Maximum diurnal trunk shrinkage is a sensitive indicator of plant water, stress in Diospyros kaki (Persimmon) trees

Persimmon tree (Diospyros kaki L.f.) is a deciduous fruit tree included in the so-called group of minor fruit tree species. Worldwide, it is not widely grown but, nowadays, Kaki culture is of some importance in the south-east of Spain because of the high fruit commercial value. Currently, neither it is known about Kaki trees water needs, nor crop responses to the irrigation regime. The objective of the present research was to assess the feasibility of using maximum diurnal trunk shrinkage (MDS) as a plant water stress indicator for Kaki trees. During two drought cycles, in trees under either full or deficit irrigation, the MDS obtained by means of LVDT sensors was compared with a reference indicator of fruit trees water status, the midday stem water potential (Ψ_stem). In addition, stomatal conductance and fruit diameter variations were also followed. As water restrictions began, there was an immediate increase in MDS, in correspondence with a decrease in Ψ_stem. Pooling data from both drought cycles and irrigation regimes, MDS and Ψ_stem were linearly correlated (r² = 0.77***). The magnitude of differences between well watered and deficit irrigated trees was much larger in the case of MDS than for Ψ_stem. However, the tree-to-tree variability of the MDS readings was three times higher than for Ψ_stem; average coefficient of variation of 14% and 38% for Ψ_stem and MDS, respectively. Overall, results reported indicated that MDS is a sensitive indicator of Kaki water status and it can be further used as an irrigation scheduling indicator for optimum irrigation management of this crop. However, the large MDS tree-to-tree variability should be taken into account when selecting the number of trees to monitor within an orchard.     

Usefulness of trunk diameter variations as continuous water stress indicators of pomegranate (Punica granatum) trees

Pomegranate trees (Punica granatum L.) is a deciduous fruit tree included in the so-called group of minor fruit tree species, not widely grown but of some importance in the south east of Spain. Pomegranate trees are considered as a culture tolerant to soil water deficit. However, very little is known about pomegranate orchard water management. The objective of this research was to asses the feasibility of using trunk diameter variation (TDV) indexes, obtained by means of LVDT sensors, as a plant water stress indicators for pomegranate trees. The experiment was carried out with mature trees grown in the field under three irrigation regimes: control well watered trees; trees continuously deficit irrigated at 50% of the control regime (SDI); and trees that had a summer water stress cycle being irrigated at 25% of the control rates only in July and August (RDI). The seasonal variations of maximum diurnal trunk shrinkage (MDS) and trunk growth rates (TGR) were compared with midday stem water potential (Ψstem) measurements. During the course of the entire season, control trees maintained lower MDS values than the SDI ones. In the RDI treatment, as water restrictions began, there was a slow increase in MDS, in correspondence with a decrease in Ψstem. When water was returned at full dosage, the RDI quickly recovered to MDS and Ψstem values similar to the control. However, lower MDS for a given Ψstem values were observed as the season advanced. The magnitude of differences between well watered and deficit irrigated trees was much larger in the case of MDS than for Ψstem. However, the tree-to-tree variability of the MDS readings was more than four times higher than for Ψstem; average coefficient of variation of 7.5 and 36% for Ψstem and MDS, respectively. On the other hand, TGR did not clearly reflect differences in tree water status. Overall, results reported indicated that MDS is a good indicator of pomegranate tree water status and it can be further used for managing irrigation. However, the seasonal changes in the MDS-Ψstem relationship should be taken into account when attempting to use threshold MDS values for scheduling irrigation.     

Determining reference values for stem water potential and maximum daily trunk shrinkage in young apple trees based on plant responses to water deficit

The use of plant water status indicators such as midday stem water potential (Ψ_stem) and maximum daily trunk shrinkage (MDS) in irrigation scheduling requires the definition of a reference or threshold value, beyond which irrigation is necessary. These reference values are generally obtained by comparing the seasonal variation of plant water status with the environmental conditions under non-limiting soil water availability. In the present study an alternative approach is presented based on the plant’s response to water deficit. A drought experiment was carried out on two apple cultivars (Malus domestica Borkh. ‘Mutsu’ and ‘Cox Orange’) in which both indicators (Ψ_stem and MDS) were related to several plant physiological responses. Sap flow rates, maximum net photosynthesis rates and daily radial stem growth (DRSG) (derived from continuous stem diameter variation measurements) were considered in the assessment of the approach. Depending on the chosen plant response in relationship with Ψ_stem or MDS, the obtained reference values varied between −1.04 and −1.46 MPa for Ψ_stem and between 0.17 and 0.28 mm for MDS. In both cultivars, the approach based on maximum photosynthesis rates resulted in less negative Ψ_stem values and smaller MDS values, compared to the approaches with sap flow and daily radial stem growth. In the well-irrigated apple trees, day-to-day variations in midday Ψ_stem and MDS were related to the evaporative demand. These variations were more substantial for MDS than for midday Ψ_stem.     

Sensitivity of root pruned ‘Conference’ pear to water deficit in a temperate climate

The present study examines the need for irrigation in pear trees (Pyrus Communis, cv. ‘Conference’) under low evaporative demand conditions, like in Belgium, in order to maintain a consistent fruit yield and high fruit size. To determine the sensitivity of the pear yield under low evaporative demand conditions three different orchards were monitored. The study shows that a Ψ_soil of −60 kPa during shoot growth has no effect on fruit yield but lower Ψ_soil values induced a decline in both fruit size and total yield. Just as for arid environments a Ψ_stem of −1.5 MPa is related to negative yield responses. In dry conditions lower Ψ_soil and Ψ_stem values were observed in root pruned trees compared to not root pruned trees in the same irrigation treatment, however without yield decline. In one orchard a biannual bearing tendency was observed after root pruning. Furthermore intensive Ψ_soil measurements show a high variation in Ψ_soil between orchards, and within an orchard. This underlines the need for irrigation management on a parcel level and the need for new irrigation scheduling techniques which take the spatial variation in the orchard into account.     

Evaluation of plant-based water status indicators in mature apple trees under field conditions

The performance of different indicators of plant water status as a tool for irrigation management was evaluated in mature field grown ‘Golden Delicious’ apple trees during the late summer of 1998. Control (C) and stress (S) treatments were studied. In the C treatment trees were irrigated daily at 100% ET_c whereas in the S treatment water was withheld during 31 days (DOY’s 236–266). Predawn water potential (Ψ_pd) and midday stem water potential (Ψ_stem) were measured several times a week during the experimental period. Three daily measurements of stomatal conductance (g_s) and stem water potential were made during five consecutive days in mid-September. Trunk diameter changes (TDC) were recorded by LVDT sensors, and from these measurements, maximum daily shrinkage (MDS), daily growth (DG), and cumulative growth (CG) were calculated. Midday Ψ_stem showed the best ratio between the response to moderate water stress and tree variability (“signal/noise” ratio) among the indicators studied here, followed closely by Ψ_pd. On the other hand, the poorest water status indicator was g_s. Due to the low trunk growth rate of the trees, and its high variability, DG and CG were not adequate indicators. MDS showed a lower sensitivity to water stress and a higher variability (CV = 0.19) than midday Ψ_stem (CV = 0.08) and Ψ_pd (CV = 0.10). However, MDS correlated well with ET₀ and with midday Ψ_stem (R ² = 0.79) thus, making this parameter an interesting and promising tool for irrigation management in apple orchards. More research needs to be done in order to define reference values for MDS and plant water potential indicators, in relation to evaporative conditions and in different phenological periods, and to quantify the relationship between water status indicators values and apple tree yield and fruit quality.     

Sustainability of regulated deficit irrigation in a mid-maturing peach cultivar

The level of irrigation restriction to apply in a deficit irrigation (DI) programme for sustainable peach (‘Baby gold 6’) production was investigated. The experiment involved four irrigation treatments over five consecutive seasons (2007–2011). They were full irrigation (control), reducing irrigation by 20 % during the first half of stage III (DI-80 %), withholding irrigation until reaching a light stress level (DI-L) and withholding irrigation until reaching a moderate stress level (DI-M). The withholding of irrigation in both DI-L and DI-M was applied only during stage II and postharvest periods and was based on midday stem water potential thresholds (Ψ _stem). For the DI-L treatment ?1.5 MPa was used in both periods, and for DI-M ?1.8 and ?2.0 MPa were used during stage II and postharvest, respectively. Average Ψ _stem values during DI periods were approximately ?1.4 and ?1.2 MPa for DI-M and DI-L, respectively. The pre-defined thresholds required to trigger irrigation were rarely reached. No significant differences between treatments were found in terms of yield in any experimental year. However, DI-M and to a lesser extent DI-L had lower final fruit fresh mass at harvest related to lower Ψ _stem after three consecutive years of the experiment (during 2010 and 2011). Therefore, in terms of fruit size, DI was not sustainable. Rather than lowering Ψ _stem thresholds, we recommend discontinuing DI after 3-year application.     

Using midday stem water potential for scheduling deficit irrigation in mid–late maturing peach trees under Mediterranean conditions

Irrigation techniques that reduce water applications are increasingly applied in areas with scarce water resources. In this study, the effect of two regulated deficit irrigation (RDI) strategies on peach [Prunus persica (L.) Batsch cv. “Catherine”] performance was studied over three growing seasons. The experimental site was located in Murcia (SE Spain), a Mediterranean region. Two RDI strategies (restricting water applications at stage II of fruit development and postharvest) based on stem water potential (Ψ_s) thresholds (?1.5 and ?1.8 MPa during fruit growth and ?1.5 and ?2.0 MPa during postharvest) were compared to a fully irrigated control. Soil water content (θ_v), Ψ_s, gas exchange parameters, vegetative growth, crop load, yield and fruit quality were determined. RDI treatments showed significantly lower values of θ_v and Ψ_s than control trees when irrigation water was restricted, causing reductions in stomatal conductance and photosynthesis rates. Vegetative growth was reduced by RDI, as lower shoot lengths and pruning weights were observed under those treatments when compared to control. However, fruit size and yield were unaffected, and fruit quality was slightly improved by RDI. Water savings from 43 to 65 % were achieved depending on the year and the RDI strategy, and no negative carryover effect was detected during the study period. In conclusion, RDI strategies using Ψ_s thresholds for scheduling irrigation in mid–late maturing peach trees under Mediterranean conditions are viable options to save water without compromising yield and even improving fruit quality.     

Early morning fluctuations in trunk diameter are highly sensitive to water stress in nectarine trees

The sensitivity to water stress of different plant water status indicators was evaluated during two consecutive years in early nectarine trees grown in a semi-arid region. Measurements were made post-harvest and two irrigation treatments were applied: a control treatment (CTL), irrigated at 120 % of crop evapotranspiration demand to achieve non-limiting water conditions, and a deficit irrigation treatment, that applied around 37 % less water than CTL during late postharvest. The plant water status indicators evaluated were midday stem water potential (Ψ _stem) and indices derived from trunk diameter fluctuations: maximum daily shrinkage (MDS), trunk daily growth rate, early daily shrinkage measured between 0900 and 1200 hours solar time (EDS), and late daily shrinkage that occurred between 1200 hours solar time and the moment that minimum trunk diameter was reached (typically 1600 hours solar time). The most sensitive [highest ratio of signal intensity (SI) to noise] indices to water stress were Ψ _stem and EDS. The SI of EDS was greater than that of Ψ _stem, although with greater variability. EDS was a better index than MDS, with higher SI and similar variability. Although MDS was linearly related to Ψ _stem down to ?1.5 MPa, it decreased thereafter with increasing water stress. In contrast, EDS was linearly related to Ψ _stem, although the slope of the regression decreased as the season progressed, as in the case of MDS. Further studies are needed to determine whether EDS is a sensitive index of water stress in a range of species.     

Assessment of trunk diameter variation derived indices as water stress indicators in mature olive trees

Plant age and size, seasonal growth patters and crop load, among other factors, have been reported to decrease the usefulness of trunk diameter variation (TDV) derived indices as water stress indicators in olive trees. Our hypothesis, however, is that indices derived from TDV records in old, big olive trees are sensitive enough to detect levels of water stress in trees of orchards under deficit irrigation that, although severe, are below the threshold for fruit shrivelling. This is of importance for the production of good quality oils, since fruit shrivelling may affect oil quality. The aim of this work was to assess different TDV-derived indices as water stress indicators in 40-year-old ‘Manzanilla’ olive trees with heavy crop load. We derived the maximum daily shrinkage (MDS), daily growth (DG) and daily recovery (DR) from TDV records taken during the 2008 dry season both in well-irrigated FAO trees and in deficit-irrigated RI trees. Measurements of volumetric soil water content (θ_v), leaf water potential (Ψ_l), stomatal conductance (g_s), net CO₂ assimilation rate (A), water and oil accumulation in the fruits and yield parameters were made for both treatments. The trunks did not grow during the experimental season, either in the FAO or RI trees, likely because of the heavy crop load. Therefore, DG was useless as water stress indicator. For MDS and DR, which were responsive to the increase of the trees’ water stress, we calculated the variability, quantified by the coefficient of variation (CV), the signal intensity (S_I) and the sensitivity (S_I/CV) values. In addition, we derived reference equations for irrigation scheduling from the relationships between MDS values in the FAO trees and main meteorological variables. Values both of S_I-MDS and S_I-DR were steady until September 9, despite of increasing differences in θ_v between treatments from early in the dry season. The Ψ_l vs θ_v values showed an outstanding capacity of the RI trees to take up water from the drying soil, and the Ψ_l vs g_s values showed a near-isohydric behaviour of those deficit-irrigated trees. These results explain, at least in part, the lack of response of MDS and DR on that period. Both S_I-MDS and S_I-DR peaked for the first time on September 9, 16 days before the appearance of fruit shrivelling. Our results suggest that using TDV-derived indices as water stress indicators for irrigation scheduling in old olive orchards with medium to low plant densities, i.e. with large root zones, may be useless in case the irrigation strategy is aimed at keeping the soil close to field capacity. Nevertheless, the MDS and DR indices may be useful indicators for the avoidance of fruit shrivelling in deficit irrigated olive orchards for the production of good quality oil. Reliable reference equations for scheduling irrigation with the signal intensity approach were obtained from the regression of MDS values vs the daily maximum values of both the air temperature and the vapour pressure deficit of the air.     

Isohydrodynamic behavior in deficit-irrigated Cabernet Sauvignon and Malbec and its relationship between yield and berry composition

Cabernet Sauvignon and Malbec grapevines were irrigated at 70 or 23 % of estimated crop evapotranspiration throughout berry development over four growing seasons. Stomatal behavior was characterized by relating predawn leaf water potential and mid-morning stomatal conductance to mid-morning leaf water potential. Seasonal average weekly midday leaf water potential was lower in Cabernet Sauvignon than Malbec despite similar irrigation amounts. Both cultivars exhibited anisohydric behavior with midday leaf water potential decreasing linearly with declining predawn leaf water potential (r ² = 0.51) and stomatal conductance (r ² = 0.42). However, both cultivars utilized hydrodynamic mechanisms to maintain a soil-to-leaf water potential gradient of ?0.62 (±0.03) MPa under standard irrigation and ?0.75 (±0.04) MPa under reduced irrigation. Berry fresh weight and titratable acidity decreased, and the concentration of total anthocyanins increased in both cultivars in response to decreases in midday leaf water potential. The slope of regression equations for seasonal mean midday leaf water potential was used to estimate cultivar-specific levels of water stress associated with changes in berry weight and berry composition at fruit maturity.     

Effects of different irrigation regimes on a super-high-density olive grove cv. “Arbequina”: vegetative growth,productivity and polyphenol content of the oil

The effects of multiple irrigation regimes on the relationships among tree water status, vegetative growth and productivity within a super-high-density (SHD) “Arbequina” olive grove (1950 tree/ha) were studied for three seasons (2008–2010). Five different irrigation levels calculated as percentage of crop irrigation requirement using FAO procedures (Allen et al. in Crop evapotranspiration. Guidelines for computing crop water requirements. Irrigation and drainage paper 56. FAO, Rome, 1998) were imposed during the growing season. Periodically during the growing season, daytime stem water potential (Ψ _STEM), inflorescences per branch, fruits per inflorescence and shoot absolute growth rate were measured. Crop yield, fruit average fresh weight and oil polyphenol content were measured after harvest. The midday Ψ _STEM ranged from ?7 to ?1.5 MPa and correlated well enough with yield efficiency, crop density and fruit fresh weight to demonstrate its utility as a precise method for determining water status in SHD olive orchards. The relationships between midday Ψ _STEM and the horticultural parameters suggest maintaining Ψ _STEM values between ?3.5 and ?2.5 MPa is optimal for moderate annual yields of good quality oil. Values below ?3.5 MPa reduced current season productivity, while values over ?2.5 MPa were less effective in increasing productivity, reduced oil quality and produced excessive crop set that strongly affected vegetative growth and fruit production the following season. On the basis of the result given here, irrigation scheduling in the new SHD orchards should be planned on a 2-year basis and corrected annually based on crop load. Collectively, these results suggest that deficit irrigation management is a viable strategy for SHD olive orchards.     

Response of citrus trees to deficit irrigation during different phenological periods in relation to yield, fruit quality, and water productivity

Four strategies of deficit irrigation based on a different water-stress ratio (WSR) applied in each phenological stage, and a control treatment were implemented in 11-year-old citrus trees (Citrus sinensis L. Osb. Cv. Navelina) grafted onto carrizo citrange (C. sinensis L. Osb. × Poncirus Trifoliata L. Osb.). The midday stem-water potential and stomatal conductance were measured during the periods considered, and these parameters were used to estimate the plant-water status. Integrated stem-water potential (Ψ_Int) and integrated stomatal conductance (g_Int) were calculated for all treatments and used as a water-stress indicator for the crop. Reference equations were formulated to quantify the relations between water-stress indicators (WSR, Ψ_Int, g_Int) and the crop response, expressed as yield, yield components, and fruit-quality parameters under limited seasonal water availability. Significant differences in yield were found in the second year of experiment between the stressed treatments and control, although these differences were evident during the first year. The main effects were detected in treatments with a water-sever stress applied during the flowering and fruit-growth phases. When this degree of stress was applied during the maturity phase, it was reflected mainly in fruit-quality parameters (total soluble solids, and titrable acidity). These results lead to the conclusion that, in mature orange trees, deficit irrigation affects yield and fruit quality, while enabling water savings of up to 1000 m³ ha⁻¹. Therefore, yield declined on average 10-12% but boosted water productivity 24% with respect to the fully irrigated treatment. Regarding the water-stress indicators used, Ψ_Int and g_Int showed highly significant correlations with the yield and fruit-quality parameters.     

Evapotranspiration, crop coefficient and growth of two young pomegranate (Punica granatum L.) varieties under salt stress

Pomegranate (Punica granatum L.) is a drought-hardy crop, suited to arid and semi-arid regions, where the use of marginal water for agriculture is on the rise. The use of saline water in irrigation affects various biochemical processes. For a number of crops, yields have been shown to decrease linearly with evapotranspiration (ET) when grown in salt-stressed environments. In the case of pomegranate, little research has been conducted regarding the effect of salt stress. Our study focused on the responses of ET, crop coefficient (K_c) and growth in pomegranate irrigated with saline water. Experiments were conducted using lysimeters with two varieties of pomegranate, P. granatum L. vars. Wonderful and SP-2. The plants were grown with irrigation water having an electrical conductivity (EC_iw) of 0.8, 1.4, 3.3, 4.8 and 8 dS m⁻¹. Plants were irrigated with 120% of average lysimeter-measured ET. Seasonal variation in ET, crop coefficient (K_c) and growth were recorded. Variation in daily ET was observed 1 month after initiation of the treatments. While significant seasonal ET variation was observed for the EC-0.8 treatment, it remained more stable for the EC-8 treatment. Salinity treatment had a significant effect on both daily ET (F = 131, p < 0.01) and total ET (F = 112.68, p = 0.001). Furthermore, the electrical conductivity of the drainage water (EC_dw) in the EC-8 treatment was five times higher than that of the EC-0.8 treatment in the peak season. Fitting the relative ET (ET_r) to the Maas and Hoffman salinity yield response function showed a 10% decrease in ET per unit increase in electrical conductivity of the saturated paste extract (EC_e) with a threshold of 1 dS m⁻¹. If these parameters hold true in the case of mature pomegranate trees, the pomegranate should be listed as a moderately sensitive crop rather than a moderately tolerant one. Fitting 30-day interval ET_r data to the Maas and Hoffman salinity yield response function showed a reduction in the slope as the season progressed. Thus using a constant slope in various models is questionable when studying crop-salinity interactions. In addition, both of the varieties showed similar responses under salt stress. Moreover, the calculated value of K_c is applicable for irrigation scheduling in young pomegranate orchards using irrigation water with various salinities.     

Effects of grapevine (Vitis vinifera L.) water status on water consumption, vegetative growth and grape quality: An irrigation scheduling application to achieve regulated deficit irrigation

Precision irrigation in grapevines could be achieved using physiologically based irrigation scheduling methods. This paper describes an investigation on the effects of three midday stem water potential (midday Ψ_S) thresholds, imposed from post-setting, over water use, vegetative growth, grape quality and yield of grapevines cv. Cabernet Sauvignon. An experiment was carried out on a vineyard located at the Isla de Maipo, Metropolitana Region, Chile, throughout the 2002/03, 2003/04 and 2004/05 growing seasons. Irrigation treatments consisted in reaching the following midday Ψ_S thresholds: −0.8 to −0.95 MPa (T1); −1.0 to −1.2 MPa (T2) and −1.25 to −1.4 MPa (T3) from post-setting to harvest. Results showed significant differences in grape quality components among treatments and seasons studied. In average, T3 produced smallest berry diameter (6% reduction compared to T1), high skin to pulp ratio (13% increment compared to T1) and significant increments in soluble solids and anthocyanins. Improvements in grape quality attributes were attributed to mild grapevine water stress due to significant reductions in water application (46% for T2 and 89% for T3 less in average, both compared to T1). This study found significant correlations between midday Ψ_S and berry quality components, no detrimental effects on yield by treatments were found in this study. This research proposes a suitable physiological index and thresholds to manage RDI and irrigation scheduling on grapevines to achieve high quality grapes on mild water stress conditions.     

Influence of crop load on maximum daily trunk shrinkage reference equations for irrigation scheduling of early maturing peach trees

The effects of high crop load (unthinned trees, 22-23 fruits cm⁻² of trunk cross-sectional area (TCSA)), commercial crop load (3-4 fruits cm⁻² of TCSA), and no crop load (all fruitlets removed) on maximum daily trunk shrinkage (MDS), trunk growth rate (TGR) and stem water potential (Ψ_stem) were studied during the fruit growth period and 20 days following harvest in fully irrigated early maturing peach trees, Prunus persica (L.) Batsch, cv. Flordastar. Even though crop load did not affect plant water status, the MDS and TGR values increased and decreased, respectively, as a result of the crop load effect. In this sense, for the same Ψ_stem value, there was a linear increase in MDS with crop load, with a slope of 6.6 μm MPa⁻¹ per unit of crop load increment. The effects of environmental conditions on daily MDS values were also dependent on crop load, suggesting that MDS reference values should be obtained by representing the relations between MDS and the climatic variables (daily mean air temperature, daily mean vapour pressure deficit and daily crop reference evapotranspiration) for a given crop load. The constancy of the relation between MDS and Ψ_stem across crop load underlined the constancy of the elastic properties of the bark tissues.     

Deficit irrigation without reducing yield or nut splitting in pistachio (Pistacia vera cv Kerman on Pistacia terebinthus L.)

An irrigation experiment involving pistachio (Pistacia vera cv Kerman on Pistacia terebinthus L. rootstocks) was performed over a four-year period in central Spain to determine the effect of regulated deficit irrigation (RDI) on nut yield and quality. The growth season was divided into three phenological stages: stage I - from sprouting until the end of rapid nut growth; stage II - from maximum nut size until the beginning of kernel growth; and stage III - from the beginning of kernel growth until harvest. Control trees were irrigated to supply their full water needs throughout the growth season, except for the post-harvest period. Sustained deficit irrigation at 65% (DI₆₅) and 50% (DI₅₀) of control irrigation was provided to two other groups of trees. The RDI provided to a further group was designed to provide a stress period during stages I and II but no water stress during stage III; the aim was to reduce water use and increase the percentage of split nuts. A fifth group of trees was maintained under rain fed conditions. Water potential and leaf conductance were affected in the DI₆₅, DI₅₀ and rain fed treatments mainly during stages II and III, with midday water potentials below −2.0 MPa. The RDI trees were only significantly water stressed during stage II, showing midday water potentials of around −1.4 MPa. On most days, leaf conductance was not significantly affected in any of the irrigation treatments. The nuts of the DI₆₅ and DI₅₀ trees were smaller in diameter and their total yield was reduced compared to the controls. However, no significant differences in kernel dry weight were observed. The RDI trees showed a total yield and percentage of split nuts similar to those of the controls, even though they received around 20% less water. The split nut yield showed a linear relationship with crop evapotranspiration. However, since the percentage of split nuts was similar in all treatments this variation was likely related to the total yield. The RDI trees did not show the normal alternate bearing pattern (which was clearly maintained in the control trees). Early splitting, a process that decreases the quality of the yield, was not related to water status but to temperatures lower than 13 °C. The results suggest that P. terebinthus L. rootstocks confer P.vera scions a degree of drought-resistance, reducing the likelihood of water stress and, therefore, allowing more severe RDI scheduling.     

Response of Clementina de Nules citrus trees to summer deficit irrigation. Yield components and fruit composition

The effects of mid-summer regulated deficit irrigation (RDI) treatments were investigated on Clementina de Nules citrus trees over three seasons. Water restrictions applied from July, once the June physiological fruit drop had finished, until mid September were compared with a Control treatment irrigated during all the season to match full crop evapotranspiration (ET_c). Two degrees of water restrictions were imposed based on previous results also obtained in Clementina de Nules trees ( [Ginestar and Castel, 1996] and [González-Altozano and Castel, 1999]). During the RDI period, deficit irrigation was applied based on given reductions over the ET_c, but also taking into account threshold values of midday stem water potential (Ψ_s) of −1.3 to −1.5 MPa for RDI-1 and of −1.5 to −1.7 MPa for RDI-2. Results showed that water savings achieved in the RDI-2 treatment impaired yield by reducing fruit size. On the contrary, the RDI-1 strategy allowed for 20% water savings, with a reduction in tree growth but without any significant reduction in yield, fruit size nor in the economic return when irrigation was resumed to normal dose about three months before harvest. Water use efficiency (WUE) in the RDI trees was similar or even higher than in Control trees. RDI improved fruit quality increasing total soluble solids (TSS) and titratable acidity (TA). In conclusion, we suggest that the RDI-1 strategy here evaluated can be applied in commercial orchards not only in case of water scarcity, but also as a tool to control vegetative growth improving fruit composition and reducing costs associated with the crop management.     

Assessment of vineyard water status variability by thermal and multispectral imagery using an unmanned aerial vehicle (UAV)

The goal of this study was to assess the water status variability of a commercial rain-fed Tempranillo vineyard (Vitis vinifera L.) by thermal and multispectral imagery using an unmanned aerial vehicle (UAV). The relationships between aerial temperatures or indices derived from the imagery and leaf stomatal conductance (g _s) and stem water potential (Ψ_stem) were determined. Aerial temperature was significantly correlated with g _s (R ²?=?0.68, p?<?0.01) and Ψ_stem (R ²?=?0.50, p?<?0.05). Furthermore, the thermal indices derived from aerial imagery were also strongly correlated with Ψ_stem and g _s. Moreover, different spectral indices were related to vineyard water status, although NDVI (normalized difference vegetation index) and TCARI/OSAVI (ratio between transformed chlorophyll absorption in reflectance and optimized soil-adjusted vegetation index) showed the highest coefficient of determination with Ψ_stem (R ²?=?0.68, p?<?0.05) and g _s (R ²?=?0.84, p?<?0.05), respectively. While the relationship with thermal imagery and water status parameters could be considered as a short-term response, NDVI and TCARI/OSAVI indices were probably reflecting the result of cumulative water deficits, hence a long-term response. In conclusion, thermal and multispectral imagery using an UAV allowed assessing and mapping spatial variability of water status within the vineyard.     

Seasonal sensitivity of stem water potential to vapour pressure deficit in grapevine

Boundary lines of stem water potential (Ψ_stem) responses to vapour pressure deficit (VPD) have been reported for several species and are generally referred to as VPD reference lines (RL). In order to study the response of Ψ_stem to VPD, RLs were determined in plants that received full (Control) and deficit (SSDI) irrigation during three consecutive years. The Control plants received irrigation equivalent to full crop water evapotranspiration minus effective rainfall and the SSDI plants were irrigated at 50% of the Control level. Ψ_stem values for the Control treatment during crop development tended to decrease, and those corresponding to SSDI plants were always lower than those for Control plants. Considering the 3-year data set, no relationship was found between Ψ_stem and VPD. However, there was a differential seasonal response between Ψ_stem and VPD, which depended on the stage of fruit development. A separate phenological analysis enabled the detection of RL for stage II (from fruit-set to veraison) and for stage III (post-veraison), whereas during stage I (from bud-break to fruit set) RL was not apparent. RL slopes increased as the season progressed and were significantly correlated to average values of Ψ_stem. The seasonal decrease in midday Ψ_stem for Control plants was interpreted as being a result of a progressive increase in canopy size and water consumption, which led to increased water depletion before each afternoon’s daily irrigation event. The apparent lack of RL during stage I was related to lower levels of water demand and high Ψ_stem.     

Influence of deficit irrigation in phase III of fruit growth on fruit quality in ‘lane late’ sweet orange

The aim of this work was to apply one strategy of deficit irrigation (DI) to improve the final fruit quality in 10-year-old ‘Lane late’ sweet orange grafted on Carrizo citrange (Citrus sinensis L. Osb. × Poncirus trifoliata L.). The experiment was carried out over 2 years in an experimental orchard located in Torre Pacheco (Murcia, south-east Spain). The deficit irrigation treatment consisted of the stopping of irrigation in phase III of fruit growth (1st October-28th February). The irrigation cut-off in phase III reduced the midday stem water potential (Ψ_md), the plant water status being heavily influenced by rainfall. In both years, the DI treatment did not alter fruit yield although mean fruit weight was slightly reduced. The main effects of DI on the final fruit quality were increases of total soluble solids (TSS) and titratable acidity (TA) and a decrease of juice percentage without altering the final maturity index. Plant water-stress integral (S_Ψ) was correlated positively with TSS and TA and negatively with juice percentage. In conclusion, a DI strategy could be useful for improving the final content of TSS and the TA, therefore allowing the harvest to be delayed.