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At least 469 records · Page 26Linked to original sources
PubMed2005

The pattern of distribution of pectin, peroxidase and lignin in the middle lamella of secondary xylem fibres in alfalfa (Medicago sativa).

BACKGROUNDS AND AIMS: Information on the micro-distribution of lignin within the middle lamella is only just beginning to emerge. This paper provides evidence of marked heterogeneity in the micro-distribution of lignin, pectin, peroxidase and hydrogen peroxide in the middle lamella of alfalfa (Medicago sativa). METHODS: Specimens from alfalfa stems were collected and processed for transmission electron microscopy. The middle lamella architecture was examined prior to and during lignification, using transmission electron microscopy in combination with pectin- and lignin-specific staining. In addition, immuno-gold labelling of peroxidase and cytochemical localization of hydrogen peroxide (H2O2) were undertaken. KEY RESULTS: Lignin showed inhomogeneity in its distribution in the middle lamella. It was found that the distribution of pectin was irregular and corresponded to the pattern of deposited lignin. Additionally, a similarity in the pattern of the deposited lignin to the pattern of distribution of peroxidase and H2O2 was also observed. CONCLUSIONS: Irregular distribution of pectin in the middle lamella may be related to subsequent inhomegeneity in lignin in this region.

Cell Wall↗
PubMed1997

Nutrition, xylem cavitation and drought resistance in hybrid poplar.

Effects of mineral nutrition on susceptibility to cavitation were examined in four hybrid poplar clones. Two drought-sensitive and two drought-resistant hybrid clones of black cottonwood (Populus trichocarpa Torr. & Gray) and eastern cottonwood (P. deltoides Bartr.) were grown at three concentrations of nitrogen (N) applied factorially with two concentrations of phosphorus (P) in a greenhouse, and subjected to varying degrees of drought stress before measurement of cavitation and of anatomical features that might affect cavitation. Mean vessel pit pore diameters were 0.132 micro m at low P, and 0.074 micro m at high P, but no other significant effects of mineral nutrition on vessel dimensions were observed. Vessel diameter and specific conductivity were greater in the drought-resistant clones than in the drought-susceptible clones. Drought-resistant clones did not reach such low water potentials as drought-sensitive clones during the cavitation induction experiments, suggesting better stomatal and cuticular control of water loss. Scanning electron microscope observations showed less damage to pit membranes, also suggesting greater membrane strength in drought-resistant clones than in drought-sensitive clones. High concentrations of N increased cavitation, whereas high concentrations of P decreased cavitation as measured by both hydraulic flow apparatus and dye perfusion techniques. For one test, cavitation was 48% at high N and low P, but only 28% at high N and high P. We consider that N fertilization may make poplars more susceptible to cavitation on dry sites, but P fertilization may reduce this effect.

Journal Article↗
PubMed2002

Influence of natural temperature gradients on measurements of xylem sap flow with thermal dissipation probes. 1. Field observations and possible remedies.

The thermal dissipation method is simple and widely used for measuring sap flow in large stems. As with several other thermal methods, natural temperature gradients are assumed to be negligible in the sapwood being measured. We studied the magnitude and variability of natural temperature gradients in sapwood of Acacia trees growing in the Sahelian zone of Senegal, analyzed their effects on sap flow measurements, and investigated possible solutions. A new measurement approach employing cyclic heating (45 minutes of heating and 15 minutes of cooling; 45/15) was also tested. Three-day measurement sequences that included 1 day without heating, a second day with continuous heating and a third day with cyclic heating were recorded during a 6.5-month period using probes installed at three azimuths in a tree trunk. Natural temperature gradients between the two probes of the sensor unit, spaced 8 to 10 cm vertically, were rarely negligible (i.e., < 0.2 degrees C): they were positive during the night and negative during the day, with an amplitude ranging from 0.3 to 3.5 degrees C depending on trunk azimuth, day and season. These temperature gradients had a direct influence on the signal from the continuously heated sensors, inducing fluctuations in the nighttime reference signal. The resulting errors in sap flow estimates can be greater than 100%. Correction protocols have been proposed in previous studies, but they were unsuitable because of the high spatial and temporal variability of the natural temperature gradients. We found that a measurement signal derived from a noncontinuous heating system could be an attractive solution because it appears to be independent of natural temperature gradients. The magnitude and variability of temperature gradients that we observed were likely exacerbated by the combination of open stand, high solar radiation and low sap flow rate. However, for all applications of the thermal dissipation method, it is wise to check regularly for natural temperature gradients by switching off the heater.

Acacia↗
PubMed2002

Influence of natural temperature gradients on measurements of xylem sap flow with thermal dissipation probes. 2. Advantages and calibration of a noncontinuous heating system.

Natural temperature gradients in stems of trees growing in open stands give rise to errors when measuring sap flow by the continuous thermal dissipation method. Previously, we obtained evidence from field measurements that a noncontinuous thermal dissipation system can prevent these errors (Do and Rocheteau 2002). Cyclic heating (i.e., 45 minutes on and 15 minutes off; 45/15) allowed the derivation of an alternate signal, defined as the difference between the temperature signal at the end of the heating period and the temperature signal at the end of the cooling period. By analogy with the continuous system, we calculated an alternate flow index. Analysis in an artificial flow system confirmed that external temperature gradients have an additive effect on the continuous signal, whereas the alternate signal is unaffected by such gradients. The response of the alternate flow index to flow densities was similar for five combinations of heating and cooling times (45/15, 40/20, 30/30, 15/15 and 10/10 min). The relationship was markedly different from Granier's (1985) calibration because measurements in cyclic systems are made under non-steady-state temperature conditions. We recommend the 15/15 min cycle, which allows two sap flow measurements per hour. We compared flow density estimates obtained from field measurements with the continuous and cyclic systems over 192 days with 1-day lags between systems. Comparisons based on daily maximum values (between 0.5 and 2.5 l dm(-2) h(-1) for the cyclic system) confirmed that differences between the continuous and cyclic systems, which could be greater than 100%, were linked to the effect of temperature gradients on the continuous system. The results demonstrate that, in situations where significant natural temperature gradients (i.e., > 0.2 degrees C) are likely, the cyclic system improves the accuracy of sap flow measurements made with thermal dissipation probes.

Acacia↗
PubMed2003

Xylem vulnerability to cavitation in Pseudotsuga menziesii and Pinus ponderosa from contrasting habitats.

In the Rocky Mountains, ponderosa pine (Pinus ponderosa (ssp.) ponderosa Dougl. ex P. Laws. & C. Laws) often co-occurs with Douglas-fir (Pseudotsuga menziesii var. glauca (Mayr) Franco). Despite previous reports showing higher shoot vulnerability to water-stress-induced cavitation in ponderosa pine, this species extends into drier habitats than Douglas-fir. We examined: (1) whether roots and shoots of ponderosa pine in riparian and slope habitats are more vulnerable to water-stress-induced cavitation than those of Douglas-fir; (2) whether species-specific differences in vulnerability translate into differences in specific conductivity in the field; and (3) whether the ability of ponderosa pine to extend into drier sites is a result of (a) greater plasticity in hydraulic properties or (b) functional or structural adjustments. Roots and shoots of ponderosa pine were significantly more vulnerable to water-stress-induced cavitation (overall mean cavitation pressure, Psi(50%) +/- SE = -3.11 +/- 0.32 MPa for shoots and -0.99 +/- 0.16 MPa for roots) than those of Douglas-fir (Psi(50%) +/- SE = -4.83 +/- 0.40 MPa for shoots and -2.12 +/- 0.35 MPa for roots). However, shoot specific conductivity did not differ between species in the field. For both species, roots were more vulnerable to cavitation than shoots. Overall, changes in vulnerability from riparian to slope habitats were small for both species. Greater declines in stomatal conductance as the summer proceeded, combined with higher allocation to sapwood and greater sapwood water storage, appeared to contribute to the ability of ponderosa pine to thrive in dry habitats despite relatively high vulnerability to water-stress-induced cavitation.

Environment↗
PubMed2004

Xylem sap flow and drought stress of Fraxinus excelsior saplings.

We report on diurnal and seasonal variations in sap flow rate and stem water potential of Fraxinus excelsior L. saplings growing at the edge of a Fraxino-Aceretum forest in western Germany. Because of shallow soil, the trees were subjected to drought in summer. When soil water availability was not limiting, sap flow rate was related to changes in solar radiation and vapor pressure deficit. Maximum transpiration rates per leaf area were 3.5-7.4 mmol m-2 s-1, and maximum daily totals were 1.7-3.3 kg m-2 day-1. Under drought conditions, stem water potential dropped to midday minima of -2.6 to -3.5 MPa and sap flow rate was strongly related to this parameter. After the drought period, reduced apparent (whole-plant) hydraulic conductance was observed, which was attributed to a continued reduction in stomatal conductance after the drought stress had ceased. A model was developed that linked sap flow rate directly to climatic variables and stem water potential. Good correlation between measured and simulated sap flow rates allowed the model to be used for data interpretation.

Circadian Rhythm↗
PubMed2001

Xylem cavitation in the leaf of Prunus laurocerasus and its impact on leaf hydraulics.

This paper reports how water stress correlates with changes in hydraulic conductivity of stems, leaf midrib, and whole leaves of Prunus laurocerasus. Water stress caused cavitation-induced dysfunction in vessels of P. laurocerasus. Cavitation was detected acoustically by counts of ultrasonic acoustic emissions and by the loss of hydraulic conductivity measured by a vacuum chamber method. Stems and midribs were approximately equally vulnerable to cavitations. Although midribs suffered a 70% loss of hydraulic conductance at leaf water potentials of -1.5 MPa, there was less than a 10% loss of hydraulic conductance in whole leaves. Cutting and sealing the midrib 20 mm from the leaf base caused only a 30% loss of conduction of the whole leaf. A high-pressure flow meter was used to measure conductance of whole leaves and as the leaf was progressively cut back from tip to base. These data were fitted to a model of hydraulic conductance of leaves that explained the above results, i.e. redundancy in hydraulic pathways whereby water can flow around embolized regions in the leaf, makes whole leaves relatively insensitive to significant changes in conductance of the midrib. The onset of cavitation events in P. laurocerasus leaves correlated with the onset of stomatal closure as found recently in studies of other species in our laboratory.

Desiccation↗
PubMed1969

Hormonal control of cell proliferation and xylem differentiation in cultured tissues of Glycine max var. Biloxi.

The relationship between tracheary element differentiation, cell proliferation and growth hormones was examined in agar-grown soybean callus. The time course of cell division and tracheary element formation in tissues grown on a medium containing 5 x 10(-7)m kinetin and 10(-5)m NAA was determined by means of maceration technique. After a slight lag period, a logarithmic increase in cell number was observed through the twelfth day of the culture period. Cell numbers increased at a considerably slower rate after the twelfth day. The rate of tracheary element formation varied with the rate of cell proliferation. Tracheary elements increased logarithmically during the log phase of growth. As the rate of cell division decreased after the twelfth day of culture, the rate of tracheary element formation also decreased. In the presence of 10(-5)m NAA, cell number increased as the kinetin concentration was increased between 10(-9) and 10(-6)m. However, tracheary element formation was not initiated unless the kinetin concentration was 5 x 10(-8)m or above. When the Biloxi callus was subcultured repeatedly on media containing 10(-8)m kinetin, a tracheary element-free population of cells was obtained. This undifferentiated tissue produced tracheary elements upon transfer to a medium containing 5 x 10(-7)m kinetin. In the presence of 5 x 10(-7)m kinetin, NAA stimulated cell proliferation between 10(-7) and 10(-5)m, but no tracheary elements were formed without auxin, or with 10(-7)m NAA. Neither NAA nor kinetin at any concentration tested stimulated tracheary element formation in the absence of an effective level of the other hormone. However, 2,4-D at 10(-7) or 10(-6)m promoted both cell proliferation and tracheary element differentiation in the absence of an exogenous cytokinin.

Cell Differentiation↗
PubMed1970

Translocation of iron citrate and phosphorus in xylem exudate of soybean.

Soybean plants, Glycine max (L.) Merrill, in standard solution received 2.5 mum ferric ethylenediamine di(o-hydroxyphenylacetate (FeEDDHA) and 0 to 128 mum phosphorus. Their stem exudates contained: 32 to 52 mum Fe, 120 to 5000 mum P, and 120 to 165 mum citrate. Electrophoresis of exudates with high P caused Fe trailing that precluded identification of any major form of Fe. Exudate with low P gave an anodic band of Fe citrate as the major Fe compound. Phosphate added to exudate in vitro depressed the Fe citrate peak and cause Fe trailing. EDDHA added to exudate in vitro pulled Fe from Fe citrate; citrate then migrated as a slower form and Fe migrated as FeEDDHA. A modified preculture system, involving 2-day renewals of 0.2 mum FeEDDHA with 3.2, 9.6, or 16 mum P and low levels of other ions, controlled pH depression and produced considerable change in citrate and P levels. The exudates contained: 45 to 57 mum Fe, 200 to 925 mum P, and 340 to 1025 mum citrate. The high citrate was from plants grown with low P. The major form of Fe in the exudates was Fe citrate. This is probably the form translocated in the plants.

Journal Article↗
PubMed1971

Effect of iron on the transport of citrate into the xylem of soybeans and tomatoes.

Iron transport in soybeans (Glycine max [L] Merr.) and tomatoes (Lycopersicum esculentum) is controlled by factors that are altered manyfold as the plant experiences an iron stress (deficiency). Depending on their response to an Fe stress, plants in this study are classed (a) Fe-inefficient or (b) Fe-efficient. The Fe-efficient plants transport more Fe and concomitantly more citrate than the Fe-inefficient plants.An available supply of Fe added to the plant root will facilitate the release of citrate to the stem exudate in both soybeans and tomatoes. Also, when the iron supply is decreased by trapping Fe(2+) at the root, Fe transport decreases with a concomitant decrease of citrate in the stem exudate. Factors other than citrate appear to affect movement of Fe from the external solution into root cells where Fe is chelated with citrate and moves thereafter as Fe citrate. This makes some of the citrate that is transported in the stem exudate dependent on the amount of Fe made available at the root.

Journal Article↗