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In South Africa, as in other semi-arid countries, sustainable production of high-value crops  requires precise management of limited water resources. We investigated daily and seasonal changes in stem and fruit growth as indicators of water stress in pear (Pyrus communis L.) trees, in the Western Cape Province. Stem and fruit growth data were collected hourly throughout the 2022–2023 growing season on 2 cultivars commonly planted in South Africa –  Packham’s Triumph and Forelle. Soil water content, tree sap flow, and orchard microclimate were also monitored. Fruit maximum daily shrinkage (MDS) was highly sensitive to soil water deficit and more sensitive than stem size changes. However, the patterns of fruit MDS for both cultivars changed as the season progressed. Early in the season (October–December), there was a strong correlation between fruit MDS and soil water deficit (R² ~ 0.72). The fruit shrunk with increasing soil water deficit as water loss through transpiration exceeded gains through xylem and phloem inflows. In contrast, daytime fruit size swelled from late December until harvest (February/March), likely because of the dominance of phloem inflows and decreased peel transpiration as the fruit matured. Correlation between fruit expansion and soil water deficit was weaker (R² ~ 0.32) during the later stage even though fruit growth continued until harvest. Stem MDS consistently showed midday shrinkage throughout the season in response to soil water deficit, but with more scatter (R²~ 0.37). Seasonal total transpiration was greater for Forelle (733 mm) than Packham’s Triumph (539 mm) because of the higher leaf area index of the Forelle and the longer growing season. This study suggests that pear fruit growth data can provide accurate estimates of tree water status, but only during the early stages of growth. Towards maturity, fruit size changes respond indirectly to water deficit, possibly through reductions in photosynthesis. [ABSTRACT FROM AUTHOR]

Top-netting technology improves crop microclimate, and its utilization in fruit tree plantations may mitigate deleterious weather effects. The technology is associated with improved tree water relations, and may result in significant water savings. Photoselective nets may provide additional, crop- and target-dependent, beneficial effects. A previous paper reported results of a five-year trial in Valencia orange, in which three nets were used, pearl, transparent and low shading red, in combination with three irrigation regimes, 100% (I100), standard commercial fertigation, and two reduced rates, 80% (I80) and 60% (I60). Among the different netting types, the combination of red nets and I80 resulted in the highest yield on a multi-annual basis. The current study provides data regarding water relations during stage II, i.e., cell expansion, of fruit development. The red net had higher radiation transmittance and canopy resistance than the pearl net. The higher resistance is an expression of the lower transpiration of the red net, monitored by sap flow, as compared to the other treatments, especially under deficient irrigation. The beneficial effect of the red net was also evidenced from multi-annual stem water potential. These data supplement the enhanced yield effect of the red net, suggesting they are associated with its spectral properties.

Studies of daily changes in tree trunk diameter provide valuable information concerning growth patterns and their relationships with varying environmental conditions. To date, very few experiments with fruit trees evaluated the effects of climate variation on trunk shrinkage and the duration of the contraction and recovery phases and of growth. In this study, electronic dendrometers continuously monitored trunk diameter and trunk water storage dynamics of drip-irrigated ‘Gala’ apple trees (Malus x domestica Borkh.) during three growing seasons, which differed significantly in temperature, precipitation, air humidity and solar irradiation. It was found that trunk diameter and meteorological variables were closely related, even when excluding the effects of soil water limitations. During each growing season, the durations of the daily contraction phase began to increase with increasing water vapour partial pressure deficit, and decreased again in autumn, when vapour partial pressure decreased. Throughout the season, the duration of the growth phase tended to change inversely to that of both contraction and recovery phase. The relationship between maximum trunk shrinkage and vapour partial pressure was higher post than preharvest for all years studied. The duration of contraction, recovery, and growth phases may provide valuable information concerning seasonal changes and environmental drivers of water storage dynamics in apple trees. [ABSTRACT FROM AUTHOR]

Precise irrigation management of grapevines in greenhouses requires a reliable method to easily quantify and monitor the grapevine water status to enable effective manipulation of the water stress of the plants. This study describes a study on stem diameter variations of grapevine planted in a greenhouse in the semi-arid area of Northwest China. In order to determine the applicability of signal intensity of stem diameter variation to evaluate the water status of grapevine and soil. The results showed that the relative variation curve of the grapevine stem diameter from the vegetative stage to the fruit expansion stage showed an overall increasing trend. The correlations of MDS (maximum daily shrinkage) and DI (daily increase) with meteorological factors were significant (p < 0.05), and the correlations with SWP, RWC and soil moisture were weak. Although MDS and DI can diagnose grapevine water status in time, SIMDS and SIDI have the advantages of sensitivity and signal intensity compared with other indicators. Compared with MDS and DI, the R² values of the regression equations of SIMDS and SIDI with SWP and RWC were high, and the correlation reached a very significant level (p < 0.01). Thus, SIMDS and SIDI are more suitable for the diagnosis of grapevine water status. The SIMDS peaked at the fruit expansion stage, reaching 0.957–1.384. The signal-to-noise ratio of SIDI was higher than that of MDS across the three treatments at the vegetative stage. The value and signal-to-noise ratio of SIDI at the flowering stage were similar to those of SIMDS, while the correlation between SIDI and the soil moisture content was higher than that of SIMDS. It can be concluded that that SIDI is suitable as an indicator of water status of grapevine and soil during the vegetative and flowering stages. In addition, the signal-to-noise ratio of SIMDS during the fruit expansion and mature stages was significantly higher than that of SIDI. Therefore, SIMDS is suitable as an indicator of the moisture status of grapevine and soil during the fruit expansion and mature stages. In general, SIMDS and SIDI were very good predictors of the plant water status during the growth stage and their continuous recording offers the promising possibility of their use in automatic irrigation scheduling in grapevine [ABSTRACT FROM AUTHOR]

• Signal MDS was unaffected for water stress conditions. • TGR was a sensitive indicator to water stress. • McCutchan and Shackel baseline could be a useful tool in full irrigated conditions. Irrigation needs in mature almond orchards are very high. Although almond trees grow in rainfed conditions, the yield response is very sensitive to irrigation. Continuous monitoring of the water status could be an adequate tool to optimize deficit irrigation. In this sense, trunk diameter fluctuations appeared as a very promising indicator at the beginning of the century, but few data have been published. The aim of this work is to check threshold values of maximum daily shrikage (MDS) and identify possible limitations to their use in commercial orchards. The experiment was performed in a commercial farm in Dos Hermanas (Seville, Spain) during the 2017 season on a 7-years-old orchard (cv Vairo). The irrigation treatments were Control (100% ETc), sustained deficit irrigation (SDI) with a maximum seasonal irrigation of 100 mm and two regulated deficit treatments (RDI). Both RDI treatments (RDI-1 and RDI-2) were scheduled using the signal of maximum daily shrinkage (signal) and the midday stem water potential (SWP). In RDI-1, full irrigation conditions were provided before kernel filling and during postharvest, using the threshold values suggested in the bibliography. During kernel filling, the water stress level was designed to be -1.5 MPa (SWP) and 1.75 (signal). RDI-2 trees were irrigated using the same scheduling as RDI-1, but target water stress values were higher in kernel filling (-2 MPa and 2.75) and with a maximum seasonal amount of water of 100 mm. SWP in Control trees was near the McCutchan and Shackel baseline for most of the season. None of the deficit treatments reached the signal values suggested. Moreover, the signal values were almost equal between treatments, with no water stress effect. The trunk growth rate (TGR) presented clear differences depending on the water status. [ABSTRACT FROM AUTHOR]

Analysis of continuous sap flux and stem growth measurements can help to elucidate how environmental conditions influence water status and stem growth in trees. Stem radius variations and sap flux densities of the drought-tolerant conifer Cedrus libani were measured using automatic point dendrometers and heat dissipation sensors, respectively. The study was conducted at the Elmalı Cedar Research Forest at 1665 m a.s.l. From April to September 2009, stem radius variation was measured hourly for nine C. libani trees. Concurrently, environmental variables (e.g., air temperature, precipitation, relative humidity, soil water content) were monitored within the study site. From the dendrometer records, maximum daily shrinkage (MDS) of the stem radius was calculated. From June to August, a time period of 2 months, four of the nine study trees were selected for hourly measurements of sap flux density (Js). Daily courses of stem radius variation and Js were very uniform and similar for all studied trees. A daily hysteresis effect was observed, reflecting the pattern of water storage and retrieval. Means of MDS and Js varied between 0.09 and 0.15 mm and between 4.3 and 10.6 g m-2 s-1, respectively. Daily maximum Js varied between 8.0 and 33 g m-2 s-1. Correlation and regression analyses between MDS and Js and environmental variables revealed that MDS and Js were most closely related to vapor pressure deficit and photosynthetic active radiation, respectively. We conclude that C. libani is well adapted to the current environmental conditions at the study site (natural habitat) and that atmospheric conditions play a primary role in transpiration and tree growth. [ABSTRACT FROM AUTHOR]

Top-netting technology improves crop microclimate, and its utilization in fruit tree plantations may mitigate deleterious weather effects. The technology is associated with improved tree water relations, and may result in significant water savings. Photoselective nets may provide additional, crop- and target-dependent, beneficial effects. A previous paper reported results of a five-year trial in Valencia orange, in which three nets were used, pearl, transparent and low shading red, in combination with three irrigation regimes, 100% (I100), standard commercial fertigation, and two reduced rates, 80% (I80) and 60% (I60). Among the different netting types, the combination of red nets and I80 resulted in the highest yield on a multi-annual basis. The current study provides data regarding water relations during stage II, i.e., cell expansion, of fruit development. The red net had higher radiation transmittance and canopy resistance than the pearl net. The higher resistance is an expression of the lower transpiration of the red net, monitored by sap flow, as compared to the other treatments, especially under deficient irrigation. The beneficial effect of the red net was also evidenced from multi-annual stem water potential. These data supplement the enhanced yield effect of the red net, suggesting they are associated with its spectral properties. • Water relations were tested in orange trees under three photo-selective top-netting. • Red net resulted in lower transpiration as compared to pearl and transparent nets. • Trees under the red net also displayed higher stem water potential values. • The above findings supplemented the enhanced yield effect of the red net. [ABSTRACT FROM AUTHOR]

Direct evidence for the link between stem diameter variations (SDV) and the daily canopy water status, i.e., daily water potentials (Ψ), is rare, particularly for tall trees. It thus remains unclear up to what degree SDV readings are useful to estimate daily canopy Ψ. We measured SDV with point dendrometers at the stem base of tall, mature individuals of six European forest tree species in a nearnatural temperate forest and compared them with daily canopy Ψ during the growing seasons of 2014 (wet) and 2015 (dry). Stem diameter variations were de-trended for growth with two different approaches leading to the so-called tree water deficit (TWD). We found that midday Ψ can be predicted from TWD, independent of the growth-de-trending procedure to obtain TWD from SDV. Further, daily TWD was a better indicator for daily midday Ψ, particularly under dry conditions, than maximum daily shrinkage, another common quantity derived from SDV. Based on data from six temperate tree species, we conclude that TWD measured at the stem base is a consistent proxy for daily canopy midday Ψ of tall trees over the entire range of measured conditions. [ABSTRACT FROM AUTHOR]

The management of regulated deficit irrigation with a continuous measurement of the water status would allow obtaining an accurate estimation of the water needs. However, although different types of sensors are available, the threshold and daily management of these data are not clearly defined. Trunk diameter fluctuations are a good example of these type of data. The trunk growth rate (TGR) is considered an early indicator in olive trees. However, the daily TGR values are very changeable, and only cumulative values of TGR show a clear trend. The number of irrigation works using this indicator is scarce. The TGR thresholds considered in these papers are the average of values over a period, and this makes it difficult to use when preparing a daily schedule. The aim of this work is to present an approach that allows using daily TGR data. During the 2015 season, an irrigation experiment was carried out in the Doña Ana farm, a table olive orchard near Seville (Spain). The trees were 30 years old with a space of 7 × 4 m and they were irrigated using two drip lines with a flow rate of 2.5 l h −1 . The experiment began in spring and involved three treatments. Control trees were irrigated to maintain the midday stem water potential values at around −1.2 MPa before pit hardening started and at −1.4 MPa after this point. The trees under a mild water stress treatment (MI) were irrigated in the same was as the Control trees, except from DOY (day of the year) 161, the beginning of pit hardening, to DOY 237, when threshold value decreased to −2.0 MPa. The Moderate water stress trees (MO) were irrigated in the same way as the Control trees, except in the same period that MI, but with a threshold value of −4.0 MPa. The midday water potential pattern and leaf conductance pattern suggested that the level of water stress in both treatments was low, slightly higher in MO than in MI at the end of the water stress period. The shoot elongation suggested a period of water stress before DOY 161 in MO trees that the midday stem water potential and leaf conductance did not detect. The maximum daily shrinkage (MDS) signal indicated water stress conditions during the drought period, but both treatments presented similar values. The trunk growth rate (TGR) indicator was useful when maximum perimeter curves, average TGR and daily TGR were used together. All the periods of water stress identified by other indicators were detected when the three TGR data were used. An approach to use these three sets of data is discussed herein. [ABSTRACT FROM AUTHOR]

Microtensiometers are plant-based sensors than can continuously measure trunk water potential (Ψ trunk). This new water status indicator, Ψ trunk , was compared with the midday stem water potential (Ψ stem) measured with a pressure chamber, the current standard for assessing water status in trees, leaf water potential, and maximum daily shrinkage (MDS) in adult 'D'Anjou' pear trees (Pyrus communis L.) irrigated following two strategies, (1) a control treatment (CTL) irrigated at 100% of crop evapotranspiration and, (2) regulated deficit irrigation (RDI). Ψ trunk , Ψ stem and MDS were directly influenced by soil water content and atmospheric demand. MDS was able to detect water stress in DI trees the earliest. However, variability was high and it was not sensitive enough to detect significant differences between irrigation treatments at the end of the season. MDS had a maximum value of 300 µm (Ψ stem =−1.4 MPa). On the other hand, variation for midday Ψ stem and Ψ trunk was low and both indicators were able to distinguish between irrigation strategies. Midday Ψ stem and Ψ trunk had a strong linear relationship similar to the identity line (R2 = 0.88). However, when Ψ stem and Ψ trunk were compared in the afternoon, Ψ trunk reported by microtensiometers was − 0.7 MPa lower than Ψ stem measured by a pressure chamber. The daily relationship between trunk diameter variations and Ψ trunk measured with the microtensiometers followed five different stages. Changes in trunk diameter were delayed relative to changes in Ψ trunk. The seasonal relationship between the MDS and Ψ trunk was strongly related at the start of deficit irrigation (R2 = 0.63), but when the complete season was considered, this relationship was weaker (R2 = 0.44). Moreover, the low coefficient of variation and high sensitivity of the midday Ψ trunk measured with the microtensiometers supports the suitability of using them in automated irrigation systems to monitor tree water status in spite of their high dependence on environmental conditions. This is one of the first studies that validates the use of microtensiometers to continuously monitor tree water status in fruit trees across two consecutive seasons under differing irrigation treatments. • Microtensiometers continuously measure trunk water potential. • Midday stem and trunk water potentials were highly correlated. • Stem water potential identified earlier water stress than trunk and leaf water potentials. • Trunk water potential was strongly dependent on VPD. • Variations in trunk diameter followed changes in trunk water potential. [ABSTRACT FROM AUTHOR]

We evaluated the size effect on stem water status and growth in Norway spruce (Picea abies (L.) Karst.) occurring at the edge of its natural range in a dry inner Alpine environment (750 m asl, Tyrol, Austria). Intra-annual dynamics of stem water deficit (ΔW), maximum daily shrinkage (MDS) and radial growth (RG) were compared among saplings (stem diameter/height: 2.2 cm/93 cm; n = 7) and mature adult trees (25 cm/12.7 m; n = 6) during 2014. ΔW, MDS and RG were extracted from stem diameter variations, which were continuously recorded by automatic dendrometers and the influence of environmental drivers was evaluated by applying moving correlation analysis (MCA). Additionally, we used Morlet wavelet analysis to assess the differences in cyclic radial stem variations between saplings and mature trees. Results indicate that saplings and mature trees were experiencing water limitation throughout the growing season. However, saplings exhibited a more strained stem water status and higher sensitivity to environmental conditions than mature trees. Hence, the significantly lower radial increments in saplings (0.16 ± 0.03 mm) compared to mature trees (0.54 ± 0.14 mm) is related to more constrained water status in the former, affecting the rate and duration of RG. The wavelet analysis consistently revealed more distinct diurnal stem variations in saplings compared to mature trees. Intra-annual RG was most closely related to climate variables that influence transpiration, i.e., vapor pressure deficit, relative air humidity, and air temperature. MCA, however, showed pronounced instability of climate-growth relationships, which masked missing temporal or significant correlations when the entire study period (April-October) was considered. We conclude that an increase in evaporative demand will impair regeneration and long-term stability of drought-prone inner Alpine Norway spruce forests.

The growing scarce water resource has promoted the scientists to develop new technologies for scheduling irrigation. Of these, the ones relying on plant-based water-stress indicators have been found to have the greatest potential. Thus, measurements of stem water content and water potential, canopy temperature, sap flow (SF), and stem diameter variation (SDV), among other variables, have proved useful not only for research purposes, but also for precise irrigation scheduling in commercial production sites. In this work, we systematically reviewed the research achievements and growing trends of precise irrigation scheduling based on SDV. Of those mentioned above, the water-stress indicator had received most attention from the scientific community in terms of its potential for irrigating commercial production sites. Apart from being capable of an early detection of water stress, even if this was mild, SDV could be continuously and automatically recorded. This was a clear advantage over conventional indicators such as stem water potential, canopy temperature and leaf gas exchange parameters (net photosynthesis rate, stomatal conductance, intercellular carbon dioxide, transpiration rate, and so on). The principle of SDV, the history of dendrometers and dendrographs, relationships between SDV-derived index and the other water-stress indicators (water potential and sap flow) were introduced and analyzed in this paper. Among the SDV-derived indices that were useful for scheduling irrigation, the maximum daily shrinkage (MDS) and stem growth rate (SGR) were the most widely used. For young trees and in their periods of rapid stem growth, SGR could be a better indicator than MDS. In periods of negligible stem growth, however, SGR could not be used as an indicator of plant water stress. SDV outputs were affected by seasonal growth patterns, crop load, plant age and size, and the other factors, apart from water stress. Thus, expert interpretation of SDV records was required before using them for scheduling irrigation, which limited their potential for automation of the irrigation. Therefore, the characteristics of an SDV-derived index for irrigation scheduling must be evaluated for the local conditions. In this study we described the characteristics that must be taken into account in the evaluation, such as variability, signal strength, sensitivity, earliness, reliability and robustness. Furthermore, methods to schedule irrigation from SDV records were summarized as using absolute values of SDV-derived indices, the signal-intensity approach and combining SDV with SF. Although SDV-derived indices showed a high variability among plants, in most cases the signal intensity was high enough to achieve an acceptable sensitivity. Accordingly, the signal intensity was important for the realization of precision, digitization and the intelligentization management of the irrigation, and was successfully used to schedule low-frequency irrigation in orchards of a variety of species, for both full-and deficit-irrigation treatments. The hot spots in this field were also proposed at last and they should be focused on the following aspects: 1) To explore more effective sensor shelters to decrease the noise produced by physical contact; 2) To determining the appropriate SDV-derived index according to the actual environment to achieve the high enough signal intensity and sensitivity; 3) To quantify the effects of other factors except water stress on SDV outputs; 4) To combine SDV with other water stress index (such as SF) to enhance the explanations for how the plants absorb and utilize the water; and 5) To combine SDV measurements with aerial or satellite imaging for scheduling irrigation in large orchards with high crop-water-stress variability. [ABSTRACT FROM AUTHOR]

We evaluated the size effect on stem water status and growth in Norway spruce (Picea abies (L.) Karst.) occurring at the edge of its natural range in a dry inner Alpine environment (750 m asl, Tyrol, Austria). Intra-annual dynamics of stem water deficit (ΔW), maximum daily shrinkage (MDS), and radial growth (RG) were compared among saplings (stem diameter/height: 2.2 cm/93 cm; n = 7) and mature adult trees (25 cm/12.7 m; n = 6) during 2014. ΔW, MDS, and RG were extracted from stem diameter variations, which were continuously recorded by automatic dendrometers and the influence of environmental drivers was evaluated by applying moving correlation analysis (MCA). Additionally, we used Morlet wavelet analysis to assess the differences in cyclic radial stem variations between saplings and mature trees. Results indicate that saplings and mature trees were experiencing water limitation throughout the growing season. However, saplings exhibited a more strained stem water status and higher sensitivity to environmental conditions than mature trees. Hence, the significantly lower radial increments in saplings (0.16 ± 0.03 mm) compared to mature trees (0.54 ± 0.14 mm) is related to more constrained water status in the former, affecting the rate and duration of RG. The wavelet analysis consistently revealed more distinct diurnal stem variations in saplings compared to mature trees. Intra-annual RG was most closely related to climate variables that influence transpiration, i.e., vapor pressure deficit, relative air humidity, and air temperature. MCA, however, showed pronounced instability of climate-growth relationships, which masked missing temporal or significant correlations when the entire study period (April-October) was considered. We conclude that an increase in evaporative demand will impair regeneration and long-term stability of drought-prone inner Alpine Norway spruce forests. [ABSTRACT FROM AUTHOR]

This experiment evaluates the potential of using parameters based on tree trunk fluctuations for detecting water deficit in citrus trees under two different water saving-irrigation strategies: sustained deficit irrigation and partial root-zone drying. Three irrigation treatments were applied: 1) Control: trees were irrigated with 100% of their evapotranspirative needs (ET); 2) 60 sustained deficit irrigation (SDI): 60% ET; and 3) partial root-zone drying (PRD): 100% ET needs, applied to only one-half of root zone. Maximum daily shrinkage (MDS), trunk growth rate (TGR), and MDS ratio (ratio between MDS of stressed trees and control trees) were determined. Day-to-day MDS values varied largely and could not be used to determine tree water deficit. TGR did not show significant differences among treatments at this level of stress. Nevertheless, the MDS ratio was a reliable indicator to measure tree water status, and it was more sensitive for detecting water deficit at the onset of a water deficit in trees under SDI than in trees under PRD. [ABSTRACT FROM AUTHOR]

The continuous monitoring of water stress will increase the accuracy of the deficit irrigation scheduling. Almonds are very sensitive to water stress conditions and an important water consumer. Recently, a novel approach to the use of trunk growth rate (TGR) data has been proposed for olive trees. These works suggested the use of TGR frequencies to evaluate water status of the trees. The aim of the current work was to compare the seasonal pattern of three different indicators derived from the daily curves of trunk diameter fluctuations with midday stem water potential. During three consecutive seasons (2017–2019), an irrigation experiment was carried out in a mature almond orchard (cv Vairo) at Dos Hermanas (Seville, Spain). Four irrigation treatments replicated in four blocks were evaluated using the daily curves of midday stem water potential and trunk diameter fluctuations. The different irrigation treatments were: Control, full irrigated conditions; RDI-1 (irrigation scheduling based on midday stem water potential with a deficit irrigation during kernel filling [values around −1.2 MPa]); RDI-2(similar to RDI-1 but with a more severe water stress [−2 MPa]) and incomplete recovery after harvest due to limitation of the seasonal amount of water (around 100 mm); SDI, sustained deficit irrigation with a seasonal applied water equal to RDI-2. Trunk diameter fluctuations were measured with a wireless band dendrometer. The daily curves were processed to obtain three different indicators. Maximum daily shrinkage (MDS) was the difference between the daily maximum and minimum. Trunk growth rate (TGR) was the difference between two consecutive daily maximums. The frequencies of several ranges of TGR were compared with the midday stem water potential. Weekly frequencies of values greater than 0.3 mm day−1 decreased with the reduction of midday stem water potential, but the pattern changed greatly in different seasons. The weekly frequency of values between − 0.1 and 0 and between 0 and 0.1 mm day−1 were steadier in different seasons. Differences between seasons were related to growth pattern and yield. • TGR frequencies were related with water stress. • Each range of TGR considered had different response to water stress. • Crown size and yield affected to the greatest TGR values. [ABSTRACT FROM AUTHOR]

Highlights: [•] Trunk growth rate measurements were similar in the branch than in the trunk locations. [•] Maximum daily shrinkage was similar in branch than in trunk in no water stress trees. [•] The influence of crown volume on indices derived from trunk diameter records was low. [•] Point and band dendrometers measurements did not fit with the expected equation. [Copyright &y& Elsevier]

Studying the variation of fruit and stem diameter is critical for optimizing irrigation schedule of tomato under alternate partial root zone drip irrigation (APRI) and improving water use efficiency in the green house. A plant physiology monitoring system (LPS-05MD) was used to measure the variation of fruit diameter and stem diameter during the fruiting stage of tomato under three different irrigation regimes: conventional drip irrigation (CDI), APRI and fixed partial root-zone irrigation (FDI). The soil under CDI were irrigated to 100% field capacity (FC) of the root-zones (from 0 m to a maximum depth of 0.40 m during the fruiting stage) by the drip irrigation system when the soil water content (SWC) in the root-zone reaching 80% FC. During the same day, the APRI and FDI treatments received about 2/3 of the irrigation amount for CDI, but only irrigated to one part of the root-zones (approximately 1/2 of the whole root-zone). For APRI, the irrigated root zone was alternatively changed from one side to another and consequently resulted in the SWC of different sides alternately high and low; whereas in FDI treatment, only the fixed side was irrigated and the other side was kept drying during the growing season. Results indicated that the variation of fruit diameter was significantly higher (P<0.05) in sunny days with high solar radiation rate than that of cloudy day with low solar radiation rate. The increase of tomato fruit diameter per day was not significantly linearly related to average SWC of the whole root zone (R²=0.30, P=0.16), but significantly related to the average solar radiation rate (R²=0.64, P=0.018). Previous studies indicate that the magnitude of MDS (maximum daily stem shrinkage) are not constant over a period of days with the same water status but different environmental conditions, and absolute MDS values registered without considering the evaporative demand might be meaningless. Based on this, there was a positive linear relationship between MDS and ET0 (R²=0.38, P<0.001), and the relationship between the value of MDS divided by ET0 (MDS/ET0) with SWC was significant different than MDS with SWC, which indicated that normalizing the absolute MDS values by environmental parameters, such as ET0, can help to indicate the soil water status precisely. Contrary to the previous results that MDS values increase in response to drought stress, in this study, MDS of APRI decreased with increasing water stress. This might be explained by the low resistance of water flow and high hydraulic capacitance of the tomato cultivar as well as the special physiological responses under APRI. For APRI, MDS/ET0 had a close relationships with SWC in the drying and wet root-zone (R²=0.60, P=0.006 and R² =0.88, P<0.001, respectively), which indicated that MDS normalized by ET0 under APRI was influenced both by SWC of drying side and wet side, and predominantly by the irrigated root-zone. Similarly, MDS/ET0 in FDI treatment was closely linearly related with the SWC in the irrigated root-zone (R²=0.61, P<0.001), but not with the fixed drying side (R²=0.02, P=0.64). This phenomenon can be explained by the decay of roots in the drying root-zone and the diminishment of the chemical signal generated from the drying roots of FDI. To the best of our knowledge, no study has been done to investigate the relationship between MDS (or MDS/ET0) and SWC of different root zone under APRI. The data in this study can help to reveal the mechanisms of variation of fruit diameter and stem diameter, as well as providing useful information for optimizing irrigation schedule of tomato under APRI grown in greenhouse. [ABSTRACT FROM AUTHOR]

Highlights: [•] Deficit irrigation has been scheduled with the data obtained in the TGR and MDS. [•] TGR values changed during pit hardening according to fruit load. [•] MDS seasonal pattern changed in the most stressed trees after the first year. [•] RDI suggested reduced water applied without severe reduction in yield. [ABSTRACT FROM AUTHOR]

Highlights: [•] Periodicity of shoot and root growth affects the trunk diameter variation (TDV). [•] Fruit load played a dominant role in determining the TDV. [•] TDV analyses may provide an integrative “holistic view” of the tree. [•] Maximum daily shrinkage cannot be used on an absolute basis for irrigation control. [ABSTRACT FROM AUTHOR]

Abstract: Olive is one of the fruit tree species for which measurements of the trunk diameter variation (TDV) has shown a lower potential both for monitoring water stress and for scheduling irrigation. This is particularly true in the case of old, big olive trees with heavy fruit load. Fernández et al. (2011, J. Environ. Exp. Bot. 72, 330–338) observed that the daily difference for maximum trunk diameter between deficit irrigated ‘Manzanilla’ olive trees and control trees growing under non-limiting soil water conditions, termed D MXTD, is more sensitive and reliable for detecting the water stress of the trees than other widely used TDV-derived indices. However, they considered their results as preliminary because of the lack of replications. The aim of this work was to evaluate the usefulness of the D MXTD index for detecting plant water stress in an orchard with 12-year-old ‘Arbequina’ olive trees with heavy fruit load. The performance of D MXTD for detecting water stress of the trees was compared to that of the signal intensity for the maximum daily shrinkage (S I-MDS) derived from the same TDV records. Results showed that S I-MDS was not useful for indicating the water stress of the trees. On the other hand, the dynamics of D MXTD mimicked those of the soil and plant water status. Four instrumented trees per treatment (replicates) were enough to reduce the uncertainty of the TDV measurements to a reasonable level. Our results show that D MXTD is a useful index to detect the onset, and severity, of water stress in mature ‘Arbequina’ olive trees with heavy fruit load. They also suggest a potential of D MXTD for scheduling low frequency deficit irrigation strategies. [Copyright &y& Elsevier]

The continuous monitoring of crop water status is key to the sustainable management of water stress situations. Two deficit irrigation (DI) treatments were studied during the maximum evapotranspirative demand period in an orange orchard (Citrus sinensis (L.) Osb. cv. Navelina): sustained deficit irrigation irrigated at 55% crop evapotranspiration (ETC), and low-frequency deficit irrigation treatment,inwhich the plants were irrigated according to stem water potential atmidday (ψ[sub stem]). Additionally, a control treatment irrigated at 100% of ETC was established. The daily canopy temperature (TC) was measured with an infrared thermometer camera together with measurements of trunk diameter fluctuations (TDF), ψ[sub stem] and stomatal conductance (gS). The time courseofallphysiological parameters andtheir relationships were analysed, confirming that canopy air temperature differential (T[sub C] -T[sub a]) variations and TDF are suitable approaches for determining the water stress. In addition, the maximum daily shrinkage (MDS) and T[sub C] - T[sub a] showed high sensitivity to water stress in comparison to ψ[sub stem] and gS. Significant relationships were found among MDS and T[sub C] -T[sub a] with ψ[sub stem] andgS, for monitoring the crop water status by means of MDS vs ψ[sub stem] and T[sub C] - Tva] vs ψ[sub stem]. Thus, the combination of these techniques would be useful for making scheduling decisions on irrigation in orchards with high variability in plant water stress. [ABSTRACT FROM AUTHOR]

The dendrometer has been proposed as a sensitive plant water indicator based on stein growth. However, studies including dendrometers have been mainly focused on fruit trees and less attention has been paid to ornamental shrubs (small plants). In the study described here, stem dendrometers were used to ascertain whether there is any relationship between water status and dendrometric indices in potted ornamental shrubs (1 to 2 cm diameter). For this purpose, three Mediterranean shrubs (Pittosporum tobira, Callistemon citrinus, and Rhamnus alaternus) were studied under water stress recovery conditions in winter, spring, and early summer. At the end of the experiment, an extreme water stress treatment, which resulted in plant death (August) was also studied. Stem diameter variations [maximum and minimum daily stein values (MXDS and MNDS, respectively), maximum daily shrinkage (MDS), and stein growth rate (SGR)], daily evapotranspiration (daily plant ET), and leaf water potential (Ψleaf) parameters were considered throughout the experiment. A regression analysis between dendrometric indices and daily plant ET showed that MXDS and MNDS were sensitive under water stress recovery conditions, especially in severe environmental conditions (spring and summer). The SGR in C. citrinus, the MDS in P. tobira, and both indices in R. alaternus were seen to be sensitive during the stress to death period. Although more studies are needed, the results confirm that the use of dendrometers in small plants may be useful to provide continuous and automated registers of the plant water status under different substrate water content and climatic conditions. However, the response of these indices may imply moderate water stress. [ABSTRACT FROM AUTHOR]

Abstract: Trunk diameter fluctuations (TDFs) have been suggested as an irrigation-scheduling tool for several fruit trees, but the works in olive trees has not obtained successful results with any of the indicators (maximum daily shrinkage (MDS) and trunk growth rate (TGR)) that are calculated from the daily TDF curves. No studies of olive trees have ever used reference trees to reduce the influence of the environment, as in work for other fruit trees. In this work, we compare different continuous and discrete water status measurements in a drought cycle. We suggest the calculation of a new and related indicator (DTGR), the difference between the TGR of stressed trees, and the TGR of reference trees. Negative DTGR values always indicate water stress conditions. The current work describes the variations of this new indicator (DTGR) in relation to water stress, and compares DTRG to the midday stem water potential, maximum leaf conductance and to the MDS. The midday stem water potential and the maximum leaf conductance describe the stress cycle clearer than the trunk diameter fluctuation indicators. No significant differences were found in the values of MDS between stressed and reference trees. On the other hand, the DTGR pattern values were near that of the stem water potential, though positive values were recorded in some dates during the water stress cycle. These variations indicate that DTGR is not a cumulative water stress indicators, as is water potential. Therefore, according to our data, water potential is a better indicator than the TDF parameters when no deficit irrigation scheduling is performed in olive trees. DTGR seems to be a good indicator of water stress from a threshold value around −1.4MPa in olive trees. In addition, higher variability of DTGR than stem water potential may also be reduced with the increase in the number of sensors. [ABSTRACT FROM AUTHOR]

Abstract: Precise irrigation is essential in arid and semi-arid areas where water is scarce. This has impelled the scientific community to develop new technologies for scheduling irrigation. Of these, the ones relying on plant-based water-stress indicators have been found to have the greatest potential. Thus, measurements of stem water content, canopy temperature, sap flow, and stem diameter variation (SDV), among other variables, have proved useful not only for research purposes, but also for precise irrigation scheduling in commercial orchards. In this work we focus on the use of SDV records for irrigation scheduling. Of those mentioned above, this is the water-stress indicator that has received most attention from the scientific community, in terms of its potential for irrigating commercial orchards. Apart from being capable of an early detection of water stress, even if this is mild, SDV can be continuously and automatically recorded. This is a clear advantage over conventional indicators such as stem water potential (Ψ stem). Among the SDV-derived indices that are useful for scheduling irrigation, the maximum daily shrinkage (MDS) and stem growth rate (SGR) are the most widely used. For young trees, and in periods of rapid stem growth, SGR could be a better indicator than MDS. In periods of negligible growth, however, SGR cannot be used as an indicator of plant water stress. Considerable differences in both MDS and SGR as a function of crop load have been reported for some species. It has been found, that SDV outputs are affected by seasonal growth patterns, crop load, plant age and size, and other factors, apart from water stress. Thus, expert interpretation of SDV records is required before using them for scheduling irrigation, which limits their potential for automating the calculation of the irrigation dose. For some species, the MDS vs Ψ stem relationships show diurnal hysteresis and seasonal changes. Some relationships also shown an increase of MDS as the plant water potential fell to a certain value, after which MDS decreases as the plant water potential became more negative. This has been reported for peach, lemon, grapevine and olive, among other species. Although SDV-derived indices show a high plant-to-plant variability, in most cases the signal intensity is high enough to achieve an acceptable sensitivity, which, for peach, lemon and pepper has been found to greater than that of Ψ stem and leaf conductance (g l). In plum, apple and grapevine, however, Ψ stem is more sensitive than MDS and SGR. In any case, the usefulness of an SDV-derived index for irrigation scheduling must be evaluated for the orchard conditions. In this work we describe the qualities that must be considered in such evaluation. One of them, the signal intensity, is being successfully used to schedule low-frequency irrigation in orchards of a variety of species, for both full- and deficit-irrigation treatments. When combined with aerial or satellite imaging, SDV measurements are useful for scheduling irrigation in large orchards with high crop-water-stress variability. [Copyright &y& Elsevier]

Different irrigation effects on stem radius variation (ΔR) and maximum daily shrinkage (MDS) in Populus deltoides ‘Dvina’ and Populus×canadensis ‘I-214’ were studied to assess differences in drought tolerance between clones. One-year-old trees growing in concrete tanks were submitted to two irrigation regimes (natural rainfall and irrigation) from 24 June to 10 August, and ΔR was monitored by automatic point dendrometers. Independently of the irrigation regime, ‘Dvina’ showed a higher stem radial increment than ‘I-214’. In both clones, the first response to changed soil water content was a significant increase in MDS, whilst ΔR decreased about 20 d later when pre-dawn leaf water potential (Ψpd) dropped below –0.4 MPa. However, they displayed different strategies to overcome drought. ‘Dvina’ maintained a positive ΔR for longer than ‘I-214’, which had lower leaf Ψpd and greater leaf abscission at the end of the drought period. After irrigation resumed, ‘Dvina’ showed a higher capacity to restore stem growth. ‘I-214’ was probably unable to recover secondary growth because of higher leaf abscission during drought stress and the production of newly expanded leaves during recovery. It is concluded that the larger radial growth of ‘Dvina’ derived from a better water use (carbon uptake versus water loss) than ‘I-214’ under limited water availability. [ABSTRACT FROM PUBLISHER]

Recovery of water status in water-stressed pistachio trees (Pistacia vera L. cv. Kerman) was investigated by subjecting trees to regulated deficit irrigation (RDI) (60% of crop evapotranspiration rate, ETc) during stages I and II of fruit development (FD) followed by full irrigation during FD stage III (kernel-filling). Trees irrigated at 100% ETc throughout FD stages I, II and III served as controls. Water-stress severity was characterized by changes in soil water content and midday stem water potential (Ψmd). Midday leaf conductance (g1) and trunk diameter variation (TDV) were also measured. In RDI trees, the lowest Ψmd value, −1.8 MPa, occurred at the end of the RDI period. The corresponding value for the control trees was around −1.1 MPa. Although the RDI treatment affected gas exchange later than Ψmd, the greatest reductions in gas exchange (60% of control values) also appeared at the end of the RDI period. There were significant differences in TDV between control and RDI trees at the end of the RDI period. Although plant water status recovered within 20 days of resuming irrigation, the TDV values indicated a longer period might be necessary for complete recovery. Recovery of g1 was faster than that of Ψmd, although differences in TDV between control and RDI trees indicated that gas exchange recovered later than Ψmd. The slow recovery of pistachio trees during FD stage III from water stress imposed during FD stages I and II suggests that irrigation should exceed 100% ETc during FD stage III or that more extensive irrigation should commence before the end of FD stage II. [ABSTRACT FROM PUBLISHER]

Radial stem size changes, measured with automated dendrometers at intra-daily resolution, offer great potential to link environmental conditions with tree physiology at the seasonal scale. Such measurements need to be time-aligned, cleaned of outliers and shifts, gap-filled and analysed for reversible (water-related) and irreversible (growth-related) fractions to obtain physiologically meaningful data. Therefore, comprehensive tools are needed for reproducible data processing and analytics of dendrometer data. Here we present a transparent method, compiled in the R package treenetproc, to turn raw dendrometer data into clean, physiologically interpretable information, i.e., stem growth, tree water deficit, growth phenological phases, mean daily shrinkage and their respective timings. The removal of errors is facilitated by additional functions and supported with graphical visualizations. To ensure reproducible data handling, the processing parameters and induced changes to the raw data are documented in the output and, thus, are a step towards a standardized processing of automatically measured stem radius time series. We discuss examples, such as the seasonality of growth or the dependence of growth on atmospheric and soil drought. The presented growth and water-related physiological variables at high temporal resolution offer novel physiological insights into the seasonally varying responses of trees to changing environmental conditions. [ABSTRACT FROM AUTHOR]

Key message: Patterns of stem radial variations showed thatCedrus libaniA. Rich. was less limited by summer drought than co-occurringJuniperus excelsaM. Bieb.Cedrus libanirecovered faster from tree water deficit and showed significantly higher radial growth rates and annual stem increments thanJ. excelsa. However, the ability ofJ. excelsato grow more hours per year may indicate a potential benefit in more extreme conditions. Context: Knowledge about species-specific drought responses is needed to manage productive forests in drought prone areas. Under water shortage, trees commonly show stem shrinkage, which is assumed to inhibit growth. Aims: We investigated whether the two co-existing conifers Juniperus excelsa M. Bieb. and Cedrus libani A. Rich. (growing at the Taurus Mountains, SW-Turkey) show differences in water relations and stem growth in order to evaluate their respective drought tolerance. Methods: Stem radius changes were hourly monitored over 2 years using high-resolution point dendrometers. Radial stem growth, tree water deficit-induced stem shrinkage, and maximum daily shrinkage were extracted from stem radius change measurements, investigated for their patterns, and related to environmental conditions. Results: Cedrus libani recovered from tree water deficit under higher temperature and vapor pressure deficit than J. excelsa. The number of hours during which stem growth occurred was higher for J. excelsa; however, growth rates and annual increments were significantly lower than in C. libani. Both species showed highest maximum daily shrinkage during the driest months indicating the ability to maintain gas exchange all year round. Conclusion: Juniperus excelsa showed a more conservative growth strategy while C. libani was less limited by summer drought and showed more annual stem increment under the conditions investigated. [ABSTRACT FROM AUTHOR]

• The seasonal growth pattern of young Populus tomentosa tree was identified. • Stage II (mid-April to mid-June) is the crucial stage for irrigation management. • Maximum growth rate occurred earlier in the well-watered trees. • The trunk growth rate is an effective indicator for assessing tree water status. High resolution measurements of trunk diameter fluctuations (TDF) provide a means to study short-term and long-term stem radial variations and response to water stress of fruit trees. There is a lack of information, however, on the performance of TDF measurements on fast-growing poplar trees. To both clarify the intra-annual and inter-annual growth patterns and to diagnose water status of young poplar trees, we monitored TDF, leaf area index, soil matric potential, midday stem water potential (Ψ stem) and main meteorological variables in a plantation with 3- and 4-year-old Populus tomentosa trees over two growing seasons (2016 and 2017). We had two irrigation treatments, well-watered (WW) and water-stressed (WS). For the first time for this species, we identified accurately and quantitatively the seasonal growth pattern and associated parameters (growth initiation and cessation time, growing season duration, maximum growth rate, etc.) of young P. tomentosa with four clear stages, based on the growth rate for both treatments. We identified that stage II (mid-April to mid-June), when trees showed the most active growth rate and less rainfall was recorded, is the most crucial stage for irrigation management, especially as trees grow larger and have a greater water demand. In the two experimental years, irrigation did not affect the timing of growth initiation and cessation. However, maximum growth rate was significantly higher and occurred earlier in the WW trees than in the WS trees. From the tested trunk diameter fluctuation indicators, trunk growth rate (TGR) showed a greater potential for assessing tree water stress than both maximum daily shrinkage (MDS) and daily growth (DG), being even more sensitive to the onset of water stress than midday Ψ stem. We conclude that high resolution trunk dendrometers are useful to track growth and determine water stress, which make them a good candidate to schedule irrigation in fast-growing poplar plantations. [ABSTRACT FROM AUTHOR]