# Grapevine, Berry and Soil Indicators to Manage Minimal Irrigation Strategy in Semi-Arid Conditions: Example of Grenache Noir (Vitis vinifera L.)

Pascal Marty, Antoine Lespès, François Colin, Anne Pellegrino, Alain Deloire

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Pascal Marty, Antoine Lespès, François Colin, Anne Pellegrino, Alain Deloire. Grapevine, Berry and Soil Indicators to Manage Minimal Irrigation Strategy in Semi-Arid Conditions: Example of Grenache Noir (Vitis vinifera L.). Vitis, 2025, 64, pp.15. 10.5073/vitis.2025.64.15. hal-05411309

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Pascal Marty¹*, Antoine Lespès², François Colin³, Anne Pellegrino⁴, Alain Deloire⁵

# Grapevine, Berry and Soil Indicators to Manage Minimal Irrigation Strategy in Semi-Arid Conditions: Example of Grenache Noir (*Vitis vinifera* L.)

## Affiliations

¹Cultivé'EAU research group, Domaine Lafage, Perpignan, France.

²Cultivé'EAU research group, Domaine Lafage, Perpignan, France.

³UMR G-EAU, University of Montpellier, INRAE, Institut Agro, CIRAD, IRD, AgroParisTech, Montpellier, France.

⁴UMR LEPSE, University of Montpellier, INRAE, Institut Agro Montpellier, Montpellier, France.

⁵University of Montpellier, Institut Agro Montpellier, Montpellier, France.

## Correspondence

*Pascal Marty: pmarty@domaine-lafage.com, Antoine Lespès: alespes@domaine-lafage.com, François Colin: francois.colin@supagro.fr,

Anne Pellegrino: anne.pellegrino@supagro.fr, Alain Deloire: alain.deloire@supagro.fr

## Summary

The combination of low rainfall, high evapotranspiration and low water availability under semi-arid conditions necessitates rethinking cultural practices including the use of irrigation. This article aims to present necessary soil-climate and vegetative-berry development variables to be considered to manage a minimal irrigation for ensuring yield consistency and avoiding potential inhibition of berry development and maturation. A two-year study (2022-2023) was conducted on 'Grenache N'. (*Vitis vinifera* L.) grafted onto 110R in commercial vineyards located in the wine region of Pyrénées-Orientales (PO), South of France. One drip-irrigated and one rainfed treatment were compared. Among the main findings of this study, the relationship between soil water content and predawn leaf water potential differed for irrigated treatment compared to the rainfed treatment. This highlights the need for a proper calibration curve between these two variables when considering drip irrigation. In addition, the monitoring of the evolutions of berry sugar and organic acids, in parallel with the evolution of berry volume (or fresh mass), permitted to evaluate the impact of water supply on vine water and carbon functioning and achieve minimal irrigation. When local water availability for irrigation is limited (less than 500 m³ per hectare per year), such soil, plant and berry measurements at key stages can make the difference and help surpass physiological limits. This study ultimately proposes an easy-to-conduct field method to help winegrowers achieve predefined yield and wine quality targets while minimizing their water footprint.

## Keywords

Precision irrigation, water deficit, berry growth, sugar accumulation, malic acid degradation.

## Introduction

The wine industry is significantly disrupted by the global climate warming and higher exposure of grapevine to extreme events such as frost, heat waves and drought (Martínez-Lüscher et al., 2020; Ollat et al., 2016). The negative impacts of those adverse conditions on yield components, berry metabolism and wine aromatic profiles threaten the vineyards sustainability and profitability. The French wine region of Pyrénées-Orientales (PO) is particularly vulnerable to climate changes because of the high inter-annual variability of rainfall amount, while evapotranspiration rate and heat waves increase. Since 2021, this region suffers from semi-arid conditions given the deficit between annual rainfall and reference evapotranspiration, which range from -739 mm to -880 mm. In order to face the adverse climate conditions and improve the water relations in the soil-plant-atmosphere continuum (SPAC), winegrowers of the PO region seek adopting different agronomic strategies. In a long-term strategy, this could involve experimentation with plant material, primarily testing different varieties and rootstock combinations that may offer physiological advantages in terms of adaptation under limited water resources (Dayer et al., 2020; Gambetta et al., 2024; Marguerit et al., 2023; Santos et al., 2021). These experiments could also help improve the vine's water and carbon balance by adjusting the fruit-to-leaf area ratio (Deloire et al., 2022), and implementing cultural practices such as precision irrigation (Romero et al., 2022), cover crops (Garcia et al., 2018), and soil amendments to improve organic matter content with biochar (Lippi et al., 2025) or compost (Ramos, 2017) to mention a few.

The water flow in the SPAC is governed by the gradient of water potential and internal grapevine regulations conditioning water absorption by the roots and water loss from aerial organs (Rogiers et al., 2009). Root depth and density are crucial for water adsorption and should be considered

<figure><img src="image_2.png" /></figure>
when determining the total transpirable soil water (Lebon et al., 2006). Root water uptake relies on the transverse osmotic pressure within the cells of the root system and on the activity of aquaporin proteins involving symplastic water transportation (Gambetta et al., 2013). Water loss, through leaf transpiration, depends on the light interception and stomatal control. Transpiration is an essential process for regulating the canopy temperature and enabling mass flow of mineral nutrients. Gas exchanges (H₂O, CO₂ and O₂) take place at the level of the stomata, located under the vine leaf. The grapevine stomatal regulation is still not fully known and depends on genotypes in combination with (micro-) environment regulations. It involves hydraulic and biochemical signals (such as abscisic acid; pH; reactive oxygen species) and genetic regulation of aquaporins (Gambetta et al., 2020). Under severe water deficit, the water flow along the conducing xylem vessels can be disrupted by cavitation, producing embolism (Wason et al., 2021). However, embolism first occurs in leaves in grapevine, thus protecting the stems and perennial organs and lowering the risk of vine death. The size of the vessels formed by the cambium on a yearly basis is also dependent on vine water status, varieties and rootstocks, allowing a supplemental regulation of the water flow in the perennial organs (De Souza et al., 2022) and it is a complex regulation as shown by Roig-Puscama et al. (2021).

Fruits also play a major role in vine water and carbon balance (Pellegrino et al., 2014). Berry development can be described by two growth phases at the population level. The first berry herbaceous growth phase starts with a period of 8 to 10 days of cell division, which is followed by cell growth (Carbonneau et al., 2020; Coombe, 1992; Ojeda et al., 2001). After a transient cessation of growth, called herbaceous plateau, the onset of berry softening (or veraison) marks the second phase of berry volume increase, due to cell enlargement, and the maturation period (Coombe and McCarthy, 2000; Dai et al., 2010; Ollat and Gaudillère, 1996). Despite the independent nature of some of its internal biochemical reactions (Gholami et al., 1995), berry growth and ripening depend on the water and nutrients supplied from the vine, through the xylem and the phloem. Xylem flow takes place very early after fruit set, but become less important from veraison onwards. The phloem flow shows the reverse trend, and represents the main source of sugar and of water after veraison (Greenspan et al., 1994).

During the herbaceous phase, the berry accumulates organic acids, i.e. tartaric and malic acids which are essential for wine quality perspectives. From the onset of berry softening, each berry starts to accumulate sugar until reaching a concentration of 1M (approximately at 18°Brix or 10% v/v potential alcohol) (Shahood et al., 2020). This threshold marks the cessation of sugar and water flows in berry. Thus, the maturation phase can be divided in two sub stages: pre- and post-plateau of berry sugar accumulation (Antalick et al., 2021; Deloire, 2013; Shahood et al., 2015; 2020). While the tartrate content in the berry is generally stable during the maturation phase, malic acid content drops as the result of a reduction of both biosynthesis and its use as a substrate via respiratory metabolism (Ruffner and Hawker, 1977; Sweetman et al., 2009; Volschenk et al., 2017). Thanks to its particular evolution through berry development and ripening, malic acid

is a relevant indicator of the dynamic of berry development and maturity (Shahood et al., 2015). In addition, berry sugar loading was reported to be closely linked to the vine leaf photosynthetic activity during ripening (Wang et al., 2003), making this process also a relevant indicator of vine functioning under different water constraint and stress conditions (Antalick et al., 2021; Bigard et al., 2022; Deloire, 2013; McCarthy and Coombe 1999; Shahood et al., 2015).

This article aims to present necessary soil, plant and berry variables to be considered at the operational level to apply minimal irrigation for ensuring yield consistency and avoiding potential alteration of berry maturation under semi-arid conditions. Notably, the relevance of two berry physiological indicators linked to vine functioning, i.e. the dynamic of berry sugar loading and the berry malic acid degradation were studied (Deloire, 2013; Rienth et al., 2016; Shahood et al., 2015; Wilhelm De Almeida et al., 2024). The 2-year survey was conducted on a vineyard planted with ‘Grenache N.’, under semi-arid conditions. One irrigated and one rainfed treatments were compared. As the local water availability in the area is extremely limited, a strategy beyond what is called “precision irrigation, PI” is required. PI involves the precise and accurate spatial and temporal management of irrigation needed to satisfy the water demand and promote crop production, sustainability and profitability (Smith and Baillie, 2009). In our case, minimal irrigation means using a maximum of 500 m³ per hectare of vineyard per year. Thus, water inputs during the vine cycle were reasoned to avoid surpassing physiological limits such as leaf loss, berry shriveling and potential inhibition of the maturation process.

## Material and Methods

### Experimental site

This study was carried out in 2022 and 2023 in the vineyard estate of Mas Moutou (42°48'15"N, 2°50'33E; Elev. 56 m) located in the Crest area of Roussillon, in the very south of France near the Spanish border.

This area is under a Mediterranean climate (Csa/Csb under the Köppen-Geiger climate classification), but reached semi-arid conditions in 2022 and 2023, with annual rainfall of respectively 305 mm and 245 mm. Daily weather records were given by Météo France from the weather station of Perpignan (42°44'14"N, 2°52'22"E; Elev. 42 m). They included rainfall, air humidity, air temperature, wind speed and solar radiation. From these parameters, reference evapotranspiration (ET₀) was calculated from the Penman-Monteith equation (Allen et al., 1998). In order to better figure out the course of each vintage, four sub-periods were defined based on the sum of Growing Degree Days, base 0 °C, from January 1 (GDD₀°C,t₁). The base temperature of 0 °C is in line with the models used to estimate budbreak and flowering in this study (Moncur et al., 1989; Parker et al., 2011). The four limit values retained were GDD₀°C,t₁ equal to 800 (GDD₁), 1800 (GDD₂), 2800 (GDD₃) and 3800 (GDD₄), because they correspond approximately to budbreak, flowering, veraison and harvest at the “Fresh Fruit” stage in this area.
The dates of budburst and flowering were expressed with the day of the year (DOY). Both dates were estimated from the Growing Degree Days, starting the first day of the year (GDD₀°C,t₁) for budbreak and the sixtieth (GDD₀°C,t₆₀) for flowering (Parker et al., 2011). The GDD to reach budburst and flowering were parameterized and validated for 'Grenache N.' using the historical data collection of the local Agriculture Institute of PO (not shown, dataset for the period 2010 to 2021).

The soil has developed from Quaternary alluvial terrace deposits, characteristic of fluvial sedimentary processes (Würm). A geophysical study (Schlumberger method) of the experimental plot realized in 2021 concluded that the depth of that material is about 5 m. Two soil pits were dug in 2022 in two different zones corresponding to the lowest and highest vegetative expression of the block defined by NDVI values (Normalized Difference Vegetation Index calculated on the 10-meter bands of a Sentinel-2 Level-2A image taken at veraison in 2021). However, the NDVI values over the block only vary from approximately 0.2 to 0.3, and the succession of layers was similar between the two soil pits as expected. The soil is composed of four distinct horizons. The texture is clay loam, sandy clay loam and sandy loam for H1 (0-25 cm), H2 (25-60 cm) and H4 (75-100 cm) respectively. The percentage of gravels of H3 is over 85%. Soil profiles show no sign of waterlogging. The vine roots reach a depth of one meter. The soil water holding capacity, calculated over this one-meter depth as the difference of volumetric moisture between the theoretical field capacity and the wilting point (Saxton and Rawls, 2006), ranged between 91 mm and 97 mm over the block, which can be considered as classical values for this type of soils (Calcaric Cambisol (Hyperskeletic)).

## Plant material and treatments

The experimental plot is located on a 4.3-ha, flat block of Grenache N. grafted onto 110R, and planted in 2009. Planting density is 4000 vines per hectare (2.5 m x 1.0 m). Row orientation is southwest-northeast. Shoots are vertically positioned thanks to one fixed wire and one pair of moving wires. The vines are spur pruned with two arms bearing four spurs each (2 buds per spur; i.e. 16 buds in total per plant). From 2017 to 2021, the average yield was 5.3 t·ha⁻¹ (σ = 0.8), equivalent to 1.3 kg per vine.

Soil management practices correspond to the local standards in organic viticulture: natural grass cover from September to February, incorporating the biomass into the soil by cultivating down to 30 cm depth pre-budbreak, followed by three shallow tillages to control weeds during the growing season.

An irrigation equipment was installed expressly for the experiment in 2022. Two treatments were defined at a commercial scale of 0.3 ha each (2 x 5 rows): IRR, which corresponds to vines watered by drip irrigation (1.6 L·h⁻¹ and a dripper each meter, Netafim AS 2022) and CONT, which is only rainfed. In 2022, IRR was irrigated three times between veraison and harvest, receiving a total volume of water per vine of 48 L (equivalent to 192 m³·ha⁻¹). In 2023, because of drier conditions in spring compared to 2022, IRR was irrigated seven times between budburst and harvest, corresponding to

107 L per vine or 429 m³·ha⁻¹. Irrigation strategy relied on the assessment of the soil water content and level of water distribution in the root zone, together with plant water status at the key stages of budburst, flowering and veraison. Water was supplied in order to avoid severe water stress (predawn leaf water potential, PLWP, higher than -0.8 MPa as defined during the characterization phase of the experimental plot in 2021). Based on previous observations of the wet bulb dimension (a circular right cylinder in shape of depth = 0.8 m and radius = 0.3 m) in relation to the duration of irrigation, each irrigation event was programmed for 10 hours.

## Plant leaf to fruit balance

The exposed canopy area (m² per m² soil) was calculated at veraison using the simplified ELA method (eq. 1) (Murisier, 1996):

$$ELA = (2H + Wc) / Wr \quad (\text{eq. 1})$$

where: H is canopy height (m), Wc is canopy width (m) and Wr is row width (m)

Harvest dates were chosen based on berry tasting and the 'per berry' Sugar Accumulation method as described by Antalick et al. (2021).

The yield at harvest was recorded for each treatment and both vintages, by weighing the total production of the treatment divided by the actual number of productive vines. This average yield indicator was used to avoid biases due to the high variability from one vine to another inherent to any plot in production.

Plant balance was described with the ratio ELA:yield, and the total sugar accumulated in berries per vine, calculated from the total number of berry times the amount of sugar per berry at harvest. Total number of berries per vine was estimated as 95% of the total yield per vine, to take into account the rachis, divided by the average individual berry weight.

## Plant and Soil Water Status

The pressure chamber technique was used to monitor the predawn leaf water potential (PLWP), which is the reference variable for addressing grapevine water status (Carbonneau, 1998; Deloire et al., 2020; Knipfer et al., 2020, Scholander et al., 1965). The measurements were performed one to two hours before sunrise on a weekly basis from flowering to harvest in 2022 and 2023. The truncated mean of six measurements (one leaf per vine), i.e. the mean discarding the highest and lowest values was then considered at each date.

One capacitive probe (AquaCheck, Corhize SAS, Estillac, France) was installed within each treatment (CONT, IRR), under the row, equidistant between a vine and a dripper. The probes measure the volumetric soil moisture every 10 cm, from 10 cm to 60 cm depth. Data are recorded, and available online, every 30 min. The volumetric soil moisture in percentage is given for specific soil type by the manufacturer.

The daily soil moisture (SMₐ) used in this study was calculated as the mean of the values at the six depths over 24h. After the full winter recharge in spring 2022, the maximum soil moisture, recorded 48 h after rainfall events
(SM<sub>max</sub>, equal to 32.7% and 33.1% for IRR and CONT respectively) was assimilated to field capacity threshold. After the long summer drought period in 2022, the minimum soil humidity (SM<sub>min</sub>, equal to 20.9% and 18.7% for IRR and CONT respectively) was assumed to correspond to the soil moisture threshold resulting from maximum root water extraction. Thus, the daily Fraction of Transpirable Soil Water (FTSW<sub>d</sub>), was calculated for each treatment as follows (eq. 2):

$$FTSW_d = (SM_d - SM_{min}) / (SM_{max} - SM_{min}) \quad (\text{eq. 2})$$

## Berry size and primary metabolites

Ten bunches per treatment were taken, following the estate standard procedure, in the vineyard on the same day as PLWP measurements, on a weekly basis from June to September in 2022 and 2023. The bunches were brought back to the laboratory where they were destemmed by cutting the berries at the pedicel level using scissors. All the berries from the ten bunches were mixed together. From the sample, three lots of a hundred berries were randomly collected.

The fresh mass of each lot of berries was weighted using a precision scale (0.1 g). In 2023, the volume of individual berries (BV, in mL) was also measured on 200 berries per sampling date using Dyostem® (Vivelys SAS, Villeneuve-les-Maguelone, France), from July to September. Then, the concentrations of malic acid and sugar (glucose and fructose) were assayed on each lot of berries by automated enzymatic method on a sequential analyser (Mindray BS800M equipment). Finally, the quantities of malic acid or of sugar per berry (in mg per berry) were estimated from their concentration (in g L<sup>-1</sup>) times berry volume (BV). For this purpose, a correlation between the BV and fresh mass (BFM, in g) on a population of two hundred berries was used (eq. 3):

$$BV = 0.706 * BFM + 0.184 (R^2 = 0.97, p < 0.001) \quad (\text{eq. 3})$$

## Statistical Analyses

Statistical analyses were conducted with R studio version 3.6.1 (RStudio: Integrated Development for R, Boston, MA, USA) for Windows. Means were calculated and compared

with the Fisher test. Pearson correlations were calculated using the same software.

The adjustments fitted for CONT and IRR for plant and/or soil variables were compared using the test of Fisher-Snedecor.

## Results

### Weather characteristics

Historical weather data show how fast the climate recently changed in the region. For instance, mean temperature over the growing season (April to September) in Perpignan was 20.5 °C in 1991-2020 (30-year average), but 21.2 °C in 2016-2020. Therefore, using the five years prior the study (2017-2021) as a reference period appears relevant to encompass the evolution of vintage weather patterns for the purpose of this study. Doing so, over the period October-March (i.e. from leaf fall to budburst), total rainfall was observed to be close to the reference period in 2022 (355 mm, +28 mm) but was reduced to 127 mm (-200 mm) in 2023. From April to September (i.e. during the growing season), total rainfall was reduced by 128 mm and 102 mm compared to the reference average (equal to 240 mm), and mean temperature increased by 1.4 °C and 0.8 °C in 2022 and 2023, respectively.

Table 1 presents for four sub-periods over the cropping seasons 2022 and 2023 a summary of the variations of six weather indicators based on temperature conditions, water supply and water demand. As an example, the value in the red-squared cell means that the mean temperature during the last sub-period in 2022, i.e. from approximately veraison to post plateau of sugar accumulation, was 2.6 °C higher than the average of 25.5 °C for the reference period. GDD<sub>1</sub> (prior budburst) was similar to the reference period in 2022 and 2023, except the rainfall was reduced to 78 mm (-36 mm) in 2023. GDD<sub>2</sub> (c. from budburst to flowering) was drier in 2022 and 2023. Total rainfall during this period was only 58 mm in 2022 (-88 mm) and 27 mm in 2023 (-119 mm). As a result, the climatic water balance deficit (i.e. the difference between rainfall and ETo) was -214 mm in 2022 and -301 mm in 2023. GDD<sub>3</sub> (ca. from flowering to veraison) was similar to the reference

<table><caption>Table 1: Summary of 2022 and 2023 weather indicators by sub-period, compared to long term average over the period 2017-2021 ("LTA5"). GDD<sub>0°C,t1</sub> = 800 corresponds to approximately budbreak, 1800 to flowering, 2800 to veraison and 3800 to harvest "Fresh Fruit". Values in italics represent difference compared to LTA5; values in bold represent particularly relevant information for the study. Abbreviations: Tmean, average mean temperature; ETo and Rain, cumulative reference evapotranspiration and precipitations; Rain ≥ 5 mm; Tmax ≥ 35 °C (days), number of days receiving 5 mm of rain or more, and number of days when the maximum temperature is above or equal to 35 °C which is the definition of a heat wave.</caption><thead><tr><th rowspan="2">GDD<sub>0°C,t1</sub></th><th colspan="3">[1;800]</th><th colspan="3">[800;1800]</th><th colspan="3">[1800;2800]</th><th colspan="3">[2800;3800]</th></tr><tr><th>LTA5</th><th>2022</th><th>2023</th><th>LTA5</th><th>2022</th><th>2023</th><th>LTA5</th><th>2022</th><th>2023</th><th>LTA5</th><th>2022</th><th>2023</th></tr></thead><tbody><tr><th>Date</th><td>Mar 20</td><td>=</td><td>+3</td><td>May 25</td><td>-3</td><td>-1</td><td>Jul 7</td><td>-5</td><td>-3</td><td>Aug 16</td><td>-8</td><td>-3</td></tr><tr><th>T<sub>mean</sub> (°C)</th><td>10.2</td><td>=</td><td>-0.4</td><td>15.2</td><td>+0.7</td><td>+1.1</td><td>23.0</td><td>+0.7</td><td>+0.8</td><td>25.5</td><td><strong>+2.6</strong></td><td>+0.1</td></tr><tr><th>ETo (mm)</th><td>196</td><td>-2</td><td>+26</td><td>278</td><td>-6</td><td>+50</td><td>283</td><td>+7</td><td>=</td><td>278</td><td>+12</td><td>+20</td></tr><tr><th>Rain (mm)</th><td>114</td><td>+19</td><td><strong>-36</strong></td><td>146</td><td><strong>-88</strong></td><td><strong>-119</strong></td><td>35</td><td>-21</td><td>-9</td><td>24</td><td>-20</td><td>+8</td></tr><tr><th>Rain ≥ 5 mm (days)</th><td>5</td><td>+1</td><td>-2</td><td>6</td><td><strong>-4</strong></td><td><strong>-4</strong></td><td>2</td><td>-1</td><td>-1</td><td>2</td><td>-2</td><td>-1</td></tr><tr><th>T<sub>max</sub> ≥ 35 °C (days)</th><td>0</td><td>=</td><td>=</td><td>0</td><td>=</td><td>=</td><td>2</td><td>+2</td><td>-2</td><td>4</td><td><strong>+6</strong></td><td>-1</td></tr><tr><th>Rain - ETo (mm)</th><td>-82</td><td>-61</td><td><strong>-144</strong></td><td>-132</td><td><strong>-214</strong></td><td><strong>-301</strong></td><td>-248</td><td>-276</td><td>-257</td><td>-254</td><td>-286</td><td>-265</td></tr></tbody></table>
rence period in 2022 and 2023. However, GDD₄ (ca. from veraison to harvest) was 2.6 °C warmer in 2022 but was equal to the reference period in 2023. Among the different indicators selected, the main climatic concern is the decrease in rainfall in winter and spring (until flowering) which was particularly drastic in 2023.

## Plant and soil water status

The evolutions of plant and soil water status varied between the two years (Figure 1). In 2022, PLWP for both treatments gradually decreased from -0.2 MPa to -0.5 MPa from flowering to veraison. Then, IRR was irrigated three times before harvest. PLWP remained above -1 MPa for both treatments over the period veraison to harvest. Finally, IRR was irrigated two more times after harvest to avoid early leaf senescence. In 2023, IRR was irrigated twice between budburst and flowering, three times between flowering and veraison and again twice between veraison and harvest. Consequently, PLWP fluctuated around -0.5 MPa up to prior veraison for IRR, while it decreased from -0.5 MPa to -0.7 MPa for CONT. The most differences between treatments were observed only from flowering to veraison. The minimum value (ca. -1 MPa) was reached for both treatments just prior to rainfall events (DOY 208 and 209) between veraison and harvest. Finally, IRR was irrigated one more time after harvest.

In 2022, the absence of rain led to a gradual decline in FTSW for CONT from maximal (1) to minimal (0) rates. Irrigation

events after veraison permitted to temporarily increase the FTSW up to 0.8. In 2023, FTSW was equivalent to ca. 0.4 at budburst for both treatments (CONT, IRR). For CONT, FTSW gradually decreased until flowering and remained between 0.1 and 0.25 until mid-September thanks to a series of efficient precipitations. Similarly to 2022, FTSW increased up to 0.8 after each irrigation event in 2023, but displayed a rapid decrease during the week following irrigation.

Leaf water potential (PLWP) was plotted as a function of the FTSW for the two treatments (CONT, IRR), considering separately the 2-month period before irrigation from the period of irrigation for the IRR treatment in 2022 (Fig. 2). PLWP was well correlated with FTSW for both IRR and CONT treatments, as reported in the literature (Lebon et al., 2003; Pellegrino et al., 2004). However, the relationship differed between the two treatments, and between the periods before vs during irrigation for the IRR treatment. Indeed, at a given calculated FTSW, PLWP was lower for the irrigated plants than for the rainfed ones. Thus, it is possible to determine the predawn leaf water potential on a daily basis (PLWP<sub>d</sub>) from FTSW<sub>d</sub> calculated from soil sensors and *vice versa*, in each considered situation.

## Vine leaf to fruit balance

In 2023, the exposed leaf area (ELA) was equivalent to 45% and 65% of that of 2022, for CONT and IRR respectively (Table 2). Similarly, the yield in 2023 for CONT and IRR reached

<figure><img src="image_3.png" /></figure>

Figure 1: Evolution of fraction of transpirable soil water (FTSW) and predawn leaf water potential (PLWP) in 2022 (A) and 2023 (B). The green arrows correspond to irrigation events and the blue ones to rain events higher than 5 mm (on a per day basis). The black bars indicate the main phenological stages. Significant mean difference of PLWP between CONT (rainfed) and IRR (irrigated) treatments on each date (p < 0.05, *; p < 0.01, **, p < 0.001, ***). DOY= day of the year.
<table><caption>Table 2: Summary of 2022 and 2023 exposed leaf area (ELA), yield and berry sugar accumulation per plant in CONT (rainfed) and IRR (irrigated) treatments. Statistical test not applicable.</caption><thead><tr><th rowspan="2">Treatment</th><th colspan="2">2022</th><th colspan="2">2023</th></tr><tr><th>CONT</th><th>IRR</th><th>CONT</th><th>IRR</th></tr></thead><tbody><tr><td>ELA (m² per m² of soil)</td><td colspan="2">1.12</td><td>0.51</td><td>0.73</td></tr><tr><td>Yield (kg per vine)</td><td>2.70</td><td>3.08</td><td>1.20</td><td>2.08</td></tr><tr><td>ELA/Yield (m²·kg⁻¹)</td><td>1.04</td><td>0.91</td><td>1.06</td><td>0.88</td></tr><tr><td>Total Sugar (g per vine)</td><td>417</td><td>488</td><td>233</td><td>387</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>Figure 2: Fraction of transpirable soil water (FTSW) plotted as a function of predawn leaf water potential (PLWP) for CONT (rainfed) and IRR (irrigated) treatments from May 01 (DOY = 121) to September 15 (DOY = 258) in 2022 and 2023. Each point is the mean of 6 PLWP values and the FTSW measured on the same day. The likelihood-ratio test shows a significant difference between the two models (p < 0.001).</figcaption></figure>

respectively 44% and 67% of the values obtained in 2022. Finally, both treatments displayed an appropriate leaf-to-fruit ratio (ELA/Yield) in 2022 and 2023, above 0.88 m²/kg. Due to the high yield differences between years, the total amount of sugar accumulated for a single vine ranged from 233 g (CONT, in 2023) to 488 g (IRR, in 2022). In both years, yield and total sugar per vine were increased in IRR.

## Berry development

The evolution of the berry fresh mass (measurements taken on berry population and presented per berry: g per berry) were fitted for both treatments and vintages (Fig. 3). Those evolutions, from two weeks before veraison to harvest, followed a characteristic S-shaped curve. The abscissa of the inflection point was 195 and 196 for CONT and IRR respectively in 2022, and 204 and 202 for CONT and IRR respectively in 2023. The 10% alcohol potentials, a berry physiological indicator of the end of water and sugar loading as proposed by Shahood et al. (2020), were reached the same day in 2022 (DOY 213) and with two days apart in 2013 (DOY 218 and 220 for CONT and IRR respectively).

<figure><img src="image_5.png" /><figcaption>Figure 3: Berry fresh mass evolution. Vertical bars indicate the standard deviation between the 3 sub-samples. The fit sigmoid curves were obtained following the nonlinear least-squared data fitting protocol. The sum of squared residuals was lower than 0.1 for each set of data. The arrows indicate 10% alcohol potential (17.5 g:1%). The likelihood-ratio test indicates no difference between the two models in 2022 but shows a significant difference in 2023 (p < 0.001). (IRR – irrigated; CONT – rainfed, 22 and 23 – 2022 and 2023 seasons). DOY – day of the year.</figcaption></figure>

In 2022, the kinetics of the evolution of berry fresh mass were similar for CONT and IRR. Thus, supplemental irrigation over the ripening period did not increase the volume of the berries. In contrast, treatments differed in 2023, with a positive effect of irrigation on berry weight (+14%). At maximum berry weight (i.e. 10% alcohol potentials) the control treatment displayed a 31% increase in 2022 compared to 2023, which was consistent with the higher PLWP from flowering to veraison in 2022 (Fig. 1). Irrigation in 2023 did not permit to achieve the berry fresh weight obtained in 2022, which remained 23% lower. Interestingly, no berry water loss was observed from maximum berry weight onwards, for both IRR and CONT treatments, despite a potential interruption of phloem functioning (Savoi et al., 2021).

<figure><img src="image_6.png" /><figcaption>Figure 4: Evolution of malic acid (AM) and sugar contents (SB) per berry, per date (2023). The grey and orange arrows indicate 10% alcohol potential (17.5 g L⁻¹:1% vol.) for CONT (rainfed) and IRR (irrigated) treatments, respectively. The likelihood-ratio test shows a significant difference between the two models for malic acid (p < 0.05) but no difference for sugar. DOY – day of the year.</figcaption></figure>
The evolution of berry sugar accumulation (mg per berry) from veraison onward in 2023 was illustrated in Fig. 4. The dynamics of berry sugar accumulation did not display any difference between IRR and CONT. They showed an increase of +10 mg per berry and day one week after the onset of veraison, which is in agreement with previous publications (Antalick et al., 2021; Deloire, 2013).

Malic acid also showed a similar evolution over time for both treatments except that the peak was higher for IRR (15 g·L⁻¹) than CONT (12 g·L⁻¹) (Figure 4). This peak was reached on DOY = 180, i.e. about 10 days prior the inception of veraison determined by the daily sugar accumulation (Δmg per berry and day). From the onset of veraison, a degradation of malic acid was observed for both treatments. The lowest amount of malic acid was reached when berry sugar concentration reached around 10% in probable alcohol (which corresponds to the “theoretical” plateau of berry sugar accumulation on a per berry basis, as shown by Shahood et al. (2020).

Discussion

Water supply efficiency on vine water status

<figure><img src="image_7.png" /></figure>

Figure 5: Effects of rain and irrigation amount on a per hectare basis on the gain of calculated Fraction of transpirable soil water (FTSW) as measured for IRR (irrigated treatment). The gain corresponds to the difference between the maximum FTSW reached after the watering event and its value before. All irrigation events (ni = 13) and all rain events higher than 5 mm (nr = 12) from June 2022 to September 2023 were considered.

The impact of water supply through a rainfall event or an irrigation on calculated FTSW rise was quantified (Fig. 5). As expressed by the equations of the trend curves, 10 m³·ha⁻¹ received by irrigation and rain led to an increase of FTSW of respectively 11.4% and 0.9%. The ratio of FTSW increase between irrigation and rain is then 12.7. However, the rewetted soil volume should be considered to interpret the efficiency of water supply on FTSW rise. Indeed, the increase of FTSW after a rainfall event concerns the total soil volume i.e. 10,000 m³ per ha, considering a one-meter depth. In contrast, FTSW increase is restricted to the volume of the wet bulb in the case of drip irrigation. The wet bulb was described in the

present study as a right cylinder of 0.226 m³ representing 905 m³ at the ha scale or 9.0% of the total soil volume.

Minimal irrigation using drip irrigation involves adding water on a per-vine basis and targeting the root zone in order to optimize water efficiency by reducing water losses though evaporation and/or runoff and avoiding watering soil zones poorly explored by the root system. The irrigation rate and duration should thus be finely reasoned to optimize the wet bulb shape and size, in regards to soil type and root architecture (Cook et al., 2003; Pisciotta et al., 2018). Quantifying the impact of irrigation event on FTSW rise as shown in Fig. 5 ultimately enables more effective decision-making for managing small vineyard zones, particularly in the context of water scarcity encountered in semi-arid wine regions with limited access to water for irrigation.

Even though the predawn leaf water potential is a stronger and more direct assessment of the plant water status and thus more reliable to manage irrigation, it is difficult to implement it at a large scale. Using a capacitive probe can be a good alternative. Fig. 2 clearly demonstrates that the relationships between the evolution of soil moisture measured with the capacitive probe and the measurement of the predawn leaf water potential (PLWP) are consistent, allowing for a reasoned irrigation management by utilizing capacitive probes once they are calibrated by PLWP. This relationship is indeed unique because the measurement of the capacitive probe is only reliable for one specific point and depends on the wet bulb size.

In practice, the wide range of soil humidity levels over an irrigated block, a fortiori with drip irrigation, requires positioning the capacitive probe in the vine strip, in between two drippers. It is then necessary to calibrate an automatic decision-making tool with the plant water status throughout its growth cycle, which depends on water supply to the root zone and capacity of roots to adsorb water, which confirms the relevance of the FTSW calculation. Moreover, it is necessary to check this calibration once a year because of the likely evolution of the root system in the wet bulb, mainly regarding the development of the fine roots.

Minimal irrigation strategy under semi-arid climates is critical to sustain grapevine performance

Irrigation events in 2022 and 2023 increased FTSW for IRR compared to CONT for a one-week period each time (Fig. 1). While irrigation between budburst and veraison (2023) also permitted to maintain PLWP to higher values for IRR than CONT, the difference between IRR and CONT were not systematic after an irrigation event over the period veraison to harvest for both vintages (2022, 2023). The minimal irrigation strategy in this vineyard context implanted with Grenache N./110R permitted to maintain PLWP above -0.5 MPa between flowering and veraison, with lower values of -0.6 MPa only reached for few days, and mostly above -0.8 MPa between veraison and harvest, with a minimum peak at -1 MPa. Although the limit of PLWP were pushed to lower levels compared to the conventional thresholds proposed for other less water-efficient grape varieties (Deloire and Pellegrino, 2021; Medrano et al., 2003), irrigation appeared efficient to avoid
major alteration of vine and/or grape development and func-
tioning such as leaf fall or berry shrivelling.

The impact of vine water status on vine and berry develop-
ment and physiology are known to depend on i) the vine wa-
ter status thresholds; ii) the phenological stage; iii) the du-
ration of the vine water constraint or stress (Alatzas et al.,
2021; Deloire and Pellegrino, 2021; Medrano et al., 2003;
Pagay, 2016). Interestingly, despite the high levels of water
deficit experienced by the vine throughout the 2023 season,
particularly for CONT, the leaf area-to-yield ratio was similar
between the two treatments and the two years and was con-
sidered balanced (Kliewer and Dokoozlian, 2005). However,
irrigation permitted to increase total yield and total sugar
accumulation in berries at the vine level for both years. The
yield gain between IRR and CONT expressed in tonnes per
ML of water supplied by irrigation, was ca. 8 tonnes ML⁻¹,
which is quite high compared to high productive vineyards
of Southern hemisphere where changes in yield from
2.3 tonnes ML⁻¹ up to 8 tonnes ML⁻¹ were reported (Knowling
et al., 2023). The gain of sugar per volume of irrigated
water (gL⁻¹) was ca. 0.7. Such yield and sugar increments for
IRR compared to CONT for both years reflect improved eco-
nomic profitability and better plant functioning, which indi-
cates better overall sustainability of the vineyard following a
minimal irrigation strategy.

Basic berry development-based indicators enable to sharpen irrigation strategies

This study confirmed the relevance of the berry physiological
indicators to supplement soil and plant water statutes indi-
cators.

The dynamics of berry sugar and malic acid evolutions were
both studied in terms of concentration as well as on a per
berry basis (mg per berry). Despite the importance of the role
of seeds in fruit development, we did not study this aspect in
our work, estimating, as shown by the results in Fig. 3, that
vine water status was the major determinant of berry volume
(Ojeda et al., 2001).

Berry volume (or berry fresh mass), berry sugar accumulation and berry malic acid degradation provided relevant information about berry water and carbon balance. Notably, berry shrivelling was not observed for any year and treatment. Thus, the restricted volume of water supplied from rain and/or irrigation permitted to compensate for water loss in the fruit. We hypothesize that this could be due to: i) the absence of water backflow from berry to vine; ii) the presence of favourable microclimate of the fruit zone maintaining correct vapor pressure deficit (VPD), thereby limiting berry water loss through evaporation (Dokoozlian and Kliewer, 1996; Reshef et al., 2018; Zhang and Keller, 2015).

As shown by previous studies on other varieties such as
'Pinot Meunier' and 'Syrah' (Shahood et al., 2015; 2020), our
results demonstrated for the first time in 'Grenache N'. that the
concentration and quantity of malic acid per berry decreased
from the onset of veraison, almost simultaneously with the
accumulation of sugar per berry, even when working with a
population of non-synchronized berries. In the warm climate

where the vineyard is located, the breakdown of malic acid
begins as the fruit accumulates sugar, with no delay between
malic acid degradation and berry sugar accumulation, unlike
in cooler climates. In cooler climates, the improved plant car-
bohydrate status is achieved by inhibiting berry respiration
more than leaf photosynthesis (Rienth et al., 2016; Luchaire
et al., 2017).

Ultimately, our results indicate that, despite the variation in
vine water status (PLWP) and fruit volume (yield) between
treatments, berry carbon functioning (malic acid degradation
and sugar accumulation) did not seem to be affected, even
for the driest year (2023). However, under water constraints
leading to the reduction in photosynthetic activity, it has been
shown that the accumulation of sugars in the berry occurs to
the detriment of starch stored in the roots (Rossouw et al.,
2017). Lower carbon reserves in perennial parts may alter the
plant's physiology the following year, mainly from bud break
to berry set. The use of reasoned irrigation, managed in such
a way that physiological blockages or carbohydrate reserves
depletion are avoided, should thus be seriously considered.
Therefore, the use of appropriate decision-making tools for
irrigation management, in the context of grower goals is cru-
cial (Levin and Nackley, 2021).

Conclusion

This 2-year study on 'Grenache N.' (Vitis Vinifera L.) grafted onto 110R was conducted in the French wine region of Pyrénées-Orientales, which has been considered a region with a high level of rainfall deficiency since 2021. The combination of low rainfall, high evapotranspiration, and low water allocation for agriculture necessitate establishing minimal irrigation strategy.

To address this challenge, our study confirmed the relevance
of a soil-plant-atmosphere approach, and in particular:

- The obligation to assess on field the basic hydraulic properties of the soil (wilting point, field capacity, saturation moisture) and to describe the vine root system in order to quantify the soil water storage capacity in the root zone.

- The relationships between the evolution of soil moisture measured with a capacitive probe and the measurement of predawn leaf water potential (PLWP) are consistent, allowing for reasoned irrigation management by utilizing capacitive probes once calibrated by PLWP.

- The relevance of using appropriate vine and berry physiological indicators, such as the dynamics of berry sugar loading and berry malic acid degradation, considering the volume of the fruit as a variable. This allows results to be obtained i) on a per berry basis (in mg per berry, indicating the stimulation or inhibition of berry compound loading, biosynthesis, and/or degradation); and ii) in concentration, which relates to the evolution of berry volume.

To conclude, it is crucial to consider the various advantages
and disadvantages of a given tool for irrigation management
in the context of growers' production goals. In this case study,
a maximum water allocation of 500 m³ ha⁻¹ per year kept vine
health, yield and fruit quality above sustainable thresholds
despite PLWP values of -0.8 MPa and lower considered in
several studies as indicators of severe water stress. However, no universal guidelines can be proposed, as calibration is needed for each specific situation (variety x soil type x root implementation x climate) for precision irrigation management using minimal water, applied on a per-vine basis, considering soil moisture (soil humectation) in the root zone.

## Acknowledgements

This study was carried out within the framework of the research project on new agroecological models in the wine regions of the Mediterranean area, created and funded by the Domaine Lafage, Roussillon, France. This project aims to respond to the growing pressure on water resources. The operational group consists in the R&D staff of Domaine Lafage, Institut Agro Montpellier and Chambre of Agriculture of the PO. The authors would like to thank Joaquín Zapata Gerstle (2022) and Nathan Hérent (2023) for their contribution in the data collection.

## Conflicts of interest

The authors declare that they do not have any conflicts of interest.

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