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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 objective of this work was to validate the trunk water potential (Ψtrunk), using emerged microtensiometer devices, as a potential biosensor to ascertain plant water status in field-grown nectarine trees. During the summer of 2022, trees were subjected to different irrigation protocols based on maximum allowed depletion (MAD), automatically managed by real-time soil water content values measured by capacitance probes. Three percentages of depletion of available soil water (α) were imposed: (i) α=10% (MAD=27.5%); (ii) α=50% (MAD=21.5%); and (iii) α=100%, no-irrigation until Ψstem reached -2.0 MPa. Thereafter, irrigation was recovered to the maximum water requirement of the crop. Seasonal and diurnal patterns of indicators of water status in the soil-plant-atmosphere continuum (SPAC) were characterised, including air and soil water potentials, pressure chamber-derived stem (Ψstem) and leaf (Ψleaf) water potentials, and leaf gas exchange, together with Ψtrunk. Continuous measurements of Ψtrunk served as a promising indicator to determine plant water status. There was a strong linear relationship between Ψtrunkvs. Ψstem (R2 = 0.86, p0.05). A mean gradient of 0.3 and 1.8 MPa was observed between Ψtrunkvs.Ψstem and Ψleaf, respectively. In addition, Ψtrunk was the best matched to the soil matric potential. The main finding of this work points to the potential use of trunk microtensiometer as a valuable biosensor for monitoring the water status of nectarine trees. Also, trunk water potential agreed with the automated soil-based irrigation protocols implemented.

This field experiment focuses on the effects of a heavy rainfall event (DANA, depresión aislada en niveles altos) that occurred on 12–14 September 2019 (DOY, Day of the year, 255–257), in southern Spain on plant water status and the thermal response of nectarine trees. Two irrigation treatments were applied during the summer–autumn postharvest period (DOY 158–329): full-irrigated (CTL) and non-irrigated (DRY). Volumetric soil water content (θv), air temperature (Ta) and canopy temperature (Tc) were monitored in real-time and the crop water stress index (CWSI) was calculated. The difference in Tc between the DRY and CTL treatments (Tc’ − Tc) is proposed as a new thermal indicator. Stem water potential (Ψstem) and leaf gas exchange measurements were recorded on representative days. During the DANA event, only the Tc measured by the infrared radiometer sensors could be monitored. Therefore, the effects of the DANA forced the soil water content sensors to be switched off, which prevented Ψstem and leaf gas exchange determinations from DOY 255 to 275. Before the DANA event, withholding irrigation caused a gradual decrease in the soil and plant water status in the DRY treatment. Significant differences appeared between treatments in the studied thermal indexes. Moreover, Tc’ − Tc was more sensitive than Tc − Ta in assessing nectarine water stress. The effects of the DANA reduced these differences, suggesting different baselines for the calculation of CWSI. In this respect, the relationship Tc − Ta vs. VPD improved the coefficient of determination after the DANA event in full-irrigated trees. Similar values of Ψstem and leaf gas exchange were found in both treatments after the DANA event, even though thermal indexes showed some significant differences. In addition, the strong relationship found between Tc − Ta and CWSI vs. Ψstem worsened after DANA occurred, revealing a lower sensitivity of Ψstem compared to canopy temperature to accurately assess nectarine water status in these saturated soil conditions. This research underlined the robustness of infrared thermography to continuously monitor plant water status under these extreme weather conditions.

The aim of this study was to test the combined effect of water stress and cropping system on yield and fruit quality in Bearss lime trees. For this purpose, two irrigation treatments were applied during stage II of fruit growth: control (well irrigated, automatically managed by soil water content sensors) and stress (non-irrigated), both under open-field and shaded conditions. Soil water status was assessed by determining soil water content and plant water status by measuring stem water potential (Ψstem), stomatal conductance (gs), and net photosynthesis (Pn). Yield parameters (kg and the number of fruits per tree and fresh mass per fruit) and fruit quality were assessed on two harvest dates. In addition, on the second harvest date, the content of metabolites and nutrients in the lime juice was analyzed. The results showed that soil water deficit induced 35% lower gs values in open-field than in shaded conditions. The highest kg and the number of fruits per tree were observed in the shaded system, especially on the first harvest date. The lowest yield was observed in stressed trees grown without netting. Slightly higher fresh mass and equatorial diameter were observed in shaded fruits than in open-field fruit. Soil water deficit increased fruit total soluble solids and decreased juice content, especially in open-field trees. Shaded conditions made the lime trees more resilient to soil water deficit, which led to higher yields and better external fruit quality traits. In addition, fruit precocity was significantly higher in the shaded system.

This work studied the sensitivity of the postharvest period of early maturing nectarine trees (Prunus persica L. Batsch, cv. Flariba) to water stresses. Along with a well-irrigated treatment (T-0), three water deficit treatments (by withholding irrigation) were applied: T-1: early postharvest (June–July), T-2: late postharvest (August–September), and T-3: the whole postharvest period (June–September). Soil water content (θv) and midday stem water potential (Ψstem) were measured throughout the study. During winter dormancy, L-arginine, starch, and phosphorus content in the roots were analyzed. Yield, fruit quality, and metabolites were determined at harvest. Ψstem reached −1.7 and −2.3 MPa at the end of the early and late postharvest periods, respectively. Total yield and number of fruits per tree were significantly reduced in all deficit treatments with respect to T-0, while no significant differences were observed in physicochemical fruit quality. The T-2 treatment showed the highest percentage of cracked fruits. Significantly, lower values of L-arginine and phosphorus were observed in the roots of T-2 trees, with respect to T-0, while they were similar in T-1 trees. Although the early postharvest stage is key for the application of RDI strategies, our results indicated that the late postharvest period was also a sensitive period to severe drought, as the accumulation of winter root reserves (L-arginine and phosphorus) was reduced, which limited yield.

The physiological performance of early-maturing nectarine trees in response to water deficits was studied during the postharvest period. Two deficit irrigation treatments were applied, moderate and severe, and these were compared with a control treatment (fully irrigated). Stem water potential and leaf gas exchange (net CO2 assimilation rate, ACO2; transpiration rate, E; and stomatal conductance, gs) were measured frequently. Drought avoidance mechanisms included a decrease in stomatal conductance, especially in the case of the severe deficit treatment, which also showed a strong dependence of ACO2 on gs. Intrinsic water-use efficiency (ACO2/gs) was more sensitive than instantaneous water-use efficiency (ACO2/E) as an indicator to detect water deficit situations in nectarine trees. However, in contrast to the results obtained for other deciduous fruit trees, a poor correlation was found between ACO2/E and ACO2/gs, despite the important relation between E and gs. ACO2/E was also weakly correlated with gs, although this relationship clearly improved when the vapor pressure deficit (VPD) was included, along with gs as the independent variable. This fact reveals that apart from stomatal closure, E depends on the boundary layer conductance (gb), which is mediated by VPD through changes in wind speed. This suggests low values of the decoupling coefficient for this water-resilient species.

Physiological plant water status indicators are useful for managing precision irrigation in regions with limited water resources. The aim of this work was to evaluate the effect of shade netting on the diurnal and seasonal variations of several plant water status indicators in young lime trees (Citrus latifolia Tan., cv. Bearss), grown at the CEBAS-CSIC experimental station in Murcia, Spain. Stem water potential (Ψstem), leaf gas exchange (net photosynthesis (Pn) and stomatal conductance (gs)), and canopy temperature (Tc) were measured on representative days of winter and summer. The Ψstem daily pattern was quite similar in both seasons under both conditions. However, the circadian rhythm of leaf gas exchange was affected by shade conditions, especially in summer, when shaded leaves showed maximum gs values for a longer time, allowing higher net photosynthesis (37%). Canopy temperature behaved similarly in both conditions, nevertheless, lower values were recorded in open-air than in shaded trees in the two seasons. The canopy-to-air temperature difference (Tc − Ta), however, was lower in shaded trees during the daylight hours, indicating the higher degree of leaf cooling that was facilitated by high gs values. The possibility of continuously recording Tc makes it (or the proposed canopy thermal index, CTI) a promising index for precise irrigation scheduling. Shade netting was seen to favour gas exchange, suggesting that it may be considered alternative to open-air for use in semi-arid areas threatened by climate change.

A growing international human population and rising living standards are increasing the demand for agricultural products. Under higher pressure over natural resources, environmental concerns are increasing as well, challenging current water use decision-making processes in irrigated agriculture. Higher agricultural productivity means water should be applied more efficiently, which requires instant information on weather, soil, and plant conditions throughout the growing season. An information-based irrigation scheduling application tightened to the spatiotemporal variability of the fields is critical for enhancing the current irrigation system and making better irrigation scheduling decisions. The aim of this study is to review current irrigation scheduling methodologies based on two case studies (woody and field crops) located in semi-arid areas of Southeast Spain. We realize that optimal irrigation programming requires consistent investment in equipment, expenditure on operation and maintenance, and qualified technical and maintenance services. These technological approaches will be worthwhile in farms with low water availability, high profitability, and significant technical-economic capacity.

The objectives of the paper were to study the pattern of root growth (measured by minirhizotrons) in relation to trunk, fruit and shoot growth and the effects of crop load on tree growth and yield in peach trees. Two crop load (commercial and low) treatments were applied in a mature early-maturing peach tree orchard growing in Mediterranean conditions. Root growth dynamics were measured using minirhizotrons during one growing season. Shoot, trunk and fruit growth were also measured. At harvest, all fruits were weighed, counted and sized. Roots grew throughout the year but at lower rates during the active fruit growth phase. Root growth was asynchronous with shoot growth, while root and trunk growth rates were highest after harvest, when the canopy was big enough to allocate the photo-assimilates to organs that would ensure the following season’s yield. Shoot and fruit growth was greater in the low crop load treatment and was accompanied by a non-significant increase in root growth. High level of fruit thinning decreased the current yield but the fruits were more marketable because of their greater size.

Monitoring plant water status is relevant for the sustainable management of irrigation under water deficit conditions. Two treatments were applied to an early-maturing nectarine orchard: control (well irrigated) and precise deficit irrigation (PDI, based on soil water content thresholds). Moderate water deficits generated by PDI were assessed by comparing terrestrial: stem water potential (Ψstem) and gas exchange parameters, with remote: canopy temperature, normalized difference vegetation (NDVI), and soil adjusted vegetation index (SAVI), plant water status indicators. The Ψstem was the only indicator that showed significant differences between treatments. NDVI and SAVI at the postharvest period were appropriate indexes for estimating winter pruning, although they did not serve well as plant stress indicator. Vapor pressure deficit along with Ψstem values were able to predict remote sensing data. Ψstem and canopy to air temperature difference values registered the highest signal intensity and NDVI the highest sensitivity for detecting water deficit situations. The results suggest that care should be taken when using instantaneous remote indicators to evaluate moderate water deficits in deciduous fruit trees; more severe/longer water stress conditions are probably needed. The proposed PDI strategy promoted water saving while maintaining yield, and could be considered a promising tool for semi-arid agrosystems.

Precision agriculture requires irrigation supported by an accurate knowledge of the crop water requirements. In this paper, a novel approach for drip irrigation scheduling of fruit trees is presented based on the results obtained during a full growing season in an early-maturing nectarine orchard growing in a clay loam soil in a Mediterranean environment. Real-time water content was monitored in the soil profile of the main root exploration zone by means of capacitance probes; in addition, plant water status (midday stem water potential and leaf gas exchange) and canopy development were frequently measured throughout the vegetative cycle. The reference evapotranspiration (ET0) values, taken from a nearby automatic meteorological station, and the measured irrigation values allowed the determination of the irrigation factors once irrigation drainage during the season was assumed to be negligible and plant water status was proved to be adequate. The proposed irrigation factors offer a hands-on approach as an easy tool for irrigation management based on suitable soil water deficits, allowing the water requirements of nectarine trees under precision irrigation to be determined in semi-arid agrosystems where water resources are limited.