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PubMed2006

Xylem structure and connectivity in grapevine (Vitis vinifera) shoots provides a passive mechanism for the spread of bacteria in grape plants.

BACKGROUND AND AIMS: Bacterial leaf scorch occurring in a number of economically important plants is caused by the xylem-limited bacterium Xylella fastidiosa (Xf). In grapevine, Xf systemic infection causes Pierce's disease and is lethal. Traditional dogma is that Xf movement between vessels requires the digestion of inter-vessel pit membranes. However, Yersinia enterocolitica (Ye) (a bacterium found in animals) and fluorescent beads moved rapidly within grapevine xylem from stem into leaf lamina, suggesting open conduits consisting of long, branched xylem vessels for passive movement. This study builds on and expands previous observations on the nature of these conduits and how they affect Xf movement. METHODS: Air, latex paint and green fluorescence protein (GFP)-Xf were loaded into leaves and followed to confirm and identify these conduits. Leaf xylem anatomy was studied to determine the basis for the free and sometimes restricted movement of Ye, beads, air, paint and GFP-Xf into the lamina. KEY RESULTS: Reverse loading experiments demonstrated that long, branched xylem vessels occurred exclusively in primary xylem. They were observed in the stem for three internodes before diverging into mature leaves. However, this stem-leaf connection was an age-dependent character and was absent for the first 10-12 leaves basal to the apical meristem. Free movement in leaf blade xylem was cell-type specific with vessels facilitating movement in the body of the blade and tracheids near the leaf margin. Air, latex paint and GFP-Xf all moved about 50-60% of the leaf length. GFP-Xf was never observed close to the leaf margin. CONCLUSIONS: The open vessels of the primary xylem offered unimpeded long distance pathways bridging stem to leaves, possibly facilitating the spread of bacterial pathogens in planta. GFP-Xf never reached the leaf margins where scorching appeared, suggesting a signal targeting specific cells or a toxic build-up at hydathodes.

Air↗
PubMed2006

Ion-mediated flow changes suppressed by minimal calcium presence in xylem sap in Chrysanthemum and Prunus laurocerasus.

After the discovery of ion-mediated changes in xylem hydraulic resistance a few years ago, a number of research papers were published that related ion-mediated flow changes in the xylem to various aspects of whole plant functioning and evolutionary diversification of vascular cells. Ion-mediated changes in xylem hydraulic resistance are commonly quantified as the percentile change in hydraulic resistance, relative to the hydraulic resistance measured using a reference fluid, usually (ultra) pure deionized water. In this research the impact was investigated of the complete absence of all ions in deionized water compared with reference fluids containing a minimal amount of free calcium on the quantification of ion-mediated flow changes in stem segments of Chrysanthemum (Dendranthemaxgrandiflorum Tzvelev) and Prunus L. (Prunus laurocerasus L.). The addition of 10 mM KCl to deionized water significantly increased flow rate in Chrysanthemum (17-24%) and Prunus L. (16%). The addition of 1 mM CaCl(2) to the reference fluid reduced this KCl-mediated increase in flow rate to 1-2% in both species. 1 mM Ca(2+) is within the lower range of Ca(2+)-concentrations normally measured in xylem sap of many plant species, and three times lower than the original Ca(2+)-concentration measured in the xylem sap of Chrysanthemum plants used for the present measurements. The present results indicate that the complete removal of cations from the xylem fluid with deionized water causes the major part of the ion-mediated flow change previously reported in the xylem of plants. It is concluded that the use of deionized water as a reference fluid should be avoided. Earlier proposed relationships between ion-mediated changes and water flow in xylem of plants should be re-evaluated if they were based on deionized water as the reference fluid.

Botany↗
PubMed2000

Xylem conductivity and vulnerability to cavitation of ponderosa pine growing in contrasting climates.

We examined the effects of increased transpiration demand on xylem hydraulic conductivity and vulnerability to cavitation of mature ponderosa pine (Pinus ponderosa Laws.) by comparing trees growing in contrasting climates. Previous studies determined that trees growing in warm and dry sites (desert) had half the leaf/sapwood area ratio (A(L)/A(S)) and more than twice the transpiration rate of trees growing in cool and moist sites (montane). We predicted that high transpiration rates would be associated with increased specific hydraulic conductivity (K(S)) and increased resistance to xylem cavitation. Desert trees had 19% higher K(S) than montane trees, primarily because of larger tracheid lumen diameters. Predawn water potential and water potential differences between the soil and the shoot were similar for desert and montane trees, suggesting that differences in tracheid anatomy, and therefore K(S), were caused primarily by temperature and evaporative demand, rather than soil drought. Vulnerability to xylem cavitation did not differ between desert and montane populations. A 50% loss in hydraulic conductivity occurred at water potentials between -2.61 and -2.65 MPa, and vulnerability to xylem cavitation did not vary with stem size. Minimum xylem tensions of desert and montane trees did not drop below -2.05 MPa. Foliage turgor loss point did not differ between climate groups and corresponded to mean minimum xylem tensions in the field. In addition to low A(L)/A(S), high K(S) in desert trees may provide a way to increase tree hydraulic conductivity in response to high evaporative demand and prevent xylem tensions from reaching values that cause catastrophic cavitation. In ponderosa pine, the flexible responses of A(L)/A(S) and K(S) to climate may preclude the existence of significant intraspecific variation in the vulnerability of xylem to cavitation.

Climate↗
PubMed2000

Effects of xylem cavitation and freezing injury on dieback of yellow birch (Betula alleghaniensis) in relation to a simulated winter thaw.

Shoot dieback, shoot growth, stem xylem cavitation, stem and root freezing injury, and root pressure were measured in 2-year-old, cold-hardened, potted yellow birch (Betula alleghaniensis Britt.) seedlings that had been subjected to a simulated winter thaw for 0, 5, 10, 19 or 27 days followed by 10 weeks at -10 degrees C. Stem xylem cavitation was determined as percent loss of hydraulic conductivity. Stem freezing injury was measured as electrolyte leakage (EL). Root freezing injury was determined by EL and by triphenyl tetrazolium chloride (TTC) reduction. Thaw duration was significantly correlated with dieback, new shoot growth, stem xylem cavitation, stem and root freezing damage, and root pressure (P < 0.05). In particular, shoot dieback was positively correlated with stem xylem cavitation (P < 0.001), residual stem xylem cavitation (P < 0.01) and root freezing injury (P < 0.010), but only weakly correlated with stem freezing damage (P < 0.05). In roots, freezing damage was negatively correlated with root pressure (P < 0.05), which, in turn, was negatively correlated with residual stem xylem cavitation after root pressure development. In stems, there was no correlation between freezing damage and xylem cavitation. We conclude that long periods of winter thaw followed by freezing resulted in freezing injury to roots concomitant with a reduction in root pressures, leading to poor recovery from freezing-induced xylem embolism.

Journal Article↗
PubMed2003

Xylem wall collapse in water-stressed pine needles.

Wall reinforcement in xylem conduits is thought to prevent wall implosion by negative pressures, but direct observations of xylem geometry during water stress are still largely lacking. In this study, we have analyzed the changes in xylem geometry during water stress in needles of four pine species (Pinus spp.). Dehydrated needles were frozen with liquid nitrogen, and xylem cross sections were observed, still frozen, with a cryo-scanning electron microscope and an epifluorescent microscope. Decrease in xylem pressure during drought provoked a progressive collapse of tracheids below a specific threshold pressure (P(collapse)) that correlates with the onset of cavitation in the stems. P(collapse) was more negative for species with smaller tracheid diameter and thicker walls, suggesting a tradeoff between xylem efficiency, xylem vulnerability to collapse, and the cost of wall stiffening. Upon severe dehydration, tracheid walls were completely collapsed, but lumens still appeared filled with sap. When dehydration proceeded further, tracheids embolized and walls relaxed. Wall collapse in dehydrated needles was rapidly reversed upon rehydration. We discuss the implications of this novel hydraulic trait on the xylem function and on the understanding of pine water relations.

Biological Transport, Active↗
PubMed1999

Direct measurement of xylem pressure in leaves of intact maize plants. A test of the cohesion-tension theory taking hydraulic architecture into consideration

The water relations of maize (Zea mays L. cv Helix) were documented in terms of hydraulic architecture and xylem pressure. A high-pressure flowmeter was used to characterize the hydraulic resistances of the root, stalk, and leaves. Xylem pressure measurements were made with a Scholander-Hammel pressure bomb and with a cell pressure probe. Evaporation rates were measured by gas exchange and by gravimetric measurements. Xylem pressure was altered by changing the light intensity, by controlling irrigation, or by gas pressure applied to the soil mass (using a root pressure bomb). Xylem pressure measured by the cell pressure probe and by the pressure bomb agreed over the entire measured range of 0 to -0.7 MPa. Experiments were consistent with the cohesion-tension theory. Xylem pressure changed rapidly and reversibly with changes in light intensity and root-bomb pressure. Increasing the root-bomb pressure increased the evaporation rate slightly when xylem pressure was negative and increased water flow rate through the shoots dramatically when xylem pressure was positive and guttation was observed. The hydraulic architecture model could predict all observed changes in water flow rate and xylem. We measured the cavitation threshold for oil- and water-filled pressure probes and provide some suggestions for improvement.

Journal Article↗
PubMed2000

Effects of iron deficiency on the composition of the leaf apoplastic fluid and xylem sap in sugar beet. Implications for iron and carbon transport.

The effects of iron deficiency on the composition of the xylem sap and leaf apoplastic fluid have been characterized in sugar beet (Beta vulgaris Monohil hybrid). pH was estimated from direct measurements in apoplastic fluid and xylem sap obtained by centrifugation and by fluorescence of leaves incubated with 5-carboxyfluorescein and fluorescein isothiocyanate-dextran. Iron deficiency caused a slight decrease in the pH of the leaf apoplast (from 6.3 down to 5.9) and xylem sap (from 6.0 down to 5.7) of sugar beet. Major organic acids found in leaf apoplastic fluid and xylem sap were malate and citrate. Total organic acid concentration in control plants was 4.3 mM in apoplastic fluid and 9.4 mM in xylem sap and increased to 12.2 and 50.4 mM, respectively, in iron-deficient plants. Inorganic cation and anion concentrations also changed with iron deficiency both in apoplastic fluid and xylem sap. Iron decreased with iron deficiency from 5.5 to 2.5 microM in apoplastic fluid and xylem sap. Major predicted iron species in both compartments were [FeCitOH](-1) in the controls and [FeCit(2)](-3) in the iron-deficient plants. Data suggest the existence of an influx of organic acids from the roots to the leaves via xylem, probably associated to an anaplerotic carbon dioxide fixation by roots.

Amino Acids↗
PubMed1966

Polarity of IAA Effect on Sieve-Tube and Xylem Regeneration in Coleus and Tomato Stems.

A technique is described for the processing of regenerated xylem and sieve tubes from the same wound area for microscopic and quantitative comparison.Regeneration was examined in internodes of 2 developmental stages in Coleus: internode 2, elongating, characteristic of primary growth; and internode 5, non-elongating, characteristic of secondary growth.Transport of indoleacetic acid (IAA) in excised number 5 internodes of Coleus is strictly polar, in a basipetal direction, judging by a regeneration bioassay involving both sieve tube strands and xylem cells. Similar results were obtained with tomato.If isolated number 5 Coleus internodes are not treated with hormone, they regenerate no xylem cells and a small number of sieve tube strands. With increasing concentrations of IAA added apically, the number of regenerated sieve tube strands (and, with higher concentrations, of xylem cells) increases progressively up to 1% IAA, the highest concentration applied.Internode 2 of Coleus regenerates fewer xylem cells or sieve tube strands than internode 5, whether on the otherwise intact plant or with a given concentration of IAA added apically. The amount of regenerated xylem increases with added apical IAA, except that the highest concentration gives no further increase. The number of xylem cells regenerated in intact plants occurs at the same interpolated IAA concentration as in number 5 internodes. No concentration of IAA tried provided replacement of intact number of sieve tube strands in internode 2.IAA can exert a regenerative stimulus on both xylem and sieve tubes in the area immediately adjacent to the site of its application.

Journal Article↗
PubMed1979

Kinetics of l-Alanine Escape from Xylem Vessels.

Labeled ((3)H or (14)C) l-alanine was perfused through the xylem vessels of isolated tomato internodes (Lycopersicon esculentum cv. Moneymaker) at various concentrations (10(-6) molar to 10(-2) molar). At each concentration the escape of l-alanine from the xylem vessels was apparently a first order process, which is in agreement with Horwitz' (1958, Plant Physiology 33:81-93) model for irreversible escape from the xylem vessels. The escape constant (K) decreased at higher concentrations of l-alanine, which implies that Horwitz' model is inappropriate to describe the kinetics of l-alanine escape, and that the escape at least partly is a saturable process. To obtain data that relate the concentration of l-alanine in the xylem vessels and the escape rate of the amino acid, average escape rates per internode were measured and the corresponding concentrations were calculated from the integrated form of the Michaelis-Menten equation.AS THE CONCENTRATION DEPENDENCE OF THE ESCAPE RATE WAS BIPHASIC, THREE POSSIBLE MECHANISMS WERE CONSIDERED, ESCAPE BEING CAUSED BY: (a) saturable amino acid uptake of cells around the xylem vessels and diffusion into the free space; (b) saturable uptake of the cells around the xylem vessels, but at higher amino acid concentrations in the xylem vessels the number of cells, that participate in the uptake, increases; (c) two, simultaneously operating, saturable uptake systems in the cells around the xylem vessels.

Journal Article↗
PubMed1980

Nitrogen Nutrition and Xylem Transport of Nitrogen in Ureide-producing Grain Legumes.

Xylem sap composition was examined in nodulated and nonnodulated cowpea (Vigna unguiculata [L.] Walp.) plants receiving a range of levels of NO(3) and in eight other ureide-forming legumes utilizing NO(3) or N(2) as sole source of nitrogen. A (15)N dilution technique determined the proportions of plant nitrogen derived from N(2) in the nodulated cowpeas fed NO(3). Xylem sap composition of NO(3)-fed, nodulated cowpea varied predictably with the relative extents to which N(2) and NO(3) were being utilized. The ratios of asparagine to glutamine (N/N) and of NO(3) to ureide (N/N) in xylem sap increased with increasing dependence on NO(3) whereas per cent of xylem nitrogen as ureide and the ratio of ureide plus glutamine to asparagine plus NO(3) (N/N) in xylem sap increased with increasing dependence on N(2) fixation. The amounts of NO(3) and ureides stored in leaflets, stems plus petioles, and roots of cowpea varied in a complex manner with level of NO(3) and the presence or absence of N(2) fixation. All species showed higher proportions of organic nitrogen as ureide and several-fold lower ratios of asparagine to glutamine in their xylem sap when relying on N(2) than when utilizing NO(3). In nodulated (minus nitrate) cowpea and mung bean (Vigna radiata [L.] Wilczek) the percentage of xylem nitrogen as ureide remained constant during growth but the ratio of asparagine to glutamine varied considerably. The biochemical significance of the above differences in xylem sap composition was discussed.

Journal Article↗
PubMed1986

Nitrogen Nutrition and Xylem Sap Composition of Peanut (Arachis hypogaea L. cv Virginia Bunch).

The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanut (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methyleneglutamine, were identified as the major components of xylem exudate collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4-methyleneglutamine. Pulse-feeding nodulated roots with (15)N(2) gas showed asparagine to be the principal nitrogen product exported from N(2)-fixing nodules. Maintaining root systems in an N(2)-deficient (argon:oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-Methyleneglutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methyleneglutamine in xylem exudate did not appear to be associated with either N(2) fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from study of changes in amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of (15)N-nitrate, and a (15)N dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N(2) or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagine:nitrate fell, while the ratio of nitrate:total amino acid rose as plants derived less of their organic nitrogen from N(2). Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N(2) and nitrate are being used in peanuts.

Journal Article↗
PubMed2003

The role of free histidine in xylem loading of nickel in Alyssum lesbiacum and Brassica juncea.

Exposure of the hyperaccumulator Alyssum lesbiacum to nickel (Ni) is known to result in a dose-dependent increase in xylem sap concentrations of Ni and the chelator free histidine (His). Addition of equimolar concentrations of exogenous L-His to an Ni-amended hydroponic rooting medium enhances Ni flux into the xylem in the nonaccumulator Alyssum montanum, and, as reported here, in Brassica juncea L. cv Vitasso. In B. juncea, reducing the entry of L-His into the root by supplying D-His instead of L-His, or L-His in the presence of a 10-fold excess of L-alanine, did not affect root Ni uptake, but reduced Ni release into the xylem. Compared with B. juncea, root His concentrations were constitutively about 4.4-fold higher in A. lesbiacum, and did not increase within 9 h of exposure to Ni. Cycloheximide did not affect root His or Ni concentrations, but strongly decreased the release of His and Ni from the root into the xylem of A. lesbiacum, whereas xylem sap concentrations of Ca and Mg remained unaffected. Near-quantitative chelation of Ni with nitrilotriacetate in the rooting medium did not enhance Ni flux into the xylem of A. lesbiacum and B. juncea, suggesting the absence of a significant apoplastic pathway for Ni entry into the xylem. The data suggest that in B. juncea roots, Ni(2+) uptake is independent of simultaneous uptake of His. In both species, enhanced release of Ni into the xylem is associated with concurrent release of His from an increased root free His pool.

Biological Transport↗
PubMed2006

Organic substances in xylem sap delivered to above-ground organs by the roots.

Squash (Cucurbita maxima) xylem sap, an apoplastic fluid, contains t-zeatin riboside, glutamine, methylglycine, myo-inositol, fructose, oligosaccharides of arabinogalactan, glucan, galacturonan, and pectins (rhamnogalacturonan-I and rhamnogalacturonan-II), as well as various proteins, including arabinogalactan and pathogen-related proteins. These substances are mainly produced in stele (xylem) parenchyma and the pericycle in the root-hair zone where ion transporter genes are expressed. Glycine-rich protein genes (CRGRPs) cloned by antiserum raised against whole xylem sap of cucumber (Cucumis sativus) were abundantly expressed in the parenchyma cells surrounding xylem vessels in the root-hair zone. CRGRP proteins accumulated and immobilized in the lignified walls of metaxylem vessels and perivascular fibers in shoots, suggesting a systemic delivery mechanism of wall materials via xylem sap. A major 30-kDa protein (XSP30) found in cucumber xylem sap was homologous to the B chains of a lectin (ricin) and bound to a nonfucosylated core N-acetylglucosamine dimer of N-linked glycoproteins abundant in leaf parenchyma cells. XSP30 gene expression, abundant in root xylem parenchyma and pericycle, and the level of XSP30 protein fluctuated diurnally under the control of a circadian clock, and the amplitude was up-regulated by gibberellic acid produced in young leaves, suggesting a long-distance control system between organs.

Cell Wall↗
PubMed2001

Haustoria in action: case studies of nitrogen acquisition by woody xylem-tapping hemiparasites from their hosts.

This review discusses studies conducted by the author and his colleagues on mistletoes and root hemiparasites native to Western Australia. Morphological characteristics of haustoria are described and their anatomical features are discussed in relation to uptake, transfer, and metabolism of xylem-borne nitrogenous solutes derived from a host. Experimental approaches used include comparisons of xylem sap composition of parasite and host(s), solute pool analyses and enzymatic properties of haustoria, host xylem feeding of 15N-labelled solutes to follow the fate of label in haustoria and body of the parasite, and studies using species-specific nonprotein amino acids to validate successful uptake from hosts or occasional backflow of xylem-borne solutes to a host. Field studies on promiscuous root hemiparasites assess frequencies of exploitation of different hosts. 15N natural abundance assays of host and parasite dry matter demonstrate marked preference by Santalum acuminatum for N2-fixing as opposed to nonfixing hosts. The ability of Olax phyllanthi to continue to exploit deep-rooted hosts ranks of importance when xylem water potentials of other hosts go out of sucking range during periods of water stress. Comparisons of xylem sap composition of parasites feeding on different hosts indicate remarkable versatility by haustoria in uptake and utilization of the different major nitrogenous solutes received from these hosts. Solute pools in parasites partly reflect metabolic transformations accentuated by haustoria while also indicating direct throughput from xylem of a host. The review concludes by showing how empirically based modelling techniques can be used to estimate proportional gains of N by parasites from single hosts and repercussions on host growth which accompany such exploitation.

Acacia↗
PubMed2002

Xylem embolism and drought-induced stomatal closure in maize.

Water relations during drought and xylem vulnerability to embolism were studied on four maize ( Zea mays L.) genotypes having contrasting grain yields under drought conditions. Drought provoked a drop in xylem pressure, leaf water potential and whole-plant transpiration. Transpiration was reduced to a minimum value when xylem pressures reached ca. -1.6 MPa. This value corresponded to the threshold xylem pressure below which xylem embolism developed to a substantial degree in leaf midribs. Therefore, xylem embolism always remained low in leaf veins, even when plants exhibited clear water-stress symptoms. This suggests that stomatal closure during drought contains xylem embolism to a minimum value. Cavitation resistance was not related to grain yield under drought conditions for the four genotypes evaluated. However, it can be speculated that an increase in cavitation resistance by cultural practices or genetic selection may increase drought survival in maize.

Acclimatization↗
PubMed2006

Characterization of the maize xylem sap proteome.

The xylem in plants has mainly been described as a conduit for water and minerals, but emerging evidence also indicates that the xylem contains protein. To study the proteins in xylem sap, we characterized the identity and composition of the maize xylem sap proteome. The composition of the xylem sap proteome in maize revealed proteins related to different phases of xylem differentiation including cell wall metabolism, secondary cell wall synthesis, and programmed cell death. Many proteins were found to be present as multiple isoforms and some of these isoforms are glycosylated. Proteins involved in defense mechanisms were also present in xylem sap and the sap proteins were shown to have antifungal activity in bioassays.

Antifungal Agents↗
PubMed2002

In situ characterization of a NO-sensitive peroxidase in the lignifying xylem of Zinnia elegans.

The lignifying xylem from Zinnia elegans stems gives an intense reaction with 3,3',5,5'-tetramethylbenzidine (TMB), a reagent previously reported to be specific for peroxidase/H2O2. However, the staining of lignifying xylem cells with TMB is apparently the result of two independent mechanisms: one, the catalase-sensitive (H2O2-dependent) peroxidase-mediated oxidation of TMB, and the other, the catalase-insensitive oxidation of TMB, probably mediated by xylem oxidases which are specific from lignifying tissues. The catalase-insensitive oxidation of TMB by the Z. elegans xylem was sensitive to sodium nitroprusside (SNP), a nitric oxide (NO)-releasing compound that, when used at 5.0 mM, is capable of sustaining NO concentrations of 6.1 &mgr;M in the aqueous phase. This effect of SNP was totally reversed by 150 &mgr;M 2-phenyl-4,4,5,5-tetramethyl imidazoline-1-oxyl-3-oxide (PTIO), an efficient NO scavenger in biological systems, so the above-mentioned effect must be ascribed to NO, and not to other nitrogen oxides. This response of the catalase-insensitive TMB-oxidase activity of the lignifying Z. elegans xylem was similar to that shown by a basic peroxidase isolated from the intercellular washing fluid, which showed TMB-oxidase activity, and which was also inhibited by 5 mM SNP, the effect of SNP also being reversed by 150 &mgr;M PTIO. These results suggest that peroxidase was the enzyme responsible for the NO-sensitive catalase-insensitive TMB-oxidase activity of the lignifying Z. elegans xylem. Further support for this statement was obtained from competitive inhibitor-dissected histochemistry, which showed that this stain responded to peroxidase-selective competitive inhibitors, such as ferulic acid and ferrocyanide, in a similar way to the Z. elegans basic peroxidase. From these results, we conclude that this NO-sensitive catalase-insensitive oxidation of TMB is apparently performed by the Z. elegans basic peroxidase, and that the regulation of this enzyme by NO may constitute an intrinsically programmed event during the differentiation and death of the xylem.

Journal Article↗
PubMed2002

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

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

Abscisic Acid↗