Do Woody Plants Operate Near the Point of Catastrophic Xylem Dysfunction Caused by Dynamic Water Stress? : Answers from a Model

We discuss the relationship between the dynamically changing tension gradients required to move water rapidly through the xylem conduits of plants and the proportion of conduits lost through embolism as a result of water tension. We consider the implications of this relationship to the water relations of trees. We have compiled quantitative data on the water relations, hydraulic architecture and vulnerability of embolism of four widely different species: Rhizophora mangle, Cassipourea elliptica, Acer saccharum, and Thuja occidentalis. Using these data, we modeled the dynamics of water flow and xylem blockage for these species. The model is specifically focused on the conditions required to generate `runaway embolism,' whereby the blockage of xylem conduits through embolism leads to reduced hydraulic conductance causing increased tension in the remaining vessels and generating more tension in a vicious circle. The model predicted that all species operate near the point of catastrophic xylem failure due to dynamic water stress. The model supports Zimmermann's plant segmentation hypothesis. Zimmermann suggested that plants are designed hydraulically to sacrifice highly vulnerable minor branches and thus improve the water balance of remaining parts. The model results are discussed in terms of the morphology, hydraulic architecture, eco-physiology, and evolution of woody plants.     

Cavitation Events in Thuja occidentalis L.? : Utrasonic Acoustic Emissions from the Sapwood Can Be Measured

Ultrasonic acoustic emissions (AE) in the frequency range of 0.1 to 1 megahertz appear to originate in the sapwood of Thuja occidentalis L. The AE are vibrations of an impulsive nature. The vibrations can be transduced to a voltage waveform and amplified. The vibrations of each AE event begin at a large amplitude which decays over 20 to 100 microseconds. Strong circumstantial evidence indicates that the ultrasonic AE result from cavitation events because: (a) they occur only when the xylem pressure potential Ψ_xp is more negative than a threshold level of about —1 megapascal; (b) the rate of AE events increases as Ψ_xp decreases and when the net rate of water loss increases; (c) the AE can be stopped by raising Ψ_xp above —1 megapascal. Ultrasonic AE have been measured in whole terminal shoots allowed to dry in the laboratory, in isolated pieces of sapwood as they dried in the laboratory, and in whole terminal shoots in a pressure bomb when Ψ_xp was decreased by lowering the gas pressure in the pressure bomb.     

Water-storage capacity ofThuja,Tsuga andAcer stems measured by dehydration isotherms

Water-storage capacity was measured inThuja occidentalis L.,Tsuga canadensis (L.) Carr., andAcer saccharum Marsh. during the dehydration of stem segments 1.5–2.5 cm in diameter. Stem water potential was measured with a temperature-corrected stem hygrometer and cavitations were detected acoustically. Water loss was measured by weight change. Dehydration isotherms consistently displayed three phases. The first phase, from water potential (Ψ) 0 to about −0.2 MPa, had a high capacitance (C>0.4kg water lost· (1 of tissue)⁻¹· MPa⁻¹) and we have attributed this high C to capillary water as defined by Zimmermann (1983, Xylem structure and the ascent of sap, Springer-Verlag). The second phase from Ψ=−0.5 to about −2.0 had the lowest C values (<0.02 kg·l⁻¹·MPa⁻¹) and was accompanied by a few cavitation events. This phase may have been a transition zone between capillary storage and water released by cavitation events as well as water drawn from living cells of the bark. The third phase also had a high C (about 0.07–0.22kg·l⁻¹·MPa⁻¹) and was associated with many cavitation events while Ψ declined below about −2.5 MPa; we presume the high capacitance was the consequence of water released by cavitation events. We discuss the ecological adaptive advantage of these three phases of water-storage in trees. In moist environments, water withdrawn from capillary storage may be an important fraction of transpiration, but may be of little adaptive advantage. For most of the growth season trees draw mainly on elastic storage, but stem elastic storage is less than leaf elastic storage and therefore unlikely to be important. In very dry environments, water relased by cavitation events might be important to the short-term survival of trees.     

Heterogeneity of heparan sulfate proteoglycans synthesized by PYS-2 cells

Antibodies to the basement membrane proteoglycan produced by the EHS tumor were used to immunoprecipitate [35S]sulfate-labeled protoglycans produced by PYS-2 cells. The immunoprecipitated proteoglycans were subsequently fractionated by CsCl density gradient centrifugation and Sepharose CL-4B chromatography. The culture medium contained a low-density proteoglycan eluting from Sepharose CL-4B at Kav = 0.18, containing heparan sulfate side chains of Mr = 35-40,000. The medium also contained a high-density proteoglycan eluting from Sepharose CL-4B at Kav = 0.23, containing heparan sulfate side chains of Mr = 30,000. The corresponding proteoglycans of the cell layer were all smaller than those in the medium. Since the antibodies used to precipitate those proteoglycans were directed against the protein core, this suggests that these proteoglycans share common antigenic features, and may be derived from a common precursor which undergoes modification by the removal of protein segments and a portion of each heparan sulfate chain.     

Drought until death do us part: a case study of the desiccation-tolerance of a tropical moist forest seedling-tree,Licania platypus (Hemsl.) Fritsch

Studies of the desiccation tolerance of 15-month-old Licania platypus (Hemsl.) Fritsch seedlings were performed on potted plants. Pots were watered to field capacity and then dehydrated for 23-46 d to reach various visible wilting stages from slightly-wilted to dead. Root hydraulic conductance, k(r), was measured with a high-pressure flow meter and whole-stem hydraulic conductance, k(ws), was measured by a vacuum chamber method. Leaf punches were harvested for measurement of leaf water potential by a thermocouple psychrometer and for measurement of fresh- and dry-weight. L. platypus was surprisingly desiccation-tolerant, suggesting that most species of central Panama may be well adapted to the seasonality of rainfall in the region. The slightly-wilted stage corresponded to leaf water potentials and relative water contents of -2.7 MPa and 0.85, respectively, but plants did not die until these values fell to -7.5 MPa and 0.14, respectively. As desiccation proceeded k(r) and k(ws) declined relative to irrigated controls, but k(ws) was more sensitive to desiccation than k(r). Values of k(ws) declined by 70-85% in slightly-wilted to dead plants, respectively. By comparison, k(r) showed no significant change in slightly-wilted plants and fell by about 50% in plants having severely-wilted to dead shoots.     

Ultrasonic acoustic emissions from the sapwood of cedar and hemlock : an examination of three hypotheses regarding cavitations

Measurements are reported of ultrasonic acoustic emissions (AEs) measured from sapwood samples of Thuja occidentalis L. and Tsuga canadensis (L.) Carr. during air dehydration. The measurements were undertaken to test the following three hypotheses: (a) Each cavitation event produces one ultrasonic AE. (b) Large tracheids are more likely to cavitate than small tracheids. (c) When stem water potentials are >−0.4 MPa, a significant fraction of the water content of sapwood is held by `capillary forces.' The last two hypotheses were recently discussed at length by M. H. Zimmermann. Experimental evidence consistent with all three hypotheses was obtained. The evidence for each hypothesis respectively is: (a) the cumulative number of AEs nearly equals the number of tracheids in small samples; (b) more water is lost per AE event at the beginning of the dehydration process than at the end, and (c) sapwood samples dehydrated from an initial water potential of 0 MPa lost significantly more water before AEs started than lost by samples dehydrated from an initial water potential of about −0.4 MPa. The extra water held by fully hydrated sapwood samples may have been capillary water as defined by Zimmerman.

We also report an improved method for the measurement of the `intensity' of ultrasonic AEs. Intensity is defined here as the area under the positive spikes of the AE signal (plotted as voltage versus time). This method was applied to produce a frequency histogram of the number of AEs versus intensity. A large fraction of the total number of AEs were of low intensity even in small samples (4 mm diameter by 10 mm length). This suggests that the effective `listening distance' for most AEs was less than 5 to 10 mm.

     

The essentials of direct xylem pressure measurement

This paper discusses the essentials of the oil‐filled pressure probe technique in the measurement of negative xylem pressures, focusing in particular on the technique and physics underlying our recent, successful experiment which has rekindled the debate on the validity of the Cohesion–Tension theory. We illustrate a number of general problems associated with the cell pressure probe and xylem pressure probe techniques, and propose appropriate criteria for micropipette construction. We enumerate factors dealing with the cavitation problem and suggest methods for eliminating air seeds in the system. We introduce reliable criteria for the successful measurement of xylem pressure, and emphasize the importance of the probe pressure relaxation test. Several problems regarding the controversy over the Cohesion–Tension theory are also discussed. We discuss the correlation between xylem pressure and the transpiration rate, the existence of absolute negative xylem pressure in intact plants, the most negative values of xylem pressure measured by the pressure probe, the agreement between the pressure probe and pressure bomb techniques, and the vulnerability to cavitation (tensile strength) of pressure probes.     

Novel Methods of Measuring Hydraulic Conductivity of Tree Root Systems and Interpretation Using AMAIZED (A Maize-Root Dynamic Model for Water and Solute Transport)

Steady-state and dynamic methods were used to measure the conductivity to water flow in large woody root systems. The methods were destructive in that the root must be excised from the shoot but do not require removal of the root from the soil. The methods involve pushing water from the excised base of the root to the apex, causing flow in a direction opposite to that during normal transpiration. Sample data are given for two tropical (Cecropia obtusifolia and Lacistema aggregatum) and two temperate species (Acer saccharum and Juglans regia cv Lara). A hysteresis was observed in the relationship between applied pressure and resulting flow during dynamic measurements. A mathematical model (AMAIZED) was derived for the dynamics of solute and water flow in roots. The model was used to interpret results obtained from steady-state and dynamic measurements. AMAIZED is mathematically identical with the equations that describe Munch pressure flow of solute and water in the phloem of leaves. Results are discussed in terms of the predictions of AMAIZED, and suggestions for the improvement of methods are made.     

Isolation of two forms of basement membrane proteoglycans

Sequential extractions of the basement membrane producing Engelbreth-Holm-Swarm tumor yielded heparan sulfate proteoglycans with different size core proteins, but the same size heparan sulfate side chains. Saline, a nondenaturing solvent, extracted a small high density proteoglycan with a heterodisperse core protein of Mr = 95,000-130,000 whereas subsequent extraction with 7 M urea, a denaturing solvent, removed a large, low density proteoglycan with a Mr = 350,000-400,000 protein core. The denaturing conditions required for extraction of the large proteoglycan suggest that it interacts strongly with other basement membrane components. Antibodies to these proteoglycans cross-react with both proteoglycans, but the large proteoglycan has additional antigenic sites not present on the small proteoglycan. These proteoglycans may be derived from the same or similar gene products.