Interpreting Leaf Water Potential Measurements with a Model of the Soil-Plant-Atmosphere Continuum

A water flux model, which assumes that the dynamic functioning of the soil-plant-atmosphere continuum may be described by a series of steady states, was examined as a means for interpreting leaf water potential measurements in ‘Valencia’ orange trees (Citrus sinensis (L.) Osbeck). According to the model, leaf water potential should be related to transpirational flux, which in this experiment was estimated by the ratio of vapor pressure deficit of the atmosphere to leaf diffusion resistance (VPD/r_leaf). Leaf water potentials decreased in a specific relationship with increasing values of VPD/r_leaf provided that soil water was adequate and soil temperature was not too low, but regardless of season of the year or climatic or edaphic differences among 3 field locations. When soil water tensions exceeded 0.3 bar or when soil temperatures were lower than 15°C, deviations from the model occurred in the form of more negative leaf water potentials than predicted by VPD/r_leaf. The model predicts from simple measurements made on intact plants that these differences were due to the modification of flow resistances by cool temperatures and the modification of both resistances and the potential of water at the source in the case of soil water depletion. The model may be a useful tool for interpreting plant water potential data under contrasting environmental conditions.     

Water Potential and Stomatal Resistance of Sunflower and Soybean Subjected to Water Stress during Various Growth Stages

Plants of two varieties of soybean (Glycine max (L.) Merr.) and two varieties of sunflower (Helianthus annuus L.) were grown in controlled environments and subjected to water stress at various stages of growth. Leaf resistances and leaf water potentials were measured as stress developed. In soybeans the upper leaf surface had a higher resistance than the lower surface at all leaf water potentials and growth stages. Resistance of the upper surface began to increase at a higher water potential and increased more than the resistance of the lower surface. Resistances returned to prestress values 4 days after rewatering. In sunflowers upper and lower leaf surfaces had similar resistances at all water potentials and growth stages. Leaf resistances were higher in sunflower plants stressed before flowering than in those stressed later. Sunflower plants stressed to −16 bars recovered their prestress leaf resistance and water potential a few days after rewatering, but leaves of sunflower plants stressed to −23 bars died. Leaves of soybean and sunflower plants stressed before flowering suffered less injury than those of older plants and sunflowers stressed after flowering suffered more injury than soybeans.     

The Use of the Pressure Chamber Technique for Measurement of the Water Potential of Transpiring Plant Organs

The existence of water potential gradients in flowering shoots and leaves of roses (Rosa sp., cv. Baccara) and along flag leaves of wheat (Triticum aestivum L.) were studied by means of the Scholander pressure chamber. In roses grown in greenhouse, the water potential measured in transpiring shoots was higher than in leaves detached from these shoots, whereas the potential differences between leaf and shoot after equilibration in the dark were small or negligible. A progressive decrease in water potential was found upon repeated measurement on the same organ; this decline was steeper in leaves than in shoots. Extrapolating this decline to excision time resulted in water potential values which, in transpiring shoots, were 3 to 5 bars higher than in leaves. Detopping the flower bud did not alter this pattern, indicating that the highest water potential in the shoot was in the stem. In field-grown wheat, the water potential measured in a whole flag leaf was about 6 bars higher than that measured in the apical one-third of the leaf, and this difference disappeared after equilibrating the detached leaf for 1 h in the dark. These potential differences indicate the presence of resistances along the water path in the organ. The results obtained by the pressure chamber represent the highest water potential in the organ, rather than the average water potential.     

The Potential Distribution and the Short-Circuiting Factor in the Sucrose Gap

The sucrose gap technique, though widely employed in many tissues, could not be used for quantitative measurements of the membrane potential, because the value of the short-circuiting factor and the influence of junction potential on the recorded potential difference were unknown. The formula that relates the recorded potential to the true resting membrane potential was found by application of the cable equations to a core conductor placed in a system with three different media, e.g. Ringer, sucrose, and KCl. The formula shows that the potential difference recorded over the sucrose insulator depends on the extracellular and the intracellular longitudinal resistances, the membrane resistance and the membrane potentials in each region, and on the junction potentials between the different media. The true membrane potential in the Ringer region can be calculated from the potential difference recorded after complete depolarization by KCl on one side of the preparation, if the longitudinal resistances, the membrane resistances, the extracellular potential in the sucrose, and the junction potential between Ringer and sucrose are determined by separate measurements.     

土壤-植物-大气连续体水流阻力分布规律的研究

本文依据田间实测资料,分析了土壤-植物-大气连续体水流阻力的相对重要性,结果表明在连续体中的水流阻力主要分布于从叶气孔腔到大气的扩散过程和根系的吸水过程。叶-气之间的水流阻力比土-根之间要大50倍。最后,讨论了控制连续体水流运动的气孔阻力的变化规律及其与环境因素之间的关系。     

A split-root technique for measuring root water potential

Water encounters various resistances in moving along a path of decreasing potential energy from the soil through the plant to the atmosphere. The reported relative magnitudes of these pathway resistances vary widely and often these results are conflicting. One reason for such inconsistency is the difficulty in measuring the potential drop across various segments of the soil-plant-atmosphere continuum. The measurement of water potentials at the soil-root interface and in the root xylem of a transpiring plant remains a challenging problem.     

叶片内水传输的模拟

根据水在介质中的流动规律和能量守恒原理,在植物叶片内建立了一个稳态的水传输模型。该模型考虑了气孔复合体内外、共质体与质外体、原生质与细胞壁在水传输上的不同,应用计算机详细地分析和计算了叶内(特别是气孔复合体内)水的传输,得到水势在叶片内近似分布的关系式。应用这些关系式对叶内的水势和水势差作了估计,并对不同解剖特征叶片内的水势差作了比较。     

Resistance to Water Transport in Shoots of Vitis vinifera L. : Relation to Growth at Low Water Potential

Apparent resistances to water transport in the liquid phase were determined from measurements of soil, root, basal shoot internode, shoot apex, and leaf water potentials and water flux in Vitis vinifera (cv White Riesling) during soil drying. Predawn water potential differences (ΔΨ) in the shoots accounted for 20% of the total ΔΨ between the soil and the shoot apex when plants were well-watered but increased to about 90% when shoot growth ceased. The ΔΨ from soil to root was essentially constant during this period. At low water potential, the ΔΨ in the shoot was persistent when transpiration was low (predawn) or completely prevented (plant bagging). The apparent hydraulic resistance between the basal shoot internode and most rapidly expanding leaf (or shoot apex) increased several-fold when water was withheld. Leaf and internode expansion both exhibited high sensitivity to increasing hydraulic resistance. Measurements of pneumatic resistance to air flow through frozen internode segments indicated progressive vapor-filling of vessels as soil drying progressed. From these observations and others in the literature, it was suggested that embolization may be a common occurrence and play an important role in the inhibition of shoot growth at moderate water deficits.     

Leaf Water Potential and Control of Water Loss in Droughted Sitka Spruce Seedlings

Sitka spruce seedlings were subjected to drought in experimentsin a growthroom, a greenhouse, and out of doors. The plantswere grown in a double chamber with the bulk of the roots inthe upper part where they dried out the soil when water waswithheld. A few new roots penetrated into the lower part inwhich the soil remained moist. The double chamber system enabledthe plant to attain a high water psotential by night and theshoot was only periodically under mild water stress. Measurementswere made on soil water potential (_solt), leaf water potential(₁), transpiration (E), and stomatal conductance (k_s). As soildecreased over a period of 4.5 d, E and k_s decreased progressively.The decline in E and k_s which indicated stomatal closure, occurredat a higher ₁ than has been reported for Sitka spruce. The behaviourof the stomata appeared to be modified by conditions at theroot, and it is proposed that differences in the response to₁,depend on Whether the latter is reduced by resistances in thexylem between root and leaf, as is known to occur in large treesin moist soil, or by stresses at the root itself.     

Interpretation of seasonal changes of xylem embolism and plant hydraulic resistance in Fagus sylvatica

The annual course of xylem embolism in twigs of adult beech trees was monitored, and compared to concurrent changes of tree water status and hydraulic resistances. Xylem embolism was quantified in 1-year-old apical twigs by the hydraulic conductivity as a percentage of the maximum measured after removal of air emboli. Tree and root hydraulic resistances were estimated from water potential differences and sap flux measurements. The considerable degree of twig embolism found in winter (up to 90% loss of hydraulic conductivity) may be attributed to the effect of freeze-thaw cycles in the xylem. A partial recovery from winter embolism occurred in spring, probably because of the production of new functional xylem. Xylem embolism fluctuated around 50% throughout the summer, without significant changes. Almost complete refilling of apical twigs was observed early in autumn. A significant negative correlation was found between xylem embolism and precipitation; thus, an active role of rainfall in embolism reversion is hypothesized. Tree and root hydraulic resistances were found to change throughout the growing period. A marked decrease of hydraulic resistance preceded the refilling of apical twigs in the autumn. Most of the decrease in total tree resistance was estimated to be located in the root compartment.     

Water Potential, Translocation and Assimilate Partitioning

Lang, A. and Thorpe, M. R. 1986. Water potential, translocationand assimilate partitioning.—J. exp. Bot. 37: 495–503. The effect of water status upon translocation and assimilatepartitioning is examined both from theory and in an experimentwith young Phaseolus plants. Theory predicts that translocationis unlikely to be directly affected by water status. However,water potential differences within plants should influence translocationflow, with regions at lower potentials attracting disproportionatelylarge shares of assimilate. This prediction is supported in the experiment with Phaseolusin which the pattern of partitioning in the root changed rapidlyin response to bathing portions of it in solutions of differentosmolarity. The relevance of these findings to the growth of plants undernatural conditions is considered and evidence is presented thatwater potential gradients may be an Important factor in thecontrol of partitioning Key words: Phloem translocation, xylem transport, partitioning, water potential, control, osmotic potential     

Root elongation, water stress, and mechanical impedance: a review of limiting stresses and beneficial root tip traits

Root elongation in drying soil is generally limited by a combination of mechanical impedance and water stress. Relationships between root elongation rate, water stress (matric potential), and mechanical impedance (penetration resistance) are reviewed, detailing the interactions between these closely related stresses. Root elongation is typically halved in repacked soils with penetrometer resistances >0.8-2?MPa, in the absence of water stress. Root elongation is halved by matric potentials drier than about -0.5?MPa in the absence of mechanical impedance. The likelihood of each stress limiting root elongation is discussed in relation to the soil strength characteristics of arable soils. A survey of 19 soils, with textures ranging from loamy sand to silty clay loam, found that ～10% of penetration resistances were >2?MPa at a matric potential of -10?kPa, rising to nearly 50% >2?MPa at - 200?kPa. This suggests that mechanical impedance is often a major limitation to root elongation in these soils even under moderately wet conditions, and is important to consider in breeding programmes for drought-resistant crops. Root tip traits that may improve root penetration are considered with respect to overcoming the external (soil) and internal (cell wall) pressures resisting elongation. The potential role of root hairs in mechanically anchoring root tips is considered theoretically, and is judged particularly relevant to roots growing in biopores or from a loose seed bed into a compacted layer of soil.     

The Forgotten Component of Plant Water Potential

Abstract: Experts in plant water relations are challenged to explain why the standard expression for the water potential of an intact, transpiring leaf omits one of the pressure components, namely the tissue pressure of the living leaf cells. Two questions are posed: A) Is there a reason for omitting this component? B) If not, what evidence exists that it may be ignored? When this component contributes significantly, the water potential cannot be interpreted in the accustomed way as measuring tension in the xylem sap.     

An electrical analogue of evaporation from,and flow of water in plants

Summary The currents generated in the analogue circuit represent vapour loss from leaves, heat loss from leaves, and liquid flow in plant and soil. The plant and soil resistances are defined in such a way that they are consistent with the resistances to transport of vapour in the atmosphere and there is continuity of potential at the analogue liquid: air interface in the leaves. The action of the environment on plant water movement is treated as an application of Thévenin's theorem of electric circuits.     

Water transport through plant tissue: the apoplasm and symplasm pathways

A detailed quantitative analysis of water flow through the apoplasm and symplasm of plant tissue is presented. The analysis results in two coupled diffusion equations which describe water transport in the two pathways. Various parameters entering the analysis identify the physical properties of the tissue which control the transport process as the resistance to water flow per cell in the two parallel pathways, the resistance per cell between pathways, and the water capacity per cell in the two pathways. Values for the several resistances and water capacities are estimated from available data, and a model problem is solved wherein a sheet of tissue at an initial water potential of — δ bars is immersed in a container of water. The resulting solutions show that depending on the values assigned to the controlling parameters, local water potential equilibrium between each cell and its cell wall may or may not obtain. In the special case of local equilibrium (water potential in the symplasm and apoplasm pathways essentially equal), the transport process can be described by a single diffusion equation which is derived along with an expression for the tissue diffusivity. It is concluded that the weakest link in the analysis is the estimated value for the permeability of the plasmodesma membrane, and that a logical extension of the theory would be to include the effects of a diffusable solute.     

A Quantitative Study of the Resistances to Transpirational Water Movement in Sunflower (Helianthus annuus L.)

Hydroponic sunflower plants were used in a quantitative studyof the relationship between total plant and leaf resistancesto transpirational water movement and transpiration rate. Theresults demonstrate that both resistances are flux-dependentand decline 5–6-fold during a comparable increase in transpiration.The resistance of excised leaves including the petiole was approximatelyhalf the total plant resistance. Subsequent analyses of the water potential gradients and transpirationalfluxes in whole plants permitted calculation of the magnitudeof the partial resistances imposed by roots, stem, petiole,and leaf. The root and leaf resistances were approximately 50%and 30% of the total resistance respectively. Stem and petiolarresistances were relatively small and both influenced watermovement to the upper leaves similarly. The values obtainedare compared with previous published results obtained usingdiverse experimental techniques.     

In Situ Estimates of Variable Plant Resistance to Water Flux in Ilex opaca Ait

Xylem pressure potentials and stomatal diffusion resistances were measured in the field in Ilex opaca Ait. during days which differed in temperature and vapor pressure deficit. Water flux into leaves was calculated by combining the field data with laboratory determinations of the relation between tissue water deficit and water potential. Estimates of apparent plant resistance were then calculated from fluxes and differences between soil water potential and xylem tension. The resistance depended strongly on water flux, dropping by a factor of over 7 from low to high water flow rates. This extends the generality of variable plant resistances measured in controlled environment studies to I. opaca as it occurs naturally in the field. The relation of apparent plant resistance to water flux as estimated in this study can be useful in simulation models which calculate water uptake to leaves as a flux driven by a difference in soil and leaf water potentials across a resistance between the bulk soil and the leaf.     

Water Flow in Beta vulgaris Storage Tissue

The relative magnitudes of the hydraulic resistances, water capacities, and water potential equilibration time constants for the single cell, for the apoplast, and for the symplast in higher plant tissue are assessed. Swelling of beetroot (Beta vulgaris, var. `Detroit Red') storage tissue sections in pure water is measured using a displacement transducer. This method of measurement avoids the difficulty of solute diffusion in the apoplast. Theoretical analysis of the experimental results shows that the main path of water flow into the tissue is the apoplast rather than the symplast, that the main resistance to water flow into the cells is usually the cell membrane rather than the apoplast, but that in some cases the apoplast resistance and water capacity can contribute significantly to the water potential equilibration time constant of the tissue.     

Seasonal Patterns of Acid Metabolism and Gas Exchange in Opuntia basilaris

Acid metabolism and gas exchange studies were conducted in situ on the cactus Opuntia basilaris Engelm. and Bigel. A pattern of significant seasonal variation was evident. The pattern was controlled by rainfall, which significantly influenced plant water potentials, total gas transfer resistances, and nocturnal organic acid synthesis. In winter and early spring, when plant water stress was mild, stomatal and mesophyll resistances remained low, permitting enhanced nocturnal assimilation of ¹⁴CO₂. The day/night accumulation of acidity was large during these seasons. In summer and fall, plant water stress was moderate, although soil water stress was severe. The nocturnal assimilation of ¹⁴CO₂ was very low during these seasons, even in stems with open stomata, indicating large mesophyll resistances restricting exogenous gas incorporation. The day/night accumulation of acidity was reduced, and a low level of acid metabolism persisted throughout this period. The rapid response to a midsummer rainfall emphasizes the importance of plant water potential as a parameter controlling over-all metabolic activity. The seasonal variations of acid metabolism and gas exchange significantly influenced the efficiency of water use and carbon dioxide assimilation. Periods of maximal efficiency followed rainfall throughout the course of the year.