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• A collaborative approach was used to compare water potential measurement methods. • Time of the day (predawn, mid-morning and noon) and mode (stem vs. leaf) were compared. • Discrimination Ratio (DR) was used to determine the discrimination ability of each. • Leaf water potential was less discriminant than predawn or stem water potential at any time. Water potential is considered to be the "gold-standard" measure for plant water status determination. However, there are some discrepancies on how and at what time of the day water potential measurements should be performed in order to obtain meaningful information. The aim of this work is to evaluate the discrimination ability of water potential measurements in grapevines depending on the time of the day and of the measurement procedure (leaf vs. stem). To do so, a meta-analysis was performed using >78,000 measurements of water potential data obtained in field irrigation experiments, provided by 13 research teams working in this subject in Spain. For each measurement day and experiment, Discrimination Ratio (DR) was calculated and used to determine the discrimination ability of each method, and then pooled for comparison. The measurement procedure with the greatest DR can be hypothesised to be the most suitable under the average working conditions. Leaf water potential showed lower DR mean values than predawn or stem water potential. The climatic conditions and the cultivar may affect to the discrimination ability, although the abovementioned trend was always maintained. Leaf water potential in vineyards should therefore be replaced, as a general rule, by either stem or predawn water potential readings, without a clear pre-eminence of the performance of predawn and stem water potential measurements. Building a common dataset and its subsequent meta-analysis has been proved to be an efficient and robust tool to compare plant measurements, and should be implemented for other species and/or measurement procedures. [ABSTRACT FROM AUTHOR]

Highlights • Spatial extrapolation of localized grapevine water status measurements is studied. • An existing empirical approach (SPIDER) is analysed using a mechanistic model. • The empirical approach is validated by the mechanistic model. • New insights on the validity domain of spatial extrapolation are found. • This work strengthens the relevancy of SPIDER for precision viticulture application. Abstract This work is devoted to precision agriculture and more precisely to the spatial monitoring of water status in viticulture. An empirical approach was introduced in 2008 based on the extrapolation across a domain (vineyard block, vineyard, region) of vine water status observations from a reference site using a simple statistical model, called SPIDER, and proved efficient in many studies. Once the extrapolation model is calibrated, this approach leads to a concentration of measurements for one site only (reference site) while providing an estimate of the grapevine water status at a larger spatial scale. It is a promising hybrid approach based both on regular (but targeted) measurements and on modelling. However, so far only empirical guidelines for its practical use have been provided. Moreover, the limits of validity (spatial, temporal, etc.) of such an approach are not known. This work intends to use a mechanistic model based on grapevine water balance modelling to study to what extent a simulated water status can be spatially extrapolated at the field scale. The water balance model was calibrated on two datasets (different cultivars and weather data) and used to analyse the performances of SPIDER. The results confirmed the relevance of the empirical approach (SPIDER) based on water status spatial extrapolation with a low error level on the two datasets studied. The use of the water balance model also helped define the validity domain of SPIDER: it confirmed the importance of having dominantly dry conditions and revealed the possibility of recovering good prediction quality after strong rainfall or irrigation. This study globally demonstrates the relevance of spatial extrapolation of the vine water status from a reference site with a linear regression model and provides new insights on the properties of the predictions for application in viticulture either at the within-field level or at larger scale. [ABSTRACT FROM AUTHOR]

Highlights • If water is not limiting, applying 100% of water requirements is recommended. • Watermelon yield is affected by the reduction in the irrigation water applied. • If water is scarce, water restrictions during fruit ripening stage are recommended. • These irrigation strategies can lead to acceptable yield and water use efficiencies. Abstract Water is an essential resource for food production, as agriculture consumes close to 70% of the total freshwater, and its shortage is becoming critical in arid and semiarid areas of the world. Therefore, it is important to use water more efficiently. The objectives of this project are to determine the productive response and the irrigation water use efficiency of seedless watermelon to three irrigation management strategies over two growing seasons. This was done by applying 100, 75 and 50% of the irrigation water requirements (IWR) the first year, in the second year added six additional treatments, of which three treatments were regulated deficit irrigation with 75% IWR during the vegetative growth, fruit development and fruit ripening stages, and the other three treatments were with 50% IWR during the same stages. The exposure of watermelon plants to severe deficit irrigation resulted in a reduction in dry biomass, total and marketable yield, average fruit weight, fruit number and harvest index, and without improvement of marketable fruit quality. The fruit ripening was the less sensitive stage to water deficits. Relative water content and cell membrane stability index decreased as the water deficit increased. Irrigation water use efficiency decreased to a lesser extend during the fruit ripening stage than when water restriction were applied during different growth stages. If water is readily available, irrigating with 100% of water requirements is recommended, but in the case of water scarcity, applying water shortage during fruit ripening stage would be advisable. [ABSTRACT FROM AUTHOR]

Signal intensity ( SI ) and maximum daily stem shrinkage ( MDS ) are indicators of the water status and irrigation schedule of fruit trees under conventional irrigation (CI). However, whether SI can reflect the water status of fruit trees under alternate partial root-zone irrigation (APRI) has rarely been reported. Field experiments were conducted on apple trees over two years with two irrigation methods (CI and APRI) and two irrigation amounts (400 mm and 500 mm) in an arid area. The followings were measured over the whole growth season: MDS , sap flow ( SF ), air temperature, net radiation, vapor pressure deficit, reference evapotranspiration, soil water content, midday stem water potential and predawn leaf water potential. The signal intensities based on MDS ( SI MDS ) and sap flow ( SI SF ) were calculated. The results show: first, MDS was significantly higher under CI at 400 mm than under APRI at 400 mm, while no difference was found between the two irrigation methods at 500 mm. MDS was significantly positively correlated with meteorological factors, while SI MDS and SI SF were not. Second, SI MDS was significantly lower under APRI than under CI, while no difference was found in SI SF between the two irrigation methods. Third, in contrast to SI SF , SI MDS was significantly correlated with soil water content as well as with midday stem water potential and predawn leaf water potential under APRI. These results show that the signal intensity based on maximum daily stem shrinkage accurately indicates the water status of apple trees under alternate partial root-zone irrigation in an arid apple production area. [ABSTRACT FROM AUTHOR]

The aim was to assess the usefulness of spectral data to detect structural and physiological changes in Citrus crops under water and saline stress. Multispectral images were acquired from a fixed-wing Unmanned Aerial Vehicle (UAV) while concomitant measurements of gas exchange, plant water status, leaf structural traits and chlorophyll were taken in a commercial farm located in southeast Spain with two Citrus species, grapefruit and mandarin irrigated for eight years with saline reclaimed water (RW) combined with regulated deficit irrigation (RDI). Measurements at leaf scale and airborne flights were carried out twice a day, at 7 and 10 GMT. Irrigation with RW decreased gas exchange and leaf dry mass per unit area (LMA) on grapefruit. However, salinity from RW resulted in an increase in pressure potential (Ψ P ) on mandarin and allowed maintaining net photosynthesis (A) and stomatal conductance (g s ) when vapour pressure deficit increased. On both crops, leaf total chlorophyll (ChlT) concentrations were significantly reduced by RW. Moreover, RDI decreased A, g s and stem water potential (Ψ s ) on grapefruit, independently of water quality. Regarding spectral data, red wavelength (R) was significantly correlated with Chl T (p < 0.001), except when mandarin was subjected to stressful climatic conditions (at 10 GMT); since R was influenced, in addition to Chl T, by the plant water and gas exchange status. Near infrared (NIR) was a useful indicator of Ψ s , A and g s on both crops. The normalized difference vegetation index (NDVI) was clearly related to gas exchange in both species and to Ψ s only on mandarin. Finally, we combined data from both Citru s species and the best indicators were NIR and R. The novelty of this study was to show that diurnal changes in physiological and structural traits of Citrus irrigated with RW combined with RDI can be determined by multispectral images from UAVs. [ABSTRACT FROM AUTHOR]

The aim of this study was to evaluate the robustness of infra-red thermometry to estimate stomatal conductance (g s) in grapefruit trees. For this purpose, the ability of several thermal indices to determine g s was tested for different: (i) irrigation strategies: full irrigation (FI) and regulated deficit irrigation (RDI), (ii) water sources: transfer water (TW) -optimal for agricultural uses- and saline reclaimed water (RW) and (iii) phenological stages: flowering-sprouting (FS) and fruit growth (FG). During two growing seasons, measurements of g s and canopy temperature (T c) were taken in 14-year-old ´Star Ruby´ grapefruit trees (Citrus paradisi Macf.,). Air temperature (T a) was also recorded and then the derived thermal indices: T c -T a , and CWSI (crop water stress index) were calculated. Subsequently, thermal-g s correlations were established at different averaged thermal data intervals, and VPD (vapour pressure deficit) and PAR (photosynthetically active radiation) conditions. The results indicated that both, water quality and deficit irrigation, affected negatively g s and T c during FG. In addition, the highest r2 values in thermal indices-g s correlations were found when i) the thermal data was averaged 60 min before the g s measurement, ii) g s was measured over a wide range of VPD (between 0 and 3.5 kPa), and iii) PAR was 1200 µmol m−2 s−1. The ability of the thermal indices to estimate g s improved under RDI and RW conditions, when compared to the control treatment (TW-FI). Moreover, the thermal indices that more accurately estimated g s were T c -T a and CWSI in FS (P < 0.001), and the absolute values of T c in FG (P < 0.001). Overall, thermal indices obtained by infra-red sensors could be a useful tool to estimate g s in grapefruit trees. However, irrigation treatment, water quality, tree phenological stage and environmental conditions (VPD and PAR) must be considered when interpreting thermal indices-g s correlations. • Infra-red sensors allowed for estimating the stomatal conductance of grapefruit trees. • Environmental conditions affected the estimation of stomatal conductance. • Crop phenological stage affected thermal indices-stomatal conductance correlations. [ABSTRACT FROM AUTHOR]

Abstract: Optimization of olive oil quantity and quality requires finely tuned water management, as increased irrigation, up to a certain level, results in increasing yield, but a certain degree of stress improves oil quality. Monitoring tools that provide accurate information regarding orchard water status would therefore be beneficial. Amongst the various existing methods, those having high resolution, either temporally (i.e., continuous) or spatially, have the maximum adoption potential. One of the commonly used spatial methods is the Crop Water Stress Index (CWSI). The objective of this research was to test the ability of the CWSI to characterize water status dynamics of olive trees as they enter into and recover from stress, and on a diurnal scale. CWSI was tested in an empirical form and in two analytical configurations. In an experiment conducted in a lysimeter facility in the northwestern Negev, Israel, irrigation was withheld for 6 days for 5 of 15 trees, while daily irrigation continued for the rest of the trees. After resuming irrigation, the trees were monitored for 5 additional days. Water status measurements and thermal imaging were conducted daily between 12:00 and 14:00. Diurnal monitoring (predawn to after dusk) of the same indicators was conducted on the day of maximum stress. Continuous meteorological data were acquired throughout the experimental period. Empirical and analytical CWSI were calculated based on canopy temperature extracted from thermal images. The empirical CWSI differentiated between well watered and stressed trees, and depicted the water status dynamics during the drought and recovery periods as well as on a diurnal scale. Analytical approaches did not perform as well at either time scale. In conclusion, the empirical CWSI seems to be promising even given its limitations, while analytical forms of CWSI still require improvement before they can be used as a water status monitoring tool for olive orchards. Practically, it is proposed to compute the wet temperature analytically and set the dry temperature to 5°C higher than air temperature. [Copyright &y& Elsevier]

Abstract: Stem sap flow (Q) and leaf turgor pressure (P c) were measured simultaneously on 4-year-old, 2.4m tall ‘Arbequina’ olive trees in a hedgerow orchard. Measurements were performed on well-watered control trees as well as on 60RDI and 30RDI trees (RDI=regulated deficit irrigation). The 60RDI trees received 59.2% of the crop water needs (ETc), and the 30RDI trees received 29.4% of ETc. P c was determined non-invasively using the magnetic leaf patch clamp pressure probe (ZIM probe). The patch pressure P p measured by the probe is inversely correlated with turgor pressure at P c > ca. 50kPa. P c is coupled with xylem pressure; thus P p yields information about the development of tension in xylem. In the case of the control trees a positive correlation between Q and P p was generally found, i.e. Q increased usually with increasing P p and decreased with decreasing P p, as expected. However, Q peaking did not always coincided with P p peaking at noon. Occasionally, Q peaking preceded or followed P p peaking with a time difference of up to 3h in both cases. Under some circumstances, the onset of Q after sunrise was greatly delayed, even though a pronounced increase of P p was observed. A delayed onset of Q after sunrise resulted in hysteresis phenomena, i.e. the linear increase of Q and P p in the morning hours did not coincide with the corresponding decrease of Q and P p in the afternoon. The development of severe water stress (P c < ca. 50kPa) associated with the increase in the intercellular spaces of the spongy tissue in the leaves resulted in inverted diurnal P p curves, i.e. minimum P p values were recorded at noon and maximum P p values during the night on the 30RDI trees. The effects were reversible as shown by re-watering. By contrast, the magnitude of Q decreased continuously from the turgescent state to the state of severe water stress; maximum Q values were still recorded around noon. The data suggests that short-range tension forces are responsible for water lifting in olive trees and that water uptake from water storage reservoirs must play an important role in the supply of the leaves with water. Furthermore, for setting of irrigation thresholds the finding of shape changes of the P p curves upon severe water stress seems to be a useful indicator. Such shape changes are detected and monitored more sensitively than changes in the magnitude of sap flow rates or of turgor pressure. [Copyright &y& Elsevier]

Abstract: Improved approaches for irrigation scheduling require specific protocols for adaptation to different growing conditions. We assessed crop intercepted radiation as the main factor for decision on irrigation scheduling. Over two growing seasons (2007–2008), apple trees growing in a large weighing lysimeter were used to measure daily canopy transpiration (T d). Seasonal patterns of daily canopy intercepted photosynthetically active radiation (IPARd) and midday stem water potential were also measured. In 2007, irrigation was withheld in two different times to study T d responses to midday stem water potential. Before harvest, under full irrigation, T d increased linearly with IPARd (R 2 =0.81 in 2007 and 0.84 in 2008). With the two year data combined, R 2 increased from 0.74 to 0.80 when VPD was considered as a second variable. When irrigation was withheld in 2007 the ratio between T d and IPARd, which is defined here as transpiratory radiation use efficiency (TRUE), decreased linearly (R 2 =0.49) as midday stem water potential decreased. Due to the highly significant effect of IPARd and VPD on T d, TRUE showed potential applications in estimating the amount of irrigation water. [Copyright &y& Elsevier]

Abstract: Drainage-weighing lysimeters allowed monitoring of water balance components of non-bearing olive (Olea europaea cv Barnea) trees over a 3-month period including short-term events of controlled but severe water stress. The objective of the study was to evaluate a variety of soil and plant-based water status and drought stress monitoring methods on the basis of tree-scale evapotranspiration (ET). As the trees entered into and recovered from water stress, meteorological data, actual ET (ETa), soil water content and changes in leaf turgor pressure were continuously monitored. Additionally, midday measurements of stem water potential, stomatal conductance, canopy temperature, and quantum yield of PSII photochemistry were conducted. Diurnal (dawn to dusk) measurements of all the above were made hourly on days of maximum stress. Shoot elongation rate was measured for periods of stress and recovery. Quantum yield of PSII photochemistry, stomatal conductance, and stem water potential all successfully indicated reductions in whole-tree water consumption beginning at moderate stress levels. These measured parameters fully recovered to the levels of non-stressed trees soon after water application was renewed. Shoot elongation was reduced 25–30% for the 10-day period during and following drought and recovered thereafter to levels of non-stressed trees. Whole-tree ETa was reduced by as much as 20% even following full recovery of the leaf level parameters, suggesting reduced canopy size and growth due to the stress period. Non-destructive, continuous (turgor pressure) and remotely sensed (canopy temperature) methods showed promising potential for monitoring effects of water stress, in spite of technological and data interpretation challenges requiring further attention. [ABSTRACT FROM AUTHOR]

Grapevines are being challenged by climate changes, forcing winemakers to implement irrigation systems to cope with excessive water stress. Previous studies focused on a small set of international varieties, and only few data are available for terroirs hosting cultivars with possibly different responses to drought stress. In this light, we monitored grapevine water status and grape's physical and chemical composition, as well as concentration and structural characteristics of grape extractable polyphenols, in ten different Refošk vineyards located in the Classical Karst terroir during 2018 and 2019. Grapevines did not suffer severe stress during the two years, but their response to water shortage periods was highly heterogeneous, as pre-dawn (Ψ pd) and minimum (Ψ min) leaf water potential significantly differed between vineyards, especially during the drier part of the season. Moreover, the timing of maximum water stress differed in the two years, as in 2019 longer water shortage periods and higher temperature occurred at flowering stage and before veraison, while in 2018 they were higher after veraison. These differences influenced berry's quality, as titratable and malic acid concentration in juice, as well as total anthocyanin, total polyphenols and higher high molecular weight proanthocyanidins (HMWP) concentration in skins, were higher in 2019 than in 2018. Regarding seed proanthocyanidins, HMWP concentration, mean degree of polymerisation (mDP) and percentage of galloylation (G) in seeds were higher in 2018 than in 2019. The differences in water status measured in spatially close-related vineyards strongly support the importance of monitoring grapevines' water status dynamics to design adequate and effective water management activities rather than relying on climate data solely. Moreover, the timing of water shortage periods also played a role in determining Refošk grape quality. Our analyses showed that the higher (but still moderate, with Ψ pd and Ψ min mean values around −0.50 and −1.25 MPa, respectively) water stress between veraison and harvest occurred in 2018 might reduce Refošk grape acidity and increase concentration, polymerisation and galloylation of seed extractable proanthocyanidins. • Climate change is forcing winemakers to implement irrigation systems in Karst. • Water status affects grape quality, but this relationship is poorly understood. • High heterogeneity in water status was found in spatially closed vineyards. • Different timing of water shortage in 2018 and 2019 influenced berry's quality. • Water management activities should be planned based on actual plant water needs. [ABSTRACT FROM AUTHOR]

Olive cultivation in Argentina has experienced an important expansion based on large-scale development of high-density (HD) olive orchards. Intensification has led to yield increases, but, because of the high annual ETo and low rainfall, olive production must rely heavily on irrigation. The aim of this work was to determine the yield response to variable water supply of a HD, tall olive orchard, and to assess the economic water productivity (WP) of the olive, growing in an environment where the water resource is extremely scarce. During three seasons in an olive orchard (cv. "Arbequina"), we evaluated seven irrigation regimes which supplied 120% (T 120), 100% (T 100), 90% (T 90), 80% (T 80), 70% (T 70), 60% (T 60) y 40% (T 40) of the estimated ETc. After the first year, which was considered of transition, the next two growing seasons exhibited a clear decline in yield in response to the irrigation decline, with the reduction in fruit yield related to both fruit numbers and fruit size. A third degree polynomial function was fitted to treatment averages (R2 = 0.99). The maximum oil production (ca = 3080 kg oil ha-1) was obtained with 860 mm of irrigation. The marginal WP reached a maximum (5.9 kg oil ha-1 mm-1) at 550 mm of irrigation and declined thereafter, reaching zero at 860 mm. In economic terms, the grower achieves maximum returns (US$ 3140 ha-1) at 850 mm of irrigation. With the cost of water increasing from the present US$ 0.04 m-3 to US$0.24 m-3, the maximum net profit would be reduced by up to 53% (US$ 1476 ha-1), however, it would be achieved at a similar irrigation level (820 mm). If in the near future water scarcity in the region makes it difficult to maintain the present levels of water supply, aimed at maximizing profits, there will be the need to impose restrictions through resource conservation policies to reach an equilibrium between economic, environmental, and sustainability goals. • There was an increase in fruit and oil yield as water supply increased. • A third degree polynomial function was fitted to the yield response to water supplied. • In economic terms, the grower achieves maximum returns at higher irrigation level (Kc ≈ 0.63). • The marginal water productivity reached a maximum with a yearly Kc of ca. 0.4. [ABSTRACT FROM AUTHOR]