The Thermodynamics of Short-distance Translocation in Plants

Flux equations derived from the thermodynamics of irreversibleprocesses are applied to calculations on the probable pathwayof short-distance translocation through non-vascular tissue.The Onsager theory predicts that a significant fraction of wateris transported over short distances via an extracellular (cellulosecell wall) pathway. It is suggested that the Onsager equationsare more comprehensive than other equations previously used,and the Onsager theory is found to support an increasing bodyof evidence for an extracellular pathway. Using the values of the Onsager coefficients measured in isolatedNitella cell walls, it is shown that an electrical gradientof I.5 mV/cm will cause the same water flux through cell wallsas a pressure gradient of I atm/cm. It is suggested that electricalgradients can contribute significantly to short-distance trans-location.     

A theoretical model of hydraulic conductivity recovery from embolism with comparison to experimental data on Acer saccharum

A theoretical model of bubble dissolution in xylem conduits of stems was designed using the finite differential method and iterative calculations via computer. The model was based on Fick's, Henry's and Charles' laws and the capillary equation. The model predicted the tempo of recovery from embolism in small diameter branches of woody plants with various xylem structures under different xylem water pressures. The model predicted the time required to recover conductivity in any position in the stem. Repeated iterative solution of the model for different situations yielded an empirical formula to calculate the time for complete recovery of conductivity in stems from a fully embolised initial state. The time, t_p, is given by: where α is a temperature coefficient; D is the coefficient of diffusion of air in wood at 25°C; r_cs is the ratio of the area of total conduit cross section to the stem cross section; Ψ_xp is the stem xylem pressure potential (Pa, where 0 Pa equals atmospheric pressure); τ is solution surface tension (0.072 N m^?1); and D_c and D_s are diameters of the conduits and the stem, respectively (m). The equation is valid only when Ψ_xp > –4τ/D_c. The model predicts no recovery of conductivity when Ψ_xp≤–4τ/D_c. The model agreed with experiments.     

New evidence for large negative xylem pressures and their measurement by the pressure chamber method

Pressure probe measurements have been interpreted as showing that xylem pressures below c. –0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting the pressure chamber technique and the existence of xylem pressures well below –0.4 MPa. We deduced xylem pressures in water-stressed stem xylem from the following experiment: (1) loss of hydraulic conductivity in hydrated stem xylem (xylem pressure = atmospheric pressure) was induced by forcing compressed air into intact xylem conduits; (2) loss of hydraulic conductivity from cavitation and embolism in dehydrating stems was measured, and (3) the xylem pressure in dehydrated stems was deduced as being equal and opposite to the air pressure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was caused by air entering the xylem conduits (air-seeding). Deduced xylem pressure showed a one-to-one correspondence with pressure chamber measurements for 12 species (woody angiosperms and gymnosperms); data extended to c. –10 MPa. The same correspondence was obtained under field conditions in Betula occidentalis Hook., where pressure differences between air- and water-filled conduits were induced by a combination of in situ xylem water pressure and applied positive air pressure. It is difficult to explain these results if xylem pressures were above –0.4 MPa, if the pressure chamber was inaccurate, and if cavitation occurred by some mechanism other than air-seeding. A probable reason why the pressure probe does not register large negative pressures is that, just as cavitation within the probe limits its calibration to pressures above c. –0.5 MPa, cavitation limits its measurement range in situ.     

Hydraulic acclimation to shading in boreal conifers of varying shade tolerance

The purpose of this study was to determine how shading affects the hydraulic and wood‐anatomical characteristics of four boreal conifers (Pinus banksiana, Pinus contorta, Picea glauca and Picea mariana) that differ in shade tolerance. Plants were grown in an open field and under a deciduous‐dominated overstory for 6 years. Sapwood‐ and leaf‐area specific conductivity, vulnerability curves, and anatomical measurements (light and scanning electron microscopy) were made on leading shoots from six to nine trees of each treatment combination. There was no difference in sapwood‐area specific conductivity between open‐grown and understory conifers, although two of four species had larger tracheid diameters in the open. Shaded conifers appeared to compensate for small diameter tracheids by changes in pit membrane structure. Scanning electron microscopy revealed that understory conifers had thinner margo strands, greater maximum pore size in the margo, and more torus extensions. All of these trends may contribute to inadequate sealing of the torus. This is supported by the fact that all species showed increased vulnerability to cavitation when grown in the understory. Although evaporative demand in an understory environment is low, a rapid change into fully exposed conditions could be detrimental for shaded conifers.     

Characterization of Photosynthetic 14Carbon Assimilation by Potamogeton lucens L.

Photosynthetic assimilation of exogenous ¹⁴CO₂ and H¹⁴CO₃^–by the aquatic angiosperm Potamogeton lucens L. is reported.Equivalent maximum rates of assimilation (1.5 µmol s^–1m^–2) were obtained in the presence of saturating levelsof ¹⁴CO₂ (1.0 mol m^–3, pH 5.3) or H¹⁴CO₃^– (1.5 molm^–3, pH, 9.2). Under subsaturating ¹⁴CO₂ levels, bothgaseous diffusion and H¹⁴CO₃^– transport were shown tooperate simultaneously, such that maximal photosynthetic rateswere established. An induction lag of approximately 3 min was observed when exogenous¹⁴CO₂ was assimilated. A longer lag of approximately 12 minwas required, however, before linear assimilation rates wereestablished when H¹⁴CO₃ acted as the carbon source. The light-activatedH¹⁴CO₃^– transport system was found to be quite labile.A brief (5 min) dark treatment returned the system to the inactivestate. Bicarbonate transport was shown to be competitively inhibitedby CO₃^2–ions. The possibility is discussed that this formof inhibition may be common to many HCO₃^– assimilators. Preliminary polar cation transport studies (from lower to upperleaf surface) indicated an almost exact one to one relationshipbetween the rates of Na⁺ influx and efflux and H¹⁴CO₃^–assimilation. The possible relationship(s) between these transportprocesses and the requirement for electrical neutrality is brieflydiscussed.     

The determination of membrane transport parameters with the cell pressure probe: theory suggests that unstirred layers have significant impact

A simulation model was written to compute the time-kinetics of turgor pressure, P, change in Chara corallina during cell pressure probe experiments. The model allowed for the contribution of a membrane plus zero, one, or two unstirred layers of any desired thickness. The hypothesis that a cell with an unstirred layer is a composite membrane that will follow the same kind of kinetics with or without unstirred layers was tested. Typical ‘osmotic pulse’ experiments yield biphasic curves with minimum or maximum pressures, P_min(max), at time t_min(max) and a solute exponential decay with halftime . These observed data were then used to compute composite membrane properties, namely the parameters L_p = the hydraulic conductance, σ = reflection coefficient and P_s = solute permeability using theoretical equations. Using the simulation model, it was possible to fit an experimental data set to the same values of P_min(max), t_min(max) and incorporating different, likely values of unstirred layer thickness, where each thickness requires a unique set of plasmalemma membrane values of L_p, σ and P_s. We conclude that it is not possible to compute plasmalemma membrane properties from cell pressure probe experiments without independent knowledge of the unstirred layer thickness.     

Water relations of Robinia pseudoacacia L.: do vessels cavitate and refill diurnally or are R‐shaped curves invalid in Robinia?

Since 2005, an unresolved debate has questioned whether R‐shaped vulnerability curves (VCs) might be an artefact of the centrifuge method of measuring VCs. VCs with R‐shape show loss of stem conductivity from approximately zero tension, and if true, this suggests that some plants either refill embolized vessels every night or function well with a high percentage of vessels permanently embolized. The R‐shaped curves occur more in species with vessels greater than half the length of the segments spun in a centrifuge. Many have hypothesized that the embolism is seeded by agents (bubbles or particles) entering the stem end and travelling towards the axis of rotation in long vessels, causing premature cavitation. VCs were measured on Robinia pseudoacacia L. by three different techniques to yield three different VCs; R‐shaped: Cavitron P₅₀ = 0.30 MPa and S‐shaped: air injection P₅₀ = 1.48 MPa and bench top dehydration P₅₀ = 3.57 MPa. Stem conductivity measured in the Cavitron was unstable and is a function of vessel length when measured repeatedly with constant tension, and this observation is discussed in terms of stability of air bubbles drawn into cut‐open vessels during repeated Cavitron measurement of conductivity; hence, R‐shaped curves measured in a Cavitron are probably invalid.     

Drought-induced leaf shedding in walnut: evidence for vulnerability segmentation

Trees of Juglans regia L. shed leaves when subjected to drought. Before shedding (when leaves are yellow), the petioles have lost 87% of their maximum hydraulic conductivity, but stems have lost only 14% of their conductivity. This is caused by the higher vulnerability of petioles than stems to water-stress induced cavitation. These data are discussed in the context of the plant segmentation hypothesis.