Changes in hydraulic conductance of citrus trees following a reduction in wetted soil volume

Abstract The effcct of the transition from fully to partially wetted soil voluine on transpiration rate and hydraulic conductance of mature citrus trees was examined in a 23-year-old, coninicrcial, sprinklerirrigated, Shanio u t i orange orchard. I rriga t i on frequency was determined by the rate of water loss from the soil, a s measured by neutron probes. The hydraulic conductance of tlic tree was coniputed from the rclationship between sap flow i n the trunk and leaf water potential. The diurnal valucs of leaf water potential and sap flow shifted towards lower levels as tlie water stored in the root zone was depleted. In the fully wetted soil volume the tree hydraulic conductance remained constant throughout the irrigation period, from June to Novcniber. However, partial wetting of the soil volume (40%) caused a reduction in the hydraulic conductance of the tree. Tlie decreased hydraulic conductance is attributed to tlie permanent interruption of water transport in part of tlie root system. Tlie rcsults of tlie experiment suggest that despite tlie increase of irrigation frequency, partial wetting intensifies water stress in tlie trees.     

A Krypton Diffusion Porometer for the Direct Field Measurement of Stomatal Resistance

A stomatal diffusion porometer is described which measures directlythe diffusion of radioactive krypton through amphistomatousleaves. The porometer is relatively small and portable and iseasily used under field conditions. It consists of a miniaturediffusion chamber above an acrylic plastic reservoir which contains1200 cm³ of air enriched with ⁸⁵Kr. Geiger tubes in the diffusionchamber and in the reservoir monitor the relative concentrationsof 85Kr. Krypton is allowed to diffuse from the larger reservoirthrough the leaf into the diffusion chamber and the time forits concentration in relation to that in the reservoir to changebetween two fixed values is recorded. When this time lapse wascalibrated against known resistances a linear relationship,independent of temperature was found. Sources of error are analysedand some experiments are described in which the porometer wasused to measure diurnal changes in stomatal resistance. Resistancesof potted sunflower plants (Helianthus annuus L.) grown in agreenhouse were measured with both the krypton diffusion porometerand a condensation-type porometer and the results were usedto calculate both cuticular and stomatal resistances. Demonstrationof field measurements with the porometer include data from eucalyptustrees {Eucalyptus camaldulensis Dehn) and from an unirrigatedcotton crop {Gossypium hirsutum var. SJ 2) growing under semi-aridconditions. Stomatal conductance of the cotton crop during theopening phase was linearly related to solar radiation.     

Photosynthesis and Diffusion Conductance of the Valencia Orange Fruit under Field Conditions

CO₂ uptake and diffusion conductance of Valencia orange fruits(Citrus sinensis L. Osbeck) were measured in the field duringthe growing season of 1977/78 to ascertain if, as in the leaf,stomata control photosynthesis and transpiration under changingenvironmental conditions. Measurements were made on 15 yearold trees grown in a sandy loam soil and receiving either adry or a wet treatment. Fruit diffusive conductance was measuredwith a modified water vapour diffusion conductance meter andgross photosynthesis was measured with a ¹⁴CO₂ uptake meter.Photosynthetically active radiation (PAR) was measured witha quantum sensor. Fruits exposed to light assimilated CO₂ ata rate which was 25–50% of that assimilated by leaves.The uptake was dependent on fruit size, PAR, chlorophyll content,and on diffusive conductance of the fruit epidermis. Epidermalconductance showed a diurnal trend which was similar in shapeto that of the leaf except in the late afternoon. Cuticularconductance of the fruit was calculated and ranged between 0.22and 0.30 mm s^–1. It was speculated that the CO₂ uptakeby the fruit could support the growth of flavedo cell layerswhen exposed to light. Dry soil caused an increase in the ¹⁴CO₂uptake by fruit possibly caused by the increased potential areaof the stomatal opening per unit of fruit surface area.     

The Partitioning of Resistances to Gaseous Diffusion in the Leaf Epidermis and the Boundary Layer

The partitioning of total leaf resistance to gaseous diffusioninto the cuticular, stomatal, and boundary-layer componentsof the upper and lower leaf surfaces was studied. A generalformula was developed, which showed that the mean stomatal oreven mean epidermal resistances may not be obtained by simplesubtraction of the boundary-layer component from the total,except in very specific cases. The formula was used to studythe relationship between the mean stomatal resistance, _s, and the residual leaf resistance, r_l (obtainedby subtracting the boundary-layer from the total resistance),in leaves differing in their degree of anisolaterality (withrespect to stomatal resistance), cuticular resistance, and totalresistance, as affected by the relative magnitude of the boundary-layercomponent. Graphical presentation of the analyses permits evaluationof the difference between _s, and r_l in awide variety of cases. It was shown experimentally that whenthe stomatal component of each leaf surface is known, the totalresistance calculated from these values according to the formulaclosely matches the measured value.     

The Partitioning of Hydraulic Conductances within Mature Orange Trees

Sap flow (F) and leaf water potential (LWP) were followed diurnallyin mature Valencia and Shamouti orange trees in an orchard.The hydraulic conductance of these trees was computed from thediurnal relationship between the LWP and F. The driving forcefor water movement was estimated from a weighted average ofsunlit and shaded LWP, assuming that leaves in the shade transpireto some extent. LWP of covered, non-transpiring leaves was alsomeasured hourly. It was assumed to represent the xylem waterpotential within the axial conduit of the trunk. Relating coveredLWP to F on an hourly basis enables the computation of the hydraulicconductance of the root system, including axial conductances.The hydraulic conductance of the transpiring crown was computed.Its magnitude was comparable to the root system hydraulic conductance. Key words: Orange trees, hydraulic conductance, sap flow, leaf water potential     

The Relationship Between Leaf Resistance to Mass Air Flow and Infiltration Score in the Cotton Crop

A close relationship was found between measurements of degreeof infiltration into stomata and their resistance to mass flowof air. The relationship was not affected by soil moisture statusover a wide range. A difference was found in the relationshipof plants growing under glasshouse and field conditions andthis was attributed to differences in the ratio of stomata onthe upper to lower epidermis (alpha ratio). The physical basisfor the relationship found is discussed.     

A Radioactive Tracer Technique for the Direct Measurement of the Diffusion Resistance of Stomata

A new method is described for measuring the stomatal resistanceto diffusion in leaves. It consists of measuring the radioactivityof air samples which have diffused through a leaf from a porometercontainer enriched with radioactive argon(⁴¹A). Details of thecalibration are given together with the methods adopted to determinethe accuracy of measurement and the optimum time of sampling.Results suggest that in the material investigated the measurementsobtained are essentially those of stomatal resistance.In onesmall experiment inter and intra plant variability in stomatalresistance was investigated; in another, the diffusion porometerwas used to compare the daily march of stomatal resistance withthe rate of transpiration in plants growing under four differentsoil-moisture regimes.