Midday Stem Water Potential Research Articles

Continuous Stem Water Potential Measurements of a Diffuse‐Porous Tree Species Offer New Insights Into Tree Water Relations

ABSTRACTWater potential is a crucial parameter for assessing tree water status and hydraulic strategies. However, methods for measuring water potential, such as the Scholander pressure chamber, are destructive, discontinuous and difficult to perform in tall forests. Consequently, important dynamics in water potentials, particularly during short‐term drought, are difficult to capture. Recent advancements have introduced low‐maintenance sensors capable of measuring continuous, high‐resolution stem water potentials.We evaluated these sensors in a temperate, diffuse‐porous species (Carpinus betulus) over a growing season marked by dry‐down periods and heat. Measurements of leaf water potential, sap flow and environmental factors (air temperature, vapour pressure deficit and soil water content) were conducted. Midday stem water potentials of C. betulus reached minimum values of −3.39 ± 0.10 MPa and exhibited pronounced seasonal fluctuations, mirroring changes in environmental conditions and sap flow.Stem water potentials correlated well with Scholander‐type measurements during predawn (R2 = 0.98) but demonstrated an offset in absolute values during midday (R2 = 0.71) and diurnal measurements. Minimum stem water potentials and maximum sap flow in the stem expressed a time lag and showed a distinct hysteresis. In this first assessment, the agreement with Scholander‐type measurements, sap flow and environmental parameters suggests the tested water potential sensors yield reliable data, especially during predawn, but need further validation during midday conditions. If applicable to other tree species, these sensors could significantly advance our understanding of tree water relations and their role in forest drought responses.

Spectral indices optimization for the assessment of midday stem water potential in grapevine

Spectral indices optimization for the assessment of midday stem water potential in grapevine

Mapping almond stem water potential using machine learning and multispectral imagery

AbstractAlmonds are a major crop in California which produces 80% of all the world’s almonds. Widespread drought and strict groundwater regulations pose significant challenges to growers. Irrigation regimes based on observed crop water status can help to optimize water use efficiency, but consistent and accurate measurement of water status can prove challenging. In almonds, crop water status is best represented by midday stem water potential measured using a pressure chamber, which despite its accuracy is impractical for growers to measure on a regular basis. This study aimed to use machine learning (ML) models to predict stem water potential in an almond orchard based on canopy spectral reflectance, soil moisture, and daily evapotranspiration. Both artificial neural network and random forest models were trained and used to produce high-resolution spatial maps of stem water potential covering the entire orchard. Also, for each ML model type, one model was trained to predict raw stem water potential values, while another was trained to predict baseline-adjusted values. Together, all models resulted in an average coefficient of correlation of R2 = 0.73 and an average root mean squared error (RMSE) of 2.5 bars. Prediction accuracy decreased significantly when models were expanded to spatial maps (R2 = 0.33, RMSE = 3.31 [avg]). These results indicate that both artificial neural networks and random forest frameworks can be used to predict stem water potential, but both approaches were unable to fully account for the spatial variability observed throughout the orchard. Overall, the most accurate maps were produced by the random forest model (raw stem water potential R2 = 0.47, RMSE = 2.71). The ability to predict stem water potential spatially can aid in the implementation of variable rate irrigation. Future studies should attempt to train similar models with larger datasets and develop a simpler faster workflow for producing stress predictions from field measurements.

Physiological and Growth Responses of W. Murcott Tangor Grafted on Four Rootstocks under Water Restriction

Citrus orchards in semi-arid regions are increasingly exposed to drought conditions due to climate change. This study compared the physiological and growth responses of ‘W. Murcott’ tangor (WM) grafted onto Citrus macrophylla (M), Swingle citrumelo (SC), C-35 citrange (C35), or bitter citrandarin (C22) rootstock subjected to two irrigation treatments: daily irrigation to replace 100% of the water lost daily by evapotranspiration (ET; control treatment) or daily irrigation to replace 75% of the water lost daily by ET (water deficit treatment). For trees in each treatment, leaf gas exchange, relative chlorophyll content, chlorophyll fluorescence, midday stem water potential, trunk cross-sectional area, and shoot length were measured 46 days after treatments were initiated. The results showed that WM on SC or C22 rootstock exhibited isohydric behavior, where decreased stomatal conductance limited transpiration in the water deficit treatment. WM on M rootstock exhibited an anisohydric response in the water deficit treatment, where there was no stomatal control of water loss by transpiration. Among the rootstocks tested for WM, the most tolerant to soil water deficit was SC, whereas trees on M rootstock were the most negatively affected by soil water deficit.

Technical Feasibility Analysis of Advanced Monitoring with a Thermal Camera on an Unmanned Aerial Vehicle and Pressure Chamber for Water Status in Vineyards

Water is a limiting factor and to adopt the most appropriate agronomic strategy it is necessary to know the water status. The objective is (i) analysing of the influence of different agronomic treatments on canopy temperature in vineyards with a thermal camera on an unmanned aerial vehicle (UAV), (ii) analysing of the influence of different agronomic treatments on vineyard water potentials with a pressure chamber, (iii) advanced technical feasibility analysis of vineyard crop monitoring. The control treatment (T07) in cv. Grenache consisted of applying 30% of reference evapotranspiration (ETo) with irrigation frequency every seven days and seven different treatments were proposed with different irrigation frequencies, pre-bud irrigation, and vine shoot distribution (T03, T15, T7A, T7V, T7P, T00, and T0P). As a result and in conclusion, the use of thermal cameras in UAVs and mid-day stem water potential allows differentiation between irrigated and unirrigated treatments, but no clear differences were shown between irrigation frequencies, pre-irrigation treatment, or vine shoot distribution. Comparing the thermal camera information in UAV and the stem water potential, certain patterns are identified with significant correlation values, the use of thermal cameras for the evaluation of plant water status is recommended, especially to obtain information in large areas.

Trunk dielectric permittivity correlates with irrigation based on soil water content in fruit trees

Time-domain reflectometry (TDR) is an electromagnetic technique that measures the dielectric permittivity (K) which is a surrogate property influenced by water content. Advances in nanoelectronics have enabled the development of a TDR probe (TDR-305 N) to monitor changes in K, bulk electrical conductivity (ECbulk) and temperature (T) in a porous medium, such as a tree trunk. The main objective of this study was to assess the effectiveness of the TDR-305 N sensors for real-time monitoring changes in water content in the trunk of nectarine trees. Throughout the summer of 2022, irrigation was automatically managed with threshold values of soil water content (θv-soil) measured with capacitance probes. Different management allowed depletion (MAD) values were set to trigger irrigation: 50 % in July (moderate water deficit), 100 % in August (severe water deficit), and recovery to well-irrigated conditions in September. Discrete measurements of midday stem water potential (Ψs,md) and leaf gas exchange were made frequently. The results showed a progressive reduction of the measured physiological parameters, as well as of K and ECbulk and θv-soil decreased. Notably, Ψs,md reached a critically low value of -2.03 MPa, coinciding with pronounced and severe stomatal closure. Both K and Ψs,md, were able to explain the variations of θv-soil by more than 75 %. Daily, a positive relationship of K and ECbulk was observed, although ECbulk exhibited a stronger dependence on Ttrunk compared to K. Furthermore, K did not return to its initial values prior to the onset of water stress, possibly influenced by xylem cavitation and a reduction in leaf area during its senescence stage. The findings suggest that trunk permittivity measurements obtained using TDR-305 N sensors could be a reliable indicator for monitoring tree water status. However, further research is needed to determine the threshold values of trunk water content under non-limiting soil water conditions for accurate irrigation scheduling.

The legacy of past droughts induces water-sparingly behaviour in Grüner Veltliner grapevines.

Drought is becoming more frequent and severe in numerous wine-growing regions. Nevertheless, limited research has examined the legacy of recurrent droughts, focusing on leaf physiology and anatomy over consecutive seasons. We investigated drought legacies (after 2 years of drought exposure) in potted grapevines, focusing on stomatal behaviour under well-watered conditions during the third year. Vines were subjected for two consecutive years to short- (SD) or long-term (LD) seasonal droughts, or well-watered conditions (WW). In the third year, all plants were grown without water limitation. Water potential and gas exchange were monitored throughout the three seasons, while leaf morpho-anatomical traits were measured at the end of the third year. During droughts (1st and 2nd year), stem water potential of SD and LD plants fell below -1.1 MPa, with a consequent 75% reduction in stomatal conductance (gs ) compared to WW. In the 3rd year, when all vines were daily irrigated to soil capacity (midday stem water potential ~ -0.3 MPa), 45% lower values of gs were observed in the ex-LD group compared to both ex-SD and ex-WW. Reduced midrib vessel diameter, lower leaf theoretical hydraulic conductivity, and smaller stomata were measured in ex-LD leaves compared to ex-SD and ex-WW, likely contributing to the reduced gas exchange. Our findings suggest that grapevines exposed to drought may adopt a more water-conserving strategy in subsequent seasons, irrespective of current soil water availability, with the degree of change influenced by the intensity and duration of past drought events.

Evaluation of canopy fraction-based vegetation indices, derived from multispectral UAV imagery, to map water status variability in a commercial vineyard

AbstractWater stress is a major factor affecting grapevine yield and quality. Standard methods for measuring water stress, such as midday stem water potential (ΨSWP), are laborious and time-consuming for intra-block variability mapping. In this study, we investigate water status variability within a 2.42-ha commercial Cabernet Sauvignon block with a standard vertical trellis system, using remote sensing (RS) tools, specifically canopy fraction-based vegetation indices (VIs) derived from multispectral unmanned aerial vehicle (UAV) imagery, as well as standard reference methods to evaluate soil and plant water status. A total of 31 target vines were monitored for ΨSWP during the whole growing season. The highest variability was at véraison when the highest atmospheric demand occurred. The ΨSWP variability present in the block was contrasted with soil water content (SWC) measurements, showing similar patterns. With spatial and temporal water stress variability confirmed for the block, the relationship between the ΨSWP measured in the field and fraction-based VIs obtained from multispectral UAV data was analysed. Four UAV flights were obtained, and five different VIs were evaluated per target vine across the vineyard. The VI correlation to ΨSWP was further evaluated by comparing VI obtained from canopy fraction (VIcanopy) versus the mean (VImean). It was found that using canopy fraction-based VIs did not significantly improve the correlation with ΨSWP (NDVIcanopyr = 0.57 and NDVImeanr = 0.53), however fractional cover (fcover) did seem to show a similar trend to plant water stress with decreasing canopy size corresponding with water stress classes. A subset of 14 target vines were further evaluated to evaluate if additional parameters (maximum temperature, relative humidity (RH), vapour pressure deficit, SWC and fractional cover) could serve as potential water stress indicators for future mapping. Results showed that the integration of NDVIcanopy and NDREmean with additional information could be used as an indicator for mapping water stress variability within a block.

Obtaining Spatial Variations in Cabernet Sauvignon (Vitis vinifera L.) Wine Flavonoid Composition and Aromatic Profiles by Studying Long-Term Plant Water Status in Hyper-Arid Seasons

The spatial variability in vineyard soil might negatively affect wine composition, leading to inhomogeneous flavonoid composition and aromatic profiles. In this study, we investigated the spatial variability in wine chemical composition in a Cabernet Sauvignon (Vitis vinifera L.) vineyard in 2019 and 2020. Because of the tight relationships with soil profiles, mid-day stem water potential integrals (Ψstem Int) were used to delineate the vineyard into two zones, including Zone 1 with relatively higher water stress and Zone 2 with relatively lower water stress. Wine from Zone 2 generally had more anthocyanins in 2019. In 2020, Zone 1 had more anthocyanins and flavonols. Zone 2 had more proanthocyanidin extension and terminal subunits as well as total proanthocyanidins in 2020. According to the Principal Component Analyses (PCA) for berry and wine chemical composition, the two zones were significantly different in the studied wine aromatic compounds. In conclusion, this study provides evidence of the possibility of managing the spatial variability of both wine flavonoid composition and aromatic profiles through connecting vineyard soil variability to grapevine season-long water status.

A Smart Crop Water Stress Index-Based IoT Solution for Precision Irrigation of Wine Grape.

The Scholander-type pressure chamber to measure midday stem water potential (MSWP) has been widely used to schedule irrigation in commercial vineyards. However, the limited number of sites that can be evaluated using the pressure chamber makes it difficult to evaluate the spatial variability of vineyard water status. As an alternative, several authors have suggested using the crop water stress index (CWSI) based on low-cost thermal infrared (TIR) sensors to estimate the MSWP. Therefore, this study aimed to develop a low-cost wireless infrared sensor network (WISN) to monitor the spatial variability of MSWPs in a drip-irrigated Cabernet Sauvignon vineyard under two levels of water stress. For this study, the MLX90614 sensor was used to measure canopy temperature (Tc), and thus compute the CWSI. The results indicated that good performance of the MLX90614 infrared thermometers was observed under laboratory and vineyard conditions with root mean square error (RMSE) and mean absolute error (MAE) values being less than 1.0 °C. Finally, a good nonlinear correlation between the MSWP and CWSI (R2 = 0.72) was observed, allowing the development of intra-vineyard spatial variability maps of MSWP using the low-cost wireless infrared sensor network.

Diversified cropping systems effect on the water status of mandarin trees under deficit irrigation

The diversified cropping systems has several environmental benefits, but there is no certainty about their effect on the water status of trees, especially in areas with permanent low availability of irrigation water. Therefore, our objective was to evaluate the effect of the establishment of diversified cropping systems in a traditional monoculture (MC) of adult mandarin trees on its water status and yield under sustained deficit irrigation (SDI) and regulated deficit irrigation (RDI). Six treatments were tested during three consecutive seasons: (i) a mandarin monoculture under SDI (MC-SDI), with permanent bare alleys and irrigated at ∼70 % ETc during the entire season (actual amount of irrigation water available in the area); (ii) a diversified mandarin crop (D1-SDI), irrigated as MC-SDI, with a rotation of fava bean and barley/vetch in the alleys; and (iii) a second diversified mandarin crop (D2-SDI), irrigated as MC-SDI, with a seasonal rotation of fava bean, purslane and cowpea. Additionally, three RDI treatments were evaluated, denoted as (iv) MC-RDI, (v) D1-RDI and (vi) D2-RDI, irrigated as MC-SDI, except during the stage II of fruit growth, when it was reduced by 50 % with an irrigation threshold of a solar midday stem water potential of ∼ –1.5 MPa. The mandarin yield was not affected by the effect of diversified cropping systems, but the RDI reduced it on average by 30.1 % with respect to SDI. The alley cropping of fava bean and barley/vetch during winter and spring under sustained deficit irrigation conditions reduced the water stress intensity of mandarin trees by 26.9 % over the entire season, while cultivation of purslane or cowpea during summer increased it by 60–144 %. Both diversifications required a greater amount of water than the monoculture, in these terms an additional reduction in the irrigation of mandarin trees, as in the RDI treatment, allowed to solve this additional water requirement, but promoted an increase in the water stress intensity of the trees, which may affect their productivity in the mid and long-term.

Combining thermal imaging and soil water content sensors to assess tree water status in pear trees.

Volumetric soil water content is commonly used for irrigation management in fruit trees. By integrating direct information on tree water status into measurements of soil water content, we can improve detection of water stress and irrigation scheduling. Thermal-based indicators can be an alternative to traditional measurements of midday stem water potential and stomatal conductance for irrigation management of pear trees (Pyrus communis L.). These indicators are easy, quick, and cost-effective. The soil and tree water status of two cultivars of pear trees 'D'Anjou' and 'Bartlett' submitted to regulated deficit irrigation was measured regularly in a pear orchard in Rock Island, WA (USA) for two seasons, 2021 and 2022. These assessments were compared to the canopy temperature (Tc), the difference between the canopy and air temperature (Tc-Ta) and the crop water stress index (CWSI). Trees under deficit irrigation had lower midday stem water potential and stomatal conductance but higher Tc, Tc-Ta, and CWSI. Tc was not a robust method to assess tree water status since it was strongly related to air temperature (R = 0.99). However, Tc-Ta and CWSI were greater than 0°C or 0.5, respectively, and were less dependent on the environmental conditions when trees were under water deficits (midday stem water potential values< -1.2 MPa). Moreover, values of Tc-Ta = 2°C and CWSI = 0.8 occurred when midday stem water potential was close to -1.5 MPa and stomatal conductance was lower than 200 mmol m-2s-1. Soil water content (SWC) was the first indicator in detecting the deficit irrigation applied, however, it was not as strongly related to the tree water status as the thermal-based indicators. Thus, the relation between the indicators studied with the stem water potential followed the order: CWSI > Tc-Ta > SWC = Tc. A multiple regression analysis is proposed that combines both soil water content and thermal-based indices to overcome limitations of individual use of each indicator.

Managing Water Stress in Olive (Olea europaea L.) Orchards Using Reference Equations for Midday Stem Water Potential

The irrigation surface of olive orchards has increased over recent decades. In zones affected, deficit irrigation scheduling is a must. The aim of this work was to study water stress management based on reference equations for midday stem water potential. An experiment was conducted over three seasons in Seville (Spain) from 2020 to 2022. A young hedgerow olive orchard (cv Manzanilla de Sevilla) was irrigated using three different treatments: Control (full irrigated), RDI, and Rainfed, in a completely randomized design (six replications). The midday stem water potential and leaf conductance were measured throughout the three seasons. Stem water potential was more sensitive to water stress than leaf conductance and showed a clearer impact and rehydration. Individual data of stem water potential were grouped according to leaf conductance reduction. The relationship of these stem water potentials and temperature or vapor pressure deficit was significant, linear, and aligned to published baselines. Scattering in these equations increased when the leaf conductance reduction was greater. These reference equations would be useful to define moderate water stress conditions in the most sensitive processes, such as vegetative or fruit growth. Definition of severe water stress conditions would be better established with constant values.

Do urban tree hydraulics limit their transpirational cooling? A comparison between temperate and hot arid climates

Evaporative cooling due to transpiration of urban trees in two contrasting climates is the subject of this study. Transpiration was studied experimentally on local tree species at ‘tree lab’ sites in Munich, Germany (temperate climate) and in Beer Sheva, Israel (hot arid climate), within various settings (park, street, square) with natural and sealed surface conditions. The comparison was based on linear relationship of midday canopy resistance to atmospheric vapor pressure deficit (VPD), where the slope is proportional to tree hydraulic conductance and midday stem water potential. Sap flux densities in all trees were similar but crown projected area (CPA) was larger in Beer Sheva, resulting in higher hydraulic resistance that limits transpirational cooling (regression slopes of 0.25–0.44 in Beer Sheva vs. 0.10–0.18 in Munich). The contribution of the transpirational cooling, studied as the proportion of latent heat to total available energy was about 40% less significant at noon hours in summer in Beer Sheva than in Munich. Results suggested that shading should be the main consideration in planting local trees in hot dry climates. The linear relationship of midday canopy resistance to VPD is proposed as an operative tool for comparing evaporative cooling between local trees in different climatic regions.

Effects of Zeowine and compost on leaf functionality and berry composition in Sangiovese grapevines

AbstractMeteorological extremes such as heatwaves and water limitations during the ripening season could negatively impact vine ecophysiology and berry metabolism resulting in lower yield per vine. This project aimed to compare two different soil managements during two growing-production seasons (2021 and 2022) with respect to control without any treatment (control). The two managements were: Zeowine (30 t/ha; a soil conditioner made with clinoptilolite and compost proceeding of industrial wine-waste) and compost (20 t/ha). The trial was organized at Col d'Orcia Estate (Montalcino, Tuscan wine region, Italy). The purpose was twofold: (1) to evaluate the effects of Zeowine treatments on leaf gas exchanges, midday stem water potential, chlorophyll fluorescence and leaf temperature (ecophysiology); and (2) to determine any repercussions on the quality of the grapes (technological and phenolics analyses). The parameters plant yield, yeast assimilable nitrogen, fractionation of anthocyanins (cyanidin, delphinidin, malvidin, peonidin and petunidin), caffeic acid, coumaric acid, gallic acid, ferulic acid, kaempferol and quercetin were also analysed. Zeowine showed higher photosynthesis, less negative midday water potential and lower leaf temperature. Essentially, no significant difference was found between the compost and the control. Furthermore, Zeowine grapevines showed higher anthocyanin accumulation and less quercetin content. In general, compost applied together with zeolite could alleviate the adverse effects of water stress and improve plant growth, yield and quality. The control management strategy proved to be the least beneficial for the well-being of the plant and the final quality of the product, confirming the need for amendments in critical years.

GrapevineXL reliably predicts multi-annual dynamics of vine water status, berry growth, and sugar accumulation in vineyards.

Climate and water availability greatly affect each season's grape yield and quality. Using models to accurately predict environment impacts on fruit productivity and quality is a huge challenge. We calibrated and validated the functional-structural model, GrapevineXL, with a data set including grapevine seasonal midday stem water potential (Ψxylem), berry dry weight (DW), fresh weight (FW), and sugar concentration per volume ([Sugar]) for a wine grape cultivar (Vitis vinifera cv. Cabernet Franc) in field conditions over 13years in Bordeaux, France. Our results showed that the model could make a fair prediction of seasonal Ψxylem and good-to-excellent predictions of berry DW, FW, [Sugar] and leaf gas exchange responses to predawn and midday leaf water potentials under diverse environmental conditions with 14 key parameters. By running virtual experiments to mimic climate change, an advanced veraison (i.e. the onset of ripening) of 14 and 28days led to significant decreases of berry FW by 2.70% and 3.22%, clear increases of berry [Sugar] by 2.90% and 4.29%, and shortened ripening duration in 8 out of 13 simulated years, respectively. Moreover, the impact of the advanced veraison varied with seasonal patterns of climate and soil water availability. Overall, the results showed that the GrapevineXL model can predict plant water use and berry growth in field conditions and could serve as a valuable tool for designing sustainable vineyard management strategies to cope with climate change.

Influence of Silicon on Tolerance to Water Deficit of Peach Trees

Water deficit in young fruit trees can reduce growth and future orchard productivity. Exogenous silicon (Si) applications have been associated with induced resistance to biotic and abiotic stresses such as water deficit, but the role of Si in fruit trees is still largely unexplored. The aim of the study was to evaluate the effect of Si applications on water status and gas exchange of young peach trees. This study comprises two experiments arranged in a factorial design with two water regimens (well-irrigated or water-stressed) and three Si concentrations (0, 10, or 20 mg⋅L−1 in the first experiment; 0, 20, or 40 mg⋅L−1 in the second experiment). Si applications via foliar spray were performed weekly after the water regimens were clearly established. Tree water status (midday stem water potential), and gas exchange parameters (CO2 assimilation, leaf transpiration, stomatal conductance, leaf water use efficiency) were measured. Si application at 10 or 20 mg⋅L−1 improved water status of water-stressed trees without affecting gas exchange, but 40 mg⋅L−1 reduced CO2 assimilation. Thus, foliar applications of Si could be a promising strategy for nonirrigated, nonbearing orchards to maintain their water status during dry periods and/or improve their recovery from water deficit.

Unveiling resilience mechanisms of Quercus ilex seedlings to severe water stress: Changes in non-structural carbohydrates, xylem hydraulic functionality and wood anatomy

Over the last few decades, extensive dieback and mortality episodes of Quercus ilex L. have been documented after severe drought events in many Mediterranean forests. However, the underlying physiological, anatomical, and biochemical mechanisms remain poorly understood. We investigated the physiological and biochemical processes linked to embolism formation and non-structural carbohydrates (NSCs) dynamics in Q. ilex seedlings exposed to severe water stress and rewatering. Measurements of leaf gas exchange, water relations, non-structural carbohydrates, drought-related gene expression, and anatomical changes in wood parenchyma were assessed. Under water stress, the midday stem water potential dropped below - 4.5 MPa corresponding to a ~ 50 % loss of hydraulic conductivity. A 70 % reduction in stomatal conductance led to a strong depletion of wood NSCs. Starch consumption, resulting from the upregulation of the β-amylase gene BAM3, together with the downregulation of glucose (GPT1) and sucrose (SUC27) transport genes, suggests glucose utilization to sustain cellular metabolism in the wood parenchyma. After rewatering, the presence of residual xylem embolism led to an incomplete recovery of leaf gas exchanges. However, the partial restoration of photosynthesis allowed the accumulation of new starch reserves in the wood parenchyma and the production of new narrower vessels. In addition, changes in the cell wall composition of the wood parenchyma fibers were observed. Our findings indicate that thirty days of rewatering were sufficient to restore the NSCs reserves and growth rates of Q. ilex seedlings and that the carryover effects of water stress were primarily caused by hydraulic dysfunction.

Cumulative response of Tempranillo vines to the crop forcing technique and pre-forcing and post-veraison water stress in terms of yield and grape and wine quality

Elevated temperatures during berry ripening are detrimental to grape quality. The crop forcing technique (summer pruning that ‘forces’ the vine to start a new cycle) increases must acidity and malic acid concentration at harvest by delaying the date of veraison. However, little information is available on the sensitivity to water stress of forced vines. A 3-year trial was conducted to test three irrigation strategies in forced vines: a minimum threshold of mid-day stem water potential (Ψs) of −0.75 MPa before forcing (DI), a minimum Ψs threshold of −1.2 MPa only after veraison (RDI), and the combination of both treatments (DI + RDI). Results were compared to a non-forced treatment with a minimum Ψs threshold of −1.2 MPa after veraison (C-RDI). Must acidity increased, and pH decreased in the forced treatments. However, yield was reduced by 35% and irrigation requirements increased by 20% when comparing forced and unforced treatments. As a result, water use efficiency was reduced in forced treatments. Only after a dry spring did the, DI (11%) and DI + RDI (30%) treatments, save water compared to the C-RDI treatment. Moreover, although Ψs before forcing never fell below −0.75 MPa, a significant negative correlation (R2 = 0.76) was found between the integral of water stress before the vines were forced and the number of forced bunches per vine. Post-veraison water stress in forced vines reduced the polyphenol content of the wine. Our findings suggest that forced vines are extremely sensitive to even mild water stress.

Response of fruit yield, fruit quality, and water productivity to different irrigation levels for a microsprinkler-irrigated apple orchard (cv. Fuji) growing under Mediterranean conditions

Irrigation management strategies based on soil–plant–climate interactions are the key to improving water productivity and maintaining optimum yield and quality for apple orchards growing under Mediterranean conditions where water availability for irrigation is significantly decreasing. Thus, an experiment was carried out to investigate the effect of different irrigation levels on the yield, fruit quality, and water status of microsprinkler-irrigated apple trees (Malus domestica ‘Fuji’) located in a Mediterranean region in the central part of Chile. Four irrigation treatments were applied to a commercial orchard during the 2011/12 and 2013/14 growing seasons as follows: 125%, 100%, 75%, and 50% of actual evapotranspiration (ETa). Fruit yield (Y), crop load (fruits per tree, FP), fruit size (fruit weight (FW) and fruit diameter (FD)), fruit quality (firmness (FF), contents of soluble solids (SS), and starch (ST)), water productivity (WP), midday stem water potential (Ψstem), and water stress integral (WSI) were measured in each growing season. The results indicate that trees at 125% and 100% ETa reached an average Ψstem near − 1.0 MPa, whereas trees at 75% and 50% ETa reached Ψstem ∼ − 1.5 MPa. With the exception of FP, irrigation treatment had significant effects on Y, FW, FD, FF, SS, ST and WP. In this regard, the apple trees irrigated at 50% ETa produced fruits with the highest SS, FF, and WP, but the Y and fruit size (FW and FD) were significantly reduced because of severe water stress (average Ψstem ranged from −1.35 to −1.88 MPa). Apple trees with the 100% and 125% ETa treatments produced higher Y, FW, and FD than those with the 75% ETa treatment. In addition, the irrigation amount for apple trees irrigated at 125% ETa was 15% greater than for those irrigated at 100% ETa, but the yield and fruit quality were similar in both treatments. Finally, this study suggested that irrigation scheduling based on 100% ETa and Ψstem closed to − 1.5 MPa could be a suitable strategy for microsprinkler-irrigated apple (cv. Fuji) orchards growing in Mediterranean climate areas.