Partial Rootzone Drying Irrigation Modulates Transpiration of Olive Trees ^†

Abstract

Water scarcity and the increasing demand for irrigation in olive orchards lead to the adoption of deficit irrigation approaches to save water. A partial rootzone drying (PRD) irrigation technique has been proposed for woody crops as an agronomic practice to improve water productivity. This study was conducted to evaluate the effects of this irrigation strategy on water relations and transpiration of olive trees (cv. Cobrançosa) under climate conditions in the northeast of Portugal during the summer season of 2014. Two irrigation treatments were used: control (FI), irrigated with 100% of the estimated crop evapotranspiration (ET) and PRD₅₀, irrigated with 50% of the control (FI) on one side and switching every two weeks. Whole tree transpiration (T) was quantified by sap flow, which was monitored within the trunks of both the control (FI) and deficit irrigated (PRD₅₀) trees using the compensation heath-pulse technique. Foliage gas exchange and water potentials were determined throughout the experimental period. During the summer, daily transpiration reached roughly 27 and 43 L d⁻¹ for PRD₅₀ and FI olive trees, respectively, with a clear reduction of 37% in PRD₅₀ olive trees. PRD₅₀ showed statistically comparable values of water potentials to the control, which appeared to prevent an excessive drop in tree water status by modulating stomatal closure.

1. Introduction

The olive tree (Olea europaea L.) assumes an important role in the Mediterranean landscape, and Vilariça valley, located in the region of Tras-os-Montes in Portugal, is no exception. The region, which contains 75,266 ha of olive groves, is assumed to be the second most important area of the country (22% of the total area). According to the Köppen-Geiger climate classification [1], this region has a Csa climate type, where summer is characterised by scarce rainfall, high temperatures and intense solar radiation, conditions that lead to the development of a high vapour pressure deficit. The olive tree is an evergreen tree known to be resistant to drought; however, as a consequence of that capacity, its photosynthesis activity decreases, limiting the growth rate and yield [2,3]. Thus, irrigation of olive trees has been adopted to overcome these negative impacts and ensure crop yield. However, increasing irrigated areas is very difficult in the olive industry due to water scarcity and increased competition with non-agricultural uses [4]. Therefore, great emphasis is placed on irrigation management in arid regions to increase water use efficiency leading to the adoption of deficit irrigation approaches to save water. Partial Rootzone Drying (PRD), derived from split-root research, is a well-documented technique of water-saving irrigation [5,6,7]. The technique was developed based on understanding the physiological mechanisms controlling plant transpiration and root-shoot signalling under water deficits. It consists in irrigating only one side of the rootzone so that the plant can be simultaneously exposed to both wet and dry soils. This technique has already been successfully tested on several trees [8,9,10,11]. In olive, different studies have shown that PRD irrigation affects water relations, namely leaf water potential and stomatal conductance [12,13,14], vegetative growth [14], with minimal impacts on yield [15] and beneficial effects on olive oil quality [16]. The evaluation of olive tree water use can be assessed by sap flow measurements using the heat compensation pulse technique [17,18,19]. This study aims to assess the effects of PRD irrigation on water relations, sap flow and transpiration of olive trees (cv. Cobrançosa) under the hot and dry climate conditions in the northeast of Portugal during the summer season of 2014.

2. Experiments

2.1. Field Conditions and Plant Material

The field trial was conducted in 2014 in a 12-year-old organic commercial olive (cv. ‘Cobrançosa’) orchard located in Vilariça valley (Trás-os-Montes, Portugal, 41.3° N, 7.0° W, 150 m altitude), a typical olive-growing area of northeast Portugal. The climate in this area is Mediterranean (IPMA, 2015), with an average rainfall of 520 mm concentrated from autumn to spring and 1130 mm of average ET₀. The soil is classified as Eutric Leptosols developed on metamorphic rocks (schists) with a sandy loam texture. Two irrigation treatments were used: Fully irrigated (FI) control, for which the water applied equalled the difference between the maximum (estimated) ET and rainfall; and PRD irrigated, partial root drying system applying the same irrigation dose as FI to one half of the root system with the irrigated and drying halves of the rootzone alternating every two weeks. Watering was performed every day from June to October, corresponding to fruit set and fruit ripening stages, respectively. The experimental design was a complete randomised block, replicated three times. Each plot contained four central olive trees surrounded by 14 border trees. All measurements were conducted on the central trees of each plot.

2.2. Sap Flow Measurements

In order to evaluate sap flow rates and transpiration, trees represented in each irrigated treatment were selected, and a set of heat-pulse probes (Tranzflo, Palmerston, New Zealand) were installed into parallel holes drilled in a radial position (north and south side) into the semi-trunk of each tree, at the height of approximately 50 cm. The heat-pulse gauge consists of a heater with a diameter of 1.8 mm and two temperature probes with the same diameter (one 15 mm downstream and the other at 5 mm upstream of the heater). Each temperature probe Copper–Constantan) had four thermocouple junctions spaced along the radius of the cross-section. In using the compensation heal-pulse technique [19], sap flow was obtained in 30 min intervals, and the transpiration of each tree was estimated as the average of the two sets of probes per tree.

2.3. Tree Water Status and Stomatal Conductance

The plant water status (predawn and midday stem water potential) was measured periodically during the 2014 cropping season using a Scholander pressure chamber (Soil Moisture Equipment Corp., PMS-1000, Corvallis, OR, USA). Predawn leaf potential (Ψ_PD) measurements were conducted before sunrise at approximately 04:30 while midday measurements of stem water potential (Ψ_stem) were recorded between at midday on a small leafy shoot near the trunk that had been covered with aluminium foil for at least 1 h before measuring. Measurements of water potential were recorded from six plants in each irrigated treatment.

Leaf stomatal conductance (gs) was measured midday with a portable porometer (Delta-T AP4, Delta-T Devices, Cambridge, UK). The device was calibrated before each use with the supplied calibration plate. The terminal part of the main leaf lobe was placed into the cup on the head unit, which was positioned to the sun. Measurements were conducted during cloudless periods on six exposed leaves/treatments around noon.

3. Results

Figure 1 shows the diurnal pattern of sap flow in conditions of high and low evaporative demand. For both treatments, sap flow diurnal patterns showed a steep morning increase leading to maximum rates achieved at about midday, when vapour pressure deficit (VPD) was at its maximum, followed by a sustained gradual decrease until late afternoon (Figure 1). However, in conditions of high evaporative demand (ETo = 7.0 mm d⁻¹), sap flow values of FI and PRD trees are similar in the first hours of the morning until 11:00. Afterwards, it fell in PRD trees, reaching a high rate of decrease at midday, being 37% lower than those of FI trees.

The seasonal evolution of daily transpiration (T) is presented in Figure 2. Averaged daily transpiration showed a progressive reduction during the season with maximum values at the end of July for both irrigated treatments when daily reference evapotranspiration was at its maximum. Such a reduction is quite similar between treatments. However, partial rootzone drying irrigated plants showed invariably lower daytime sap flow rates than full irrigated plants. Values of transpiration in FI plants ranged from 16 to 39.9 L d⁻¹ at DOY 265 (ETo = 1.9 mm d⁻¹) and 209 (ETo = 7.6 mm d⁻¹), while in PRD plants, a minimum of 13.1 L d⁻¹ and a maximum of 27.3 L d⁻¹ was attained on the same dates. Good agreement between measurements for daily water use and daily reference evapotranspiration was observed for both treatments (p < 0.05) with a coefficient of determination (r²) of 0.74 for FI and 0.68 for PRD; although, the slope of the regression was significantly higher in FI plants compared to PRD trees.

Predawn values of water potential showed slight differences between FI and PRD treatments only in DOY 231, in which Ψ_PD of PRDI plants attained a minimum of −1.16 MPa (Figure 3). For FI trees, values were higher at −0.65 MPa, whereas, for PRD plants, they were usually higher than −0.80 MPa. Stem water potential was not significantly affected by PRD with values similar to FI plants, generally higher than −2.0 MPa.

The evolution of leaf gs measured throughout the olive growing season (

Table 1. Midday values of leaf stomatal conductance (gs, mm s ¹) for the two irrigation treatments, during three representative days of the season. Values are means (±SE) of six replicates.

Irrigation Treatment	24 July	18 August	15 September
FI 1	6.1 ± 0.17	6.5 ± 0.13	7.2 ± 0.12
PRD 1	4.1 ± 0.32	3.6 ± 0.19	4.7 ± 0.74

¹ FI—full irrigated; PRD—partial rootzone drying.

) showed that control plants had significantly higher gs values than plants exposed to PRD, with a reduction of 33–45%.

4. Discussion

The values of Ψ_PD for FI plants are in accordance with those observed for other environmental conditions and cultivars in absence of water stress. The minimum Ψ_PD of −1.16 MPa indicated that plants did not undergo full recovery and hydration during the night, indicating a mild water stress. The absence of differences in midday values of Ψ_stem between FI e PRD treatments associated with lower values of gs in PRD plants indicated a more conservative use of water than FI olive trees, preventing excessive water loss and avoiding leaf dehydration. A similar gs response was observed for other cultivars in field growing conditions and this decrease did not limit the overall photosynthesis process [12,13,14]. Some studies described a clear decrease in PRD olive trees’ water status (Ψ_stem and relative water content), and others reported no differences [20], which appeared to prevent an excessive decline in tree water status by modulating stomatal closure. In our previous studies with this cultivar [21], we observed large fluctuations in midday gs with midday leaf water potential that were relatively stable and higher than −3 MPa; a near-isohydric behaviour was also identified for cv. “Cobrançosa”. The behaviour of PRD plants could be explained by the roots of the well-watered side, which maintained the plant water status while dehydrating roots; this may be responsible for inducing stomatal closure by sending chemical signals to the shoots through the xylem [22]. The PRD irrigated olive trees exhibited daily sap flow and transpiration consistently lower than FI plants throughout the season, which is further evidence that these plants effectively regulate water lost by stomatal closure, showing a more conservative water use strategy.

5. Conclusions

Preliminary results of this study performed in field-grown olive trees showed that partial rootzone drying irrigation did not affect plant water relations, as expressed by bulk leaf water potential when the total amount of water supplied to these adult olive trees was 50% of that supplied to control plants. The stomatal closure observed in PRD plants affects water use in PRD₅₀ plants as shown by a clear reduction of sap flow and transpiration of mature olive trees. The coordinated adjustment in stomatal responses may represent an adaptive advantage in water deficit conditions induced by PRD irrigation. Further research is needed to understand the long-term yield and water use efficiency response of Cv. Cobrançosa to this irrigation strategy.