Xylem Function Research Articles

The role of waterlogging andPhytophthora cinnamomi in the decline and death ofEucalyptus marginata in Western Australia

About 14% of the jarrah (Eucalyptus marginata) forest in Western Australia is infested with the soil-borne pathogenic fungusPhytophthora cinnamomi. P. cinnamomi can infect jarrah roots causing cankers in the phloem. It also modified forest sites, however, by killing many understorey species. Jarrah trees die in greater numbers on poorly drained, infested sites following exceptionally heavy rainfall. Experiments have shown that when jarrah seedlings are waterlogged the number of functional xylem vessels in the tap root is reduced; as transpiration continues, the plants wilt and die. In anoxic conditionsP. cinnamomi will not sporulate, but zoospores will germinate and grow. Infection of jarrah roots by zoospores can be increased by soil saturation. Thus the drainage characteristics of a site will affect both the ability of jarrah to grow on that site as well as its infection byP. cinnamomi.

Water relations of cut greenhouse roses: The relationships between stem water potential, hydraulic conductance and cavitation

The stems of cut greenhouse roses, Rosa hybrida ‘Samantha’, were fully hydrated and then allowed to dehydrate while measurements were made of water loss, stem water potential, hydraulic conductance and the onset and extent of xylem cavitation (i.e. disruption of xylem water continuity). The inter-relationships of these parameters were followed through the full range of plant water status. Xylem cavitation was initiated at stem water potentials of −0.2 to −0.4 MPa. Rates of cavitation increased as water potential declined. This was associated with a corresponding decline in stem hydraulic conductance. The data indicated that a loss of about 50% of the stem water content at full hydration resulted in a total loss of water-conducting capacity. This corresponded with a stem water potential of about −4.0 MPa. The results indicate that the shelf-life of a cut flower may, in many instances, be relative to the proportion of functional xylem remaining in the stem.

Influence of Phony Disease of Peach on Stem Hydraulic Conductivity and Leaf Xylem Pressure Potential

Abstract Stem hydraulic conductivity of peach [Prunus persica (L.) Batsch] was lower in trees with phony disease than in healthy trees. This lower conductivity occurred in 1- to 4-year stems, in five cultivars, in two pruning systems, and from June through October. Leaf xylem pressure potential was lower in trees with phony disease than in healthy trees in each of the five cultivars tested and from June through September. The reduction in pressure potential in diseased trees exceeded any variations in pressure potential with cultivar or month. The area of functional xylem stained by dye was visibly smaller in stems from diseased trees than in healthy trees. These results were all consistent with the theory that symptoms of phony disease of peach are caused by xylem blockage.

Pathological anatomy of black stain root disease of Douglas-fir

Twenty, black stain root disease infected Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) from the Oregon Cascade and Coast ranges were dissected; macroscopic and microscopic patterns of fungal colonization are described. Ceratocystis wageneri Goheen et Cobb colonized secondary xylem, disrupting xylem transport. Histopathological studies revealed that the tracheid-limited pattern of colonization exhibited by this fungus is unique among Ceratocystis wilt pathogens. Axial colonization by hyphae was confined to xylem tracheids and radial development of mycelia in ray tracheids was limited. Parenchyma and cambial necroses did not result from direct hyphal attack. Living host cells, although not invaded, were responsive to hyphal attack of adjacent functional xylem. Secondary phloem was never invaded by hyphae, but some dysfunction was evident. Host responses to infection were unsuccessful in curtailing the spread of the pathogen.

Elm leaf scorch: abnormal physiology in American elms infected with fastidious, xylem-inhabiting bacteria

Stem elongation, leaf area, xylem function, water relations, and stem starch reserves were compared between leaf scorch affected American elms (Ulmusamericana L.) infected with fastidious, xylem-inhabiting bacteria and symptomless trees. Stem elongation was significantly lower in elm leaf scorch affected trees than in symptomless trees, while no differences occurred in leaf area. Stem hydraulic conductivity in affected trees decreased from June through September, but increased during the same period in symptomless trees. Dye infiltration rates and number of functional vessels in stems collected from leaf scorch affected trees in September were significantly lower than in symptomless trees. Changes in stem water potential paralleled xylem infiltration rates. No differences in any measured parameters occurred between elm leaf scorch affected and symptomless trees in June prior to symptom onset. Once symptoms developed, stem water potential was more negative in affected trees than in symptomless trees. Mean stem starch levels were significantly higher in symptomless trees than in symptomatic trees. Results confirm that water deficit in leaf scorch affected trees may be attributed to xylem dysfunction possibly caused by bacterial occlusion. Growth reduction can be associated with decreased starch reserves induced by reduced water translocation and reduced effective photosynthetic area caused by the foliar necrosis of elm leaf scorch.

Rate of acropetal transport of the xylem stream in phaseolus vulgaris

Abstract Hate of acropetal movement of the xylem stream was measured in the petioles, epicotyls, and hypocotyls of bean plants. Loss of water by intact, transpiring plants was first determined gravimetrically. Next, volume of the functional xylem in various sections of the stems was determined using radioisotope dilution techniques. Rate at which the xylem stream renewed itself to account for the overall water loss of the plant served as the basis for flow rate values.

Relations between water content, potential and permeability in stems of conifers.

Abstract. The influence of sapwood water content on the conductivity of sapwood to water was measured on stem sections of Pinus contorta. A reduction in relative water content from 100 to 90% caused permeability to fall to about 10% of the saturated value. Pressure–volume curves of branchwood and stem sapwood of Pinus contorta and Picea sitchensis have been analysed to definè the tissue capacitance and the time constant and resistance for water movement between stored water and the functional xylem as functions of tissue water potential. Three phases in water loss were discernible. In the initial phase at high water potentials (> –0.5 MPa), the capacitance was large, the time constant long and the resistance to flow large in comparison with intermediate water potentials (−0.5 to −1.5 MPa). At still lower water potentials (−1.5 to −3.0 MPa), the time constant and resistance declined still further but the capacitance had a tendency to increase again, especially in the stemwood of Sitka spruce. Typical values in the second phase were for the time constant 5 s, for the resistance 4 × 10−13 N s m−5 and for the capacitance (change in relative water content per unit change in potential) 1×10−11 m3 Pa−1. These parameters define the availability of stored water and are being used in a dynamic model of water transport in trees.

Ethephon-Induced Gummosis in Sour Cherry (Prunus cerasus L.)

Ethephon, (2-chloroethyl)phosphonic acid, was sprayed at concentrations up to 69.2 millimolar to enhance gum formation in 1-year-old shoots of mature Prunus cerasus L. cv Montmorency trees. Gum accumulation caused rupturing of the shoot periderm, followed by gum extrusion. Lower ethephon concentrations were required to induce gum formation in spring and early summer (1.7-3.5 millimolar) then in late summer and fall (13.8-69.2 millimolar). The number of functional vessels, shoot hydraulic conductance, and water potential of both leaf and internode tissue decreased as gum content of shoots increased. Nontreated control shoots also contained small quantities of gum. There was no difference in neutral sugar composition of gum exuded by the tree, obtained from aqueous shoot extracts, or flushed from the vessels of shoots, whether induced by ethephon or not. Severe decrease in shoot and leaf water potential was associated with shoot die-back. Recovery of xylem function may occur where gummosis is less severe. Discrepancy between measured and predicted hydraulic conductance increased as shoot gum content increased, suggesting that decrease in number of functional vessels alone was not sufficient to explain the effects of gum on loss of shoot hydraulic conductance. Increased gum content in those vessels remaining functional would increase vessel sap viscosity and further reduce hydraulic conductance. The viscosities necessary to account for discrepancy between measured and predicted hydraulic conductance were calculated. Gum concentration less than 1.0% (w/v) would produce these viscosities.

Apical Dominance is not Due to a Lack of Functional Xylem and Phloem in Inhibited Buds

Apical Dominance is not Due to a Lack of Functional Xylem and Phloem in Inhibited Buds

Lateral Movement of Water and Sugar Across Xylem in Sugarcane Stalks

Laterally connected vascular bundles in the nodes of sugarcane (Saccharum species cv. Pindar) stalks allow a rapid redistribution of water across the stalk should the vascular continuity be partly disrupted. Tritiated water supplied to the roots exchanged rapidly between the xylem and storage tissue so that net movement up the stalk was slow. The half-time for exchange in a labeled stalk was about 4 hours so that the entire water content of a sugarcane stalk can turn over at least once in a single day. No rapid flux of sugar between xylem and phloem or xylem and storage tissue was detected. Functional xylem contained only low sugar concentrations: less than 0.3% w/v in the stalk and less than 0.02% w/v in the leaf. Previous reports of high sugar levels (9% w/v) in sugarcane stalk xylem reflect some degree of xylem blockage followed by a slow equilibration with free space sugars in the storage tissue.

Pathways of auxin transport in the intact pea seedling (Pisum sativum L.)

When small colonies of the pea aphid [Acyrthosiphon pisum (Harris)] were established on the stem of Meteor Dwarf Pea seedlings (Pisum sativum L.), (14)C was found in the honeydew 4.5 h after applying IAA-1-(14)C to a fully-expanded foliage leaf. In contrast, no activity was found in the honeydew or aphids 4.5 h after the application of IAA-1-(14)C to the intact apical bud even though the internode upon which the aphids were feeding contained high levels of (14)C. The lack of radio-activity in aphids feeding on stems to which IAA-1-(14)C was applied via the apical bud was found not to be influenced by the internode position or by the transport interval allowed (up to 24 h).Radioactivity derived from either foliar or apical applications of IAA-1-(14)C was not transported through stem tissues killed by heat treatment. Xylem function was shown not to be impared by the heat treatment employed.It was concluded that the long-distance transport of IAA from the apical bud of intact pea seedlings does not take place in the phloem sieve tubes involved in the transport of metabolites from foliage leaves, or in the non-living tissues of the xylem.

Anatomical factors in resistance to Dutch elm disease

The localizing of pathogen (Ceratocystis ulmi) establishment in Dutch elm disease-resistant hosts is hypothesized as the major mechanism of resistance to this disease. Four factors are proposed that, singly or in all possible combinations, could regulate this general mechanism. We tested one of these factors, vessel size and number of contiguous vessels in the functional xylem at the point of natural inoculation. Using 23 selections from the Ulmaceae, we examined a constant area of second-year xylem of 2-year-old twigs. These anatomical data indicate that a correlation exists between increasing disease susceptibility and increasing vessel group size (the product of average vessel diameter and average number of contiguous vessels). Other evidence suggests that lateral pathogen movement is at first confined to the vessel group or groups of initial inoculation. Therefore, in a given limited time period, tylosis production could more effectively block vertical pathogen movement in vessel groups of small size. Relationships between vessel group size and the extent of sapwood discoloration are discussed. Vessel size and arrangement also is discussed in relationship to the resistance reported for young, greenhouse-grown trees. The use of vessel group size is suggested in screening programs for disease resistance. Recent research on oak suggests chemical modification of xylem tissue to lessen elm susceptibility.

THE RELATIONSHIP OF XYLEM MATURATION TO THE ABSORPTION AND TRANSLOCATION OF P32

Techniques were developed to supply radioactive phosphorus to specific regions of the roots of corn and to ascertain the extent of functional xylem. The data support the view that all regions of the root are effective in absorbing phosphorus but translocation of the absorbed phosphorus to the shoot was correlated with the presence of functional xylem in the region where phosphorus was supplied.

The Relationship of Xylem Maturation to the Absorption and Translocation of P 32

Techniques were developed to supply radioactive phosphorus to specific regions of the roots of corn and to ascertain the extent of functional xylem. The data support the view that all regions of the root are effective in absorbing phosphorus but translocation of the absorbed phosphorus to the shoot was correlated with the presence of functional xylem in the region where phosphorus was supplied.

Transport of Rubidium Absorbed by the Apex of the Bean Root

Rubidium-86 was applied to the terminal 5.0 mm region of intact roots for a 6 hour period. The plants were then harvested after 0, 18, 24, 48, 72, or 96 hours additional growth in a rubidium-free continuously renewed nutrient solution. A substantial portion of the rubidium absorbed in a region of the root devoid of functional xylem tissue was ultimately transported to more basipetal regions.

Beobachtungen über die Besiedlung von Gletschervorfeldern in den Schweizeralpen

Tree-rings are precious natural archives to assess ecosystem variability over time. Xylem anatomy in woody tissue is a promising source of information in tree-rings since it is closely linked to tree hydraulics and carbon fixation. However, despite the rising interest for cell anatomy in dendrochronology, still little is known about the interpretation of the variability of cell anatomical responses observed across different environments and species.Here we analyze cell anatomical responses to increasing summer drought on 18 trees from 3 conifer species (Picea obovata, Pinus sylvestris, and Larix sibirica) growing in the transition zone between forest and steppe in the Republic of Khakassia (Russia). Analyses include the comparison of tracheid size distributions along climatic gradients and contrasting micro-topography from 1986 to 2008.Results indicate an overall decrease of earlywood tracheid lumen and cell wall thickness to high temperature and drought regardless of species and site conditions. In particular an increase of one degree Celsius during the summer caused up to 5% reduction of earlywood cell lumen and wall thickness. These anatomical shifts suggest that a downscaling of hydraulic efficiency is not paralleled by increased hydraulic safety, presumably due to carbon limitation.Based on the results of this case study, we suggest that increasing drought stress might hamper the formation of a functional xylem structure, thus being a possible trigger for a miss-acclimation causing long-term decline and higher exposure to hydraulic failures. Despite the promising study approach, more studies including more data (trees, years) and broader climatic gradients would be needed to further improve our mechanistic understanding.

Translocation of the Floral Stimulus in Xanthium

1. The floral stimulus in Xanthium pennsylvanicum crossed an inarch contact between an induced donor plant and a receptor plant only when tissue union was established and direct tissue contact was uninterrupted for more than 4 days. 2. The floral stimulus failed to be translocated downward from an induced leaf through dead petiole tissue or through functional xylem cells. 3. The floral stimulus failed to be translocated from an induced donor branch of a two-branched plant to a receptor branch when the bark was removed from the donor branch below the induced leaves or from the receptor branch immediately above the point of branching. 4. The floral stimulus is translocated chiefly in the bark, both up and down the stem.