The objective of the study reported here was to determine if transpiration inhibition develops in maize sap collected from well-watered plants when the sap is stored under various conditions. It was found that storage of maize sap collected from well-watered plants for only 1 d at -20°C resulted in the development of substantial transpiration inhibition in bioassay leaves. Storage of sap at 4°C resulted in the development of the effect after 2 weeks, while storage at ?86°C showed only small transpiration inhibition after 3 weeks. The major source of the transpiration inhibition was the development of a substance in the stored sap that resulted in physical blockage of the transpiration stream in bioassay leaves. However, a small signal component may also have developed in the stored sap. Because of the possibility of ionic activity under freezing conditions at ?20°C, calcium was studied for its potential involvement in the transpiration inhibition. However, the calcium concentrations found to inhibit transpiration were nearly an order of magnitude larger than the calcium concentrations observed in xylem sap.