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At least 325 records · Page 18Linked to original sources
PubMed1998

Effects of xylem pH on transpiration from wild-type and flacca tomato leaves. A vital role for abscisic acid in preventing excessive water loss even from well-watered plants

The pH of xylem sap from tomato (Lycopersicon esculentum) plants increased from pH 5.0 to 8.0 as the soil dried. Detached wild-type but not flacca leaves exhibited reduced transpiration rates when the artificial xylem sap (AS) pH was increased. When a well-watered concentration of abscisic acid (0.03 &mgr;m) was provided in the AS, the wild-type transpirational response to pH was restored to flacca leaves. Transpiration from flacca but not from wild-type leaves actually increased in some cases when the pH of the AS was increased from 6.75 to 7.75, demonstrating an absolute requirement for abscisic acid in preventing stomatal opening and excessive water loss from plants growing in many different environments.

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PubMed2000

Subcellular localization of expansin mRNA in xylem cells.

Terminal differentiation of many vascular cells involves cell wall changes. Cells first elongate their primary wall, then lay down a lignified secondary wall, which is often followed by digestion of the primary wall. Expansins are wall proteins that regulate wall changes, but little is known about the specific functions of the many individual expansin isoforms. An in vitro cell culture of synchronously differentiating tracheary elements was used to identify three new expansins and to compare their expression kinetics with the timing of wall changes. The genes encoding these expansins from zinnia (Zinnia elegans), designated ZeExp1, ZeExp2, ZeExp3, are expressed during cell elongation. ZeExp1 and ZeExp2 mRNA decrease at the early stage of secondary wall formation, whereas ZeExp3 does not. In planta, all three ZeExp mRNAs are found predominantly in a single flank of cells adjacent to protoxylem and metaxylem vessels and in cells roughly at the radial position of the fasicular and interfasicular cambium. Furthermore, within these cells, Exp mRNA is localized exclusively either to the apical or basipetal end of cells depending on the expansin gene and organ, providing the first evidence for polar localization of mRNA in plant cells. ZeExp1 and ZeExp3 mRNA are localized at the apical tip, whereas ZeExp2 mRNA is found in the basal tip. These observations indicate that these three expansins are xylem cell specific and possibly involved in the intrusive growth of the primary walls of differentiating xylem cells.

Amino Acid Sequence↗
PubMed2000

Increases in water potential gradient reduce xylem conductivity in whole plants. Evidence from a low-pressure conductivity method.

A new method using hydrostatic suctions (less than 0.02 MPa) was used to measure whole-root conductivity (K(r)) in saplings of two angiosperm pioneer trees (Eucalyptus regnans and Toona australis) and two rainforest conifers (Dacrycarpus dacrydioides and Nageia fleurii). The resultant K(r) was combined with measurements of stem and leaf hydraulic conductivity to calculate whole-plant conductivity and to predict leaf water potential (Psi(l)) during transpiration. At normal soil temperatures there was good agreement between measured and predicted Psi(l) during transpiration in all species. Changes in the soil-to-leaf water potential gradient were produced by root chilling, and in three of the four species, changes in Psi(l) corresponded to those expected by the effect of increased water viscosity on K(r). In one species, however, root chilling produced severe plant wilting and a decline in Psi(l) significantly below the predicted value. In this species Psi(l) decreased to a value close to, or below, the Psi(l) at 50% xylem cavitation. It is concluded that decreased whole-plant conductivity in T. australis resulted from a decrease in xylem conductivity due to stress-induced cavitation.

Algorithms↗
PubMed1967

Freezing of xylem sap without cavitation.

Freezing of stem sections and entire twigs of hemlock (Tsuga canadensis) has been demonstrated to occur without increasing the resistance to the movement of water through the frozen part after rewarming. This was interpreted to mean that freezing did not produce cavitation in the xylem sap even though A) the sap was unquestionably frozen; B) it contained dissolved gases; and C) it was under tension before freezing and after. Freezing stem sections of some other evergreen gymnosperms during the summer again produced no evidence for cavitation of the xylem sap. On the other hand, freezing stem sections of some angiosperms invariably increased the resistance to sap flow leading to wilting and death in a few hours when the sap tension was at normal daytime values at the time of freezing. These results were interpreted to mean that the bordered pits on the tracheids of gymnosperms function to isolate the freezing sap in each tracheid so that the expansion of water upon freezing not only eliminates any existing tension but also develops positive pressure in the sap. Dissolved gases frozen out of solution may then be redissolved under this positive pressure as melting occurs. As the bubbles are reduced in size by this ice pressure developed in an isolated tracheid, further pressure is applied by the surface tension of the water against air. If the bubbles are redissolved or are reduced to sufficient small size by the time the tension returns to the sap as the last ice crystals melt, then the internal pressure from surface tension in any existing small bubbles may exceed the hydrostatic tension of the melted sap and the bubbles cannot expand and will continue to dissolve.

Journal Article↗
PubMed1972

Radial transport of sodium and chloride into tomato root xylem.

Transport of Na and Cl across exuding tomato (Lycopersicon esculentum Mill.) roots was determined as a function of ambient NaCl concentrations in the ranges of both systems 1 and 2. Kinetics of radial transport under steady-state conditions and the effect of dinitrophenol indicate that Na and Cl were transported by two different mechanisms. Sodium was neither accumulated against a concentration gradient nor directly inhibited by dinitrophenol from diffusing into the xylem. Chloride was accumulated in the xylem and its transport was nearly completely blocked by dinitrophenol. A comparison of the radial transport isotherms for Na and Cl for intact and decapitated plants indicates that the separate mechanisms were not unique to excised roots. It is concluded that radial Na transport in tomatoes was facilitated by a passive convective type process with the rate-limiting barrier located at the outer cortical plasmalemma. Chloride transport in both concentration ranges involved, either directly or indirectly, a metabolic mechanism. Absorption and retention of Na in the root tissue was negligible. Chloride was accumulated by the tissue but was unaffected by dinitrophenol.

Journal Article↗
PubMed1973

The mechanism of freezing injury in xylem of winter apple twigs.

In acclimated winter twigs of Haralson apple (Pyrus Malus L.), a lag in temperature during cooling at a constant rate was observed at about -41 C by differential thermal analysis. The temperature at which this low temperature exotherm occurred was essentially unaffected by the cooling rate. During thawing there was no lag in temperature (endotherm) near the temperature at which the low temperature exotherm occurred, but upon subsequent refreezing the exotherm reappeared at a somewhat higher temperature when twigs were rewarmed to at least -5 C before refreezing. These observations indicate that a small fraction of water may remain unfrozen to as low as -42 C after freezing of the bulk water in stems. The low temperature exotherm was not present in twigs freeze-dried to a water content below 8.5% (per unit fresh weight), but it reappeared when twigs were rehydrated to 20% water. When freeze-dried twigs were ground to a fine powder prior to rehydration, no exotherm was observed. Previous work has shown that the low temperature exotherm arises from xylem and pith tissues, and that injury to living cells in these tissues invariably occurs only when twigs are cooled below, but not above the temperature of the low temperature exotherm. This study revealed that the low temperature exotherm resulted from the freezing of a water fraction, that the freezing of this water was independent of the freezing of the bulk water, that the exotherm was associated with some gross structural feature but not the viability of the tissue, and that injury to living cells in the xylem and pith was closely and perhaps causally related to the initial freezing of this water.

Journal Article↗
PubMed1973

The velocities of ion transport into and through the xylem of roots: findings with a two-point application pulse-chase technique.

Excised corn roots, Zea mays, had radioactively labeled solution applied at two points along their length, for 1 minute, and were then kept in dilute, unlabeled nutrient solution. During this "chase" period, exudate was collected at 1-minute intervals, and its content of radioions was determined. Two pulses of label appeared in succession, originating at the points of application near the cut end of the root and farther from the cut end, respectively. Calculation yields the velocity at which the ions moved radially across the root into the xylem (v(r)) and the velocity at which they moved longitudinally within the xylem (v(l)). For Rb(+) labeled with (86)Rb, v(r) was 1.8 and v(l), 35 cm/hr. For Br(-) labeled with (82)Br, v(r) was 1.4 and v(l), 103 cm/hr.

Journal Article↗
PubMed1978

Cation pretreatment effects on nitrate uptake, xylem exudate, and malate levels in wheat seedlings.

Week-old wheat seedlings absorbed at least 40% NO(3) (-) from NaNO(3) when preloaded with K(+) than when preloaded with Na(+) or Ca(2+). Cultures of Triticum vulgare L. cv. Arthur were grown for 5 days on 0.2 mm CaSO(4), pretreated for 48 hours with either 1 mm CaSO(4), K(2)SO(4), or Na(2)SO(4), and then transferred to 1 mm NaNO(3). All solutions contained 0.2 mm CaSO(4). Shoots of K(+)-preloaded plants accumulated three times more NO(3) (-) than shoots of the other two treatments. Initially, the K(+)-preloaded plants contained 10-fold more malate than either Na(+)- or Ca(2+)-preloaded seedlings. During the 48-hour treatment with NaNO(3), malate in both roots and shoots of the K(+)-preloaded seedlings decreased. Seedlings preloaded with K(+) reduced 25% more NO(3) (-) than those preloaded with either Na(+) or Ca(2+). These experiments indicate that K(+) enhanced NO(3) (-) uptake and reduction even though the absorption of K(+) and NO(3) (-) were separated in time. Xylem exudate of K(+)-pretreated plants contained roughly equivalent concentrations of K(+) and NO(3) (-), but exudate from Na(+) and Ca(2+)-pretreated plants contained two to four times more NO(3) (-) than K(+). Therefore K(+) is not an obligatory counterion for NO(3) (-) transport in xylem.

Journal Article↗
PubMed1978

Influence of Varied Phosphorus Supply on Growth and Xylem Sap Cytokinin Level of Sycamore (Platanus occidentalis L.) Seedlings.

Effects of varying levels of phosphorus (P) on sycamore (Platanus occidentalis L.) growth and on the endogenous cytokinin levels in the xylem sap were tested. The seedlings were grown in aerated nutrient solutions in the greenhouse. Photoperiod was held constant at 16 hours. Four P concentrations, 0.02, 0.10, 0.50, and 2 mm, were used. There were no P deficiency symptoms even at the lowest P concentration. The maximum growth occurred in seedlings grown in 0.10 mm P. The greatest dry matter yield as well as stem height, stem diameter, and leaf area were produced at 0.10 or 0.50 mm P. The amount of cytokinin in xylem sap for each harvest during P nutrition followed closely the dry matter values, stem height, stem diameter, and leaf area.

Journal Article↗
PubMed1980

Occurrence of nicotinamide adenine dinucleotide phosphatelinked glyoxylate reductase in nonphotosynthetic xylem tissue of perennials.

Xylem extracts of poplar tree contained glyoxylate reductase specific for NADPH. By isoelectric focusing in the pH ranges 3.5 to 10 or 4 to 6, the enzyme exhibited a single peak of activity at pH 5.4. The enzyme showed essentially no activity toward hydroxypyruvate, pyruvate, or NADH. The reaction was optimal at pH 6.0 in phosphate buffer and the activity profile exhibited a sharp and narrow pH profile with half-maximal velocities at about pH 7.0. The K(m) of the enzyme for glyoxylate was 0.11 millimolar. The xylem tissue of poplar tree exhibited high levels of enzyme activity (30 micromoles per gram dry weight per hour) even in the wintering stage and a slight change in activity occurred in spring and fall at the time when metabolism transition occurs.

Journal Article↗
PubMed1980

Abscisic Acid promotes both volume flow and ion release to the xylem in sunflower roots.

The effect of abscisic acid (ABA) on the exudation process in excised sunflower root was investigated. A promotion of both ion flux and volume exudation rate was observed. Cutting off the external supply of ions to the roots markedly increased the magnitude of the ABA effect. The promotive effect of ABA on exudation rate was extremely rapid (less than 6 minutes) and seemed to be biphasic. Reasons are given for relating the ABA effect to two separate actions, namely: (a) on water permeability; and (b) on the release of ions both from vacuoles to the cytoplasm and from symplasm to the xylem. Kinetin inhibited movement of ions to the xylem and its effect did not appear to be related to that of the ABA.

Journal Article↗
PubMed1980

Organic Acids and Ionic Balance in Xylem Exudate of Wheat during Nitrate or Sulfate Absorption.

Experiments were designed to study the importance of organic acids as counterions for K(+) translocation in the xylem during excess cation uptake. A comparison was made of xylem exudate from wheat seedlings treated 72 hours with either 1.0 millimolar KNO(3) or 0.5 millimolar K(2)SO(4), both in the presence of 0.2 millimolar CaSO(4). Exudation from KNO(3) plants had twice the volume and twice the K(+) and Ca(2+) fluxes or rate of delivery to shoots, as K(2)SO(4) plants. Malate flux was 25% higher in K(2)SO(4) than in KNO(3) exudate. Malate was the principal anion accompanying K(+) or Ca(2+) in K(2)SO(4) treatment, while in the KNO(3) treatment, NO(3) (-) was the principal anion. The contribution of SO(4) (2-) was negligible in both treatments. In a second experiment, exudate was collected every 4 hours during the daytime throughout a 72-hour treatment with KNO(3). Malate was the only anion present in exudate at first, just after the CaSO(4) pretreatment had ended. Malate concentration decreased and NO(3) (-) concentration increased with time and these concentrations were negatively correlated. By 62 hours, NO(3) (-) represented 80% of exudate anions. K(+) and NO(3) (-) concentrations in exudate were strongly correlated with K(+) and NO(3) (-) uptake, respectively. The first 36 hours of absorption from KNO(3) solution resembled the continuous absorption of K(2)SO(4), in that malate was the principal counterion for translocation of K(+).

Journal Article↗
PubMed1982

Transport, metabolism, and redistribution of xylem-borne amino acids in developing pea shoots.

Amino acid metabolism and transport was investigated in the leaves of 3-week-old nonnodulated seedlings of Pisum sativum L. Xylem sap entering the shoot contained nitrate (about 5 millimolar), and amino compounds (11 millimolar) of which 70% was asparagine plus glutamine; aspartate and homoserine were also present. Mature leaves showed stable nitrogen levels and incoming nitrogen was redistributed to growing leaves. Younger leaves, still enclosed in the stipules, showed negligible rates of transpiration, suggesting that most of their nitrogen must arrive in the phloem.(14)C-Labeled amides and amino acids were supplied to detached shoots through the xylem, and metabolism and redistribution were followed over 12 hours in light. Asparagine entering mature leaves was reexported directly to young leaves, with relatively little metabolic conversion. Substantial amounts of glutamine were converted to glutamate, which was exported (with unchanged amide) with little further conversion. The pattern of redistribution was confirmed when (14)C-labeled amino acids were applied directly to the under surface of mature leaves. Labeled compounds were found in the phloem exudate from treated leaves, and the composition resembled the pattern of labeling in the compounds arriving in the young developing leaves.

Journal Article↗
PubMed1984

Regulatory effect of cytokinin on secondary xylem fiber formation in an in vivo system.

The regulatory effect of cytokinin on the formation of secondary xylem fibers was studied in the hypocotyl of young Helianthus annuus L. plants. Positive correlation was found between the kinetin supplied (0.25-0.5 micrograms/gram) to the growth medium and the rate of fiber formation within and between the vascular bundles. Reducing the root originated cytokinin supply, either by root removal or by lowering the transpiration rate, diminished the number of newly formed secondary xylem fibers. This decrease was considerably reversed in the presence of 0.5 microgram/gram kinetin. Early pulse exposure of kinetin had a temporary promoting effect on fiber differentiation at low concentrations and a permanent inhibitory effect at high concentration.

Journal Article↗
PubMed1986

Changes in Cytokinin Concentrations in Xylem Extrudate following Infection of Eucalyptus marginata Donn ex Sm with Phytophthora cinnamomi Rands.

The concentrations of zeatin-type and isopentenyladenine-type cytokinins were reduced in the xylem extrudate collected from seedlings of Eucalyptus species following infection by Phytophthora cinnamomi Rands. The use of an enzyme-linked immunosorbent assay (ELISA) allowed the detection of these cytokinins over the range of 0.3 to 7 picomoles for the isopentenyladenine-type and 1 to 1000 picomoles for the zeatin-type. Isopentenyladenine-type cytokinins occurred in concentrations less than 10% of the zeatin-type, but they could be readily detected and measured. This is the first report of their presence in xylem. The sensitivity of the assay allowed a short collection period (30 minutes) reducing any confusion with trauma-induced changes. Infection of the susceptible species Eucalyptus marginata Donn. ex Sm. resulted in significant reduction of zeatin-type cytokinins within 3 days of infection, and at 14 days postinfection the concentration of both cytokinin types was reduced to 26% of uninoculated controls. No reduction in cytokinins occurred with the field resistant Eucalyptus calophylla R. Br. It is suggested that failure of cytokinin transport from the root system may be responsible for the failure in water transport and symptoms of P. cinnamomi infection observed in infected susceptible eucalypts.

Journal Article↗
PubMed1986

Detection of Xylem Cavitation in Corn under Field Conditions.

We report the detection of cavitation events in corn (Zea mays) plants growing under field conditions in Greeley, CO. To our knowledge this study reports the first successful attempt to monitor continuously for long periods the cavitation events of a crop plant using acoustic detection techniques. Cavitation events occur in corn plants using acoustic detection techniques. Cavitation events occur in corn plants irrigated daily when the xylem pressure potentials fall below about -1.0 megapascals. In unirrigated corn we estimate that approximately half of all vessels cavitate on any one day when xylem pressure potentials fall below about -1.8 megapascals. We postulate that root pressure developed every night in irrigated and unirrigated corn is adequate to rejoin cavitated water columns.

Journal Article↗
PubMed1988

Organic Constituents and Complexation of Nickel(II), Iron(III), Cadmium(II), and plutonium(IV) in Soybean Xylem Exudates.

The xylem exudates of soybean (Glycine max cv Williams), provided with fixed N, were characterized as to their organic constituents and in vivo and in vitro complexation of plutonium, iron, cadmium, and nickel. Ion exchange fractionation of whole exudates into their compound classes (organic acid, neutral, amino acid, and polyphosphate), followed by thinlayer electrophoresis, permitted evaluation of the types of ligands which stabilize each element. The polyvalent elements plutonium(IV) and iron(III) are found primarily as organic acid complexes, while the divalent elements nickel(II) and cadmium(II) are associated primarily with components of the amino acid/peptide fraction. For plutonium and cadmium, it was not possible to fully duplicate complexes formed in vivo by back reaction with whole exudates or individual class fractions, indicating the possible importance of plant induction processes, reaction kinetics, and/or the formation of mixed ligand complexes. The number and distribution of specific iron- and nickel-containing complexes varies with plant age and appears to be related to the relative concentration of organic acids and amino acids/peptides being produced and transported in the xylem as the plant matures.

Journal Article↗
PubMed1991

Effect of Macerase, Oxalic Acid, and EGTA on Deep Supercooling and Pit Membrane Structure of Xylem Parenchyma of Peach.

The object of this study was to determine if calcium cross-linking of pectin in the pit membrane of xylem parenchyma restricts water movement which results in deep supercooling. Current year shoots of ;Loring' peach (Prunus persica) were infiltrated with oxalic acid or EGTA solutions for 24 or 48 hours and then either prepared for ultrastructural analysis or subjected to differential thermal analysis. The effect of 0.25 to 1.0% pectinase (weight/volume) on deep supercooling was also investigated. The use of 5 to 50 millimolar oxalic acid and pectinase resulted in a significant reduction (flattening) of the low temperature exotherm and a distinct swelling and partial degradation of the pit membrane. EGTA (10 millimolar) for 24 or 48 hours shifted the low temperature exotherm to warmer temperatures and effected the outermost layer of the pit membrane. A hypothesis is presented on pectin-mediated regulation of deep supercooling of xylem parenchyma.

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