Rapid Oscillations in Plant Water Potential Measured with a Stem Psychrometer

Oscillations in water potential were measured for the firsttime in the plant stem. The phase of the oscillations in waterpotential (measured in Brussels sprout) lagged behind that oftranspiration by less than 5 min. The cycles ranged from 22to 48 min. The results support the view that oscillations inwater potential in the plant stem and interconnecting xylemtissues would synchronize oscillations in transpiration andstomatal opening throughout the plant. Brassica oleraceae L., Brussels sprout, oscillations, stem water potential, transpiration, stem psychrometer     

Effect of Brown Stem Rot on Water Relations of Soybean

To investigate the effects of brown stem rot, a vascular disease of soybean (Glycine max) induced by Phialophora gregata, on the water relations of diseased plants, stems of greenhouse-grown plants of susceptible (Pride B216) and resistant (BSR 201) cultivars were injected with the pathogen at vegetative growth stage VI. Plants of both cultivars developed internal stem browning, but those of Pride B216 developed more severe symptoms of water stress (reduced leaf water potential and stem conductance). Inoculated plants of both cultivars also had reduced stem conductance and increased stomatal conductance and transpiration. Disease-related water stress can be attributed to the combined effects of reduced stem conductance and increased water loss resulting from increased stomatal conductance.     

Stem flow and porometer measurements of transpiration from honey mesquite (Prosopis glandulosa)

The objective of this study was to compare stem flow and porometermethods of measuring transpiration of honey mesquite (Prosopisglandulosa) trees on a semiarid site. Stem flow was measuredusing heat balance stem flow gauges. Porometer measurementsof leaf stomatal conductance (g_s) were made within foliage layersof each stem and scaled to transpiration values for the entirestem (E_stem) using stem leaf area. Simultaneous measurementsusing both methods were made diurnally and under artificiallyimposed stem shading or defoliation in June and October 1990.Stem flow and E_stem had similar diumal patterns except on 2d in June when E_stem increased during the afternoon while stemflow declined relative to midday values. During October, E_stemwas greater than stem flow throughout the day. This was attributedto sampling error in which only undamaged leaves were used forporometer measurements yet, by this time in the growing season,many leaves on each stem were damaged from insects or wind andlikely had lower transpiration rates. A regression coefficientbetween E_stem and stem flow of 0.79 in June and 0.91 in Octobersuggested the two methods were comparable, but there was considerablevariation between methods during peak transpiration rates. Bothtechniques detected that artificial shading or defoliation causedsimilar relative declines in transpiration. Results imply thatestimates of stem transpiration can be obtained by scaling porometermeasurements of leaves but accuracy declines at higher transpirationrates. Key words: Sap flow, evapotranspiration, stomatal conductance, scaling, water relations     

Diurnal Changes in Stem Diameter Depend Upon Variations in Water Content: Direct Evidence in Peach Trees

Three to five-year-old peach trees (Prunus persica (L.) Batschcv. ‘Maycrest’) grafted on P. ‘Damas 1869’grown in a sand trench were removed in the spring and grownhydroponically for several months. The system comprised twobalances continuously recording the mass of the nutrient solutionand that of the tree, so as to estimate transpiration and wateruptake rates separately. Diurnal variation in plant water content(transpiration minus water uptake) was observed, with rapidlydecreasing values when the solar radiation increased, whilethe reverse occurred when radiation decreased. Changes in stemdiameter were continuously recorded using linear variable differentialtransducers. Data collected over several days of contrastingclimatic conditions revealed that rapid changes in the stemdiameter occurred throughout the day and were closely relatedto plant water content. A lag-time not exceeding 10 min wasfound between changes in stem diameter and plant water content.These results are discussed in relation to the use of micromorphometricmethods to control irrigation in fruit trees. Moreover, we givevalues for the water stored in the shoots which may contributeto the transpiration stream. Key words: Prunus persica, stem shrinkage, plant water storage, water uptake, transpiration     

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.     

Sap flow as an indicator of transpiration and the water status of young apricot trees

The relationship between water loss via transpiration and stem sap flow in young apricot trees was studied under different environmental conditions and different levels of soil water status. The experiment was carried out in a greenhouse over a 2-week period (November 2–14, 1997) using three-year-old apricot trees (Prunus armeniaca cv. Búlida) growing in pots. Diurnal courses of leaf water potential, leaf conductance and leaf turgor potential also were recorded throughout the experiment. Data from four days of different enviromental conditions and soil water availability have been selected for analysis. On each of the selected days the leaf water potential and the mean transpiration rates were well correlated. The slope of the linear regression of this correlation, taken to indicate the total hydraulic resistance of the tree, confirmed an increasing hydraulic resistance under drought conditions. When the trees were not drought stressed the diurnal courses of sap flow and transpiration were very similar. However, when the trees were droughted, measured of sap flow slightly underestimated actual transpiration. Our heat-pulse measurements suggest the amount of readily available water stored in the stem and leaf tissues of young apricot trees is sufficient to sustain the peak transpiration rates for about 1 hour. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Relationship between Stem Diameter and Water Potentials in Stems of Young Cabbage Plants

Continuous measurements were made of stem shrinkage, stem waterpotential (₃) and transpiration rate (T) in young, pot-growncabbage plants subjected to cycling evaporative demands. Sequencesof increasing evaporative demands induced increases in T anddecreases in both ₃ and stem diameter and conversely, wheneverevaporative demand decreased, T declined and ₃ and stem diameterrose. Over short periods, stem water potentials and stem shrinkagewere virtually parallel even when rapid oscillations were induced.Over longer periods the effects of growth were important comparedwith those of water stress on stem diameter when the moisturecontent of the soil was high. Growth, however, ceased when theplant was subjected to relatively mild water stress (₃ = –0.4MPa). Stem diameters, after correction for growth, were linearlyrelated to plant water potential. The results suggest that stemshrinkage and only a few calibration measurements might be usedto provide continuous estimates of water potentials in fieldcrops. Key words: Cabbage, Stem diameter, Stem water potential     

Water potential components in growing citrus fruits

Growing navel orange fruits (Citrus sinensis) 5.4 to 5.7 centimeters in diameter were used as a model system to determine the effects of transpiration and carbohydrate translocation on water and osmotic potentials in fruit tissues. Evidence supported the hypothesis that osmotic potential in the vesicles would be affected little by changes in transpiration or carbohydrate translocation because the vesicles are anatomically isolated from the transpiration stream and are at the end of the carbohydrate translocation pathway. In the mesocarp tissue, which contains a vascular network, osmotic potential decreased during the daytime when environmental conditions favored transpiration and increased at night. Exocarp water potential followed a similar pattern. Girdling of the stem above the fruits 5 days before sampling caused an increase of osmotic potential in the mesocarp but had no effect on exocarp water potential. Neither diurnal changes in transpiration nor girdling of the stem affected the osmotic potential of the vesicles.     

Stem hydraulic capacitance decreases with drought stress: implications for modelling tree hydraulics in the Mediterranean oak Quercus ilex

Hydraulic modelling is a primary tool to predict plant performance in future drier scenarios. However, as most tree models are validated under non‐stress conditions, they may fail when water becomes limiting. To simulate tree hydraulic functioning under moist and dry conditions, the current version of a water flow and storage mechanistic model was further developed by implementing equations that describe variation in xylem hydraulic resistance (R_X) and stem hydraulic capacitance (C_S) with predawn water potential (Ψ_PD). The model was applied in a Mediterranean forest experiencing intense summer drought, where six Quercus ilex trees were instrumented to monitor stem diameter variations and sap flow, concurrently with measurements of predawn and midday leaf water potential. Best model performance was observed when C_S was allowed to decrease with decreasing Ψ_PD. Hydraulic capacitance decreased from 62 to 25 kg m^?3 MPa^?1 across the growing season. In parallel, tree transpiration decreased to a greater extent than the capacitive water release and the contribution of stored water to transpiration increased from 2.0 to 5.1%. Our results demonstrate the importance of stored water and seasonality in C_S for tree hydraulic functioning, and they suggest that C_S should be considered to predict the drought response of trees with models.     

Stem diameter changes before bud opening in <Emphasis Type="Italic">Zelkova serrata</Emphasis> saplings

It is well known that stems of woody plants shrink and swell diurnally. These fluctuations of stem diameter are induced mainly by the changes of water contents in plants, which are caused by the combination of leaf transpiration and root absorption of water. This implies that dormant-like deciduous broadleaved trees in a leafless state should show no or less changes in stem diameter. However, some physiological activities in woody plants are also known to precede their winter bud opening. Whether and how diameter changes occur in deciduous tree stems during winter was investigated using Zelkova serrata saplings in a leafless state. Measurements of stem diameter changes were done for more than 4 months continuously. The saplings showed distinct diameter changes with periodicities from diurnal to a few weeks, and these changes were initiated 2 months before winter bud opening. These results indicate that some physiological and/or developmental activities occur in the stem of deciduous trees before winter bud opening, and do not correspond to changes in water relations as a result of leaf transpiration. These internal activities cause fluctuations in stem diameter prior to winter bud opening in deciduous trees. Electronic Publication     

Stem radius changes and their relation to stored water in stems of young Norway spruce trees

Changes in the stem radius of young Norway spruce [Picea abies (L.) Karst.] were related to changes in stem water content in order to investigate the relationship between diurnal stem size fluctuations and internally stored water. Experiments were performed on living trees and on cut stem segments. The defoliated stem segments were dried under room conditions and weight (W), volume (V), and xylem water potential (O_s) were continuously monitored for 95 h. Additionally, photos of cross-sections of fresh and air-dried stem segments were taken. For stem segments we found that the change in V was linearly correlated to the change in W as long as O_s was >-2.3ǂ.3 MPa (phase transition point). Stem contraction occurred almost solely in the elastic tissues of the bark (cambium, phloem, and parenchyma), and the stem radius changes were closely coupled to bark water content. For living trees, it is therefore possible to estimate the daily contribution of "bark water" to transpiration from knowledge of the stem size and continuous measurements of the stem radius fluctuations. When O_s reaches the phase-transition point, water is also withdrawn from the inelastic tissue of the stem (xylem), which - in the experiment with stem segments - was indicated by an increasing ratio between (V and (W. We assume that for O_s below the transition point, air is sucked into the tracheids (cavitation) and water is also withdrawn from the xylem. Due to the fact that in living P. abies O_s rarely falls below -2.3ǂ.3 MPa and the xylem size is almost unaffected by radius fluctuations, dendrometers are useful instruments with which to derive the diurnal changes in the bark water contents of Norway spruce trees.     

The Relationship Between Plant Water Potential and Transpiration Rate in Young Cabbage Plants Growing in Wet Soil

Water potentials in the plant stem were monitored together withtranspiration rates in young cabbage plants growing in potsof soil and subjected to cyclic transpirational demands. Whenthe soil was very wet plant water potential was approximatelyproportional to transpiration rate. At lower soil water contentswater potentials remained almost constant despite considerablechanges in transpiration rate. When the transpiration rate washigh, water potentials in plants were lower in wet soil thanin soil having a slightly lower moisture content. It is arguedthat the results can best be explained in terms of root conductivityincreasing as the plant becomes more water stressed when thegradient between soil and plant water potentials steepens. Key words: Cabbage, Stem water potential, Transpiration rate     

Quantitative analysis of transpiration stream dynamics in an intact cucumber stem by a heat flux control method

Water flux of transpiration stream in an intact stem of the 10 leaf stage cucumber plant (Cucumis sativus L. cv. Chojitsu-Ochiai) was measured by a novel system of heat flux control method with a resolution of 1 × 10⁻³ grams per second and a time constant of 1 minute; two heat flux control sensors were attached to the seventh internode and the stem base. The transpiration stream responded clearly to leaf transpiration and root water absorption when the plant was exposed to light, and the water flux at the stem base corresponded to the transpiration rate per plant in steady state. Root water absorption lagged about 10 minutes behind leaf transpiration. Dynamics of water fluxes were affected by the lag of water absorption in roots, and temporary water loss caused by rapid increase in leaf transpiration was buffered by about 5% of the water content in the stem.     

Analysis of Growth and Water Relations of Tomato Fruits in Relation to Air Vapor Pressure Deficit and Plant Fruit Load

The influence of air vapor pressure deficit (VPD) and plant fruit load on the expansion and water relations of young tomato fruits grown in a glasshouse were evaluated under summer Mediterranean conditions. The contributions of phloem, xylem and transpiration fluxes to the fruit volume increase were estimated at an hourly scale from the growth curves of intact, heat-girdled and detached fruits, measured using displacement transducers. High VPD conditions reduced the xylem influx and increased the fruit transpiration, but hardly affected the phloem influx. Net water accumulation and growth rate were reduced, and a xylem efflux even occurred during the warmest and driest hours of the day. Changes in xylem flux could be explained by variations in the gradient of water potential between stem and fruit, due to changes in stem water potential. Misting reduced air VPD and alleviated the reduction in fruit volume increase through an increase in xylem influx and a decrease in fruit transpiration. Under low fruit load, the competition for assimilates being likely reduced, the phloem flux to fruits increased, similarly to the xylem and transpiration fluxes, without any changes in the fruit water potential. However, different diurnal dynamics among treatments assume variable contributions of turgor and osmotic pressure in F3 and F6 fruits, and hypothetical short-term variations in the water potential gradient between stem and fruit, preventing xylem efflux in F3 fruits.     

亚低温与干旱胁迫对番茄植株水分传输和形态解剖结构的影响

为探讨亚低温和干旱对植株水分传输的影响机制,以番茄幼苗为试材,利用人工气候室设置常温(昼25 ℃/夜18 ℃)和亚低温(昼15 ℃/夜8 ℃)环境,采用盆栽进行正常灌水(75%～85%田间持水量)和干旱处理(55%～65%田间持水量),分析了温度和土壤水分对番茄植株水分传输、气孔和木质部导管形态解剖结构的影响。结果表明: 与常温正常灌水处理相比,干旱处理使番茄叶水势、蒸腾速率、气孔导度、水力导度、茎流速率、气孔长度和叶、茎、根导管直径显著减小,而使叶、茎、根导管细胞壁厚度和抗栓塞能力增强;亚低温处理下番茄叶水势、蒸腾速率、气孔导度、水力导度和叶、茎、根导管直径显著降低,但气孔变大,叶、根导管细胞壁厚度和叶、茎、根抗栓塞能力显著升高。亚低温条件下土壤水分状况对番茄叶水势、蒸腾速率、气孔导度、水力导度、气孔形态、叶、根导管结构均无显著影响。总之,干旱处理下番茄通过协同调控叶、茎、根结构使植株水分关系重新达到稳态;亚低温处理下番茄植株水分关系的调控主要通过改变叶和根导管结构实现,且受土壤水分状况的影响较小。     

EFFECT OF SOIL MOISTURE STRESS ON THE CORRELATION BETWEEN HEAT PULSE VELOCITY AND TRANSPIRATION

(1) Heat pulse velocity in the stem of pine and sour orangeseedling, measured by HUBER's heat pulse method, was correlatedwith transpiration as measured by determining the humidity ofair passed over the plant. This correlation was used to calculatetranspiration under natural conditions, when heat pulse velocityonly was measured. (2) It was shown that heat pulse velocity at a given transpirationrate was related to water stress. When water potential in thesoil decreased heat pulse velocity increased in pine seedlingsand decreased in sour orange. This effect of water stress didnot appear to be completely reversible. Under conditions ofvery low water potential in the soil, no useful correlationbetween transpiration and heat pulse velocity could be obtainedeither in pine or sour orange. (3) From the data obtained it is concluded that transpirationcan be reliably calculated from heat pulse velocity measurementsonly if the calibration curve relating transpiration to heatpulse velocity is determined shortly beforehand under similarconditions of soil moisture and potential evapotranspiration.Even so, only total daily transpiration can be estimated bythis method. Hourly figures of transpiration are unreliable,especially under conditions conducive to low water potentialin the plant. (Received June 24, 1964; )     

Changes in Alfalfa Stem Conductance Induced by Corynebacterium insidiosum Toxin

A toxin involved in bacterial wilt of alfalfa (Medicago sativa L.) has been isolated from cultures of the pathogen, Corynebacterium insidiosum, as well as from diseased plants (S. M. Ries and G. A. Strobel. 1972. Physiological Plant Pathology 2: 133-142). The influence of this toxin, a glycopeptide with a molecular weight of 5 × 10⁶, on the water relations of alfalfa was examined. It was found that very small amounts of the toxin (2 μg) significantly reduced stem conductance through 15-cm long stems. This decrease in stem conductance caused by the toxin best explains the rapid decrease in transpiration and stomatal conductance and the resultant wilting after alfalfa cuttings have been in 200 μg ml⁻¹ toxin for 2 hours. Membrane damage resulting in water leakage was ruled out as a factor in the wilting during the 2-hour period. It is postulated that the toxin acts by interfering with water movement through pit membranes.     

Propagation in Vicia faba Stem of a Potential Variation Induced by Wounding

Heating of leaf blade in Vicia faba triggered bioelectricalevents ("variation potential") spreading throughout the shoot,preferentially in the acropetal direction. The spreading ofa variation potential, previously described in Mimosa and otherplants, is related to a concomitant transport of a stimulatingsubstance in the transpiration stream. This conclusion is supportedby the observation in Vicia faba stem that conduction in thebasipetal direction is promoted when the transpiration streamis inverted. The conduction is promoted in the vascular bundles in directrelation with the burnt lobe of the leaf, but a lateral conductionmay exist, as electrical variations are recorded in isolatedbundles of the orthostichy opposite to the wounded leaf. A local cooling (1C) of the stem has no influence on the conductionrate but it completely inhibits the bioelectrical variationsin the cooled zone. This last result reinforces earlier demonstrationsthat the substance is transported in the vessels, accordingto a physical process, but that the electrical events requiremetabolic integrality in other cellular members. (Received May 18, 1985; Accepted July 19, 1985)     

Frequency and Time-Domain Dielectric Measurements of Stem Water Content in the Arborescent Palm, Sabal palmetto

Two methods for monitoring stem water content in the arborescentpalm, Sabal palmetto by determining its dielectric constantwerecompared. The first approach used an oscillating circuit whosefrequency (40 to 70 kHz) was determined by a parallel-platecapacitor that sandwiched a portion of the stem. The secondtechnique was based on measurement of the velocity of an electromagneticpulse (frequency range of 500 kHz and 1 GHz) propagating withina wave-guide embedded in the stem (Time-Domain Reflectometry,TDR). There was basic agreement in the apparent dielectric constantas determined by the two techniques; both resulted in valuesof approximately 90 when the plant was fully hydrated, fallingto values near 50 when water was withheld for one week. The capacitance technique was non-invasive, but was influencedby temperature fluctuations, and we were unable to calibrateit accurately against stem volumetric water content. Insertionof TDR probes did not lead to tissue damage and determinationof an empirical relationship to volumetric water content allowedquantitative estimates of stem water content. Sensitivity ofTDR to small changes in stem water content was restricted bythe fact that attenuation of the pulse within the stem necessitatedthe use of short (0·125 m) wave guides. Despite this,during periods of high transpiration (>10kg plant^–1d^–1) bi-hourly changes in stemmoisture content were detectable. Key words: Dielectric constant, Sabal palmetto, stem capacitance, time-domain reflectometry, water storage     

Water flow and water storage in Agave deserti: osmotic implications of crassulacean acid metabolism

Abstract Water flow and water storage were investigated for Agave deserti, a desert succulent showing crassulacean acid metabolism (CAM). The anatomy and water relations of the peripheral chlorenchyma, where CAM occurs, and the central water-storage parenchyma were investigated for its massive leaves so that these tissues could be incorporated as discrete elements into an electrical-circuit analogue of the whole plant. The daily cycling of osmotic pressure was represented by voltage sources in series with the storage capacitors. With soil water potential and leaf transpiration rate as input variables, axial water flow through the vascular bundles and radial flows into and out of storage during the day/night cycle were determined. The predominantly nocturnal transpiration was coincident with increases in cell osmotic pressure and in titratable acid of the leaf chlorenchyma. In the outer layers of the chlorenchyma, water potential was most negative at the beginning of the night when transpiration was maximum, while the water-storage parenchyma reached its minimal water potential 9 h later. The roots plus stem contributed 7% and the leaves contributed 50% to the total water flow during maximal transpiration; peak water flow from the soil to the roots occurred at dawn and was only 58% of the maximal transpiration rate. Over each 24-h period, 39% of the water lost from the plant was derived from storage, with flow into storage occurring mainly during the daytime. Simulations showed that the acid accumulation rhythm of CAM had little impact on water uptake from the soil under the conditions employed. In the outer chlorenchyma, water potential and water flows were more sensitive to the day/night changes in transpiration than in osmotic pressure. Nevertheless, cell osmotic pressure had a large influence on turgor pressure in this tissue and determined the extent to which storage was recharged during the latter part of the night.