Response of young super-high density table olive orchard (Manzanilla de Sevilla) to different water stress levels considering an accurate determination of endocarp development

Response of young super-high density table olive orchard (Manzanilla de Sevilla) to different water stress levels considering an accurate determination of endocarp development Marta S´ anchez-Pi˜ nero a , Mireia Corell a,b , Alfonso Moriana a,b,* , David P´ erez-L´ opez c , Laura L. de Sosa b,d , Noemí Medina-Zurita a,b , Pedro Castro-Valdecantos a , María Jos´ e Martín-Palomo a,b a Agronomy Department, ETSIA, Universidad de Sevilla, Crta de Utrera Km 1, Sevilla E-41013, Spain b Unit associated to the CSIC de Uso Sostenible del Suelo y el Agua en la Agricultura (US-IRNAS), Crta de Utrera Km 1, Sevilla E-41013, Spain c Agricultural Production Department, CEIGRAM-Universidad Polit´ ecnica de Madrid, Av. Puerta de Hierro, 2, Madrid 28040, Spain d Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS-CSIC), Av. Reina Mercedes 10, Sevilla 41012, Spain ARTICLE INFO Keywords: Fruit size Inflorescence set Midday stem water potential Pit hardening Stress integral ABSTRACT Olive trees are extremely drought-resistant species. This particularity could be used to reduce water needs in regulated deficit irrigation scheduling. However, the yield response to water deficit is linked to the moment and the level of water stress imposed. Pit hardening has been commonly used as the stage to decrease irrigation, but it is not measured under field conditions. The aim of this work is to describe the impact of water stress conditions on yield response for table olive trees, considering an accurate determination of the endocarp development. The experiment was performed from the 2020 to the 2023 seasons on a two-year-old super-high density table olive orchard (cv Manzanilla de Sevilla). The three irrigation treatments (six repetitions) were: Control, no water stress conditions throughout the season in 2020 and 2021, and mild water stress to reduce vigour in 2022 and 2023; Regulated deficit irrigation (RDI), based on a moderate to severe water stress during pit hardening, using midday stem water potential measurements; and Rainfed. Endocarp development was established using the pit-breaking pressure throughout the 2021, 2022 and 2023 seasons. Water relations, vegetative growth, number of inflorescence and fruits, yield and fruit quality were measured during the experiment. Crown volume limited yield in Rainfed treatments but not in RDI, compared to Control. The number of inflorescences and inflorescence set was not affected in RDI and Rainfed, only reduced in severe water stress in 2023. RDI presented a similar yield response to Control, with a reduction of 50 % on applied water and a significant more negative midday stem water potential. The Rainfed trees were unsuitable for table production because rehydration did not recover fruit size enough to be commercial. Endocarp maintained the capacity of growing until the end of the season. 1. Introduction The scarcity of water resources limits agricultural activity around the world (Steduto et al., 2012). Olive (Olea europaea L.) was typically a rainfed fruit tree, but nowadays it is frequently irrigated to increase productivity. According to Steduto et al. (2012) the relative increase in crop evapotranspiration of irrigated olive orchard from 50 % to 90 % would enhance relative fruit yield from 40 % to 100 %. Therefore, irrigated olive orchards started spreading widely from the 90’s of last century, when tree density increased greatly. This change in the cultivation system was strongly based on a reduction in harvest costs and the increase of the surface dedicated to super-high density olive orchards. Super-high density orchards would reduce harvest costs around 35 % in comparison with high density orchards (Connor et al., 2014). Therefore, this fruit tree competed for water resources, and, in some areas, it was one of the most important petitioner of irrigation, e.g. in Spain, with more than 400,000 ha of irrigated olive orchards (MAPA, 2024). However, olive trees are considered a drought-resistant species in which an irrigation scheduling based on plant water status could improve yield response to water stress (Ben-Gal et al., 2021). * Corresponding author at: Agronomy Department, ETSIA, Universidad de Sevilla, Crta de Utrera Km 1, Sevilla E-41013, Spain. E-mail address: [email protected] (A. Moriana). Contents lists available at ScienceDirect Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat https://doi.org/10.1016/j.agwat.2024.109044 Received 30 May 2024; Received in revised form 7 August 2024; Accepted 31 August 2024 Agricultural Water Management 303 (2024) 109044 Available online 6 September 2024 0378-3774/© 2024 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). Regulated deficit irrigation can be used to optimize water resources, adjusting water inputs to the phenological periods in which a water deficit does not affect yield and quality of the crop (i.e. Moriana et al., 2003). The duration and timing of the water stress cycle determines the plant vegetative and productive response (Hsiao, 1990). It is, therefore, necessary to establish the different phenological stages of the olive trees to understand the impact of water deficit on the yield response (Fern´ andez, 2014). Phenological stages of olive trees for deficit irrigation scheduling were commonly determined based on endocarp (pit) development. Pit hardening was considered the most resistant period to water stress (Goldhamer, 1999; Moriana et al., 2003; Fern´ andez, 2014, among others). However, it has not been easy to determine this period under field conditions, although it usually occurs in the middle of the summer season. Recently, S´ anchez-Pi˜ nero et al. (2022) reported endocarp development of different cultivars using the description of the pit-breaking pressure pattern throughout the season, concluding that the beginning of this pit hardening period, Period II, could be estimated as 49 days after full bloom. However, the length of this Period II was not constant and changed according to cultivar and thermal time (S´ anchez-Pi˜ nero et al., 2022). These results imply that deficit irrigation scheduling based on a constant period of water stress around mid-Summer could include different phenological stages, therefore, it could change the yield response in different experiments. The most drought sensitive period for olive trees was from shoot sprouting to fruit set, which included the first Phase of endocarp development. Relative low water stress levels in this period (midday stem water potential around −2 MPa) caused a great decrease in yield (Moriana et al., 2003; Bey´ a-Marshall et al., 2018; Hueso et al., 2021) or a permanent reduced fruit size, even if trees recovered for the rest of the season (Rapoport et al., 2004; Gucci et al., 2007; Caruso et al., 2013; G´ omez del Campo, 2013a; G´ omez del Campo et al., 2014; Gucci et al., 2019). Only under conditions of very low fruit load (0.5 kg tree −1 ), water stress did not affect fruit development in this period (S´ anchez-Pi˜ nero et al., 2024). Water stress conditions from Period II of endocarp development would reduce yield, but only with severe water stress levels (Fern´ andez et al., 2013; G´ omez del Campo, 2013a; Moriana et al., 2003; Ahumada-Orellana et al., 2017; Ben-Gal et al., 2021). Recently, suggested that cumulative water stress indicators in olive trees would describe more accurately the yield response than a management based on threshold values (Corell et al., 2022; S´ anchez-Pi˜ nero et al., 2024). Accurate determination of endocarp development would provide the real end of Period II. Otherwise, some dates considered in this phenological stage could be, in fact, part of the next one. Some authors reported that oil accumulation started in this Period II (Fern´ andez et al., 2013; G´ omez del Campo, 2013a; S´ anchez-Pi˜ nero et al., 2024). Moreover, the reduction in oil rate accumulation during Period III, when this rate was maximum (Fern´ andez et al., 2013), was affected by the water stress imposed during Period II (50 % less with water potential more negative than −3 MPa, S´ anchez-Pi˜ nero et al., 2024). On the other hand, table olive trees need a rehydration period before harvest as an essential requirement for maximum fruit size (i.e. Corell et al., 2020), therefore the accurate determination of this phenological stages is critical for the strategy. Moreover, S´ anchez-Pi˜ nero et al. (2024) reported that endocarp development was less affected for water stress than total fruit growth or oil accumulation. This recovery at the end of fruit development was commonly reported in this and other fruit trees (Steduto et al., 2012) and, in olive fruits was related with the phase of cell growth (Rapoport et al., 2004). Super-high density olive orchards are a relatively recent production system, more so for table olives (first orchards around 2010, Morales-Sillero et al., 2014) than in oil production (from 90’s of the last century, Connor et al., 2014). The increase in the number of trees means that the water stress management would be more difficult to apply in comparison to traditional orchards because water stress would be greater with the increase in the number of trees. For instance, S´ anchez-Pi˜ nero et al. (2024) reported great changes in the level and duration of water stress in two different densities of rainfed orchards. However, in recent years, there has been an increase of the surface dedicated to this production system under rainfed conditions to reduce the harvest costs, although these also reduced strongly the yield. Such systems would be even more constraining in young orchards and in semi-arid conditions where crown volume (Connor et al., 2014; Pierantozzi et al., 2020) and, even survival, could be severely reduced. Pierantozzi et al. (2020) reported a decrease from 15 to 5 m 3 in canopy volume will decrease yield in a 50 % in a 5 year-old olive orchard. P´ erez-L´ opez et al. (2007) in the first commercial yield of a young olive orchard also reported a decrease around 20 % in yield when different irrigation scheduling for full irrigated conditions was compared. This latter author concluded that was crown volume the main difference between them. On the other hand, from our knowledge, there are no data about tree survival in super-high density young olive orchard. This rainfed super-high-density systems were probably unsuitable for table olive production, in which fruit size is an important yield feature. Final fruit size was strongly linked to a great rehydration before harvest (Corell et al., 2020) that allowed an almost complete recovery when the water stress was not imposed before Phase II (G´ omez del Campo, 2013a; G´ omez del Campo et al., 2014; Gucci et al., 2019). As far as we know, there is limited information about the response of super-high density table olive orchards in rainfed and deficit irrigation systems (S´ anchez-Pi˜ nero et al., 2024; Morales-Sillero et al., 2024). Moreover, there are a few references in relation to the response of yield component to water stress level, from our knowledge no one in table production. In addition, the moment of water stress was not commonly described and commonly only fixed periods along the season were considered. The initial hypothesis of the current work was that the main constraints in yield would be crown volume and fruit set in the most severe water stress level. In addition, endocarp development would present a similar pattern in all the treatments but with a shorter period of massive pit hardening in the most severe water stress conditions. The aim of this work was to describe the yield response considered these two mainly factors of the regulated deficit irrigation the moment and the level of the water stress in super-high density table olive orchard. This aim was in the frame of young orchard, which were commonly less studied, in table production, which needs a more accurate water management, and with an extremely severe water stress treatment. 2. Material and methods 2.1. Site description and experimental design The experiment was conducted from 2020 to 2023 in the experimental farm “La Hampa” (IRNAS-CSIC), located in Coria del Rio (Seville, Spain) (37º17’N, 6º 3’W, 30 m asl). The orchard was 2 years-old at the beginning of the experiment and the first yield was obtained in the first season of the current work. The growing system was a hedgerow olive (Olea europaea L.) orchard (4×1.5 m apart, 1667 tree ha −1 ) of the cultivar Manzanilla de Sevilla. The soil was sandy loam with a depth greater than 1 m. Volumetric water content was estimated as 0.21 m 3 m −3 at field capacity and 0.1 m 3 m −3 at permanent wilting point (Moreno et al., 1983). The amount of organic matter was 0.8 % which is low and could reduce the capacity of soil water retention. Extractable Phosphorus and Potasium in the soil was also low (Olsen P 10 mg kg −1 ; available potassium 200 mg kg −1 ). In order to eliminate the effect of this low soil fertility, all treatments were fertilized with Nitrofoska perfect (15−5−20) at a rate of 105 kg ha −1 at the end of Winter and with 3–4 foliar application of 1 % urea solution along the season. The drip irrigation system consisted of a single line of emitters (2.2 L h −1 ) per line of trees, separated 0.5 m. The statistical design was made of completely randomized plots with 6 repetitions of 3 treatments. Each plot included 5 lines of 13 trees and 2 side rows, and the first trees in each row was considered as the edge. Differential irrigation M. S´ anchez-Pi˜ nero et al. Agricultural Water Management 303 (2024) 109044 2 treatments started from June 2020 because of the coronavirus pandemic. The three treatments were: •Control. The objective of this water management was to optimize vegetative growth based on the orchard stage. •Regulated Deficit Irrigation (RDI). The objective of this treatment was to optimize the water stress management. •Rainfed. This treatment was included because of the great increase in new orchards under such conditions. These new orchards hypothesized that the reduction in yield could be compensated with the reduction in harvest costs. The objective of this treatment was to optimize rainfall and change the harvest date to match the beginning of the rainy period after Summer to maximize fruit size at harvest Treatments were a combination of the level and time point of water stress. Pit hardening period was the main reference for determining the different periods of water stress application. The beginning of this period was estimated according to Rapoport et al. (2013). The fruit length was measured weekly from the first week after full bloom. The starting date of pit hardening occurred when the growth in fruit length decreased. The end of pit hardening was considered as a fix date on the third week of August. The level of water stress was estimated according to midday stem water potential (SWP), using constant or relative threshold values. Relative values were obtained as the distance below the baseline equation by Corell et al. (2016). Irrigation scheduling in each treatment was described in S´ anchez-Pi˜ nero et al. (2024). Control irrigation was commonly greater than FAO method, because P´ erez-L´ opez et al. (2007) reported that this method underestimated water needs. In this way, reduction coefficient (Kr) was considered as 1 because estimation at the beginning of the season were around 0.85, then to secure full irrigated conditions all the season it was also increased from the beginning. In any of the experimental seasons, Control trees did not present hypoxic symptom. Here is presented a brief summary: •Control. In 2020 and 2021, irrigation was scheduled for maximum vegetative growth. From 2022, orchard was considered as mature, and moderate vigour control was included in this treatment. In 2023, the applied water was 100 % ETc and threshold values were relative to the baseline with a decrease of 0.25 MPa before and after pit hardening and 0.5 MPa during pit hardening (according to S´ anchez-Pi˜ nero et al., 2023). •Regulated Deficit Irrigation (RDI). In 2023, thresholds were the same as in 2022 but with a constant value of −3 MPa during pit hardening. •Rainfed. During 2020, treatments started in the middle of the season, and this treatment was irrigated with 58 mm until June (before started the experiment) and 45 mm just one week before harvest. Irrigation before harvest was provided to evaluate a potential rainy period on the common harvest date for table olives. After that, trees were not irrigated, and harvest was delayed until a date when the cumulative rain was approximately 40 mm. Then, the harvest date in 2021was on the 2nd of November, after a total rain of 51 mm; in 2022, the harvest date was on the 27th of October, after a total rain of 18 mm; and in 2023, the harvest date was the 6th of September, after a total rain of 47 mm. In 2022, the harvest date occurred earlier because of the extremely low amounts of rain received over that period. The harvest date coincided with those of the irrigated treatments in 2020 and 2023. 2.2. Measurements The beginning of the experiment was delayed until June 2020, when the irrigation season had already started, due to the coronavirus pandemic. This caused some measurements to be missing in this season. Some of the measurements described (mainly the ones from 2020, 2021, and 2022) had already been previously published (S´ anchez-Pi˜ nero et al., 2023, 2024), but they will still be included in the discussion section. Climate data were obtained from the climatic station “La Puebla del Rio” of the Andalusian climatic network (37º14’N, 6º 6’W, SIAR, 2024) around 7 Km far and they are described in Results section. The impact of water stress was determined by monitoring the endocarp development pattern. This pattern was slightly different from the irrigation scheduling described above. Endocarp development was outlined using the pattern of the pit-breaking pressure (PBP) from the 2021, 2022, and 2023 seasons (only the last one is included in the current work). PBP was measured according to Rapoport et al. (2013) with a device where the pit was broken and store the maximum pressure data. This methodology has several potential sources of error. This device used a load cell (Model 6967, BCM Sensors, Schoten-Antwerpen, Belgium) to estimate the PBP, which was calibrated every season. The device has a hole where sample was placed, this permitted that the breaking point was the same, around the longest transversal diameter of the fruit, and maximum breaking force was the ones of the sample. Samples of 10 fruits per treatment and repetition were collected periodically from two weeks after full bloom until harvest. To secure a similar conditions of measure, all samples were stored for one day and cooled (10ºC) and then they were measured in laboratory conditions (around 25ºC) for the same operator. PBP data were compared between treatments and adjusted to a sigmoid equation: PBP =a 1+e−b(DAFB−c)+d(1) Where:PBP is the pit-breaking pressure (MPa)a is the range from the minimum to the maximum PBPb is the slope of the inflection pointc is the date when the inflection point occurredd is the minimum PBPDAFB are the days after full bloom. When this equation was obtained, three periods of endocarp development were defined according to Rapoport et al. (2013): •Period I, slow hardness increase, almost constant values. •Period II, fast increase of PBP data. The starting point was dated when 10 % of maximum value (a+d) was reached. The end point was estimated when PBP was around 95 % of the maximum value (a+d). •Period III, maximum hardness value, almost constant values. Midday stem water potential (SWP) was measured weekly at midday using a pressure chamber (PMS 1000, PMS, USA) in 2023 from February to October. Total expanded leaves near the main trunk were covered with an aluminium bag at least two hours before the measurements were taken. The stress integral was estimated according to Myers (1988) but considered, on each date, the baseline value from Corell et al. (2016) as a reference for full irrigated conditions: SI =|∑(SWP −(BL)) * n | (2) Where:SI is the stress integral (MPa*day)SWP is the midday stem water potential (MPa)BL is the value estimated on the baseline (MPa)n number of days (days) Gas exchange was measured weekly using an IRGA (CI-340, CID BioScience, USA) in 2023. Measurements were carried out at midday in a sunny, completely expanded leaf per treatment and repetition from mid-June until October. Vegetative growth was described using crown volume and shoot expansion. Crown volume was estimated in three different directions of each plot every month and it was assumed to present a prism shape. Height was measured as the difference between the total height and the height where the canopy started. Ten shoots per plot were marked when inflorescences were observed before full bloom. Only shoots that presented inflorescence at that time were selected. The length of the shoot and the number of inflorescences per shoot were measured periodically. The percentage of inflorescence set and maximum number of inflorescences were estimated considering only the date when the maximum number was measured. The number of fruits per inflorescence M. S´ anchez-Pi˜ nero et al. Agricultural Water Management 303 (2024) 109044 3 in these shoots was counted from the beginning of pit hardening until harvest. These latter data only included those shoots in which fruits were observed at the beginning of pit hardening. Shoot expansion, inflorescences and fruit number were not measured during the 2020 season. Each plot was harvested for green style preparation in early September, except Rainfed treatments in 2021 and 2022. The three lines of each plot inside the edge lines (13 trees each) were hand harvested and weighted. In each plot, a sample of 2 kg was collected for quality determination. These samples were processed in the laboratory. Around a subsample of 0.5 kg of fruits was counted to obtain an estimation of the fruit size (number of fruits per kilogram, USDA, 2019). One hundred fruits were selected according to a colour maturity index (IM, Hermoso et al., 1997). In addition, three subsamples of 10 fruits per plot were randomly collected to estimate the pulp to pit ratio in fresh and dry weight (60ºC until constant weight). Fruit texture was determined for 10 fruit per plot by measuring the maximum peak force of the first compression (Szychowski et al., 2015) using a force gauge (FM 200, PCE Instruments, Spain). Applied water was measured in the main pipe at the beginning of each plot with a water-meter (Hydrojet, Hidroconta, Spain). At the end of each season, the number of trees in each plot was counted to obtain the percentage of tree survival. 2.3. Statistical analysis In the endocarp development, the Sigmoid parameters were estimated with nonlinear regression tools with the package “nls2” in R environment (R Core team, 2013). The statistical analysis in most of the measurements was conducted with ANOVA and a separation of means (Tukey Test) using the Statistic (SX, v 8.0) program. Significant differences with a p-level <0.05 were considered in both tests. The assessment of the water stress effect on inflorescence set was done with multivariate and simple analyses. Outliner data in this relationship was estimated with the relationship between error and expected values. Data which error were higher than 4-fold the average of each season was consider outliner. Comparison between fits considered R 2 , standard error and mean square error to discuss the best relationship. 3. Results 3.1. Climate Reference evapotranspiration (ETo) and rainfall presented the typical pattern of a Mediterranean climate, although there were some extreme seasons (Fig. 1). The climatic conditions showed a warm winter with an extremely hot and dry summer. Seasonal ETo was 1273 mm only in 2020, while it was 1373 mm in 2022 and 1365 mm in 2023. Maximum daily ETo was greater than 7.5 mm in all seasons and greater than 8 mm in 2022 and 2023. The period in which daily ETo was higher than 5 mm was usually from the beginning of May until mid-September, although in 2022, it started in mid-April and, in 2023, on the 3rd of April (Fig. 1). Maximum temperatures showed a similar pattern to ETo. Absolute seasonal maximum temperature varied between 39.9 ºC in 2020 and 44.2ºC in 2022, with values higher than 43ºC in the other two seasons (44.2ºC in 2021 and 43.7ºC in 2023). All of them were greater than the historical seasonal average of maximum temperatures (36.0 ºC, AEMET, 2024). In the period when fruit set occurred, from April to the end of June, maximum daily temperature oscillated between 36.6ºC in 2021 and 42.9º C in 2023. The average maximum temperatures in this period showed the lowest values in 2020 (26.7ºC) while 2023 was the hottest (30.8ºC), with intermediate values in the other two years, around 28ºC. The distribution and amount of rain also presented extreme values throughout the 4 seasons of the experiment (Fig. 1). All years experienced a lower amount of seasonal rain than the 30-year average (539 mm., AEMET, 2024). Only in 2020 this reduction was lower than 10 % (493 mm), but in the rest of the seasons, the decrease was greater than 30 % (357 mm, 2021; 362 mm, 2022; 289 mm, 2023). The distribution was also different in comparison to the average season (Fig. 1). Summer was the driest period every year, but the interval from January to April was received more rain than the average year. In this period, more than 50 % of the seasonal rain occurred and the volume was greater than the historical average (186 mm, average year vs 268 mm, 2020; 210 mm, 2021, 194 mm, 2022), except in 2023, when only 51 mm were measured (27 % of the seasonal average). The beginning of the rainy period, typically from September onwards, started on those dates in all seasons, except in 2022, when it was delayed until mid-October. However, the amount of rain in September, when harvest Fig. 1. Reference evapotranspiration (ETo), rainfall and maximum temperature throughout the 4 years. Source: La Puebla del Rio Station, SIAR (2024). M. S´ anchez-Pi˜ nero et al. Agricultural Water Management 303 (2024) 109044 4 occurred, was lower than the average year for this month (27 mm, AEMET, 2024) in 2020 (16 mm) and 2022 (4 mm). Only in 2023 season the amount of rains were greater (54 mm) with a unique event of 47 mm at the beginning of the month. 3.2. Endocarp development The pit-breaking pressure (PBP) of the endocarp showed the three different phases in which water relations data will be discussed (Fig. 2). Fig. 2 depicts data from the 2023 seasons. Control and RDI presented a similar pattern throughout the 2023 season. PBP was almost constant in the three treatments until around 50 days after full bloom (DAFB), but the Rainfed curve changed from that date on, marking the beginning of Phase II. While irrigated trees showed a fast increase of PBP in this period, Rainfed ones displayed a significantly lower hardness at 56 and 70 DAFB, with a recovery of PBP from 70 DAFB. At the beginning of Phase III, when PBP was at its maximum level, Rainfed trees presented lower values than irrigated trees, significantly different by 112 DAFB. Only on the last date of measurement, after a heavy rain, hardness was almost the same in all treatments. The adjustment of these data to a sigmoid equation is presented by the parameters of Table 1. Only maximum hardness (parameters a and a+d) was significantly different between Rainfed and the irrigated trees. In Phase III, fruit from Rainfed presented PBP values around 10 % lower than irrigated trees. In addition, the determination coefficient (R 2 ) was also lower, on average, in Rainfed trees. This decrease in R 2 was likely related to the recovery pattern during Phase II. The rate of increase during Phase II (parameter b) was equal in all treatments. However, the date when 50 % of PBP occurred (parameter c) took place with approximately 1 day later in RDI and Rainfed versus Control trees, although such differences were not significant. 3.3. Water relations The SWP pattern throughout 2023 showed strong changes in the most stressed trees due to the rain period (Fig. 3 and Fig. 1 around DOY 150). Significant differences were measured on most dates between Rainfed and Control, while RDI was often near Control. Control SWP was near to the baseline equation in Phase I and during pre-flowering period. From Phase II, this treatment presented a small decrease respect to this equation. The decrease in SWP was sharp in Rainfed and RDI, although in the former it was disrupted by several rain events that allowed an almost complete recovery (on DOY 144, 34 DAFB; DOY 166, 56 DAFB; DOY 249, 139 DAFB). Minimum values in Rainfed SWP were around −6 MPa from DOY 186 until harvest when a previous rain almost recover this treatment. On the other hand, RDI was irrigated to be able to reach a maximum water stress level of approximately −3 MPa at Phase II. Differences between RDI and Control were significant mainly in the period from DOY 151–193. In addition, during Phase I, an irrigation problem increased RDI water stress more than expected in that period with significant more negative values than Control and around −3 MPa though irrigation permitted a completely recover with near value to the baseline equation. Table 2 showed data of minimum SWP and SI in four different phases Fig. 2. Pattern of endocarp pit-breaking pressure during the 2023 season. Each symbol is the average of 3 data. Vertical bars represent the standard error. Vertical lines define the different phases. Control, blue circle; RDI, green diamond; Rainfed, red square. Asterisks indicate significant differences between treatments on that date. Table 1 Parameters (average±standard error) of the fit adjusted in the individual pitbreaking pressure curves for the 2023 season. Sigmoid equations for modelling the pit-breaking pressure (PBP) according to the equation PBP = a 1+e−b(DAFB−c)+d. Each data is the average of 3 plots. Different letters in the same column indicate significant differences (Tukey Test, p<0.05). a*b c d a+d R 2 Control 156.4 ±3.7a 0.1 ±0.0 72.2 ±1.1 5.1 ±1.2 161.5 ±2.9a 0.96 RDI 161.4 ±4.8a 0.1 ±0.0 73.6 ±0.5 3.5 ±1.4 164.9 ±4.6a 0.91 Rainfed 138.2 ±3.7b 0.1 ±0.0 73.5 ±0.9 5.9 ±1.1 144.1 ±3.7b 0.73 * a, Maximum pit-breaking pressure (MPa); b: rate of pit-breaking pressure increase (MPa day −1 ); c. days after full bloom for 50 % hardening; d, minimum pit-breaking pressure (MPa). M. S´ anchez-Pi˜ nero et al. Agricultural Water Management 303 (2024) 109044 5 of the 2023 season. These phases were delimited by the PBP curves (Fig. 2 and Material and Methods). Only in the pre-flowering phase there were no important differences between treatments. This period presented the lowest water stress level in all trees, although in Control, SI data were near to the ones estimated in Phase II or Phase III. Greater differences than previously were found in the rest of phases in both indicators (Min SWP and SI). Minimum SWP values on Control trees were always considerably more positive than RDI, and this, more so than in Rainfed. These values decreased in all treatments throughout the season, and the most negative SWP were measured in the Phase III of endocarp development. Similarly, SI values of Control were significantly lower than RDI and Rainfed in all phases. The maximum SI was estimated in Phase II in all treatments using the greatest value in Rainfed (more than 200 MPa*day, only in this period). Gas exchange was considerably reduced in Rainfed trees during summer of the 2023 season (Fig. 4). These measurements started from the beginning of June, just after a period of rain (Fig. 1). Under those conditions, trees presented a similar midday net photosynthesis (Fig. 4). From this date, all treatments trended to reduce their values. Photosynthesis in Control and RDI treatments had a similar pattern with values between 4 and 6 μ mol m −2 s −1 . In that period, from DOY 172–242, maximum temperature was between 30 and 40 ºC with ETo values higher than 6 mm day −1 (Fig. 1). The last data of these treatments (DOY 258), after harvest, reached their maximum photosynthesis values, between 7 and 8 μ mol m −2 s −1 in cooler conditions with a maximum temperature of 29.4 ºC and ETo around 4.5 mm day −1 (Fig. 1). In the hottest period, DOY 172–242, Rainfed net photosynthesis sharply decreased and reached values near zero. These minimum values, were measured in conditions of very high temperature (40ºC, DOY 200). The last date of measurements was performed after harvest and a great rain even on DOY 246 (46.7 mm, Fig. 1). Such conditions permitted the Fig. 3. Pattern of Stem water potential (SWP) throughout the 2023 season. Each point is the average of six data. Vertical bars represent the standard error. Solid lines indicate full bloom (left) and harvest (right). Dash lines limit the Phase II of fruit development. Control, blue circle; RDI, green diamond; Rainfed, red square. Solid line represented Corell et al. (2016) baseline. Asterisks indicate significant differences between treatments on that date (p<0.05, Tukey Test). Table 2 Minimum midday stem water potential (SWP, MPa) and Stress Integral (MPa*day) in pre-flowering and the three phenological phases according to endocarp development throughout the 2023 season. Each value is the average of 6 data. Different letters in the same row indicate significant differences (p<0.05, Tukey Test). Control RDI Rainfed Min SWP PreF*−1.38±0.03a −1.51 ±0.08a −1.77±0.30a Stress Int. PreF 21.6±1.4a 23.0 ±2.1a 30.3±5.7a Min SWP Phase I −1.62±0.13a −3.02 ±0.35b −4.85±0.27c Stress Int. Phase I 9.8±1.9a 35.1 ±3.3b 82.0±7.2c Min SWP Phase II −2.61±0.19a −3.91 ±0.30b −6±0c Stress Int. Phase II 27.1±2.6a 71.1±5.8b 219.1±3.0c Min SWP Phase III −3.34 ±0.28a −4.10 ±0.21b −6.5±0c Stress Int. Phase III 25.0 ±2.8a 50.1±3.5b 104.5 ±0.8c * PreF, Pre-flowering Fig. 4. Pattern of Midday Net Photosynthesis during the 2023 season. Each symbol is the average of 6 data. Vertical lines represent the standard error. Control, blue circle; RDI, green diamond; Rainfed, red square. Asterisks indicate significant differences between treatments on that date (p<0.05, Tukey Test). M. S´ anchez-Pi˜ nero et al. Agricultural Water Management 303 (2024) 109044 6 recovery of Rainfed photosynthesis which were near to irrigated trees and around the maximum values of the dataset. 3.4. Vegetative growth The crown volume patter was similar between seasons, with the greatest differences at the end of the growth period (Fig. 5). In the first season (2020), significant differences were found between all treatments from September onwards, just after harvest. The decrease of this measurement (the two last data point) in this season was related to pruning just after harvest. After this season, more frequent and earlier measurements were possible. Only in 2021, the three treatments were significantly different from the beginning and throughout the season. On the first date, RDI presented around 20 % and Rainfed around 50 % less than Control. The seasonal pattern was also different between treatments, Control presented a greater increase of crown volume than RDI and Rainfed. The latter displayed a very low growth throughout the season, only around 5 % increase from the beginning of the season. The two irrigated treatments increased their differences at the end of 2021 season up to 27 %. The last two seasons, 2022 and 2023, Control and RDI showed the common pattern of mature orchards. Crown volume at the beginning was clearly lower than at the end of the previous season because of the winter pruning. Such decrease caused, in 2022 and 2023, both irrigated treatments to be very similar at the beginning, approximately 10 % lower in RDI, with no significant differences. On the contrary, Rainfed did not reduce crown volume between seasons because pruning was almost null. Control and RDI presented a continuous growth throughout 2022 and 2023 but at a lower rate in RDI than in Control. This pattern caused important differences between treatments at the end of the season, with a decrease of around 25 % in RDI versus Control. Conversely, Rainfed presented a concentrated growth period during the spring rainfalls, greater in 2022 than in 2023, and with significantly lower crown volume values than both irrigated treatments. The pattern of shoot expansion changed greatly between seasons (Fig. 6). Each year, the beginning of data collection varied between DOY 80–120. In all the tree years, the seasonal pattern was similar with an important increase of expansion on the first dates and a strong reduction from the beginning of pit hardening. The year 2021 presented the lowest rate of increase, showing no significant differences between treatments. In this season, Control started with a lower shoot expansion than Rainfed and RDI, but, in the last measurements, Control had a higher shoot growth rate than the rest, although without significant differences. Spring rainfall in this season was around 98.6 mm, greater than the 10 years-average (95.4 mm,AEMET, 2024 ). In the year 2022, the three treatments showed a very fast expansion before pit hardening, but there was also a sharp change around this date. Rainfed treatment had a significantly greater expansion than the other treatments on the first two dates and remained higher during all the measurements. Control and RDI treatments had a similar expansion and did not stop from pit hardening onwards, as Rainfed did. This was the year with the highest spring rainfall, around 209 mm. In 2023, again the shoot growth presented a very pronounced initial growth, followed by slow growth around the beginning of pit hardening. In this case, Control treatment had the greatest expansion, followed by the RDI, and lastly the Rainfed. This season was the driest, with only 68 mm concentrated around the beginning of pit hardening. 3.5. Reproductive development Fig. 7 shows the maximum number of inflorescences per shoot for each year. The 2021 season presented the highest number of inflorescences in the three treatments. Control and RDI were significantly higher than Rainfed, which presented values half the others. Conversely, in 2022, there were no considerable differences between treatments, with a strong reduction in Control and RDI versus the previous season. All treatments presented similar number of inflorescences, around 5 per shoot, which was the number obtained in Rainfed in the previous season. In the last year, Rainfed treatment had significantly lower number of inflorescences than RDI but not than Control. However, the trend was a clear reduction in Rainfed, with values around half those in Control and RDI. The number of inflorescences in the two irrigated treatments was very similar, around 5 per shoot, to the previous season. The changes in inflorescence set along the season is presented in Fig. 8. The seasonal pattern was similar in all years, with a very large initial decrease followed by a period with steady numbers at the beginning of the pit hardening. In 2021, the three treatments showed the same pattern without significant differences, except for the second Fig. 5. Pattern of crown volume and rainfall throughout the four seasons of the experiment. Each point is the average of six measurements. Vertical lines indicate the standard error. Dash lines limit each season. Control, blue circle; RDI, green triangle; Rainfed, red square. Asterisks show the date when statistical differences were found (p<0.05, Tukey Test). M. S´ anchez-Pi˜ nero et al. Agricultural Water Management 303 (2024) 109044 7 measure. Approximately 60 % of inflorescences fell off after the first three measurements, around 50 days after full bloom. After this initial drop, all treatments were steady during the pit hardening period, with values around 40 % of inflorescence set. No reduction was observed in any of the treatments during this period. The pattern in 2022 was similar to the one in the previous season but with changes in the final inflorescence set values. The initial decrease in the number of inflorescences per shoot was again observed, shorter (around 35 days) and smaller (around 55 %) than in the previous season. From DOY 153 onwards, the number of inflorescences became stable in Control and RDI treatments, while it continued to decrease in Rainfed treatment until DOY 175, from when inflorescence set was significantly lower than the rest (40 % vs 55 %). No reduction was measured in any of the treatments from this date onwards. In the last year, there was a more pronounced initial decrease in RDI and Rainfed around 33 days after full bloom (DOY 124), which presented an inflorescence set of around 20 %, while Control was significantly higher (around 50 %). Then, all treatments showed a slight decrease until DOY 165, when inflorescence set was steady at around 40 % in Control, 20 % in RDI and 15 % in Rainfed. These values were considerably different between Control and the rest of treatments. After this date, there were no changes in any of the treatments. The steady values obtained in all seasons and shown in Fig. 8, were related to different water status measurements in the period from full bloom to pit hardening and with maximum temperature the week before (Table 3). The best fit was obtained with the Stress Integral (Fig. 9), which presented an exponential relationship (Table 3). The two data out of the regression at Fig. 9 presented an error 4-fold greater than the average of their season. Fig. 9 shows that inflorescence set decreased with the increase of the SI, therefore, with the increase of water stress conditions. Data from the 2022 (the highest inflorescence set) and 2021 seasons presented values of SI lower than 20 MPa*day, with an inflorescence set between 40 % and 70 %. Rainfed plots in 2021 and 2023, and also RDI in 2023, presented a clear decrease below 30 % of inflorescence set from an SI greater than 20 MPa*day. Two Rainfed plots in 2023 were not considered in any of the regressions (Table 3) because the SI was measured in a tree with a lower fruit load than the average of the plot. The simple regression between inflorescence set and minimum water potential or average maximum temperatures was clearly weaker that the one with the SI (Table 3). Multivariable regression improved the simple regression results (Table 3). Fits which included two variables were similar in accuracy, with a slightly higher determination coefficient than the simple fits with SI, while the three-variable model was the best fit. In all equations, inflorescence set decreased when SI increased (Table 3). However, the relationship with the average maximum temperature the week before (Tmax) proved that there was an increase in inflorescence set with the increase in temperature. These results were unexpected, as was the one from the SWP, which was variable. In the simple fit (Table 3), inflorescence set decreased when SWP was more negative, meaning the stress conditions were harder. Nevertheless, in the multivariable fits, the relation with SWP was the opposite, and inflorescence set decreased when SWP increased, therefore, the more positive the values, the less severe the water stress conditions. Therefore, this latter relationship and the one with Tmax were not considered The number of fruits per inflorescence were around 1.3 in most seasons and treatments from the beginning of pit hardening (Fig. 10). In 2021 and 2023, there were no marked differences between treatments on any of the dates considered. Only in 2022, RDI presented significantly higher values than Rainfed on both sample dates, and Control was an intermediate treatment. In this latter season, RDI presented the maximum values of the three years, while Control and Rainfed were very similar. There were no differences between dates within the same season in any of the three treatments. 3.6. Quantity and quality of harvest Table 4 presents the data related to yield (quality and quantity), seasonal applied water and tree survival throughout the four seasons of the experiment. The pattern of the yield during the 4 years showed the changes from a young to a mature orchard. The first commercial yield was harvested in the 2020 season and was the lowest of all seasons in the three treatments. From 2021, an alternate bearing pattern was observed in irrigated trees, with the highest yield in 2021 and the lowest in 2022. Rainfed treatment presented a significant reduction in yield in comparison to Control in all seasons, except 2022. These reductions were important and varied from around 55 % in the lowest yield seasons (2020 and 2022) to 93 % in the two greatest yield ones (2021 and 2023). Differences between RDI and Rainfed were lower than the ones with Control and only significant in 2021 and 2023. Differences in yield between RDI and Control were important only in 2021, with a reduction of around 27 %. In the rest of the seasons, RDI also tended to lower values with reductions between 12 % in 2020 and 29 % in 2022. Fruit size is one the most important features of yield quality (Table 4). Rainfed trees presented significantly higher values of fruits Fig. 6. Pattern of shoot expansion and rainfall throughout the three seasons of the experiment. Each point is the average of six measurements. Vertical lines indicate the standard error. Control, blue circle; RDI, green triangle; Rainfed, red square. Vertical dash lines limit the period of pit hardening. Asterisks show the date when statistical differences were found (p<0.05, Tukey Test). M. S´ anchez-Pi˜ nero et al. Agricultural Water Management 303 (2024) 109044 8 per kilogram, therefore lower fruit size, than the other two irrigated treatments. These differences were obtained even though these trees were harvested always after a short period of rehydration with irrigation (2020 season) or rain (rest of seasons). The values obtained in Rainfed trees were higher than 350 fruits per kilogram in all seasons (Petite class, USDA, 2019), and the fruits were considered unsuitable for the table industry. RDI and Control values presented no meaningful differences in fruit size. Minimum values in Control were measured in 2022 (199 fruits kg −1 , extra-large class, USDA, 2019), while in RDI were obtained in 2023 (196 fruits per kg −1 , extra-large class, USDA, 2019). The worst fruit sizes in both treatments were harvested in 2021, the highest yield season (Control, 233, and RDI, 251 fruits kg −1 , Medium class, USDA, 2019). In all seasons, RDI tended to smaller fruit size than Control, with a reduction between 8 % and 15 %, except in 2023 when RDI produced the largest fruits. Irrigation management produced meaningful greater differences in applied water (Table 4). Rainfed trees were not irrigated from the end of 2020. During this first season, this treatment was irrigated before the beginning of the experiment and the week before harvest. RDI reduced the water applied in comparison to Control by 52 % (2020), 57 % (2021), 72 % (2022) and 43 % (2023). Despite these large differences in water applied, the percentage of tree survival in each treatment was not significant differences from 2020 to 2022. However, Control presented the highest percentage of trees (around 95 %) and RDI the lowest (88 %) in the irrigated treatments. Only from 2022, there was a decreasing trend in Rainfed, which presented significantly lower percentage of tree survival in 2023 (84 vs 88 and 94 %). Data related with several fruit features are presented at Table 5. Results of fruit texture and maturity index (MI) were usually related. When fruit ripening progressed, fruit texture decreased and MI increased (Table 5). Control and RDI data were not markedly different in any of the seasons. In 2020, when harvest was delayed, fruit texture was considerably lower in Control and RDI than Rainfed because the MI was slightly, but significantly, greater in the former treatment. In the following two seasons, 2021 and 2022, Control and RDI were harvested for green style preparation, but Rainfed trees were harvested later, after the first rains. Rainfed presented a significantly lower values of fruit texture because they were in a more advanced MI (around 3, fruit veraison, vs around 1, green fruit). In 2023, all treatments were harvested on the same date because rain occurred earlier than in previous seasons. This year, fruit texture was almost equal in all treatments and similar to the values obtained in the green fruit stage of previous seasons (around 50 N), but MI was significantly different. Rainfed presented values greater than 3, while irrigated trees were around 1, the optimum for green style preparation. Pulp/stone ratio (PS) is another important feature in yield quality. No differences were found in PS in fresh or dry weight between Control and RDI (Table 5). Although PS in fresh weight in RDI tended to be lower than in Control. On the other hand, PS was significantly reduced in Rainfed in comparison to irrigated trees. Only in the 2021 season and in dry weight, those differences were not significant. Such reductions were more severe in the 2022 and 2023 seasons, when Rainfed was around half the PS than irrigated trees. 4. Discussion Yield response in fruit trees is related to several factors, which change their relevance at different stages in the orchard’s lifecycle. The current work presented data from the transition from a young to a mature orchard, when these changes occurred. Crown volume was the main factor in the yield respond in 2021, when irrigated trees reached yield levels close to the maximum (Fig. 5, Table 4). Tree size was probably the main limitation that strongly reduced Rainfed yield throughout the experiment but did not delay the moment in which the first yield was harvested. Similar results in relation to the beginning of flowering were reported in irrigated hedgerow orchards (G´ omez del Campo, 2010) and would suggest a strong drought-resistance of the flower induction process. In fact, shoot and crown volume growth have been reported as the main limitations of yield response in olive trees (Connor et al., 2014; Pierantozzi et al., 2020). However, because the vegetative growth was very sensitive to water stress (Hsiao, 1990), the amount of irrigation to achieve maximum crown volume as soon as possible would be considerable (as in current work, Table 4) and not Fig. 7. Maximum number of inflorescences throughout the three seasons of the experiment. Each bar is the average of six measurements. Vertical lines indicate the standard error. Control, blue circle; RDI, green triangle; Rainfed, red square. Different letters indicate statistical differences between treatments. (p<0.05, Tukey Test). M. S´ anchez-Pi˜ nero et al. Agricultural Water Management 303 (2024) 109044 9