A new method of measuring water potential in tree stems by water injection

Abstract. A new method of measuring xylem water potential in the stems of trees is described. The flow rate of water injected into the xylem at two or more known pressures is measured. The xylem water potential is derived either graphically from the relationship between flow rate and applied pressure, or from the solution of simultaneous flow equations.     

A method for measuring radial differences in water content of intact tree stems by attenuation of gamma radiation

Abstract. A technique is described for the nondestructive measurement of water content variations in the stems of trees.
The density of intact tree stems in the forest was estimated using attenuation of gamma radiation. Water content was calculated after subsequent derivation of the volume fraction of solid matrix.
A radial sequence of such measurements was obtained by measuring a sequence of chords, and assuming homogeneity within concentric tori. Analysis of the source of errors showed that the system was a close approximation of an ideal, rigid-geometry, mono-energetic source/detector system, with basic precision dependent on the quantity of gamma radiation measured. Correlation coefficients between the gamma-attenuation technique and subsequent gravimetric estimates of water content for two field experiments reported here were 0.882 and 0.938.
The technique was shown to be capable of describing the radial and diurnal variation in water content at two heights in the stem of a 20-year-old tree of Pinus contorta .     

Relationship between cavitation and water uptake in rose stems

Cavitation in rose stems ( Rosa hybrida L.) was assessed in both intact plants and excised flowers, by measurement of ultrasonic acoustic emissions at the stem surface and determination of the air-conductivity of 2.5-cm segments that were attached at one end to air at low pressure (0.01 MPa). On sunny days the stems of intact rose plants showed acoustic emissions and conductivity to air, starting early in the morning. In Cara Mia and Sonia rose plants the cavitations were repaired during the late afternoon; in Madelon plants this repair only occurred overnight. Water flow was seriously impaired in stems of Cara Mia roses cut around midday, on sunny days.
During dehydration of cut roses in air the onset of a high rate of acoustic emissions coincided with a low rate of water uptake when stems were subsequently placed in water. High emission frequency occurred after 2.4 ± 0.7 h, 6.8 ± 4.3 h and 19.8 ± 9.0 h of exposure to air in Cara Mia, Madelon and Sonia roses, respectively. A low rate of water uptake in excised stems placed in water was found after 3–4, 9–12 and 24–36 h of desiccation in air, respectively. The onset of the high emission frequency corresponded with a water potential of −1.7, −2.9 and −3.8 MPa in the three cultivars, respectively.
It is concluded that a high number of cavitations may occur in noncut stems of rose plants, leading to low water uptake immediately after excision, depending on the weather and the cultivar, and that the low rate of water uptake after a period of dry storage, among the three rose cultivars investigated, is correlated with the presence of a high number of cavitated xylem elements.     

Diurnal changes in water content of the stems of apple trees, as influenced by irrigation

Abstract Groups of apple trees within an orchard were irrigated by either releasing a fine mist within the canopy or spraying water on the soil. Diurnal changes in the water content of the xylem were inferred from measurements of density made with a gamma probe. Transpiration rates were estimated from leaf temperatures, and water potentials and changes in radial dimension of the stem were recorded. Xylem water content and water potential declined each morning and recovered in the afternoon. Radial dimensions of the xylem followed a similar pattern, but the percentage change was very small in relation to the change in water content of the same tissue. The decline in water content is unlikely to have been caused by cavitation, as it was readily reversed in the afternoon. It is more likely to have been caused by changes in the volume of water occupying already-cavitated fibres or intercellular spaces. Water potential and xylem water content were highest in the mist-irrigated trees and least in the unirrigated controls.     

Water transport through tall trees: A vertically explicit,analytical model of xylem hydraulic conductance in stems

Trees grow by vertically extending their stems, so accurate stem hydraulic models are fundamental to understanding the hydraulic challenges faced by tall trees. Using a literature survey, we showed that many tree species exhibit continuous vertical variation in hydraulic traits. To examine the effects of this variation on hydraulic function, we developed a spatially explicit, analytical water transport model for stems. Our model allows Huber ratio, stem‐saturated conductivity, pressure at 50% loss of conductivity, leaf area, and transpiration rate to vary continuously along the hydraulic path. Predictions from our model differ from a matric flux potential model parameterized with uniform traits. Analyses show that cavitation is a whole‐stem emergent property resulting from non‐linear pressure‐conductivity feedbacks that, with gravity, cause impaired water transport to accumulate along the path. Because of the compounding effects of vertical trait variation on hydraulic function, growing proportionally more sapwood and building tapered xylem with height, as well as reducing xylem vulnerability only at branch tips while maintaining transport capacity at the stem base, can compensate for these effects. We therefore conclude that the adaptive significance of vertical variation in stem hydraulic traits is to allow trees to grow tall and tolerate operating near their hydraulic limits.     

Osmotic water permeability of isolated vacuoles

We measured the osmotic water permeability (P _os) of vacuoles isolated from onion (Allium cepa L.), rape (Brassica napus L.), petunia (Petunia hybrida Hook.) and red beet (Beta vulgaris L.). For all the vacuolar types investigated, P _os values were in the range 200–1000 μm s⁻¹. The change in membrane surface area induced by an osmotic gradient was smaller than 2–6%. The vacuolar P _os values for red beet and onion were reduced by 1 mM HgCl₂, to 14% and 30% of the control values, respectively, but were partially restored to 51% and 76% by 5 mM β-mercaptoethanol. These results suggest that aquaporins were present in all the vacuoles tested. In HgCl₂-treated onion vacuoles, the reduced P _os (56 μm s⁻¹) had a low activation energy (approx. 6 kJ mol⁻¹), indicating that water permeation was still occurring mainly via aquaporins, and that the water permeability of the lipid part of the vacuolar membrane is probably very low. Received: 18 February 1999 / Accepted: 21 June 1999     

Phase and amplitude relations between transpiration, water potential and stem shrinkage

Abstract An electrical analogue describing the phase and amplitude relations between transpiration, water potential and stem shrinkage for trees was developed. Observations of shrinking and swelling at various heights up a Pinus radiata tree were obtained over several weeks in summer and autumn. The relative amplitude in shrinkage increased by a factor of two up the stem, but phase lags were small. The data obtained were used in conjunction with the electrical analogue of the How pathway to obtain an estimate of the relative magnitude of the capacitance of the living bark and the sapwood, and to predict phase lags between transpiration and leaf water potential, and lags in transpirational flux up the stem. The results suggest that if water potentials recover by dawn, phase lags in water potential down tree stems arc small and that the exchange between water stored in the sapwood and the transpiration stream is irreversible over the diurnal time scale.     

夏玉米叶片水分变化与光合作用和土壤水分的关系

叶片是光合作用的重要器官,其含水量的变化必将影响光合作用,但关于叶片水分变化对光合作用的影响报道较少。以华北夏玉米为研究对象,利用三叶期不同水分梯度的持续干旱模拟试验资料,分析夏玉米叶片水分变化及其与叶片净光合速率和土壤水分的关系。结果表明:夏玉米叶片净光合速率对叶片水分变化的响应显著且呈二次曲线关系,叶片含水量约为70.30%时,叶片净光合速率为零;叶片含水量与土壤相对湿度呈非直角双曲线关系,叶片最大含水量约为85.14%。研究结果可为准确描述叶片水分变化对光合作用的影响及客观辨识夏玉米干旱的发生发展及监测预警提供参考。     

Osmotic water permeability in glycoprotein containing liposomes

The kinetics of osmotic water permeability in proteoliposomes containing ₁-acid glycoprotein was investigated by means of stopped-flow spectrophotometry. A biphasic time-course of scattered light with time was registered. The rate constants calculated from fits to an exponential function in the first phase were proportional to the final medium osmolarity. The apparent second order rate constants K_app (Osm^-1 sec^-1) were determined at different glycoprotein concentrations in the original mixture for preparation of proteoliposomes. The value of K_app at lipid:glycoprotein weight ratio = 1 was plotted in Arrhenius coordinates. The calculated activation energy for water permeation through the lipid bilayer suggests that eventual channel mechanism may be involved due to the presence of glycoprotein molecule in the liposomes.     

Characteristic moisture curves and maximum water content of two crop residues

Limited data are available relating water potential () to crop residue water content (), although this relationship is important to study decomposition and moisture retention of the residue layer in no-till systems and other agricultural situations where residues are used. The objectives of this study were (i) to determine the characteristic moisture curves of rye (Secale cereale L.) and clover residues (Trifolium incarnatum L.), and (ii) to determine residue characteristics that can predict maximum water content of crop residues. Air-dried residues were separated into leaves and stems, cut into 0.5 cm length pieces and saturated with distilled water. Pieces of the drained residues were dried to various water contents in the laboratory and then transferred into thermocouple psychrometer chambers. Characteristic moisture functions of the type = a ^–b, where a and b are empirical constants, were fitted to the data. The characteristic moisture curves had a similar shape to that of a Cecil sandy loam soil used as an example; however, while plant residues were able to retain up to 4.3 g H₂O g^–1, the mineral soil retained only 0.22 g H₂O g^–1. Soluble carbohydrate concentration can be used as a practical index to estimate maximum water content of residues, given the good relationship between both variables (R ² = 0.92).     

Relationship between transpiration and water potential in grafted scions of Picea

Total water and osmotic potential, turgor pressure and transpiration rate were measured on scions of Picea pungens (Englemann) during union development. In controlled environments, declines in water potential were correlated with lower transpiration rates to about −2.0 MPa. Water potentials below −2.0 MPa resulted in graft failure and were associated with sharply increased transpiration rates. Bulk turgor pressures remained high in the needles during this period of declining water potential and increasing transpiration. Transpiration rates of successful and unsuccessful greenhouse grafts were not significantly different during union development. Transpiration rates of these grafts were highest around dawn, then declined throughout the day only to increase again after sunset. High bulk needle turgor values (1.3 MPa), maintained by osmotic adjustment, may prevent stomatal closure of Picea scions at water potentials below −2.0 MPa.     

Effects of water stress on growth, osmotic potential and abscisic acid content of maize roots

Under water stress conditions, induced by mannitol solutions (0 to 0.66 M ) applied to the apical 12 mm of intact roots of Zea mays L. (cv. LG 11), a growth inhibition, a decrease in the osmotic potential of the cell sap and a significant accumulation of abscisic acid (ABA) were observed. When the roots were placed in a humid atmosphere after the stress, the growth rate increased again, even if elongation had been totally inhibited. Under a stress corresponding to an osmotic potential of -1.09 MPa in the solution, growth was totally inhibited, which means that the root cell turgor pressure was reduced to the yield threshold. These conditions led to the largest accumulation of ABA. The effect of water stress on the level of ABA was studied for three parts of the root. The greatest increase in ABA (about 10 fold) was obtained in the growth zone and this increase was apparently independent of the hydrolysis of the conjugated form. With a mannitol treatment of 1 h equivalent to a stress level of -1.39 MPa, a 4-fold increase in ABA efflux into the medium was obtained. These results suggest that there are interactions between water stress, root growth, osmotic potential and the ABA level. The growth under conditions of stress and the role of endogenous ABA in the control of plant metabolism, specially in the growth zone, are discussed.     

华北落叶松树体储水利用及其对土壤水分和潜在蒸散的响应: 基于模型模拟的分析

 树体储水在树木水分传输中具有重要的作用, 不仅为蒸腾提供水分来源, 还具有缓冲作用, 可防止木质部导管水势过低以至于水分传输的失败。树体储水动态及其利用的研究对于认识树木对水分胁迫的响应机制具有重要意义。该研究构建了包含树体储水释放-补充作用的树干水分传输模型, 可模拟计算林分小时尺度的冠层蒸腾、边材液流、树体储水与木质部导管水流交换过程, 并以六盘山北侧的华北落叶松(Larix principis-rupprechtii)人工林为例, 在林分水平分析树体储水利用及其 与土壤水分和潜在蒸散之间的关系。检验结果表明, 该模型能够精确地模拟出林分边材液流的日变化特征, 模拟与观测的小时液流速率决定系数R²为0.91 (n = 2 352)。模拟结果表明, 在典型晴朗天气下, 在日出时树体储水利用启动, 至9:00左右达到峰值(0.14 mm?h^–1), 午间降至0, 下午降为负值直至午夜, 即进入树体补水阶段; 树体储水日使用量(DJ_z)为0.04–0.58 mm?d^–1, 与日蒸腾量(DT_r)成正相关(R² = 0.91), 对蒸腾的贡献为25.6%。分析结果表明, 当潜在蒸散(ET_p)低于4.9 mm?d^–1时, ET_p是华北落叶松树体储水利用的主要驱动因子, DJ_z与ETp成正相关(R² = 0.68); 当ET_p高于4.9 mm?d^–1时, DJ_z随着ET_p的增加呈现降低趋势; DJ_z与土壤水势没有显著相关关系(p > 0.05), 但最大树体储水日使用量(DJ_zmax)与土壤水分含量成正相关(R² = 0.79), 说明土壤水分是树体储水利用的限制因子。     

Survival of Lactobacillus helveticus entrapped in Ca-alginate in relation to water content, storage and rehydration

Lactobacillus helveticus CNRZ 303 entrapped in Ca-alginate gel beads was investigated for improved survival and stability during fluidized-bed drying, storage and rehydration. Addition of protective solutes was very important. Studies of the conditions showed that inactivation of entrapped L. helveticus started when the water content exceeded 0.3–0.4 g H₂O (g dry wt)⁻¹ for adonitol, glycerol and reconstituted non fat milk solids (NFSM). With Ringer’s solution (control) and betaine, the fall in viability was evident above 1 g H₂O (g dry wt)⁻¹. Drying down to 0.2 g H₂O (g dry wt)⁻¹ required the removal of 98.5–98.9% of the water. The best survival rate with the least injured cells among survivors was experienced with adonitol and NFMS, respectively, 71% and 57% (compared to the initial) immediately after dehydration. Adonitol and NFMS were also best for survival during storage. The highest cell recovery was obtained by rehydrating the cells in cheese whey permeate between 20–30°C done at pH 6.0–7.0, satisfying the demands for cell survival, repair and slow swelling (adaptions). Received 04 January 1999/ Accepted in revised form 29 April 1999     

Effect of PEG-6000 imposed drought stress on RNA content,relative water content (RWC), and chlorophyll content in peanut leaves and roots

Drought, one of the environmental stresses, plays crucial role in reduction in plant production on majority of agricultural fields of world, In order to evaluate drought stress on RNA content Relative water content (RWC), and chlorophyll content, Water deficit was induced by Polyethylene glycol (PEG) in peanut (Arachis hypogaea), accession number ICGV 91114. In this current study we evaluate RNA content and Relative water content (RWC) both in leaves and roots and chlorophyll content in leaf. The present study was undertaken with the aim to investigate the effect of water deficit imposed by PEG-6000, 40 old day seedlings were treated with varying concentrations of polyethylene glycol-6000 (PEG-6000; w/v-5%, 10%, 15% & 20%) for 24 h. The results showed that RNA content and Relative water content (RWC) content was significantly reduced in both leaves and roots with increased concentration of PEG, In leaves, a concentration dependent decline in chlorophyll content with increasing concentration of polyethylene glycol-6000 (PEG-6000). Reduction in chlorophyll ‘a’ level was to a greater extent than the chlorophyll ‘b’. Thus, this attributes can be used as screening tool for drought tolerance in peanut.     

The expansion of maize root-cap mucilage during hydration. 3. Changes in water potential and water content

Root-cap mucilage from aerial nodal roots of maize has been found to have water potential values of −11 MPa or lower when air dried. The value approaches 0 MPa within 2 min of hydration in distilled water. In this time the expanding gel absorbs only about 0.3% of the water content of fully expanded mucilage. It is concluded that the root-cap mucilage per se has almost no capacity to retain water in the rhizosphere. Any function that it may play in the slowing of root desiccation would be indirect. For example, mucilage might decrease pore size between and within soil aggregates by pulling the particles together in a cycle of nocturnal efflux of water from the root surface, and diumal dyring during transpiration.     

Stem radial growth and water storage responses to heat and drought vary between conifers with differing hydraulic strategies

We investigated stem radial growth and water storage dynamics of 2 conifer species differing in hydraulic carbon strategies, Juniperus monosperma and Pinus edulis, under conditions of ambient, drought (～45% reduction in precipitation), heat (～4.8 °C temperature increase), and the combination of drought + heat, in 2013 and 2014. Juniper maintained low growth across all treatments. Overall, the relatively isohydric piñon pine showed significantly greater growth and water storage recharge than the relatively anisohydric juniper across all treatments in the average climate year (2014) but no differences in the regionally dry year (2013). Piñon pine ceased growth at a constant predawn water potential across all treatments and at a less negative water potential threshold than juniper. Heat has a greater negative impact on piñon pines' growth and water storage than drought, whereas juniper was, in contrast, unaffected by heat but strongly impacted by drought. The whole‐plant hydraulic carbon strategies, in this case captured using the isohydric/anisohydric concept, translate into alternative growth and water storage strategies under drought and heat conditions.     

The use of tree root suckers to estimate root water potential

Abstract. This paper describes a simple method to estimate the root water potential of trees using the root suckers as xylemic probes. On the day before the water potentials were measured in pressure chamber, all the sucker shoots were enclosed in plastic bags to prevent transpiration. Under the premise that the potential at two points in a flow system would be equal if there were no flux between these points, the sucker shoot water potential estimates the root water potential. The results are not dependent on the sucker architecture, are consistent with the soil-plant-atmosphere continuum concept, and are supported by psychrometric measurements.     

土壤含水量与苹果叶片水分利用效率的关系

以盆栽苹果幼树为试材研究了土壤相对含水量与叶片水分利用效率（WUE）的关系,探讨了引起WUE变化的原因,结果表明,土壤相对含水量（SWC）52.0％时WUE最高。SEC从77.2%降至52.0％时,气孔导度下降,并使蒸腾速率（Tr）的于净光合速率（Pn）的降幅而导致WUE升高;SEC从52.0％降至20.1%时,WUE降低的根本原因在于羧化效率下降使Pn大为降低 水后WUE回升,但至复水第7天仍低于对照,土壤渍水当天WUE下降,第3天回升至对照水平,之后随渍水期延长WUE逐渐降低,淹水第6天羧化效率开始下降。     

Chloride-activated water permeability in the frog corneal epithelium

We have previously reported that the isolated frog corneal epithelium (a Cl^–-secreting epithelium) has a large diffusional water permeability (P_dw 1.8×10^–4 cm/s). We now report that the presence of Cl^– in the apical-side bathing solution increases the diffusional water flux, J_dw (in both directions) by 63% from 11.3 to 18.4 l min^–1 · cm^–2 with 60 mm [Cl] exerting the maximum effect. The presence of Cl^– in the basolateral-side bathing solution had no effect on the water flux. In Cl^–-free solutions amphotericin B increased J_dw by 29% but only by 3% in Cl^–-rich apical-side bathing solution, suggesting that in Cl^–-rich apical side bathing solution, the apical barrier is no longer rate limiting. Apical Br^– (75 mm) also increased J_dw by 68%. The effect of Cl^– on J_dw was observed within 1 min after its addition to the apicalside bathing solution. HgCl₂ (0.5 mm) reduced the Cl^–-increased P_dw by 31%. The osmotic permeability (P_f) was also measured under an osmotic gradient yielding values of 0.34 and 2.88 (x 10^–3 cm/s) in Cl^–-free and Cl^–-rich apical-side bathing solutions respectively. It seems that apical Cl^–, or Cl^– secretion into the apical bath could activate normally present but inactive water channels. In the absence of Cl^–, water permeability of the apical membrane seems to be limited to the permeability of the lipid bilayer.This work was supported by National Eye Institute grants EY-00160 and EY-01867.