Low Vapor Pressure Deficit Research Articles

Direct Penman–Monteith parameterization for estimating stomatal conductance and modeling sap flow

The novel approach for direct parameterization of the Penman–Monteith equation was developed to compute diurnal courses of stand canopy conductance from sap flow. The Penman–Monteith equation of evaporation is often combined with sap flow measurements to describe canopy transpiration and stomatal conductance. The traditional approach involves a two-step calculation. In the first step, stomatal conductance is computed using an inverted form of Penman–Monteith equation. The second step correlates these values with environmental factors. In this work, we present an improved approach for direct parameterization of the Penman–Monteith equation developed to compute diurnal courses of stand canopy conductance (g c) from sap flow. The main advantages of this proposed approach versus using the classical approach are: (1) the calculation process is faster and involves fewer steps, (2) parameterization provides realistic values of canopy conductance, including conditions of low atmospheric vapor pressure deficit (D), whereas the traditional approach tends to yield unrealistic values for low D and (3) the new calculation method does not require enveloping curves to describe dependence of g c on D and thus avoids subjective data selection but it still allows to visualize separable responses of g c to environmental drivers (i.e., global radiation and vapor pressure deficit). The proposed approach was tested to calculate g c and to model the sap flow of a high mountain Pinus canariensis forest. The new calculation method permitted us to describe the stand canopy conductance and stand sap flow in sub-hour resolution for both day and night conditions. Direct parameterization of the Penman–Monteith approach as implemented in this study proved sufficiently sensitive for detecting diurnal variation in g c and for predicting sap flow from environmental variables under various atmospheric evapotranspirative demands and differing levels of soil water availability.

Bundle-sheath leakiness and intrinsic water use efficiency of a perennial C4 grass are increased at high vapour pressure deficit during growth.

Bundle-sheath leakiness (ϕ) is a key parameter of the CO2-concentrating mechanism of C4 photosynthesis and is related to leaf-level intrinsic water use efficiency (WUEi). This work studied short-term dynamic responses of ϕ to alterations of atmospheric CO2 concentration in Cleistogenes squarrosa, a perennial grass, grown at high (1.6 kPa) or low (0.6 kPa) vapour pressure deficit (VPD) combined with high or low N supply in controlled environment experiments. ϕ was determined by concurrent measurements of photosynthetic gas exchange and on-line carbon isotope discrimination, using a new protocol. Growth at high VPD led to an increase of ϕ by 0.13 and a concurrent increase of WUEi by 14%, with similar effects at both N levels. ϕ responded dynamically to intercellular CO2 concentration (C i), increasing with C i Across treatments, ϕ was negatively correlated to the ratio of CO2 saturated assimilation rate to carboxylation efficiency (a proxy of the relative activities of Rubisco and phosphoenolpyruvate carboxylase) indicating that the long-term environmental effect on ϕ was related to the balance between C3 and C4 cycles. Our study revealed considerable dynamic and long-term variation in ϕ of C. squarrosa, suggesting that ϕ should be determined when carbon isotope discrimination is used to assess WUEi Also, the data indicate a trade-off between WUEi and energetic efficiency in C. squarrosa.

High atmospheric demand for water can limit forest carbon uptake and transpiration as severely as dry soil

AbstractWhen stressed by low soil water content (SWC) or high vapor pressure deficit (VPD), plants close stomata, reducing transpiration and photosynthesis. However, it has historically been difficult to disentangle the magnitudes of VPD compared to SWC limitations on ecosystem‐scale fluxes. We used a 13 year record of eddy covariance measurements from a forest in south central Indiana, USA, to quantify how transpiration and photosynthesis respond to fluctuations in VPD versus SWC. High VPD and low SWC both explained reductions in photosynthesis relative to its long‐term mean, as well as reductions in transpiration relative to potential transpiration estimated with the Penman‐Monteith equation. Flux responses to typical fluctuations in SWC and VPD had similar magnitudes. Integrated over the year, VPD fluctuations accounted for significant reductions of GPP in both nondrought and drought years. Our results suggest that increasing VPD under climatic warming could reduce forest CO2 uptake regardless of changes in SWC.

Physiological regulation and efficient xylem water transport regulate diurnal water and carbon balances of tropical lianas

Summary Tropical lianas deploy most of their leaves towards the top of the forest canopy, whereas trees exhibit a more stratified crown. Forest canopies are often exposed to hot and windy conditions, and how lianas cope with the extremely high transpirational demands under these environments remains unknown. We investigated stem hydraulic properties, leaf drought tolerance, diurnal changes in leaf and stem water potentials (Ψleaf and Ψstem), stomatal conductance (gs), photosynthetic rate, sap flow and stem native percentage loss of conductivity (PLC) for four liana species in a tropical forest in southwest China. Five co‐occurring tree species were also selected for comparison. Lianas reached maximal transpiration at a relatively lower vapour pressure deficit (<1 kPa) than did co‐occurring trees, suggesting vigorous photosynthesis during the morning. However, liana gs declined markedly over the day, with low gs at midday and afternoon. Lianas generally had higher stem sapwood‐specific conductivity and maximum sap flux density but were less tolerant to drought‐induced cavitation than were evergreen trees. Both lianas and trees lost leaf turgor in the top canopy at midday, but lianas lost leaf turgor earlier (∼2 h) than trees. Seven of eight species exhibited midday increases in PLC when xylem tensions were released to −0·3 to −0·5 MPa for PLC measurements. On average, lianas experienced high PLC (35·9%), along with a greater degree of disequilibrium between leaf and stem water potentials than trees (ΔΨstem–leaf: 1·37 MPa vs. 0·75 MPa) during the day. Earlier stomatal closure and efficient water transport may help lianas maintain higher Ψstem than trees despite having similar Ψleaf. Our results provide evidence that physiological regulation and efficient water transport mediate daily water relations in tropical lianas and may explain how lianas operate efficiently in tropical seasonal forests. Further studies involving a broader range of species are needed to confirm our findings.

Climate predicts the proportion of divaricate plant species in New Zealand arborescent assemblages

AbstractAimPuzzling features of plants are sometimes explained as legacies of co‐evolution with extinct herbivores. One example is the convergent evolution of a small‐leaved, twiggy ‘divaricate’ form in > 50 woody species in New Zealand. This growth form was first interpreted as a response to the Plio‐Pleistocene onset of frosty, droughty environments, but opinion now favours the hypothesis that it arose as a defence against large herbivorous birds (moa). It has been argued that the extinction of moa during the last millennium has left divaricate plants at risk of being out‐competed by faster growing broadleaved competitors, yet their current abundance in some habitats suggests this growth form might confer other advantages besides protection against avian browsing. We aimed to clarify the adaptive significance of the divaricate growth form by identifying environmental correlates of its geographic distribution in New Zealand.LocationThe main islands of the New Zealand archipelago, spanning latitude c. 34.5–47° S.MethodsWe used generalized additive models to examine the environmental correlates of the proportion of local tree and shrub assemblages contributed by divaricate species, on 1152 0.04 ha plots distributed across a wide range of climate and landforms.ResultsA model with four climatic variables explained 37% of the observed variation. Mean minimum temperature of the coldest month was the single best predictor of the proportion of divaricate species in arborescent assemblages, with annual solar radiation, vapour pressure deficit and annual water deficit also contributing to the final model. Divaricates made their largest contributions to assemblages in the rain shadow of the Southern Alps, on sites with cold winters, low solar radiation and significant vapour pressure deficits, where potential evapotranspiration exceeds precipitation by > 130 mm. No physiographic variables (e.g. slope) contributed to the final model.Main conclusionsThe distribution of divaricate plants suggests they cope with frosty, droughty environments better than most other elements of New Zealand's arborescent flora. This may reflect selection for both (1) physiological resistance to frost and drought and (2) deterrence of herbivores, as climatic constraints on plant growth rates probably leave seedlings exposed for longer to ground‐dwelling browsers. However, the widespread persistence of divaricate plants during the c. 450‐year hiatus between the extinction of moa and the onset of widespread browsing by introduced ungulates suggests that even in the absence of large herbivores, divaricates are well adapted to frosty, droughty environments.

Low vapour pressure deficit affects nitrogen nutrition and foliar metabolites in silver birch.

Air humidity indicated as vapour pressure deficit (VPD) is directly related to transpiration and stomatal function of plants. We studied the effects of VPD and nitrogen (N) supply on leaf metabolites, plant growth, and mineral nutrition with young micropropagated silver birches (Betula pendula Roth.) in a growth chamber experiment. Plants that were grown under low VPD for 26 d had higher biomass, larger stem diameter, more leaves, fewer fallen leaves, and larger total leaf area than plants that were grown under high VPD. Initially, low VPD increased height growth rate and stomatal conductance; however, the effect was transient and the differences between low and high VPD plants became smaller with time. Metabolic adjustment to low VPD reflected N deficiency. The concentrations of N, iron, chlorophyll, amino acids, and soluble carbohydrates were lower and the levels of starch, quercetin glycosides, and raffinose were higher in the leaves that had developed under low VPD compared with high VPD. Additional N supply did not fully overcome the negative effect of low VPD on nutrient status but it diminished the effects of low VPD on leaf metabolism. Thus, with high N supply, the glutamine to glutamate ratio and starch production under low VPD became comparable with the levels under high VPD. The present study demonstrates that low VPD affects carbon and nutrient homeostasis and modifies N allocation of plants.

Convergent approaches to determine an ecosystem's transpiration fraction

AbstractThe transpiration (T) fraction of total terrestrial evapotranspiration (ET), T/ET, can vary across ecosystems between 20–95% with a global average of ∼60%. The wide range may either reflect true heterogeneity between ecosystems and/or uncertainties in the techniques used to derive this property. Here we compared independent approaches to estimate T/ET at two needleleaf forested sites with a factor of 3 difference in leaf area index (LAI). The first method utilized water vapor isotope profiles and the second derived transpiration through its functional relationship with gross primary production. We found strong agreement between T/ET values from these two independent approaches although we noted a discrepancy at low vapor pressure deficits (VPD). We hypothesize that this divergence arises because stomatal conductance is independent of humidity at low VPD. Overall, we document significant synoptic‐scale T/ET variability but minimal growing season‐scale variability. This result indicates a high sensitivity of T/ET to passing weather but convergence toward a stable mean state, which is set by LAI. While changes in T/ET could emerge from a myriad of processes, including aboveground (LAI) or belowground (rooting depth) changes, there was only minimal interannual variability and no secular trend in our analysis of T/ET from the 15 year eddy covariance time series at Niwot Ridge. If the lack of trend observed here is apparent elsewhere, it suggests that the processes controlling the T and E fluxes are coupled in a way to maintain a stable ratio.

Gravimetric phenotyping of whole plant transpiration responses to atmospheric vapour pressure deficit identifies genotypic variation in water use efficiency

There is increasing interest in rapidly identifying genotypes with improved water use efficiency, exemplified by the development of whole plant phenotyping platforms that automatically measure plant growth and water use. Transpirational responses to atmospheric vapour pressure deficit (VPD) and whole plant water use efficiency (WUE, defined as the accumulation of above ground biomass per unit of water used) were measured in 100 maize (Zea mays L.) genotypes. Using a glasshouse based phenotyping platform with naturally varying VPD (1.5–3.8kPa), a 2-fold variation in WUE was identified in well-watered plants. Regression analysis of transpiration versus VPD under these conditions, and subsequent whole plant gas exchange at imposed VPDs (0.8–3.4kPa) showed identical responses in specific genotypes. Genotype response of transpiration versus VPD fell into two categories: 1) a linear increase in transpiration rate with VPD with low (high WUE) or high (low WUE) transpiration rate at all VPDs, 2) a non-linear response with a pronounced change point at low VPD (high WUE) or high VPD (low WUE). In the latter group, high WUE genotypes required a significantly lower VPD before transpiration was restricted, and had a significantly lower rate of transpiration in response to VPD after this point, when compared to low WUE genotypes. Change point values were significantly positively correlated with stomatal sensitivity to VPD. A change point in stomatal response to VPD may explain why some genotypes show contradictory WUE rankings according to whether they are measured under glasshouse or field conditions. Furthermore, this novel use of a high throughput phenotyping platform successfully reproduced the gas exchange responses of individuals measured in whole plant chambers, accelerating the identification of plants with high WUE.

Stomatal characteristics and desiccation response of leaves of cut chrysanthemum (Chrysanthemum morifolium) flowers grown at high air humidity

Although it is well known that, as a short-term response, stomata close at low relative humidity (RH) (high Vapour Pressure Deficit) and open at high RH (low Vapour Pressure Deficit), effects of long-term exposure to different Vapour Pressure Deficits (VPD’s) have only been studied in a few economically important horticultural crops, especially cut roses. To save energy costs, often low VPD conditions are present in greenhouses, due to low ventilation. We studied stomatal and desiccation responses of leaves of cut chrysanthemums after the plants had been exposed to different VPD conditions. While lower transpiration rate (E) was recorded for plants growing at low VPD (L-plants) in comparison with growth at moderate VPD (M-plants), higher stomatal conductance (gs) was found in L-plants compared to M-plants. Bigger size of stomata and higher stomatal density were observed in L-plants. Besides differences in gs, E and stomatal characteristics during growth of the plants at different VPDs, stomatal closure response to desiccation (under the same VPD condition) was different when plants had been grown before at different VPD’s. Following desiccation, leaves of L-plants showed higher E compared with M-plants. Also E of leaves grown at moderate VPD but exposed to low VPD for only 5 days (M→L-plants) decreased less as response to desiccation than E of M-plants. After rapid desiccation, leaves of L and M→L plants had larger stomatal openings compared with leaves that were grown continuous at M condition, as indicated by their PSII efficiencies under non-photorespiratory conditions (ΦPSII). In conclusion, control of VPD during growth will be important to prevent subsequent wilting of leaves of chrysanthemum cut flowers.

Environmental influence on the inter- and intraspecific variation in the density and morphology of stomata and trichomes of epiphytic bromeliads of the Yucatan Peninsula

We set out to test whether trichome and stomata variation in epiphytic bromeliads in the field would support the hypothesis that trichomes confer protection from excess light, increase water absorption and protect the stomata. We characterized the intra- and interspecific plasticity in the morphology and density of trichomes and stomata of 14 epiphytic bromeliad species found in six contrasting vegetation types (coastal sand dune scrub, scrub mangrove, peten mangrove, and deciduous, semi-deciduous and semi-evergreen forest) on the Yucatan Peninsula along a precipitation gradient from 500 to 1500 mm (mean annual precipitation). We performed leaf impressions to determine trichome and stomatal density and to characterize morphology. Trichome morphology did not show variation between sites, but was distinctive for the species. Our data supported the existing hypothesis that trichome density and area were higher in the atmospheric species that inhabited the drier sites, compared with the wetter extreme, which may confer higher protection against photoinhibition and desiccation. Stomatal density increased with precipitation in tank species, and negatively correlated with vapour pressure deficit during the dry season. Fewer trichomes and greater numbers of stomata than expected by rainfall were found in mangroves that maintained low vapour pressure deficit during the dry season.

High transpiration efficiency increases pod yield under intermittent drought in dry and hot atmospheric conditions but less so under wetter and cooler conditions in groundnut (Arachis hypogaea (L.))

Water limitation is a major yield limiting factor in groundnut and transpiration efficiency (TE) is considered the main target for improvement, but TE being difficult to measure it has mostly been screened with surrogates. The paper re-explore the contribution of TE to grain yield in peanut by using a novel experimental approach in which TE is measured gravimetrically throughout the crop life cycle, in addition to measurement of TE surrogates. Experimentation was carried out with the groundnut reference collection (n=288), across seasons varying for the evaporative demand (vapor pressure deficit, VPD) and across both fully irrigated and intermittent water stress conditions. There was large genotypic variation for TE under water stress in both low and high VPD season but the range was larger (5-fold) in the high- than in the low-VPD season (2-fold). Under water stress in both seasons, yield was closely related to the harvest index (HI) while TE related directly to yield only in the high VPD season. After discounting the direct HI effect on yield, TE explained a large portion of the remaining yield variations in both seasons, although marginally in the low VPD season. By contrast, the total water extracted from the soil profile, which varied between genotypes, did not relate directly to pod yield and neither to the yield residuals unexplained by HI. Surrogates for TE (specific leaf area, SLA, and SPAD chlorophyll meter readings, SCMR) never showed any significant correlation to TE measurements. Therefore, TE is an important factor explaining yield differences in groundnut under high VPD environments, suggesting that stomatal regulation under high VPD, rather than high photosynthetic rate as proposed earlier, may have a key role to play in the large TE differences found, which open new opportunities to breed improved groundnut for high VPD.

Slope position influences vegetation-atmosphere interactions in a tropical montane cloud forest

Abstract Throughout a single day, the microclimatic conditions in tropical montane cloud forests vary from strong solar radiation with simultaneous strong atmospheric water demand (high vapor pressure deficit, VPD, values), followed quickly by clouds and fog events drastically reducing both of these environmental variables. Due to the complex topography and weather patterns in these regions, microclimate, including fog events, can vary strongly across very small spatial scales as well, leading to a highly dynamic and compelling environment to examine how environmental variables influence tree water use across space and time. Due to this variation, the present study examines how environmental drivers of tree water use varies across three slope positions (upslope, midslope, low-slope) in a tropical montane cloud forest in Veracruz, Mexico. Measurements of sap flow using the heat ratio method were conducted on three dominant canopy species along with simultaneous measurements of microclimate within each site. To assess the relative importance of microclimatic variables in explaining tree water use across diurnal periods, data were separated into day and night periods and fog and clear events. Multiple regression models were conducted for each tree with input variables of VPD, solar radiation, air temperature, shallow soil moisture, deep soil moisture, and leaf wetness. We found that VPD explained a large majority of the variation in tree water use during daytime fog periods, particularly at the upslope and midslope sites. During nighttime periods, VPD was the dominant driver of water use variation during clear periods while a combination of VPD and leaf wetness explained variation during night, fog periods. Additionally, tree water use was more decoupled from environmental variables at the low-slope site. Finally, a separation of model components into fog and clear periods improved model outputs particularly at low flow conditions, highlighting the differential interactions between tree water use and environment during night and fog periods. Results from this study provide new insight into the importance of fog events, low VPD, and leaf surface wetting at controlling tree water use in cloud forests. The variation in drivers of water use across short spatial scales demonstrate the importance of considering individual and species level variation across fog and clear periods in predicting physiological responses of species to climate in cloud forests.

Preharvest environmental conditions affect the vase life of winter-cut roses grown under different commercial greenhouses

The vase life of cut flowers is not only determined by differences between cultivars, but also by growth conditions. For instance, the vase life of cut roses that are grown hydroponically during the winter period often ends at an early stage of maturation due to petal wilting or neck bending. In addition, the vase life of cut roses from different growers varies markedly despite identical postharvest conditions. To elucidate the underlying mechanisms, the relationship between preharvest environmental factors, postharvest morphological and physiological factors, and the vase life of cut roses were determined. Cut roses (Rosa hybrid L. cv. Fuego) that were grown hydroponically during winter were harvested from three commercial greenhouses in Korea, and the interrelations between the above-mentioned factors and the vase life of the cut flowers were analyzed using principal component analysis (PCA). The vase life of roses varied by 70% among growers in January when the vapor pressure deficit (VPD) differed the most. PCA revealed that preharvest environmental conditions strongly correlated with changes in morphological and physiological characteristics and the vase life of the cut roses. Roses grown under high relative humidity (RH) and low VPD during the winter had less functional stomata and, consequently, higher transpiration from leaves after harvest. This increased transpiration caused shorter vase life in the cut roses. In addition, supplementary lighting, high photosynthetic photon flux, and CO2 supplementation also increased transpiration and, consequently, shortened vase life, although these conditions improved the external quality of the cut roses. Thus, the external quality, i.e., the appearance of the cut flowers, did not directly correspond with the length of vase life of the flowers. This study improves our understanding of the relationship between preharvest conditions and morphological and physiological characteristics and vase life of winter-cut roses.

Substrate Effects on Plant Transpiration Rate under Several Vapour Pressure Deficit (VPD) Levels

Vapour pressure deficit (VPD) is considered as an important environmental factor that affects transpiration rate (TR) in plants. VPD is the difference between the amount of moisture in the air and the amount of moisture the air can hold when saturated. Since VPD increases, the drying ability of air increases. Plants transpire more, requiring more water to be drawn from the roots. As the guinea grass are well adapted in various types of soils, therefore in this study, Panicum (Panicum maximum cv. tanzania) was used with 4 different substrates (hydroponic, organic, sand and mineral) subjected to low (0.50-1.50) and high VPD (2.50-3.90) environments to study their substrate effects on plant transpiration rate in 3 different growth stages (31, 37 and 43 DAS for organic, sand and mineral soil and 25, 31, 38 DAS for hydroponic respectively). The highest transpiration rate of Panicum was measured in hydroponic condition (5.44) under higher VPD level and lower leaf area. The lowest TR measured in sandy soil (0.17) with lower VPD level and large leaf area. The results showed that sand substrate has the lowest transpiration rate and hydroponic condition showed highest transpiration rate. Other substrates rate in between two of them. However, the results showed that overall transpiration rate was significantly decreasing along the growth stages. Adding more substrates and tying a small around all the exposed leaves would be the way to obtain the exact amount of water transpired from the water source, which makes the results more comparable among the substrates.

Climatic influences on leaf phenology, xylogenesis and radial stem changes at hourly to monthly scales in two tropical dry forests

Assessing the effects of climate on leaf phenology and xylem development will improve our understanding of how wood formation and carbon uptake occur in tropical tree species. Wood formation depends on how tree stems enlarge, shrink or swell at multiple temporal scales. To address these issues we analyzed climate data, leaf phenology, xylogenesis and radial increment at hourly to monthly scales of ten tree species in two tropical dry forests with different drought seasonality and severity (the wet Tuluá site is located in Colombia and the dry INPA site is situated in Bolivia). Leaf flushing and radial growth occurred during the wet season at both sites, reflecting the influence of high precipitation, a positive water balance and low vapor pressure deficit on the development of new leaves and xylem cells. In Tuluá, the xylem growing season was associated with low air temperatures, while in INPA it was related to high air temperatures. At both sites, the high air temperatures registered throughout the day negatively affected radial-increment rates at hourly scales, probably by rising vapor pressure deficit and enhancing evapotranspiration rates. Tree species could face adverse dry conditions by growing in periods of the day when temperatures decrease and water loss due to evapotranspiration is reduced, particularly at dry INPA site. Stem shrinkage and swelling were observed at hourly to daily scales in all tree species, but most INPA species also registered strong reversible shrinkage at monthly scales. The strength of the positive association between leaf flushing and radial-increment rates was species-specific and it related to sapwood density. Thus, Cedrela fissilis, a pioneer deciduous species with low sapwood density showed the strongest associations between leaf flushing and radial-increment rates, whereas Acosmium cardenasii, a shade-tolerant deciduous species showed the reverse characteristic. The time-dependent growth responses of tree species to water availability should be explicitly considered to properly forecast their responses to climate warming and to evaluate their relevance as carbon sinks under warmer and drier conditions.

Mapping Atmospheric Moisture Climatologies across the Conterminous United States

Spatial climate datasets of 1981–2010 long-term mean monthly average dew point and minimum and maximum vapor pressure deficit were developed for the conterminous United States at 30-arcsec (~800m) resolution. Interpolation of long-term averages (twelve monthly values per variable) was performed using PRISM (Parameter-elevation Relationships on Independent Slopes Model). Surface stations available for analysis numbered only 4,000 for dew point and 3,500 for vapor pressure deficit, compared to 16,000 for previously-developed grids of 1981–2010 long-term mean monthly minimum and maximum temperature. Therefore, a form of Climatologically-Aided Interpolation (CAI) was used, in which the 1981–2010 temperature grids were used as predictor grids. For each grid cell, PRISM calculated a local regression function between the interpolated climate variable and the predictor grid. Nearby stations entering the regression were assigned weights based on the physiographic similarity of the station to the grid cell that included the effects of distance, elevation, coastal proximity, vertical atmospheric layer, and topographic position. Interpolation uncertainties were estimated using cross-validation exercises. Given that CAI interpolation was used, a new method was developed to allow uncertainties in predictor grids to be accounted for in estimating the total interpolation error. Local land use/land cover properties had noticeable effects on the spatial patterns of atmospheric moisture content and deficit. An example of this was relatively high dew points and low vapor pressure deficits at stations located in or near irrigated fields. The new grids, in combination with existing temperature grids, enable the user to derive a full suite of atmospheric moisture variables, such as minimum and maximum relative humidity, vapor pressure, and dew point depression, with accompanying assumptions. All of these grids are available online at http://prism.oregonstate.edu, and include 800-m and 4-km resolution data, images, metadata, pedigree information, and station inventory files.

Vineyard irrigation scheduling based on airborne thermal imagery and water potential thresholds

Background and Aims Mapping the spatial variability of vine water status within a vineyard is necessary for the efficient management of irrigation water. The objective of this study was to determine whether estimates of remotely sensed leaf water potential (Ψrem) could be employed as a precise tool for scheduling irrigation at the irrigation sector level throughout the season. Methods and Results Three irrigation treatments were applied in a 16-ha commercial vineyard to analyse the performance of the proposed methodology for monitoring regulated deficit irrigation strategies, and to evaluate the required frequency of the acquisition of thermal images for irrigation scheduling. An aircraft equipped with a thermal sensor flew over the vineyard throughout the season, and the averaged Ψrem of each irrigation sector was used as the irrigation trigger. The acquisition of about five or six Ψrem maps over the season is recommended. The starting date for acquiring thermal images depends on canopy vegetation size and on the difficulty of extracting pure vegetation pixels. The effect of acquiring thermal imagery on days after rainfall or with low vapour pressure deficits affected the estimation of Ψrem, and these constraints need to be considered for feasible irrigation purposes. Conclusions Remotely sensed leaf water potential was successfully used as an irrigation trigger to adopt regulated deficit irrigation strategies without any negative effect on yield and wine composition. Significance of the Study This study presented a promising and powerful method for scheduling irrigation throughout the season at vineyard level based on estimates of remotely sensed leaf water potential.

Physiological and biochemical plasticity of Lepidium latifolium as 'sleeper weed' in Western Himalayas.

To understand the spread of native populations of Lepidium latifolium growing in different altitudes in Ladakh region of Western Himalayas, photosynthetic and fluorescence characteristics were evaluated in relation to their micro-environment. Three sites representing sparsely populated (SPS), moderately populated (MPS) and densely populated site (DPS) were selected. Results showed that the DPS had higher photosynthetic accumulation than MPS and SPS. The higher transpiration rate at DPS despite lower vapor pressure deficit and higher relative humidity suggest the regulation of its leaf temperature by evaporative cooling. Intrinsic soil parameters such as water holding capacity and nutrient availability also play crucial role in higher biomass here. The quantum efficiency of PSII photochemistry (Fv/Fm, non-photochemical quenching (NPQ), ΦPSII) and light curve at various PPFDs suggests better light harvesting potential and light compensation point at DPS than the other two sites. Concomitantly, plants at SPS had significantly higher lipid peroxidation, suggesting a stressful environment, and higher induction of antioxidative enzymes. Metabolic content of reduced glutathione also suggests an efficient mechanism in DPS and MPS than SPS. High light intensities at MPS are managed by specialized contrive of carotenoid pigments and PsbS gene product. Large pool of violaxanthin and lutein plays an important role in this response. It is suggested that L. latifolium is present as 'sleeper weed' that has inherent biochemical plasticity involving multiple processes in Western Himalayas. Its potential spread is linked to site-specific micro-environment, whereby, it prefers flat valley bottoms with alluvial fills having high water availability, and has little or no altitudinal effect.

Understory CO2, sensible heat, and latent heat fluxes in a black spruce forest in interior Alaska

An open black spruce forest, the most common ecosystem in interior Alaska, is characterized by patchy canopy gaps where the forest understory is exposed. This study measured CO2, sensible heat, and latent heat fluxes with eddy covariance (EC) in one of those large canopy gaps, and estimated understory fluxes in a black spruce forest in 2011–2014. Then understory fluxes and ecosystem fluxes were compared. The understory fluxes during the snow-free seasons were determined by two approaches. The first approach determined understory fluxes as the fluxes from the canopy gap, assuming that fluxes under the canopy crown also had the same magnitude as the canopy gap fluxes. The second approach determined the understory fluxes by scaling canopy gap fluxes with a canopy gap fraction, assuming that only canopy gaps, which mostly constitutes the forest floor, contribute to fluxes. The true understory fluxes would be in between these two estimates. Overall, the understory accounted for 53% (39–66%), 61% (45–77%), 63% (45–80%), 73% (56–90%), and 79% (59–98%) of the total net ecosystem productivity (NEP), gross primary productivity (GPP), ecosystem respiration (RE), sensible heat flux (H), and latent heat flux (LE), respectively. The ratio of understory NEP (NEPU) to the ecosystem NEP (NEPE) and similarly calculated LEU/LEE during the daytime increased with vapor pressure deficit (VPD) at low VPD conditions (∼2000Pa) at half-hourly temporal scale. At high VPD conditions, however, NEPU/NEPE decreased with VPD, whereas LEU/LEE was maintained at the high level even at high VPD conditions. Despite large ranges of the estimates for the understory contributions, we conclude that the understory plays an important role in the carbon and energy balances of the black spruce ecosystem, and their contribution highly depends on the level of VPD.

Effects of nitrogen and vapour pressure deficit on phytomer growth and development in a C4 grass.

Phytomers are basic morphological units of plants. Knowledge of phytomer development is essential for understanding morphological plasticity, functional-structural modelling of plant growth and the usage of leaf characteristics to indicate growth conditions at the time of production (e.g. stable isotope signals). Yet, systematic analysis on the process of phytomer development is unavailable for wild or perennial C4 grasses. Also, effects of environmental factors, such as nitrogen nutrition or vapour pressure deficit (VPD), on coordination events of developmental processes of C4 grasses have not been studied. This study investigates phytomer growth and development in Cleistogenes squarrosa, a predominant C4 grass in the Eurasian steppe, grown at low (0.63 kPa) or high (1.58 kPa) VPD with low or high nitrogen supply in controlled environments. Elongation of phytomers on marked tillers was measured daily for 13 days. Then lengths of immature and mature phytomer components (blade, sheath and internode) of all phytomers were measured following dissection. Nitrogen nutrition and VPD had no effects on coordination of growth within and between phytomers: phytomer tips emerged when phytomers reached 26 % of their final length, coincident with the acceleration phase of its elongation; blade elongation stopped when phytomers reached ∼75 % of their final length and elongation of the preceding phytomer was confined to the internode. The relationship between fraction of final phytomer length and days after tip emergence for all treatments was well described by a sigmoidal function: y = 1/{1 + exp[(1.82 - x)/1.81]}. C. squarrosa exhibited little morphological plasticity at phytomer-level in response to nitrogen supply and VPD, but a clear increase in tillering under high N supply. Also, the invariant coordination of elongation within and between phytomers was a stable developmental feature, thus the quantitative coordination rules are applicable for predicting morphological development of C. squarrosa under contrasting levels of nitrogen nutrition or VPD.