Vapor Concentration Difference Research Articles

The role of thermodiffusion in transpiration.

Plant leaf temperatures can differ from ambient air temperatures. A temperature gradient in a gas mixture gives rise to a phenomenon known as thermodiffusion, which operates in addition to ordinary diffusion. Whilst transpiration is generally understood to be driven solely by the ordinary diffusion of water vapour along a concentration gradient, we consider the implications of thermodiffusion for transpiration. We develop a new modelling framework that introduces the effects of thermodiffusion on the transpiration rate, E. By applying this framework, we quantify the proportion of E attributable to thermodiffusion for a set of physiological and environmental conditions, varied over a wide range. Thermodiffusion is found to be most significant (in some cases > 30% of E) when a leaf-to-air temperature difference coincides with a relatively small water vapour concentration difference across the boundary layer; a boundary layer conductance that is large as compared to the stomatal conductance; or a relatively low transpiration rate. Thermodiffusion also alters the conditions required for the onset of reverse transpiration, and the rate at which this water vapour uptake occurs.

Examining indicators and methods for quantifying ozone exposure to vegetation

The U.S. National Park Service uses W126 to assess critical levels of ozone (O3) exposure to sensitive plant species, whereas the scientific community considers the stomatal flux-based metric, phytotoxic O3 dose (POD), more relevant in foliar risk evaluation. This study found a decreasing trend of 0.15 ppm-hrs/yr (p = 0.049) in W126 at a Yellowstone National Park monitoring site during 1997–2021 and no trend at a Grand Teton National Park site over 2012–2021 but no trends at both sites in POD calculated following the United Nations Economic Commission for Europe Convention on Long-range Transboundary Air Pollution (CLRTAP) (2017). To evaluate the CLRTAP (2017) method, 2019 summer POD was calculated for comparison using the Big-leaf model and was found 20%–100% less than the CLRTAP (2017)-calculated leaf-level one already, let alone compared to the latter scaled up to canopy-level, owing to the much smaller stomatal O3 flux (SOF) estimates under most conditions. Further analysis revealed exceptions where larger SOF values (>0.02 mmol/m2/hr), obtained from using the Big-leaf model with measured sensible heat and water vapor flux included in input, were not captured by the CLRTAP (2017) method and the Big-leaf model with input of regular meteorological variables only. Those large SOF values resulted under cooler, less windy, more humid, and cloudier conditions, when the approximate water vapor concentration difference between the ambient and leaf saturation level was found <1 mol/m3. A multivariate function was developed, through machine learning, by regressing SOF on air temperature, wind speed, relative humidity, phytosynthetically active radiation, and O3 concentration. Its testing and evaluation demonstrated improved estimates of the large variability in SOF. This new parameterization showed promising capability of assessing critical levels of O3 exposure in park management, and it could be further improved with long-term flux measurement data becoming available and evapotranspiration better represented.

Ultrathin MgO Nanosheets Fabricated by Thermal Evaporation Method in Air at Atmospheric Pressure

Ultrathin MgO nanosheets were successfully synthesized by thermal evaporation of a mixture of Mg and graphite powders as the source material. The synthesis was performed at 1000 oC in air. Scanning electron microscopy showed that the two-dimensional MgO nanosheets had widths of several micrometers and the thickness of less than 20 nm. X-ray diffraction analysis revealed that the MgO nanosheets had a cubic crystal structure and high purity. Zero-dimensional MgO nanocubes were formed at temperatures below 1000 oC and one-dimensional MgO nanowires were grown at a temperature higher than 1000 oC. As the synthesis temperature increased, the morphology of the Mg nanocrystals changed from cube to sheet and then wire. The experimental results suggested that the difference in Mg vapor concentration could be responsible for the morphological change in the MgO nanocrystals. When Mg vapor concentration was low, MgO nanocrystals were grown with a cubic shape. A relatively high concentration of Mg vapor led to the growth of sheet-like MgO nanocrystals. A very high Mg vapor concentration favored the growth of MgO nanowires. The growth mechanism is discussed based on the Mg vapor concentration and the crystal structures of Mg and MgO. Visible emissions, which were attributed to lattice defects such as oxygen vacancies, were observed in the cathodoluminescence spectra of the MgO nanocrystals.

Evaporation and Electrowetting of Sessile Droplets on Slippery Liquid-Like Surfaces and Slippery Liquid-Infused Porous Surfaces (SLIPS).

Sessile droplet evaporation underpins a wide range of applications from inkjet printing to coating. However, drying times can be variable and contact-line pinning often leads to undesirable effects, such as ring stain formation. Here, we show voltage programmable control of contact angles during evaporation on two pinning-free surfaces. We use an electrowetting-on-dielectric approach and Slippery Liquid-Infused Porous (SLIP) and Slippery Omniphobic Covalently Attached Liquid-Like (SOCAL) surfaces to achieve a constant contact angle mode of evaporation. We report evaporation sequences and droplet lifetimes across a broad range of contact angles from 105°–67°. The values of the contact angles during evaporation are consistent with expectations from electrowetting and the Young-Lippman equation. The droplet contact areas reduce linearly in time, and this provides estimates of diffusion coefficients close to the expected literature value. We further find that the total time of evaporation over the broad contact angle range studied is only weakly dependent on the value of the contact angle. We conclude that on these types of slippery surfaces, droplet lifetimes can be predicted and controlled by the droplet’s volume and physical properties (density, diffusion coefficient, and vapor concentration difference to the vapor phase) largely independent of the precise value of contact angle. These results are relevant to applications, such as printing, spraying, coating, and other processes, where controlling droplet evaporation and drying is important.

Experimental study of drop shape and wake effects on particle scavenging for non-evaporating drops using ultrasonic levitation

Single drop particle scavenging was experimentally measured for a non-evaporating drop using an ultrasonic levitation technique. This technique enabled measurements of scavenging efficiency, E, for individual drops, and allowed for control of drop axis ratio, α, drop shape oscillations, and Reynolds number, Re, independently from drop diameter. Non-evaporating drops were used which resulted in essentially zero temperature and vapor concentration difference between the drop surface and the surrounding air, virtually eliminating the possibility of confounding phoretic effects. Plots of E versus Stokes number, Stk, became independent of α when Stk was calculated using the Sauter mean diameter (as opposed to the equivolume diameter). Furthermore, E was shown to be insensitive to both Re and drop shape oscillations, suggesting that wake effects do not have a measurable impact on E. Finally, a method was developed to relate E for spherical drops, which are assumed for existing scavenging model predictions, to E for arbitrarily deformed drops, such as those occurring in rain. Of note, these are the first measurements of droplet scavenging using ultrasonic levitation.

Effects of leaf temperature on initial stomatal opening and their roles in overall and biochemical photosynthetic induction

The more stomata affect photosynthetic induction courses, the stronger the influence of leaf temperature on the induction parameters becomes. Initial stomatal conductances relative to species-specific stomatal thresholds explain seemingly contradictory response patterns. We analyzed the effects of leaf temperature, T leaf (at simultaneously altered leaf-to-air vapor concentration difference, Δw) on “overall” and “biochemical” photosynthetic induction as modified by stomata and in response to single, rectangular steps from darkness to saturating light. Studies were performed in sun leaves of shade-intolerant Betula pubescens Ehrh. and shade-tolerant Fagus sylvatica L. In general, induction proceeded faster with higher T leaf. This pattern was clearly visible when limitations to induction were dominated by stomata, which was the case at low initial stomatal conductance (g ini), as well as when there was a long stomatal lag time in response to the light-step (t lag) and slow stomatal opening (long time to reach 90% of full stomatal conductance, t 90%g). t lag and t 90%g became shorter with rising T leaf despite strongly increasing Δw (temperature-driven), while g ini was not affected. Species-specific thresholds, namely g ini(crit), above which halftimes of induction are no longer related to g ini, were higher in Betula (≈35 mmol m−2 s−1) and most likely not temperature related, while they decreased with T leaf (from 15 to 35 °C) in Fagus from about 30 to 10 mmol m−2 s−1. In Betula, g ini values were typically above and in Fagus typically below g ini(crit). Induction states 60 s after the light-step (IS60) rose with temperature while becoming more sensitive to g ini with higher T leaf. Induction courses which began either above or below g ini(crit) resulted in entirely different dependencies of induction halftimes on T leaf, confirming the importance of g ini(crit). This must be kept in mind when comparing different species. Implications for modeling are discussed.

A steady-state stomatal model of balanced leaf gas exchange, hydraulics and maximal source-sink flux.

Trees must simultaneously balance their CO2 uptake rate via stomata, photosynthesis, the transport rate of sugars and rate of sugar utilization in sinks while maintaining a favourable water and carbon balance. We demonstrate using a numerical model that it is possible to understand stomatal functioning from the viewpoint of maximizing the simultaneous photosynthetic production, phloem transport and sink sugar utilization rate under the limitation that the transpiration-driven hydrostatic pressure gradient sets for those processes. A key feature in our model is that non-stomatal limitations to photosynthesis increase with decreasing leaf water potential and/or increasing leaf sugar concentration and are thus coupled to stomatal conductance. Maximizing the photosynthetic production rate using a numerical steady-state model leads to stomatal behaviour that is able to reproduce the well-known trends of stomatal behaviour in response to, e.g., light, vapour concentration difference, ambient CO2 concentration, soil water status, sink strength and xylem and phloem hydraulic conductance. We show that our results for stomatal behaviour are very similar to the solutions given by the earlier models of stomatal conductance derived solely from gas exchange considerations. Our modelling results also demonstrate how the 'marginal cost of water' in the unified stomatal conductance model and the optimal stomatal model could be related to plant structural and physiological traits, most importantly, the soil-to-leaf hydraulic conductance and soil moisture.

Short-term stable isotopic composition variations of near-surface atmospheric water vapor in four semiarid areas (Binxian, Guyuan, Wujiachuan, Yuzhong) in interior northwestern China

This study used an in situ method to monitor short-term atmospheric water vapor concentrations and stable isotopic composition (δ18Ov and δDv) variations at four sites in semiarid areas of northwestern China during 2014. On a spatial scale, we distinguish a south-to-north difference in atmospheric water vapor concentration. The spatial patterns of water vapor lines (WVLs) may be associated with regional setting. Smaller slopes of WVLs in Guyuan and Yuzhong may reflect dew evaporation, and a steeper slope of WVL in Wujiachuan may be representative of atmospheric water vapor isotopic composition in equilibrium with that of precipitation. On a temporal scale, δ18Ov, δDv, and dv exhibited variable patterns, indicating various influences of entrainment, local evapotranspiration (ET), and dew evaporation. A deceasing trend of atmospheric water vapor δ18Ov and δDv in Wujiachuan indicates that the entrainment effect surpassed that of evaporation; an increasing trend in Guyuan and Yuzhong may be attributed to the effect of evaporation surpassing that of entrainment. Based on a precipitation event at Guyuan, the lateral air mass transported from Wujiachuan may be the reason for rapid recovery of atmospheric vapor isotopic composition to the baseline value and a looping pattern between atmospheric water vapor mixing ratio (w) and δ18Ov observed at Guyuan. Linear correlation coefficients of δ18Ov with w varied from 0.02 to 0.88 at four study sites. Location differences may be the reason for the capability of w to explain why the variability of atmospheric water vapor isotopic composition was greater at Wujiachuan than at Binxian. Dew evaporation after the precipitation event may be the major reason for eliminating the relationship between w and δ18Ov at Guyuan and Yuzhong. Although synoptic conditions were similar at the four sites, δv variations may be controlled by ET, entrainment, and dew formation at different sites and times.

Pedicel Transpiration in Sweet Cherry Fruit: Mechanisms, Pathways, and Factors

Pedicel appearance is a good indicator of freshness in sweet cherries (Prunus avium L.). Fruit with shriveled, discolored pedicels have reduced market value. Shriveled pedicels are thought to result from postharvest water loss due to transpiration. The objectives of our study were to 1) quantify the transpiration permeances of fruit and pedicel surfaces; 2) determine the role of the fruit in pedicel transpiration; and 3) identify the effects of selected factors on pedicel transpiration. Fruit with and without pedicels were incubated under controlled conditions [usually 22 °C, 75% relative humidity (RH)] and their mass losses determined gravimetrically. Pedicel transpiration was calculated by subtracting measured transpiration of fruit without pedicels from that of fruit with pedicels. Cumulative pedicel transpiration increased with time. Rates of pedicel transpiration were essentially constant over the first 0 to 1.5 hours but declined thereafter, approaching an asymptote over the subsequent period of 1.5 to 96 hours over which measurements were made. Cumulative pedicel transpiration exceeded the amount of water in the pedicel, indicating that at least some of the transpired water originated from the fruit. There was no significant effect of steam girdling on pedicel transpiration suggesting that water moved from the fruit to the pedicel through the xylem (steaming prevents phloem conduction). Abrading the cuticular membrane (CM) from a pedicel surface or extracting the cuticular wax by dipping pedicels once or five times in chloroform/methanol (1:1 v/v) increased rates of transpiration 12-, 3-, and 5-fold, respectively. The water vapor permeance of the pedicel surface determined under steady-state conditions (8.7 ± 0.4 × 10−4 m·s−1) exceeded that of the fruit (2.1 ± 0.1 × 10−4 m·s−1), possibly because of a more permeable CM and/or a higher stomatal density (38.5 ± 1.3 stomata/mm2 for pedicels vs. 1.1 ± 0.0 stomata/mm2 for fruit). Treatments known to affect stomatal opening (incubation in buffered abscisic acid at 0.1 mm or in CO2- or N2-atmospheres) had no effects on pedicel transpiration. Rates of transpiration were negatively correlated with RH but positively with temperature. There was no effect of RH and/or temperature on the permeances of pedicel or fruit surfaces. From our results it is inferred that 1) pedicel transpiration is a physical process governed by Fick’s law of diffusion, where cuticle and wax in particular represent the major rate-limiting barriers; 2) the permeances of pedicel surfaces exceed those of fruit surfaces; and 3) pedicel transpiration can be minimized by minimizing the driving force (difference in water vapor concentration) during postharvest handling and storage.

Analysis of Moisture Transfer During the Drying of Clay Tiles with Particular Reference to an Estimation of the Time-Dependent Effective Diffusivity

The description of the drying process was reduced to the establishment of a series of theoretical and empirical drying models. The complex processes of simultaneous moisture and heat transfer, which are often nonstationary, and the distinct nature and properties of the material to be dried further complicate the description of the drying process. Three theories—diffusion theory, capillary flow theory, and evaporation–condensation theory—have won general recognition for the explanation of moisture transfer in porous media. The mechanisms of moisture movement during drying in the constant and especially in the falling drying period are rather complex and, hitherto, there have been no generally accepted explanations that could identify the exact transition between possible drying mechanisms, such as liquid movement due to capillary forces, liquid diffusion due to concentration gradients, liquid and vapor flow due to differences in total pressure, vapor diffusion due to difference in vapor concentration, vapor diffusion due to partial vapor pressure gradients, Knudsen diffusion, thermodiffusion, and the evaporation–condensation mechanism. The goal of this study was to find a way to better understand the different drying mechanisms, to identify the exact transition between them, and to estimate the time-dependent effective diffusivity. The results presented in this article confirmed that the effective diffusivity represents an overall mass transport property of moisture that includes all possible moisture transport mechanisms that are simultaneously controlling the moisture migration process in a material during drying. The experimental investigations were performed on clay tiles in a laboratory recirculation dryer, for which the drying parameters (humidity, temperature, and velocity) could be programmed, controlled, and monitored during drying.

Experimental and numerical investigations of moisture permeation through membranes

Experimental and numerical combined studies were carried out to investigate the effects of membrane properties and operating condition on water vapor (moisture) permeation through membranes. Experiments were conducted with water vapor transferring from a highly to a less humid air across membrane due to the water vapor concentration difference between the two sides of membrane, and numerical simulations were performed to simulate such process. The transmembrane moisture transfer was characterized using the moisture transfer resistance through membrane as well as the total moisture resistance, which include the membrane resistance and the boundary layer resistance on the two sides of membrane. The uniqueness of this research was a systematic examination of the effects of various membrane parameters and operating condition on the moisture permeation through membranes by combining the experiments and simulations. Tests were done on two membranes including the PVDF and PES membranes. The moisture diffusivities in these membranes were determined by comparing the experimental and numerical total moisture resistances. The results show that the moisture diffusivities in the PVDF and PES membranes are in the order of 10 −6 kg m −1 s −1, with the PVDF yielding a larger diffusivity than the PES membrane. The moisture diffusivity in membrane, the maximum water uptake of membrane, and the sorption constant of membrane all have significant effects on the membrane resistance, with a high diffusivity, a large water uptake, and a proper sorption constant leading to a small membrane resistance, while the effects of the air entering humidity and airflow rate on the membrane resistance are dependent on the sorption constant. These results may help for the selection of the membrane materials.

Performance evaluation of a solar still in the Eastern Province of Saudi Arabia—an improved analysis

The performance of a solar still is predicted though an improved and updated mathematical model. The model is based on classical energy balance equations; however, variable properties were considered in addition to a more reliable correlation for convection heat transfer from the seawater surface and the still glass cover. The effect of density variation within the solar still due to water vapor concentration difference is taken into consideration through a modified expression of the temperature difference (ΔT') and a more reliable expression for the heat transfer coefficient within the still that was developed for tilted covers. The performance of the still is validated against the published experimental values. In addition, the (distillate) productivity was reported for two days that represent both summer and winter conditions in Dhahran, Saudi Arabia. A parametric study was performed to identify the effect of the most influential parameters on the still performance. In addition, a sensitivity analysis ...

Heat and Mass Transfers and their Mutual Effects in Membrane Processes

A mathematical model was developed to describe the coupled heat and mass transfers in membrane processes. Equations for the heat and mass transfer resistances were derived and the coupling effects of the heat and mass transfer were analyzed. With taking the membrane separation process of moist air as an example, the effects of air temperature and water vapor concentration on the heat and moisture transfer process were investigated. The results show that neither the thermal resistance nor the moisture resistance are constant, they are affected by not only the membrane parameters but also the air state. As the temperature difference between the two airstreams separated by the membrane increases, both the thermal and moisture resistances decrease, causing an improved heat and mass transfer. As the average temperature of the two airstreams increases, the thermal resistance remains almost constant while the moisture resistance decreases significantly. Further, as the water vapor concentration difference between the two airstreams increases, both the thermal and moisture resistances increase. As the average water vapor concentration of the two airstreams increases, the thermal resistance remains unchanged while the moisture resistance decreases.

Analysis of Heat and Mass Transfer during High-Temperature Drying of Pinus radiata

This article describes the analysis of heat and mass transfer coefficients for a single board of Pinus radiata (D. Don) timber over a range of high temperature and superheated steam drying conditions. The calculated heat transfer coefficients were in the range 20 to 60 W m−2 K−1. The mass transfer coefficients were of the order of 2 × 10−8 to 3 × 10−7 kg m−2 s−1, based on the vapor pressure difference, and of the order of 0.002 to 0.04 m s−1 (expressed in terms of mass transfer velocity) based on vapor concentration difference between the surface of the board and the bulk drying medium.

Epidermal Segments: A Useful Model System for Studying Water Transport through Fruit Surfaces

Water conductance of the cuticle of mature fruit of apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf., `Golden Delicious' Reinders/`Malling 9' (M.9)], sweet cherry (Prunus avium L., `Sam'/`Alkavo'), grape (Vitis vinifera L.), pepper (Capsicum annuum L. var. annuum Fasciculatum Group, `Jive'), and tomato (Lycopersicon esculentum Mill.) was de ter mined using excised epidermal segments (consisting of epidermis, hypodermis, and some cell layers of parenchyma) and enzymatically isolated cuticular membranes (CM) from the same sample of fruit. Segments or CM were mounted in diffusion cells and transpiration was monitored gravimetrically. Conductance (m·s-1) was calculated by dividing the flux of water per unit segment or CM area (kg·m-2·s-1) by the difference in water vapor concentration (kg·m-3) across segments or CM. Transpiration through segments and through CM increased with time. Conductance of segments was consistently lower than that of newly isolated CM (3 days or less). Conductance decreased with increasing time after isolation for apple, grape, or sweet cherry CM, and for sweet cherry CM with increasing temperature during storage (5 to 33 °C for 4 days). There was no significant effect of duration of storage of CM on conductance in pepper or tomato fruit. Following storage of CM for more than 30 days, differences in conductance between isolated CM and excised segments decreased in apple, grape, and sweet cherry, but not in pepper or tomato. Use of metabolic inhibitors (1 mm NaN3 or 0.1 mm CCCP), or pretreatment of segments by freezing (-19 °C for 18 hours), or vacuum infiltration with water, had no effect on conductance of apple fruit segments. Our results suggest that living cells present on excised segments do not affect conductance and that epidermal segments provide a useful model system for quantifying conductance without the need for isolating the CM. Chemical names used: sodium azide (NaN3); carbonylcyanide m-chlorophenylhydrazone (CCCP).

Experiment and Analysis of Combined Heat and Water Vapor Transfer Through Clothes with Condensation

A series of experiments is performed on the combined heat and water vapor transfer through fabric at simulated high altitudes. A considerable water vapor concentration difference is imposed between the water vapor source and the environment, so that condensation takes place in the specimen. A new analytical expression for the rate of condensation in textile materials is derived by considering the combined heat and mass transfer between the specimen and the environment. Although the amount of condensation in textiles can not be correlated well by primitive parameters such as temperature or water vapor concentration differences, good agreement can be obtained between the condensa tion mass flux measured experimentally and that calculated by means of the derived expression.

Studies on water transport through the sweet cherry fruit surface: II. Conductance of the cuticle in relation to fruit development.

Water conductance of the cuticular membrane (CM) of sweet cherry (Prunus avium L. cv. Sam) fruit during stages II and III (31-78 days after full bloom, DAFB) was investigated by gravimetrically monitoring water loss through segments of the exocarp. Segments were mounted in stainless-steel diffusion cells, filled with 0.5 ml of deionized water and incubated for 8 h at 25 +/- 2 degrees C over dry silica. Conductance was calculated by dividing the amount of water transpired per unit surface area and time by the difference in water vapor concentration across the segment (23.07 g m(-3) at 25 degrees C). Fruit mass and fruit surface area increased 4.9- and 2.8-fold between 31 and 78 DAFB, respectively. However, CM mass per unit area decreased from 3.9 to 1.5 g m(-2) and percentage of total wax content remained constant at about 31%. Stomatal density decreased from 0.8 to 0.2 mm(-2) (31-78 DAFB). Total conductance of the CM on the fruit cheek (gtot.) remained constant during stage II of development (approx. 1.38 x 10(-4) m s(-1) from 31 to 37 DAFB), increased to 1.73 x 10(-4) m s(-1) during early stage III of fruit growth (43-64 DAFB) then decreased to 0.95 x 10(-4) m s(-1) at maturity (78 DAFB). Partitioning gtot. into cuticular (gcut.) and stomatal conductance (gsto.) revealed that the relative contribution of gcut. to gtot. increased linearly from 30% to 87% of gtot. between 31 and 78 DAFB. respectively. On a whole-fruit basis, g,tot. and gcut. consistently increased up to 64 DAFB, and decreased thereafter. A significant negative linear relationship was obtained between gcut. and CM thickness, but not between the permeability coefficient (p) and CM thickness. Further, p was positively related to strain rate, suggesting that strain associated with expansion of the fruit surface increased p.

A METHOD FOR MEASURING UPPER RESPIRATORY TRACT VAPOR UPTAKE AND ITS APPLICABILITY TO QUANTITATIVE INHALATION RISK ASSESSMENT

A thorough understanding of the toxicity of any substance requires knowledge of the relationships between exposure concentration and dose delivered to the critical target site. This is particularly true for inhalation exposures because inspired particles and vapors do not deposit uniformly in the respiratory tract. The current report describes in detail a methodology for measuring upper respiratory tract (URT) uptake of vapors in the rat. A urethane-anesthetized animal model is utilized in which two endotracheal tubes are inserted: one leading toward the lung to facilitate respiration, and the other toward the nose to allow air sampling through the nasal passages. The animal is placed in a nose-only exposure chamber and test vapor is drawn through the nose for periods up to 1 h. Uptake efficiency is calculated from the difference in vapor concentration between the inspired (chamber) air and air exiting the URT. Uptake data are provided for acetaldehyde and nicotine vapors, and a suggested experimental design that includes multiple air flow regimes and inspired concentrations is described. The data obtained by this methodology are not necessarily reflective of uptake efficiencies in normally breathing animals due to the nonphysiologic airflow regimes and the invasiveness of the procedure. The data so obtained are best utilized to support and validate state-ofthe-art mathematical simulation models of regional vapor uptake. These models increase scientific rigor and reduce uncertainty in quantitative risk assessments for inhaled materials.

Gas exchange and its factorial dependency in field-grown Brassica napus L.

The purpose of the present field study was to derive factorial dependencies (including sub models) between gas exchange (conductance and photosynthesis) and plant water potential and micrometeorological factors based on physiological measurements in rape ( Brassica napus L.). In canopy models, such relationships are useful for scaling up photosynthesis and transpiration from leaf or organ to crop level. The factorial dependencies were derived from midday measurements during soil drying causing different levels of plant water stress and then tested against other sets of diurnal measurements. In leaves and pods, net photosynthesis ( A n) as a function of light intensity was well described by empirical logarithmic functions on the basis of parameters characterizing initial slope and maximum photosynthesis. By relating A n at light saturation to conductance ( g H 2O ) for leaves and pods, our data indicated that in non-water stressed plants, stomates regulated CO 2 diffusion rate, so that the internal carbon dioxide concentration ( C i ) during photosynthesis was close to the normal C 3 plant CO 2 transition point of 250 μl l −1. However, water stress caused a further decrease in C i, which significantly increased the slope of A n over g H 2O and caused an increase in the `instantaneous WUE'. The factorial dependency of g H 2O of leaves and pods on their water potential ( ψ), photosynthetic active radiation (PAR) and leaf- or pod-to-air water vapour concentration difference ( D), was combined in empirical factorial stomatal models. In pods, a close agreement between measured and predicted g H 2O values was found. However, a closer agreement for short-term fluctuations could be obtained if the level of photosynthesis was taken into account as a factorial parameter. This might reflect a coordination between levels of CO 2 assimilation and stomatal conductance controlled by C i. In leaves, substantial discrepancies occurred at a high evaporative demand mainly because a low water potential in fully watered plants simulated stomatal closure. The need to relate stomatal responses to soil water status when simulating stomatal behaviour is discussed.

SIMULATION OF TRANSPIRATION OF CITRUS GOVERNED BY LEAF CONDUCTANCE

A water flow model was developed which uses irradiance, leaf-to-air vapor concentration difference, and soil water potential to establish stomatal conductance. Water flow to the roots was computed using a linear approximation of radial flow through the soil toward the axis of the roots across concentric shells. Root length density and soil rooting volume within four separate layers or compartments were included in the model. The simulation was executed in small time step iterations. A small increment of transpiration was translated to a water content deficit at the root and then sequentially through the concentric shells to simulate water uptake and change of soil water potential. The change in soil water potential was used to increment changes in stomatal conductance and transpiration. The output of the model simulated the pattern of diurnal stomatal behavior observed in other types of experiments, as well as the total soil water extraction patterns of young potted citrus trees.