Vapor Deficit Research Articles

Response of Transpiration Characteristics and Water Use Efficiency of Setaria viridis to the Enhancement of Simulated Photosynthetic Radiation and CO2 Enrichment

To determine how transpiration characteristics and water use efficiency of annual €4 plants respond to simulated photosynthetic radiation enhancement and CO2 enrichment, we used LT-6400 Portable Photosynthesis System to examine photosynthesis rate ( Pn ) , transpiration rate ( Tr ) , intercellular CO2 concentration ( Ci) , stomatal conductance ( Gs ) and vapor deficit at the leaf surface ( Vpdl) of Setaria viridis under simulated photosynthetic radiation (SPR) intensities from 0 to 2 400 jumol · m -2 · s-1 and CO2 concentration from 300 to 1 000 μmol · mol -1. Pn, Tr and Gs increased with enhanced SPR intensity. The increase in Pn, Tr and Gs with each unit SPR added was reduced as SPR intensity increased. At last, these physiological parameters tend to dynamic balance. Water use efficiency ( WUE) increased with enhanced SPR, then plateaued as SPR intensity exceeded 1 200 μmol·m-2 ·s-1 . For photosynthesis process demands C02, so Ci intensity enhancement. Vpdl also decreased with SPR intensity increase, then reached dynamic balance as SPR intensity exceeded 600 μmol · m- 2 · s-1. Pn increased with CO2 concentration between 300 and 600μmol · mol -1, and then decreased as C02 concentration increased from 600 to 1 000 μmol· mol-1. Ci, Vpdl and WUE of Setaria viridis rose with the elevation of CO2 concentration. Both Gs and Tr dropped as the CO2 concentration was elevated . We conclude that Pn of annual C4 plants was not sensitive to C02 concentration variation. The response of Tr and WUE to CO2 enrichment was most significant. It is obvious that the function of instantaneous simulated CO2 enrichment to increased photosynthesis rate was gradually diminished. However, it could enhance primary production by improving the use efficiency of existing water.

Surface energy fluxes and control of evapotranspiration from a Swedish Sphagnum mire

The components of the energy budget for an open Swedish boreal bog were quantified over two growing seasons and an analysis of the influence of factors such as peat wetness, vapour deficit and radiative forcing was made, in order to understand how they can be represented by physically-based models. The presence of a pronounced micro-topography caused a variation of the surface wetness as well as the cover of vascular plants. This was reflected in the spatial variation in soil temperatures and the ground heat flux was almost twice as much in hollows as in ridges. A theoretical function of peat thermal conductance with porosity and water content agreed with calculated damping depths. The surface roughness was low (z 0=2–3 cm) and did not change during the season. Bluff-rough effects had to be considered for heat and vapour fluxes. Albedo changed little but significantly with development of vascular plants (from 0.11 to 0.14). The Bowen ratio did not change with temporal fluctuations in peat wetness but depended mainly on net radiation, air temperature and relative humidity. Consequently, there was a trend of falling Bowen ratio both during the day and during the season from May (monthly value 0.9) to September (monthly value 0.6). The bulk surface resistance ( r s) to evapotranspiration was seldom close to zero and varied little (mean r s±S.D.=160±70 s m −1). Its variation depended mainly on vapour pressure deficit and was not directly correlated to peat wetness. At least half the evapotranspiration was estimated to originate from the moss surface. The evapotranspiration was 60–80% of the Penman potential evaporation. The relatively large amount of non-transpiring biomass above ground gave an effect with a relatively high Bowen ratio and a large but stable r s. It was concluded that there exists a canopy layer, within which the distribution of heat and vapour sources may vary with time. As the sources of evapotranspiration probably vary, the stable surface resistance is not caused by a constant regulation of moss evaporation but by small-scale advection effects. The low coupling to the ambient vapour deficit resulted in the evapotranspiration fitting well with the Priestly–Taylor model, with an α value of 0.8. Simple models may then be very effective for a surface type like this. However, the influence of different surface properties on the parameterisation needs to be investigated further. The use of more complicated, but physically sound, models demands more detailed studies of both physical properties of mosses and effects of differences in surface cover and micro-topography.

Development and validation of model for estimating temperature within maize ear

We present a three-dimensional computer model that simulates ear temperatures under field conditions for both daytime and night-time. The meteorological data used are total and diffuse radiation, wind speed, air temperature and humidity (or wet bulb temperature). The model is based on the energy variation of volume elements on ear surface. It takes into account, net radiation, sensible and latent heat exchange and heat diffusion within the ear. The model performs a radiation balance that separates direct, diffuse and scattering components. The husk stomatal resistance was parameterised as a function of water vapour deficit and solar radiation deduced from our experimental data. The model was tested in two stages: first, the calculated flux of downward and upward all wave radiation, at ear level, was compared with real measurements. Second, the calculated grain temperatures were compared with air temperature, and with data collected, for different polar positions around the cob, in two experiments conducted in 1997, in Morocco, and 1998, in Belgium. The agreement was satisfactory; the average difference between the model estimates and measurements of grain temperature were 0.5°C in Belgium and 0.6°C in Morocco, whereas using air temperature as the simplest estimate of the grain temperature gave average differences against the measured grain temperature of 1.1 and 1.8°C, respectively.

IMPACT OF THERMAL STRESS AND HIGH VPD ON GAS EXCHANGE AND CHLOROPHYLL FLUORESCENCE OF CITRUS GRANDIS UNDER DESERT CONDITIONS

The photosynthetic response of Citrus grandis to high light intensities, low air humidity and high temperature stress was investigated under dese rt conditions in the southern AravaValley (Israel). During summer, a typical midday stomatal closure was observed even in well-watered trees due to the dramatic increase of the leaf-to-a ir water vapour deficit. As a result of the reduced transpirational cooling, leaf temperatures increased up to 11 °C above ambient air temperature. The combination of heat stress and pho toinhibition resulted in a reversible decrease of photosynthetic activity of Citrus grandis under the extreme summer conditions.

A model to estimate the temperature of a maize apex from meteorological data

During early growth, when the apical meristem of a maize plant is close to the soil surface, its temperature may be very different from air temperature. A model is proposed to estimate apex temperature from meteorological data, when the leaf area index is less than 0.5. This model is based on the energy balance of the apical meristem, considered as a vertical cylinder close to the soil surface, called ‘apex’. Soil surface temperature was calculated from an energy and water balance of the soil. Input data were hourly standard meteorological data and soil texture. Stomatal conductance was calculated from solar radiation and water vapor deficit. Five field experiments with different soil and climatic conditions were conducted to calibrate and validate the model. The roughness length of the soil surface was used as a calibrating factor. The selected value was 0.3 mm, and was used on all datasets. The agreement between observed and calculated apex temperatures was fairly good, with residual standard deviations between 0.8 and 1.9 K in five experiments, while apex temperature was generally higher than air temperature at screen level by more than 5–7 K during the day. This study showed that the main problem to overcome in estimating apex temperature, is to calculate air temperature at apex height, i.e. at several centimeters from the soil surface. This requires development of precise soil surface temperature models.

Surprising implications of the optimality hypothesis of stomatal regulation gain support in a field test

Our previous reformulation of Cowan and Farquhar’s optimality hypothesis of stomatal reg-ulation has resulted in models for photosynthesis and transpiration which have been readily testable against field data. When analysing the water use efficiency implied by our previous reformulation of the optimality hypothesis of stomatal regulation, we discovered an unexpected property: when stomatal reg-ulation is active, a linear relationship could be found between transpiration and a term involving water vapour deficit and photosynthesis. This prediction gives rise to a novel test which requires no parameter estimation. We conducted such a test in Scots pine, utilising ca 10 000 measurements of CO2 exchange, transpiration, temperature, PAR, and water vapour concentration, taken at the SMEAR I measuring station in Finnish Lapland. As predicted, on clear and sunny days the correlation coefficient of the linear relationship was as high as 0.99, corroborating our formulation of the optimality hypothesis.

Surprising implications of the optimality hypothesis of stomatal regulation gain support in a field test

Our previous reformulation of Cowan and Farquhar’s optimality hypothesis of stomatal reg-ulation has resulted in models for photosynthesis and transpiration which have been readily testable against field data. When analysing the water use efficiency implied by our previous reformulation of the optimality hypothesis of stomatal regulation, we discovered an unexpected property: when stomatal reg-ulation is active, a linear relationship could be found between transpiration and a term involving water vapour deficit and photosynthesis. This prediction gives rise to a novel test which requires no parameter estimation. We conducted such a test in Scots pine, utilising ca 10 000 measurements of CO2 exchange, transpiration, temperature, PAR, and water vapour concentration, taken at the SMEAR I measuring station in Finnish Lapland. As predicted, on clear and sunny days the correlation coefficient of the linear relationship was as high as 0.99, corroborating our formulation of the optimality hypothesis.

Seasonal variation of boreal forest surface conductance and evaporation

Long term measurements (June 1994 to December 1996) of evaporation were made in a boreal forest in central Sweden. Fluxes were measured continuously with eddy-correlation systems from a 100m tower. Surface conductance and potential evaporation were estimated using the Penman–Monteith equation. The overall average evaporation during the 947-days observation period was 1.07mmd−1. The average evaporation from June to December 1994 was 1.25mmd−1, 1.07mmd−1 during 1995, and 0.97mmd−1 during 1996. Maximum daily rates were typically 4mmd−1 around mid-summer in 1994 and 1995 and slightly less in 1996. During the winter period from November to March, the evaporation sometimes reached 0.5mmd−1. Generally, the actual evaporation followed the dynamics of the potential evaporation fairly well. As a total for the entire period, the actual evaporation accounted for 918mm, or 69% of the potential evaporation (1332mm). There were three major time periods with a considerable evaporation deficit: the late summer (July–August) 1994, the late summer (August) 1995, and the spring to early summer (March–June) 1996 (Table 1Table 1Sums of actual and potential evaporation in NorundaPeriodE (mm)Epot (mm)E/Epot (%)July 1994 to October 1996916133169July–August 199413218073August 19956510463March–May 199611624348December 1994 to January 19954.670.15December 1995 to January 19962.690.80). The first two periods were correlated with low soil moisture while the last probably was caused by low winter temperatures. A surface conductance model showed little dependence on soil moisture. Radiation and vapour pressure deficit were the most important factors.When soil water was not limiting, the diurnal courses of surface conductance showed a steep increase in the morning followed by an almost linear decrease, the coniferous trend caused by stomatal control. During periods of water stress a midday depression occurred after the morning maximum. During wintertime, the surface conductance hardly ever approached to zero.A simple model for surface conductance based on global radiation and vapour pressure deficit fitted the estimated data fairly well except for calm nights with poor turbulent mixing. This was apparently due to water vapour storage within and above the canopy. Including the friction velocity in the model parameterisation improved the fitting considerably.

A hydrometeorological model for basin‐wide seasonal evapotranspiration

A new methodology is proposed to capture the seasonal behavior of evapotranspiration from precipitation and streamflow data and to develop hydrometeorological evapotranspiration models tailored for each basin. The water budget method for determining evapotranspiration is downscaled to periods between 15 and 160 days that occur between well‐marked hydrological recessions. Using these uneven time periods, the error associated with the unknown soil moisture storage is minimized, whereas groundwater storage changes are estimated by means of a classical linear groundwater reservoir whose time constant is obtained by recession analysis. This seasonal water budget (SWB) method is able to reproduce the seasonal signal of evapotranspiration even when it is absent from the precipitation and streamflow records. The estimates are also compatible with calculated monthly net radiation. By selecting short enough water budget periods it is possible to check the relationship between SWB evapotranspiration estimates and net radiation, Penman and Priestley‐Taylor potential evaporation, precipitation minus outflow, water vapor deficit, and basin storage. The ratio of SWB evapotranspiration to an upper limit value represented by either net radiation or potential evaporation is well correlated with precipitation minus outflow, water vapor deficit, or both but is very poorly related to basin storage. The calculated regressions lead to a family of hydrometeorological evapotranspiration monthly (HEM) models fitted to the basins in question, in a way analogous to the calibration of rainfall‐runoff models. In the two watersheds where the methodology was applied the HEM models were able to preserve mass, with total accumulated differences no larger than 0.25 mm d−1 and root‐mean‐square errors of the order of 0.7 mm d−1.

Microphysical and Large-Scale Dependencies of Temporal Rainfall Variability over a Tropical Ocean

Abstract The focus of this paper is the elucidation of the physical origins of the observed extreme rainfall variability over tropical oceans. A simple statistical–dynamical model, suitable for use in repetitive Monte Carlo experiments, is formulated as a diagnostic tool for this purpose. The model is based on three partial differential equations that describe airmass, water substance, and vertical momentum conservation in a column of air extending from the ocean surface to the top of the storm clouds. Tropospheric conditions are specified for the model state variables (such as updraft–downdraft velocity, precipitation water and cloud content, or saturation vapor deficit) in accordance with past observations in oceanic convection, to allow for vertical integration of the model equations and the formulation of a computationally efficient diagnostic tool. Large-scale forcing is represented by stochastic processes with temporal structure and parameters estimated from observed large-scale data. This model for...

Characteristics of energy and water budgets over wet sedge and tussock tundra ecosystems at North Slope in Alaska

Energy budgets were measured at Arctic tundra ecosystems in Alaska, USA. Measurements were carried out over coastal tundra at Prudhoe Bay in 1994, and at wet sedge, moist and dry tussock tundra around Happy Valley in 1995. Sensible heat flux, H, and latent heat flux, lE, were determined by an eddy correlation technique. Over the Arctic coastal tundra, the energy partition changed remarkably as a result of air mass conditions. Under cold and humid wild (onshore) from the Arctic Ocean, the temperature gradient over the tundra ecosystem increased, which resulted in a high level of H. Under warm and dry air mass on clear days from the inland tundra (offshore), air and soil temperatures and the water vapour deficit increased, which resulted in an increase in lE. The same effect or air mass condition on energy partition was observed at the tundra ecosystem in Happy Valley located 135 km inland from the Arctic coast. The energy budget at wet sedge tundra was characterized as the heat storage within the water layer on a daily basis, which was partitioned mainly to lE in daytime and lE and H at night. Latent heat flux, lE, was largest at wet sedge tundra and lowest at dry tussock tundra. The Bowen ratio decreased with air temperature over wet sedge and dry tussock tundra ecosystems. The aerodynamic resistance ra, and canopy resistance, rc, over the wet sedge tundra at Happy Valley had different levels over four major wind directions. These levels decreased with increase in wind speed. ra was slightly higher than over other vegetation, which seemed to be owing to a relatively lower wind speed, while the canopy resistance rc was lower than over other vegetation. Global warming will affect the energy partition of the Arctic tundra ecosystem, and the warming and drying climate will increase the lE of tundra, especially in flooded areas. Expanding dry tundra increases H rather than lE, which enhances the warming of the tundra ecosystem. Copyright © 1998 John Wiley & Sons, Ltd.

Evapotranspiration of Melaleuca Forest in South Florida

Melaleuca forests cover extensive areas of south Florida, and are widely assumed to have extraordinarily high evapotranspiration (ET) rates. This investigation indicates that the transpiration of the melaleuca forest is dominated by vapor deficits, with net radiation playing a secondary role. Evaporation of intercepted rainfall contributes between 12 and 23% to the monthly ET, with lower contributions during the dry season and higher contributions during the wet season. Annual ET from the melaleuca forest is estimated to be approximately 1,600 mm, with 300 mm contributed by intercepted rainfall. Comparison of these results with predictions of other forest-ET models indicate that the Shuttleworth model overestimates the ET of melaleuca forest by about 5% in a typical month as a result of neglecting the canopy wetness effect. The Clader-Newson model, which equates forest transpiration to short-vegetation transpiration, produces annual ET estimates that are within 6% of the annual ET estimated in this study. However, the discrepancies in monthly ET range between -8 and +17%.

Transpiration rate of Prunus avium L. seedlings inside an unventilated treeshelter

Transpiration rate was measured on one-year-old sheltered and control non sheltered Prunus avium L. seedlings using the stem heat balance method. The sheltered trees, protected by unventilated 2.1-m-tall plastic tubes, had an average transpiration rate 6 times less than control trees, although the leaves had a high conductance throughout the day. Lower transpiration rates in shelters may be explained by low values of net radiation, extremely small water vapour deficit, and low windspeed resulting in a high boundary layer resistance to water transfer. Consequences of such low transpiration are discussed, and recommendations regarding the optimal use of treeshelters are deduced. The removal or sudden ventilation of a treeshelter during the growing season should be avoided, and weeding the sheltered trees is essential.

Water relations and gas exchange in young coconut palm (Cocos nuciferaL.) as influenced by water deficit

Drought is the main climatic limitation to coconut palm (Cocos nucifera L.) production. To identify early screening parameters for drought resistance, physiological responses to water deficit were investigated for the first time using containerized young plants grown outside. Three varieties were studied: 'West Coast Tall' (drought avoiding), 'Malayan Yellow Dwarf' (drought susceptible), and their progeny, the hybrid 'PB 121' (drought resistant). Leaf water status (relative water content, leaf water potential) and leaf gas exchange parameters (stomatal conductance to water vapor, net photosynthetic rate, and leaf internal CO2concentration) were measured throughout a drying cycle induced by withholding watering. On fully hydrated plants, the sensitivity of stomata to atmospheric water vapor deficit was also investigated. In the three varieties, stomatal conductance to water vapor declined before leaf water status parameters were affected. The existence of a root-to-shoot communication system was proposed. Net photosynthetic rate was highly dependent on stomatal aperture, but nonstomatal factors also participated in the reduction of CO2fixation. Since gas exchange rates were equally sensitive to drought in the three varieties, they could not be used as screening parameters for drought resistance. Under severe drought stress, both relative water content and leaf water potential differed significantly among populations. 'West Coast Tall' maintained a higher leaf water status than 'Malayan Yellow Dwarf'. Remarkably, the water status parameters of 'PB 121' were intermediate between those of the two parents. Similar ranking was obtained in experiments with excised leaflets. The high reproductibility of the results suggests that leaf water status parameters might be useful as early selection criteria for drought resistance in coconut palm. Key words: Cocos nucifera L., gas exchange, leaf water status parameters, water deficit, water relations.

アラスカ州プルドーベイの北極域ツンドラにおける盛夏期のＣＯ２収支に及ぼす微気象の影響

The characteristics of micrometeorology and the CO2 flux of the Arctic wet tundra ecosystem were measured during the mid-summer of 1994 at Prudhoe Bay, Alaska. CO2 flux was measured by the eddy correlation technique with a sonic anemometer and an open path IRGA.Over the coastal Arctic tundra, the micrometeorology of the surface boundary layer changed remarkably according to wind direction. Wind from the Polar Sea brought cold and moist air, and southern wind brought warm and dry air with higher water vapor deficit, which resulted different CO2 flux over the wet tundra ecosystem.A simple canopy resistance model was applied in order to analyze the relationship between the CO2 flux and the micrometeorology, in which parameters were determined using measured data. The ecosystem respiration increased with water temperature, and the rate was larger under higher wind speed. The photosynthesis rate decreased with water vapor deficit between leaves and the atmosphere, and its decrement was larger under higher wind speed conditions.The effects of micrometeorology on the CO2 flux were examined using the model, which showed that the level of ecosystem respiration due to high water temperature and that of photosynthetic decrease due to high water vapor deficit were the same orders as the potential of photosynthesis measured over the coastal Arctic tundra at Prudhou Bay, Alaska in mid summer. The magnitude of the ecosystem respiration and the decrement of photosynthesis were large enough to diminish the downward CO2 flux to zero. The CO2 budget over the coastal Arctic wet tundra was much affected by the ecosystem respiration and the contribution of water stress to plant photosynthesis caused by higher temperature and drying.

Involvement of malate and mannitol in the diurnal regulation of the water status in members of Oleaceae

This study examines water status regulation in plants of the Oleaceae family and in some other co-occurring species that are exposed to high solar radiation, in the same habitat. Fraxinus excelsior L., one of the most studied Oleaceae in this field exhibited, during the growing season, a close relationship between diurnal variations in leaf water potential and changes in malate, mannitol and K+ levels, depending on the weather conditions. On sunny days, similar variations can be observed in leaves of the other Oleaceae, with a concomitant decrease in the osmotic potential between predawn and solar noon. Malate, mannitol and the well-known osmoticum K+, contribute greatly to the osmotic potential decrease. This mechanism, which can be related to the osmotic adjustment described for both drought and salt-affected plants, appears as a general response in plants of the Oleaceae family. Among the other co-occurring species investigated, only Quercus robur L. displayed a similar mechanism under the same environmental conditions, but two other organic compounds, quinic and shikimic acids, are presumably involved. Alnus glutinosa (L.) Gaertn. and Robinia pseudacacia L. responded to a vapor deficit by partial stomatal closure, as transpiration progressed through the morning.

Measuring and Modeling Stomatal and Aerodynamic Conductances of Mountain Birch: Implications for Treeline Dynamics

Stomatal conductances of mountain birch leaves were determined by field measurements using a steady-state porometer under varying weather and radiation conditions. The stomatal conductances were related to atmospheric environmental parameters (water vapor deficit, air temperature, soil moisture potential, and photoactive radiation) by a nonlinear regression equation where the fit to the data set was optimized. By applying a 3-layer canopy model for evapotranspiration, with the actual measurements of the stomatal conductance inserted, characteristic bulk aerodynamic conductances for the exchange between the birch trees and the ambient air were determined empirically by comparison with the evapotranspiration from cut birches using a potometric technique. Finally, the 3-layer evapotranspiration model, with the stomatal submodel incorporated, was tested against the evapotranspiration measurements using a total of 27 cut birches on 54 d. Water vapor pressure deficit has a dominating effect on the variations of stomatal conductance. For air temperatures below about + 13'C, there is a significant decreasing trend of the conductance. The ecological implications of stomatal conductance and the seasonal transpiration near the altitudinal treeline are discussed.

Simulation of diurnal transpiration and photosynthesis of a water stressed soybean crop

The diurnal course of photosynthesis and transpiration of different plants can exhibit a dissymmetric behaviour between the morning and the afternoon, a photosynthetic midday depression and a transpiration ‘plateau’. A field experiment conducted in a meditenanean climate allows us to identify these features for a soybean crop undergoing water stress. The experimental data also show that, before and after the depression and plateau times, photosynthetic and transpiration fluxes are similar from one day to another. The duration of the period of depression and plateau increases as the soil dries out. Two soil-vegetation-atmosphere transfer models are used to simulate these features. Both are able to capture the midday depression and the plateau because they impose a direct relationship between leaf water potential, stomatal conductance and photosynthesis. When the leaf water potential reaches a `critical' value, feedback mechanisms tend to regulate the leaf water potential such that the transpiration becomes nearly constant. The stomatal conductance and the photosynthesis keep decreasing in response to an increase of water vapour deficit. The dissymmetry depends on the dissymmetry of the diumal course of saturation deficit in the atmosphere, and on the evolution of the water properties of the soil during the day. We conclude: that, even if they do not include formulations of photosynthesis and stomata] conductance based on hormonal regulations of stomatal conductance and inter-regulations between photosynthesis and conductance, such models remain useful for the simulation of canopy processes.

Optimal Control of Gas Exchange during Drought: Theoretical Analysis

An optimal strategy of stomatal control during a drought period, in plants adapted to a humid climate, is derived by maximizing the photosynthetic production during the expected duration of drought. The expected duration of drought is calculated from the probability that rain occurs during a certain period, which is assumed constant. The underlying plant model describes photosynthetic production and the consumption of water from the soil, with a given initial soil water content. Water is consumed through transpiration at a rate dependent on water vapour deficit, temperature and stomatal conductance and carbon is assimilated at a rate dependent on light intensity and stomatal conductance. The optimization problem is solved with driving variables and the probability of rain corresponding to a Fenno-Scandian climate. The resulting optimal stomatal control consists of two processes with different time constants: (1) daily variation depending on the driving variables, and (2) a declining trend as a function of the initial soil water content and the probability of rain. The result allows for a physical interpretation of the so-called ‘cost of water’ used in similar optimization studies. An approximate model is derived from the optimal solution, such that the ‘cost of water’ is a function of the soil water content.

Optimal Control of Gas Exchange during Drought: Empirical Evidence

The optimal regulation model by Mäkelä, Berninger and Hari ( Annals of Botany 77: 461–467, 1996) was applied to data for photosynthesis and transpiration of Scots pine during a 22-d drought period. There was a clear decrease in photosynthesis and transpiration during that period. The agreement between model and photosynthesis data was good. The residuals of photosynthesis were not systematic with respect to temperature, irradiance or water vapour deficit. However, the model initially overestimated transpiration by 50%, although there was a clear linear relationship between measured and estimated values. The results suggest that there was no decrease in photosynthetic capacity during the period, but a decrease in stomatal conductance was responsible for the changes in photosynthesis and transpiration. The observations are similar to results in the literature.