Growth chamber studies showed that a relationship exists between transpiration and dry matter production of spring wheat (Tritiaum Aestivum L. var . Thatcher). A temperature of 27 C for a 16-hour day,and 21 C at night were used throughout the experiment. Relative humidities (RH) of 12, 25, 71, and 83 percent and matric suctions of 1, 3, and 9 bars were used a l ong with six fertility levels and a 20-day growing period. An equation was developed from previous equations by De Wit and Arkley to describe the transpiration ratio (Tr = mass of water transpired/mass of dry matter produced) as it relates to evaporative demand conditions measured by humidity and pan evaporation. Time and fertility effects were not included because of insufficient data. As humidity both increases and decreases from 25 percent, the transpiration ratio decreases. Increasing levels of matric suction had an effect on Tr only at 25 percent RH. As fertility increased, Tr decreased toward some minimum level. Tr seems to reach a stable maximum as plants mature under steady state conditions.

Cuticles cover all above-ground primary plant organs and are lipoid in nature consisting of a cutin matrix with cuticular waxes embedded within or deposited on its surface. The foremost function of the plant cuticle is the limitation of transpirational water loss into the surrounding atmosphere. Transpiration of water vapour from plants differs between stomatal and cuticular transpiration. Stomatal closure minimises the stomatal water loss and the remaining, much lower water transpiration occurs through the plant cuticle. Temperature influence on the transpiration barrier properties of intact leaves is not yet known, despite the importance of the cuticular transpiration especially under drought and heat conditions. The present study focuses on the temperature-dependent minimum water permeability of whole leaves, in comparison to the temperature effect on the cuticular permeance of isolated, astomatous cuticles (Chapter I - III). The minimum water permeability was determined gravimetrically from leaf drying curves and represents the cuticular water permeability of intact, stomatous leaves under conditions of complete stomatal closure. The temperature effect on the transpiration barrier of the desert plant Rhazya stricta and the Mediterranean sclerophyll Nerium oleander exposed a continuous increase of minimum water permeabilities with an increase in temperature. In contrast to other published studies, no abrupt and steep increase of the water permeability at high temperatures was detected. This steep increase indicates structural changes of the barrier properties of isolated cuticular membranes with a drastic decrease of efficiency. A stabilising impact of the cell wall on the plant cuticle of intact leaves was proposed. This steadying effect was confirmed with different experimental approaches measuring the cuticular water permeability of Prunus laurocerasus intact leaves. Physiological analysis of water transport on isolated, astomatous leaf cuticles indicated a drastic decline of the barrier properties at elevated temperatures for Prunus laurocerasus but not for Nerium oleander. Cuticular components were quantitatively and qualitatively analysed by gas chromatography with a flame ionisation detector and a mass spectrometric detector, respectively. A high accumulation of pentacyclic triterpenoids as cuticular wax components in relation to the cutin monomer coverage was detected for Nerium oleander and for Rhazya stricta leaves, too. Accordingly, reinforcing of the cutin matrix by triterpenoids was proposed to improve the mechanical strength and to reduce the extensibility of plant cuticles. Thus, structural changes of the cuticular barrier properties were potentially suppressed at elevated temperatures. The function of the cuticular wax amount and/or wax composition and its relation with the cuticular water permeability remains to be elucidated. In the second part of this work the cuticular wax quantity and quality as well as its impact on the transpiration barrier properties was analysed in order to deduce a potential relation between chemistry and function of plant cuticles (Chapter IV - V). Chemical analyses of the cuticular wax components of a wide range of plant species, including one tropical (Vanilla planifolia), temperate (Juglans regia, Plantago lanceolata), Mediterranean (Nerium oleander, Olea europaea) and one desert (Rhazya stricta) plant species, were conducted. The cuticular wax compositions of nine characteristic plant species from xeric limestone sites naturally located in Franconia (Southern Germany) were determined for the first time. The corresponding minimum or cuticular water permeabilities of both stomatous and astomatous leaf surfaces were measured to detect a potential relationship between the cuticular wax amount, wax composition and the cuticular barrier properties. It was demonstrated that abundant cuticular wax amounts did not constitute more efficient transpiration barriers. However, 55% of the cuticular barrier function can be attributed to the very-long-chain aliphatic wax coverages. These new findings provide evidence that the acyclic wax constituents play a pivotal role establishing efficient transpiration barriers. Additionally, these findings strengthen the hypothesis that cyclic components, such as pentacyclic triterpenoids, do not hinder the water diffusion through plant cuticles as effectively as acyclic constituents. For the first time a relationship between the cuticular wax composition and the transpiration barrier properties of a wide range of plant species proved insights into the potential relation between chemistry and function of plant cuticles. / Die Kutikula bedeckt die Epidermis aller primären oberirdischen Pflanzenorgane. Diese lipophile Membran besteht aus dem Polymer Kutin und ein- bzw. aufgelagerten kutikulären Wachsen. Die wichtigste Aufgabe der Kutikula ist der Schutz der Pflanze vor Austrocknung, indem der unkontrollierte Wasserverlust in die Atmosphäre reduziert wird. Spaltöffnungen unterbrechen die kontinuierliche Schutzschicht, wobei die stomatäre Transpiration durch Spaltenschluss minimiert wird und die verbleibende, stark reduzierte Transpiration ausschließlich durch die pflanzliche Kutikula erfolgt. Der Temperatureinfluss auf die Transportbarriere intakter Blätter ist bislang unerforscht, obwohl die kutikuläre Transpiration vor allem an trockenen und heißen Standorten eine wichtige Rolle spielt. Im Rahmen dieser Dissertation wurde die temperaturabhängige kutikuläre Wasserpermeabilität ganzer Blätter und isolierter Kutikularmembranen verglichen (Kapitel I - III). Die minimale Wasserpermeabilität wurde gravimetrisch mittels Blattaustrocknungskurven bestimmt. Sie ist ein Maß für die kutikuläre Wasserdurchlässigkeit intakter, stomatärer Blätter bei geschlossenen Spaltöffnungen. Die minimale Wasserpermeabilität intakter Blätter von Rhazya stricta und Nerium oleander zeigte einen kontinuierlichen Anstieg mit zunehmender Temperatur. Im Gegensatz zu anderen Veröffentlichungen wurde kein abrupter, steiler Anstieg der Wasserpermeabilität bei erhöhten Temperaturen detektiert, welcher auf strukturelle Veränderungen der Transpirationsbarriere isolierter Kutikularmembranen und auf eine damit einhergehende, stark verminderte Effizienz hindeutet. Dies kann auf einen stabilisierenden Einfluss der Zellwand auf die pflanzliche Kutikula zurückgeführt werden. Verschiedene experimentelle Ansätze zur Bestimmung der temperaturabhängigen kutikulären Wasserpermeabilität von Prunus laurocerasus Blättern konnten dies bestätigen. Bei erhöhten Temperaturen wiesen die isolierten, astomatären Kutikularmembranen von Prunus laurocerasus Blättern eine starke Abnahme der Barrierefunktion auf, die isolierten Kutikularmembranen von Nerium oleander Blättern jedoch nicht. Die kutikulären Wachs- und Kutinkomponenten wurden quantitativ mittels Gaschromtograph mit Flammenionisationsdetektor und qualitativ mittels Gaschromatograph gekoppelt mit Massenspektrometer analysiert. Ein sehr hoher Gehalt an pentazyklischen Triterpenoiden im kutikulären Wachs in Bezug auf den Kutingehalt wurde sowohl für die Blätter von Nerium oleander als auch für Rhazya stricta bestimmt. Triterpenoide erhöhen möglicherweise die mechanische Festigkeit und reduzieren die Dehnbarkeit der Kutikula, indem sie die Kutinmatrix verstärken. Hierdurch könnten strukturelle Veränderungen der Transpirationsbarriere bei erhöhten Temperaturen herabgesetzt werden. Die weit verbreitete Ansicht, dass die Wasserpermeabilität von der kutikulären Wachsmenge und/oder der Wachszusammensetzung bestimmt wird, konnte bislang nicht bestätigt werden. Im zweiten Teil der vorliegenden Arbeit wurden chemisch-analytische Methoden angewandt, um den Einfluss der Wachskomponenten auf die Transpirationsbarriere zu ermitteln, und somit einen potentiellen Zusammenhang zwischen der Chemie und der Funktion der pflanzlichen Kutikula abzuleiten (Kapitel IV - V). Um Hinweise auf die Auswirkung der chemischen Zusammensetzung der Kutikula auf die Transpirationsbarriere zu erhalten, wurden die kutikulären Wachse eines breiten Artenspektrums analysiert, darunter eine tropische Pflanzenart (Vanilla planifolia), mediterrane Arten (Nerium oleander, Olea europaea), Pflanzenarten der gemäßigten Zone (Juglans regia, Plantago lanceolata) und eine Wüstenpflanze (Rhazya stricta). Zusätzlich wurde die kutikuläre Wachszusammensetzung von neun charakteristischen Pflanzenarten des Mainfränkischen Trockenrasens erstmals untersucht. Die entsprechende minimale oder kutikuläre Wasserpermeabilität von stomatären und astomatären Blattoberflächen dieser Pflanzenarten wurde bestimmt, um einen möglichen Zusammenhang zwischen der Wachschemie mit der Barrierefunktion aufzuklären. Es konnte gezeigt werden, dass hohe Wachsmengen keine effizienteren Transpirationsbarrieren bilden. Jedoch konnten rund 55% der Barrierefunktion dem Anteil an langkettigen aliphatischen Komponenten zugeordnet werden. Diese neuen Erkenntnisse erbringen den Nachweis, dass die kutikuläre Transpirationsbarriere entscheidend von azyklischen Wachskomponenten beeinflusst wird. Zudem konnte bestätigt werden, dass zyklische Wachskomponenten die Wasserpermeabilität weniger stark beeinflussen als azyklische Bestandteile. Diese Ergebnisse zeigen zum ersten Mal einen Zusammenhang zwischen der chemischen Zusammensetzung der kutikulären Wachse und der kutikulären Transportbarriere anhand eines breiten Artenspektrums.

Tens of millions of dollars have been spent to control Tamarix (saltcedar) trees along waterways in the Southwestern United States for the purpose of increasing streamflow yet no increase in streamflow has been demonstrated. The Pecos River Ecosystem Project (PREP) served as a case study to characterize surface and groundwater interaction along the Pecos River in Texas, assess the influence of saltcedar transpiration on stream stage and water table fluctuations, and evaluate the impacts of large-scale saltcedar control on baseflows. This is the first study that has investigated the influence of saltcedar transpiration on surface and groundwater interaction and the first to provide a mechanistic explanation for the lack of measurable increase in streamflow. Neither saltcedar transpiration nor saltcedar removal influenced hydraulic gradients, streambank seepage, or stream elevations. The results of the plot scale studies indicate saltcedar transpiration along the Pecos River is lower than reported elsewhere and therefore may not yield detectable increases in baseflow. To extend the study to a much larger scale, we analyzed annual baseflows at the downstream end of 340 km river reach from 1999 (pretreatment) through 2009. Surprisingly, baseflows declined for four years after the project began despite additional acreages of saltcedar treatment each year. However, baseflow surged in 2005 and remained higher than the pretreatment year (1999) through 2009. Additional detailed analyses of reservoir release and delivery records and rainfall are needed to better understand contributions of rainfall and flow regulation to this increase. Tracer based studies to determine the relative contributions of releases and groundwater would also enable a better interpretation of the change in baseflows. We did not investigate any other reported benefits, such as restoration of native plant species, or reduced soil salinity, of saltcedar control.

Balancing plant growth between vegetative and reproductive status is crucial for producing high quality greenhouse tomatoes while maintaining high productivity in long crop production seasons. In the tomato industry, certain plant morphological characteristics are used to classify plant growth status as vegetative, reproductive or balanced. Each growth status has been associated with distinct greenhouse environments which reduce or enhance transpiration.The effect of different transpiration on vegetative, reproductive or balanced plant growth status as defined by a set of plant morphological characteristics was investigated. To validate the practical significance of such classification, growth status was quantified as the relationship between variations in morphological characteristics and the fresh weight distributed between reproductive and vegetative organs.Two electrical conductivity (EC) levels of the nutrient solution, high and standard EC, were combined with two potential transpiration environments, low and high potential transpiration. All treatment combinations were contrasted with a reference greenhouse environment similar to the industry standard.Electrical conductivity had the greatest effect on morphological characteristics which were reduced in size with high EC. For each EC level, the response decreased for increasing potential transpiration. Stem diameter had the greatest sensitivity to the different treatment combinations. For the standard EC and for the range of potential transpirations achieved, stem diameter varied within a relatively narrow range, close to the industry standard 'threshold' used to classify a balanced tomato plant. A reproductive plant growth status, as evaluated by a smaller value than this threshold, was observed only with high EC. No vegetative plants were produced within any potential transpiration or EC treatment combination.High EC decreased the cumulative total fresh weight production by the same magnitude for all potential transpirations. Potential transpiration had a minimal effect on the total fresh weight production or on its components. As a result, the fresh weight ratio between reproductive and vegetative plant organs was similar for most potential transpiration environments, regardless of variations in stem diameter. Therefore, within the range of potential transpiration environments achieved, the distinction between vegetative and reproductive growth status as an indicator of fresh weight distribution and fruit yields could not be quantitatively validated.

Universiẗat, Diss., 2005--Bonn.

Thesis (MTech (Horticulture))--Cape Peninsula University of Technology, 2017 / Transpiration serves in leaf cooling, maintaining turgor pressure, promoting xylem transport of nutrient solutes from roots to shoots and delivering mobile soil nutrients to root surfaces. Soil availability of nitrogen can modulate transpiration rates, consequently powering nutrient delivery to the root surfaces (‗mass-flow'). Although such knowledge on N-regulation of transpiration is available, it remains unknown, however, whether it is NO3- or NH4+ that regulates transpiration. Given that both nitrogen forms co-occur in soils, it is not known how they interact at varying ratios in modulating stomatal behaviour. To test the functional role of NO3- and NH4+ in regulating water fluxes for mass-flow nutrient acquisition, P. vulgaris L. plants were grown with NO3- or NH4+ placed at one of four distances behind a nylon mesh, which prevented direct root access to nitrogen, whilst control plants intercepted the nitrogen source (Chapter 3). Day- and night-time stomatal conductance and transpiration, measured using Infra-Red Gas Analyser (IRGA) declined in NO3- fed plants with the increased distance behind a nylon mesh, with maximum water fluxes at the closest distance (ca. 0 mm), demonstrating a regulatory role of NO3- on stomata closure. An opposite trend was displayed by NH4+ -fed plants, which indicated the incapacity of NH4+ to down-regulate water fluxes and ammoniacal syndrome at high concentrations. To test how different [NO3-] and [NH4+] regulate day- and night-time stomatal conductance and transpiration (Chapter 4), P. vulgaris was fed with six concentrations (0, 0.25, 0.5, 1, 2, 4 and 8 mM) of each nitrogen form. A biphasic trend emerged, as postulated in previous studies (Wilkinson et al., 2007; Matimati et al., 2013), characterized by an increase in stomatal conductance and transpiration as [NO3-] increased, attaining a maximum before declining with higher [NO3-]. Plants displayed 2-fold higher photosynthetic rates, 2.2-fold higher stomatal conductance and 2.3-fold higher transpiration rates at 4 mM than at 0.25 mM of [NO3-]. The lowest [NO3-] up-regulated night-time stomatal conductance and transpiration, indicating that NO3- -fed plants opened their stomata at night-time, but reduced night-time water loss at higher [NO3-]. NH4+-fed plants had the incapacity to regulate day- and night-time water fluxes, but rather displayed wilting and stress known as ‗ammoniacal syndrome'. Thus, under NO3- deprived soil conditions P. vulgaris may be opportunistic in their water uptake, transpiring more when water is available in order to draw nutrients through ‗mass-flow'. This thesis explored and confirmed the functional role of NO3- in regulating day- and night-time water fluxes as a mechanism for increasing ‗mass-flow' acquisition of N and possibly other nutrients, signalling a down-regulation of day-time and night-time water fluxes when [NO3-] is replete (Chapter 3 & 4). Where both NO3- and NH4+ are present in soils, it is the [NO3-] and not [NH4+] that regulated stomatal conductance and transpiration. Since organic nitrogen forms such as amino acids also occur in soils, there is a need for further work on their role in stomatal behaviour. Using amino acids laced with 15N isotopes as a nitrogen source can allow their acquisition and role on stomatal behaviour to be discovered. Current trends in research are focussed around developing real-time in-situ sensing of soil nitrogen status to promote enhanced nitrogen and water use efficiency in agricultural systems. This thesis provides the vital literature on stomatal regulation by [NO3-].

Physical root water uptake models can provide more insight into the mechanism, but their physical plant hydraulic parameters are hardly-ever available, making them less attractive in practical applications. Conversely, empirical root water uptake modes are more readily used because of their simplicity and lower data requirements, but their empirical parameters and ability in describing the dynamics of root water uptake need further investigation. Combining physical and empirical models might be an effective way to address these issues. In this thesis, it is tested the feasibility of deriving parameters for empirical root water uptake models by using predictions performed by an enhanced mechanistic root water uptake model. It is also reviewed the major root water uptake models that have been used together with larger eco-hydrological models and some alternatives are also presented. All these models are analyzed for different scenarios concerning soil type, atmospheric demand and root length density. Evaluation was performed by optimizing their empirical parameters so that the best fitting with the physical model is achieved. At last, further analyzes are performed for an empirical model pointed at the previous analyzes, and the empirical parameters for this model are provided for different scenarios regarding soil type, root length density R, rooting depth and potential transpiration Tp as well as for three levels of radial root hydraulic conductivity. It is shown that (i) the largely-used Feddes empirical root water uptake model performs well only under circumstances of low R -- that is for the scenarios of low root water uptake \"compensation\"-- and from medium to hight R, the model can not mimic properly the root uptake dynamics as predicted by the physical model; (ii) the Jarvis model provides good predictions only for low and medium R scenarios and for high R the model can not mimic the uptake patterns predicted by the physical model; Using the proposed reduction function in Jarvis model, that is the JMm model, helps to improve water uptake predictions; (iii) the proposed models are capable of predicting similar root water uptake patterns by the physical model and the statistical indices point them as the best alternatives to mimic root water uptake predictions by the physical model; (iv) the parameters of empirical models can be retrieved in a single experiment of soil drying-out by defining the objective function in terms of root water uptake; (v) the empirical parameters provided by the proposed model varies with the scenarios as well as its overall performance. / Embora modelos físicos de extração de água do solo sejam importantes para analisar detalhes mecanísticos do sistema, seus parâmetros hidráulicos não são facilmente disponíveis, e assim são menos utilizados em situações práticas. Entretanto, modelos empíricos são facilmente aplicados devido a sua simplicidade e baixo requerimento de dados, porém seus parâmetros empíricos e habilidade em descrever a dinâmica da extração de água do solo precisa ser mais investigada. O uso combinado de modelos empíricos e físicos pode ser útil nesse contexto. O objetivo geral deste trabalho é testar se os parâmetros de modelos empíricos de extração de água do solo podem ser determinados através de simulações feitas como um modelo físico de extração de água do solo. Fez-se uma revisão sobre os principais modelos empíricos usados em modelos hidrológicos, assim como algumas alternativas foram apresentadas. Alguns desses modelos foram analisados para diferentes cenários de tipo de solo, demanda atmosférica e densidade de comprimento de raiz R. A análise foi feita otimizando-se os parâmetros empíricos dos modelos a fim de obter o melhor ajuste com o modelo físico. Em seguida, fez-me uma análise mais detalhada sobre o desempenho de um modelo empírico sugerido nas analises anteriores, como o objetivo de fornecer os valores de seus parâmetros empíricos para diferentes cenários de tipo de solo, R, profundidade do sistema radicular e transpiração potencial. Analisou-se também a variação desses parâmetros empíricos em função da condutividade hidráulica da raiz. Os resultados mostraram que (i) o modelo empírico de Feddes, que é largamente utilizado, só apresenta bom desempenho em cenários de baixo R -- ou seja, para cenários com baixa compensação de extração de água do solo-- e, para cenários de médio a alto R, o modelo não é capaz de representar adequadamente o dinâmica de extração de água do solo simulada pelo modelo físico; (ii) O modelo de Jarvis só apresenta desempenho adequado em cenários de baixo R e, para alto R, o modelo não é capaz de representar adequadamente a distribuição de extração simulada pelo modelo físico; (iii) inserindo-se a função de redução proposta no presente trabalho no modelo de Jarvis, ou seja o modelo JMm, proporciona melhores estimativas da distribuição de extração de água do solo; (iv) Os modelos propostos apresentam o melhor desempenho em descrever as predições feitas pelo modelo físico; (v) os parâmetros dos modelos empíricos podem ser obtidos em um único experimento de secagem do solo, definindo-se a função objetivo em função da extração de água do solo; (vi) Os parâmetros empíricos do modelo proposto variam em função dos cenários avaliados.

Increased frequency of severe drought events, coupled with rising air temperatures and vapor pressure deficits (VPD), pose a great threat to Mediterranean forests. Pinus halepensis Mill. is one of the most widespread species in the countries surrounding the Mediterranean basin. Thus, water use of this species plays a critical role in the regions water balance. Studying transpiration patterns and the mechanisms behind stomatal responses to the combined effects of changing VPD and soil moisture can help us improve estimation of forest water use in a changing climate. To improve the estimation of forest evapotranspiration in the Mediterranean basin, the objective of this thesis is to evaluate the transpiration patterns of Pinus halepensis and the role of this species in the soil-water balance under different environmental conditions. Two pine forests in the Turia river basin, eastern Spain, were monitored over a two-year period. The two locations were selected at contrasting altitudes and distances to the sea but within the same hydrological basin, to evaluate if this placement would change the relationship between environmental conditions and the water use of the pines. Sap flow measurements were obtained on a 30-minute interval together with soil moisture measurements and meteorological variables. A soil-water balance was performed on a forest plot-level using an eco-hydrological model in combination with the transpiration data, to assess the contribution of pine transpiration to actual evapotranspiration. Transpiration was dictated by changes in VPD, relative extractable water (REW) and the interaction between these two variables at both sites, indicating that the pines depended on water in the shallow soil layers, and this was restricted during large parts of the year. Except for low winter temperatures having a decreasing effect on transpiration only at the inland site, no significant differences were found in the relationship between environmental drivers and transpiration patterns between the two sites. Using a predictive model, sap flow was shown to be restricted on days of mean VPD values of 2.5 kPa, even when soil moisture levels were relatively sufficient (REW = 0.30), indicating a VPD threshold that decreases pine transpiration. This could potentially affect performance and survival of the species with predicted increases in air VPD. Transpiration levels were highly restricted throughout the first year demonstrating that physiological stresses were not limited to summer months. Using two-year old seedlings in an experiment under controlled conditions confirmed that high levels of VPD can have a decreasing effect on transpiration of P. halepensis (in response to instant changes from 1.5 kPa to 2.7 kPa), while there is an intermediate VPD range that increases transpiration (between 1.0 kPa – 1.5 kPa). This demonstrate how important it is to incorporate VPD changes when predicting forest water use under future climatic changes. Combining transpiration data with eco-hydrological modelling demonstrated that transpiration levels accounted for 62% of total ETa levels during two years of study. Interception levels where 32% of gross precipitation, representing a large water loss to the forest ecosystem. With increased frequency of drought events, soil moisture levels are predicted to become even more limited. Together with a rise in temperatures and consequently VPD, transpiration and growth are likely to be constrained.

Transpiration cooling as potential replacement of multi-hole effusion cooling for gas turbine engines combustion liner application is investigated by comparing their cooling film effectiveness based on the mass transfer analogy (CFEM). Pressure sensitive paint was used to measure CFEM over PM surfaces which was found to be on average 40% higher than multi-hole effusion cooling. High porosity PM with low resistance to flow movement were found to offer uneven distribution of exiting coolant, with large amounts leaving the trailing edge, leading to lopsided CFEM. Design of anisotropic PM based on PM properties (porosity, permeability, and inertia coefficient) were investigated using numerical models to obtain more uniform CFEM. Heat transfer analysis of different PM showed that anisotropic samples offered better thermal protection over isotropic PM for the same porosity. Comparison between cooling film effectiveness obtained from temperatures CFET against CFEM revealed large differences in the predicted protection. This is attributed to the assumptions made to apply CFEM, nonetheless, CFEM remains a good proxy to study and improve transpiration cooling. A method for creating a CAD model of designed PM is proposed based on critical characteristics of transpiration cooling for future use in 3D printing manufacturing.

Canopy transpiration is a major factor determining crop evapotranspiration and energy budgets. Unfortunately the development of robust models of canopy transpiration is hindered by a lack of reliable data due to the difficulties of making canopy-scale measurements. However, measurements of canopy water vapor and carbon fluxes via gas exchange techniques are possible in controlled environments. Simultaneous measurements of transpiration, photosynthesis, and canopy temperature were made in wheat and soybean communities. These data were used to calculate chamber aerodynamic and canopy stomata! conductances, and to model the response of canopy transpiration to CO2concentration and vapor pressure deficit. Canopy stomata! conductance was found to decrease diurnally by 20-30% in well-watered crops grown under constant environmental conditions. The magnitude of this diurnal decrease in the canopy stomata! conductance of wheat and soybean decreased with increasing ambient CO2 concentrations. Eight models describing how canopy stomatal conductance responds to environmental changes were incorporated into a canopy transpiration model. The results and methods developed in this study will allow future physiologically-based canopy transpiration models to incorporate these models for predicting the response of transpiration rates in controlled environments.