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The stem heat balance (SHB) method is a widely used sap flow technique to determine the transpiration and the water demands of herbaceous and woody plants, especially those with small diameters ( e.g. climbers). The accuracy of the sap flow derived by this method ( Q S ) depends on correction of the total measured heat input ( Q t ) by subtracting unintended heat losses; these heat losses are referred to as “fictitious flow” ( Q fic ) ( Q S = Q t − Qfic ). We developed a physically consistent baseline-correction approach using minimum nocturnal water vapour pressure deficits ( VPD ). This VPD approach was compared to the so-called “night value subtraction” (NVS) approach and direct gravimetric determination for potted climbing plants and an outdoor climbing plant stand. In addition, performance tests were also conducted on artificial model stems and cut plant stems. In the tests with the outdoor climbing plant stand, sap flow corrected by the NVS approach underestimated daily transpiration by up to 33% compared to direct gravimetric determination. In contrast, the newly developed VPD approach underestimated or overestimated transpiration by only 5%–10%. The VPD approach makes use of the direct dependence of sap flow on VPD during zero-radiation conditions (night). This means, Q fic is the constant of the linear regression of the VPD and the lowest recorded Q t at night. Therefore, the correction is based on all recorded sap flow data from the measurement period itself, which in turn accounts for all factors influencing Q fic , including RH and T air ; these latter parameters are often recorded in any case. This also means that this method can be subsequently applied to currently available data sets in order to improve their quality. Our results suggest that when the raw data are corrected appropriately, the SHB method is viable when attempting to determine transpiration rates of climbing plants. This is especially true for urban areas, with their illumination, typically high VPD s and increased T air at night. [ABSTRACT FROM AUTHOR]

Transpiration and to a lesser degree respiration are two well-known processes of water loss from fresh agricultural products, associated with visual and texture degradation. Neglecting respiration as a water loss mechanism leads to erroneous results at saturation where, although the water vapour pressure deficit is zero and therefore water loss should be zero, on the contrary a finite water loss exists. In this context an experiment was set up to analyse the water loss associated with transpiration and respiration in pears ( Pyrus communis L., Kontoula ) at 0, 10 and 20 °C and 70%, 80% and 95% RH, as well as the air humidity of the cold storage. The choice of pears was based on the fact that they rank third among the most important tree fruits. The estimated transpiration rates ranged between 0.03 and 0.28 mg cm −2 h −1 for water vapour pressure deficit range of 0.0–0.52 kPa. The mean respiration rates were calculated at 0, 10 and 20 °C as 0.48 ± 0.1, 1.27 ± 0.2 and 3.48 ± 1.1 mL[CO 2 ] 100 g −1 h −1 . Quantification of the two sources of water loss showed that, close to saturation (20 °C and 95% RH), the water loss due to respiration accounts for 39% of the water loss due to water vapour pressure deficit while, on average, the water loss due to respiration accounts for 8%, 14% and 23% of the water loss due to water vapour deficit at 0, 10 and 20 °C. These findings justify why water loss due to respiration should not be neglected under certain environmental conditions. [ABSTRACT FROM AUTHOR]

Estimation of reference evapotranspiration (ET o ) is crucial in crop production practices and other hydrological processes. The hourly FAO-56 Penman-Monteith (ET oPM ) method was used to calculate ET o at Bergville, Bethlehem and Harrismith in the eastern Free State, South Africa. Priestley-Taylor evapotranspiration coefficients (PT c ) were estimated using ET oPM and equilibrium evapotranspiration. The study establishes that for the mountainous semi-arid areas, aerodynamic conditions are major contributors of evapotranspiration. The aerodynamic component of ET oPM is generally a dominant contributor varying between 50% and 70% of the total ET oPM depending on the season and location. Dry and windy atmospheric conditions that reach their highest levels during the spring season cause water vapour pressure deficit to be a parameter with greater influence on ET o . The study further shows that the impact of solar irradiance on ET o decreases with altitude in the study area. More than 87% of ET oPM from aerodynamic resistance is experienced during daytime. Agricultural water resources management activities need to consider the importance of nocturnal ET oPM which contributes between 10–14% of total daily ET o depending on the station. Geometry of a surface affects total solar irradiance received at a place and the associated ET o . The commonly used average Priestley-Taylor constant of 1.26 under-estimates evaporation rates in the semi-arid environments. This study shows that the coefficients are highly variable during winter and spring, less variable in autumn, and that topoclimate has a significant impact on the PT c variability. This variability reflects a direct influence that the dry atmospheric environments of the semi-arid regions have on PT c , which is a limitation of the use of the 1.26 coefficient. [ABSTRACT FROM AUTHOR]

Transpiration is a well known physiological process of water loss from fresh products, associated with visual and texture degradation and loss of market value. A loss of 3-5% of the initial mass may cause in fresh products loss of freshness and visual attractiveness. Grape tomato has been increasingly accepted by consumers as "snacking tomato" and as an ingredient in mixed salads of fresh-cut vegetables. An experimental procedure was developed to record the associated with transpiration, water loss in grape tomatoes (Solanum lycopersicum, Lobello F1), at temperatures 10, 15 and 20 °C and relative humidity 70, 80 and 92%. Water activity was calculated and correlated with the respective mass loss; its average value was found 0.988 ± 0.01. The mean transpiration rates ranged between 0.012 and 0.058 mg cm-2 h-1 for water vapour pressure deficit range of 0.061-0.662 kPa. A semi-empirical and an analytical model were developed to correlate the mass loss of grape tomatoes with the storage conditions (temperature and relative humidity) and storage time. Both provided satisfactory fit to the experimental data. Finally, the air-film mass transfer coefficient (ka) and skin mass transfer coefficient (ks) were calculated and the ks coefficient correlated efficiently with an exponential equation with the respective water vapour pressure deficit. [ABSTRACT FROM AUTHOR]

To better understand the effects of ambient relative humidity (RH) on physiological variables and the implications of RH- correcting evaporative water loss (EWL) data for marsupials, we examined the effect of RH on EWL, body temperature (Tb), metabolic rate (MR) and thermal conductance (c1 of the brushtail possum (Trichosurus vulpecula), a medium-sized marsupial. Correcting EWL data for 27 species of marsupial for water vapour pressure deficit (▵WVP) in the chamber during measurement significantly increased, rather than decreased, the variability of the allometric relationship for EWL. For the brushtail possum, both ambient temperature (T[suba]) and RH significantly affected EWL. At Ta=25°C, EWL was independent of RH at "63% RH, but decreased linearly at higher RH values. At Ta=30°C, EWL was significantly related to RH from 26% to 92% RH. There was a significant effect of Ta on Tb and dry thermal conductance (Cd[subdry]; higher at 30°C), but no effect of RH. For MR and wet thermal conductance (C[subwet]) there was a significant effect of T[suba] (MR higher and C[subwet] lower at 25°C), and RH at T[suba]=30°C (MR higher and C[subwet] lower at the lowest RH) but not at 25°C. Our results indicate that brushtail possums do not necessarily show the linear relationship between ambient RH and EWL expected for an endotherm, possibly because of behavioural modification of their immediate microclimate. This may account for the failure of WVP deficit correction to improve the allometric EWL relationship for marsupials. Chamber RH is an important environmental factor to be considered when measuring standard physiological variables such as MR and C[subwet]. [ABSTRACT FROM AUTHOR]

Abstract: Hydrangea macrophylla cv. Leuchfeuer is grown in the greenhouse under two different conditions of relative humidity (vapour pressure deficits (vpd) of 1.6 and 0.6kPa). The relative humidity does not affect the initiation or the release of new pairs of leaves. However, it modifies the size of the plant and the total leaf area. After 79 days of growth, a decrease in the relative humidity from 80% to 50% leads to a 38% decrease in height growth and a 30% decrease in leaf growth. The effect of relative humidity on the growth of H. macrophylla influences the management of the size of the plant and, therefore, the quality of the product marketed. It is likely that it will have an effect on floral development as well. [Copyright &y& Elsevier]

It is widely acknowledged that the possible impacts of ozone on forest trees are more closely related to ozone flux through the stomata than to external ozone exposure. However, the application of the flux approach on a European scale requires the availability of appropriate models, such as the European Monitoring and Evaluation Programme (EMEP) ozone deposition model, for estimating ozone flux and cumulative ozone uptake. Within this model stomatal conductance is the key variable, since it determines the amount of ozone absorbed by the leaves. This paper describes the suitability of the existing EMEP ozone deposition model parameterisation and formulation to represent stomatal behaviour determined from field measurements on adult Norway spruce (Picea abies (L.) Karst.) trees in the Central European Alps. Parameters affecting maximum stomatal conductance (e.g. seasonal phenology, needle position, needle age, nutrient deficiency and ozone itself) and stomatal response functions to temperature, irradiance, vapour pressure deficit, and soil water content are investigated. Finally, current limitations and possible alterations of the EMEP model will be discussed with respect to spatial scales of available input data for future flux modelling. [Copyright &y& Elsevier]

Internal pressurization and convective gas flow in emergent wetland plants is a function of the water vapour pressure deficit (WPD) and stomatal conductance (Gs) separating the external atmosphere from the internal aerenchyma. We have compared the effects of WPD and Gs under a range of light intensities on static pressures and convective flows in Phragmites australis, Typha orientalis and Baumea articulata. The capacity of the three species to generate flows per unit leaf area differed, being greatest in P. australis and lowest in B. articulata. In all three species, decreasing light intensity from full sunlight (2200 μmol m-2 s-1 photosynthetically active photon flux density (PPFD)) to < 200 and < 10 μmol m-2 s-1PPFD caused immediate decreases in photosynthetic assimilation, followed by more gradual decreases in transpiration and Gs. However, internal pressures and flows in the two low light intensities remained similar to values recorded in full sunlight. WPD was more significantly related to pressures and flows in P. australis and T. orientalis than Gs. In B. articulata, pressures increased at low Gs values but flow rates were unaffected, as predicted by earlier models describing pore size effects on pressures and flows. The data suggest that emergent macrophytes can maintain significant internal convection even at low light intensities, and this may be beneficial for nocturnal aeration, particularly in arid climates where the atmospheric humidity at night is low. [ABSTRACT FROM AUTHOR]

SummaryFuture increases in air temperature resulting from human activities may increase the water vapour pressure deficit (VPD) of the atmosphere. Understanding the responses of trees to spatial variation in VPD can strengthen our ability to predict how trees will respond to temporal changes in this important variable. Using published values, we tested the theoretical prediction that conifers decrease their investment in photosynthetic tissue (leaves) relative to water-conducting tissue in the stem (sapwood) as VPD increases. The ratio of leaf/sapwood area (AL/AS) decreased significantly with increasing VPD in Pinus species but not in Abies, Pseudotsuga, Tsuga and Picea, and the average AL/AS was significantly lower for pines than other conifers (pines: 0.17 m2 cm-2; nonpines: 0.44 m2 cm-2). Thus, pines adjusted to increasing aridity by altering above-ground morphology while nonpine conifers did not. The average water potential causing a 50% loss of hydraulic conductivity was -3.28 MPa for pines and -4.52 MPa for nonpine conifers, suggesting that pines are more vulnerable to xylem embolism than other conifers. For Pinus ponderosa the decrease in AL/AS with high VPD increases the capacity to provide water to foliage without escalating the risk of xylem embolism. Low AL/AS and plasticity in this variable may enhance drought tolerance in pines. However, lower AL/AS with increasing VPD and an associated shift in biomass allocation from foliage to stems suggests that pines may expend more photosynthate constructing and supporting structural mass and carry less leaf area as the climate warms. [ABSTRACT FROM AUTHOR]

Features of Crassulacean acid metabolism (CAM) were studied in a variety of different succulents in response to climatic conditions between March 1977 and October 1983 in the southern Namib desert (Richtersveld). A screening in 1977 and 1978 revealed that nearly all investigated succulents performed a CAM, but overnight accumulation of malate declined gradually with decreasing soil water potential, tissue osmotic potential, and leaf water content. This was further substantiated by an extended period of insufficient rainfall in 1979 and 1980 which damaged the evergreen CAM succulents between 80 and 100%. In most of the species still living, neither CO-gas exchange nor diurnal acid fluctuation, indicative of CAM, could be detected unless an abundant rainfall restored both CAM features. Plants persisted in a stage of latent life. Water supply is one necessary prerequisite for CAM in the Richtersveld. But even well-watered plants with CAM were sensitive to short-term water stress caused by high water-vapour partialpressure deficit (VPD) in the night, which reduced or prevented CO uptake and resulted in a linear relation between overnight accumulated malate and VPD. The results do not support the opinion that, for the Namib succulents, CAM is an adaptive mechanism to water stress since long-term and short-term water stress stopped nocturnal malate synthesis, but instead lead to the conclusion that nocuturnal CO fixation is only performed when the water status of the plant can be improved simultaneously. [ABSTRACT FROM AUTHOR]

Pattern and magnitude of stem gas exchange were studied under controlled conditions on two CAM species of the genus Cissus differing in morphological features. In the cactus-like liana Cissus quadrangularis, at water vapour deficit of the air (VPD) lower than 400 mPa Pa during daytime, under 24/16°C and 27/16°C temperature regime, CO uptake occurred during daytime and nighttime but night fixation was responsible for 74 and 77% respectively of the CO fixed during the whole diurnal cycle; the contribution of night fixation increased up to 84% at VPD of 590 mPa Pa under 27/15°C. In Cissus sp., a slightly succulent xerophytic liana with mesophytic deciduous leaves, at 27°C day temperature and VPD of 520-540 mPa Pa, under both 12 and 16°C night temperature, CO uptake occurred exclusively during the night; however at lower day temperatures (18-20°C) and lower VPD (169-269 mPa Pa) substantial CO uptake was observed in the light. Transpiration was higher in Cissus sp. than in Cissus quadrangularis under all of the conditions applied; moreover night transpiration in Cissus sp. contributed more to total day transpiration as compared with Cissus quadrangularis. The results support the opinion that the nocturnal gas exchange has a preeminent role in Cissus sp. as compared with Cissus quadrangularis. Indeed under conditions which enhance evaporative demand even well-watered plants of Cissus sp. resort totally to night fixation. In contrast Cissus quadrangularis which shows a larger resistance to water loss and possesses an abundant water storing parenchyma as compared with Cissus sp. resorts totally to night CO fixation only under drought stress. [ABSTRACT FROM AUTHOR]

It is a central paradigm of comparative physiology that the effect of humidity on evaporative water loss (EWL) is determined for most mammals and birds, in and below thermoneutrality, essentially by physics and is not under physiological regulation. Fick's law predicts that EWL should be inversely proportional to ambient relative humidity (RH) and linearly proportional to the water vapour pressure deficit (Δwvp) between animal and air. However, we show here for a small dasyurid marsupial, the little kaluta (Dasykaluta rosamondae), that EWL is essentially independent of RH (and Δwvp) at low RH (as are metabolic rate and thermal conductance). These results suggest regulation of a constant EWL independent of RH, a hitherto unappreciated capacity of endothermic vertebrates. Independence of EWL from RH conserves water and heat at low RH, and avoids physiological adjustments to changes in evaporative heat loss such as thermoregulation. Re-evaluation of previously published data for mammals and birds suggests that a lesser dependence of EWL on RH is observed more commonly than previously thought, suggesting that physiological independence of EWL of RH is not just an unusual capacity of a few species, such as the little kaluta, but a more general capability of many mammals and birds. [ABSTRACT FROM AUTHOR]