Effect of Differentiated Irrigation Availability on the Postharvest Quality Parameters of Actinidia chinensis Fruits

1. Introduction

Water availability is one of the most influential environmental factors shaping productivity, physiological performance, and fruit quality in horticultural crops. Deficit irrigation strategies, including regulated deficit irrigation (RDI), have been widely implemented to optimize water use efficiency (WUE) while maintaining or improving fruit quality under water-limited conditions. These approaches apply irrigation below full crop evapotranspiration (ETc) during phenological stages that are less sensitive to water stress [1]. RDI has been shown to enhance WUE and reduce water waste while preserving or even improving crop quality in several horticultural systems, particularly in woody perennials and fruit crops, by imposing controlled water deficits tailored to specific developmental phases [2,3]. Over recent decades, climate change has intensified water scarcity across major horticultural regions, especially in Mediterranean and temperate climates, due to more frequent and severe droughts, altered rainfall patterns, and increased evaporative demand [4]. Under these conditions, efficient irrigation management is essential to sustain productivity and fruit quality while conserving limited water resources. Fruit trees are particularly sensitive to soil water availability because of their high transpiration rates and relatively shallow root systems. Water deficits can trigger complex physiological responses, including stomatal regulation, hydraulic adjustments, and metabolic reprogramming, that collectively influence growth and quality outcomes [5,6]. In kiwifruit (Actinidia spp.), water stress affects gas exchange, carbohydrate partitioning, and fruit physiology, with consequences for key quality traits such as soluble solids concentration (SSC), titratable acidity (TA), and dry matter content [7,8,9]. Classic studies demonstrated that withholding irrigation late in the season can markedly increase soluble carbohydrate concentration without negatively affecting firmness or storability, and that these quality responses persist during postharvest storage [10,11,12]. More recent physiological research confirms that Actinidia chinensis is highly responsive to changes in soil water potential, with prolonged deficits reducing gas exchange and vegetative growth, underscoring the importance of irrigation timing and magnitude [9,13]. Mediterranean environments, characterized by hot, dry summers and irregular rainfall, place additional pressure on irrigation resources and require optimized water management to sustain profitable production. Moderate deficit irrigation strategies are increasingly adopted to balance water conservation with fruit quality objectives; however, their influence on postharvest quality dynamics remains insufficiently understood. While numerous studies have examined the effects of irrigation regimes on yield and harvest-time quality, fewer have evaluated how preharvest water restriction affects fruit behavior during cold storage, particularly in high-yielding and commercially important cultivars such as ‘Gold3’ [14,15]. Recent work on A. chinensis has shown that late-season RDI can enhance intrinsic WUE, soluble solids content, and sensory attributes without compromising yield or commercial quality. For instance, short-term RDI applied during late ripening increased intrinsic WUE and improved fruit quality while maintaining firmness and yield in cv. Soreli [3]. Similarly, deficit drip irrigation under rain-shelter cultivation improved firmness, SSC, TA, and water productivity without adverse effects on yield [12]. These findings highlight the potential of RDI as a sustainable strategy to conserve water while maintaining or enhancing fruit quality in kiwifruit production systems. +Understanding whether moderate preharvest irrigation reduction induces physiological changes that influence postharvest storage behavior and quality evolution is therefore essential. In this context, the present study evaluated the effects of controlled water deficit on A. chinensis cv. ‘Gold3’, assessing how differentiated irrigation volumes affect morphometric, physicochemical, colorimetric, and nutraceutical parameters, with particular attention to quality changes emerging during postharvest cold storage.

2. Materials and Methods

2.1. Experimental Site and Irrigation Treatments

The experiment was conducted during the 2024–2025 biennium in the Gioia Tauro Plain (Reggio Calabria, Italy), within the municipality of Polistena, an area highly suited to Actinidia cultivation (Adornato Farm; 38°25'14.9"N, 16°01'56.9"E). The study focused on Actinidia chinensis (Planch.) cv. Gold3. The orchard was established in 2015, with vines trained to a pergola system and spaced 4 m within rows and 5 m between rows (500 vines ha⁻¹). A male-to-female ratio of 1:7 was adopted using the pollinizer ‘Bélen’. Standard cultural practices, including bud load regulation and winter and summer pruning, were applied. Irrigation was supplied through an overhead sprinkler system operating daily from late April to late October.

2.2. Climatic and Pedological Characterization

According to the Köppen–Geiger classification, the study area corresponds to a Mediterranean climate (Csa), characterized by winter-concentrated precipitation and hot, dry summers. Soil characteristics were obtained from the Soil Map of Calabria (ARSAC) and classified within Pedological Subsystem 3.2 for the municipality of Polistena. Mean reference evapotranspiration (ET₀) from April to October averaged 4.93 mm day⁻¹ in 2025 and 4.84 mm day⁻¹ in 2024, while mean vapor pressure deficit (VPD) for the same period was 0.68 kPa. The highest mean maximum temperature occurred in July (32 °C), and rainfall was mainly concentrated during autumn and winter. Meteorological data were recorded using an on-farm weather station connected to the WinEnet agrometeorological network (METOS Italia S.r.l., Bolzano, Italy), providing continuous measurements of temperature, relative humidity, solar radiation, wind speed, and rainfall.

2.3. Irrigation Treatments, Crop Evapotranspiration Estimation and Fruit Sampling

Ten vines per irrigation treatment were selected based on comparable vegetative development and fruit load. Three irrigation regimes were applied: 120, 90, and 75 L plant⁻¹ day⁻¹. The 120 L treatment represented standard farm practice, whereas the other two levels were imposed as reduced-water regimes. Crop evapotranspiration (ETc) was estimated for July, August, and September using a crop coefficient (Kc) of 1.15, following FAO guidelines [1]. Average ETc values were 7.14 mm day⁻¹ in July, 6.31 mm day⁻¹ in August, and 4.77 mm day⁻¹ in September. Irrigation volumes corresponded to 80%, 60%, and 50% of ETc in July; 100%, 80%, and 60% in August; and 110%, 85%, and 70% in September.

2.4. Fruit Selection and Storage

At harvest (second decade of October; 164 DAFB), yield per vine and morpho-biometric parameters were recorded on 100 randomly collected fruits per treatment. Quality attributes were measured on half of each sample, while the remaining fruits were stored at 4 °C for 60 days under daily air renewal. After storage, fruits were reweighed and analyzed to assess postharvest changes.

2.4.1. Fresh Weight

Fresh weight (FW) was measured using an analytical balance (Precisa BJ 610C) and expressed in grams.

2.4.2. Polar and Equatorial Diameters

Polar diameter (PD) and equatorial diameter (ED) were measured using a precision digital caliper. These measurements were used to calculate the equatorial ratio (minimum/maximum ED), relative length (RL; PD/mean ED), and fruit symmetry index (ratio between the two equatorial diameters).

2.4.3. Flesh Firmness

Flesh firmness (FF), expressed in kg cm⁻², was measured on two opposite sides of 30 fruits per treatment using a penetrometer (PCE FM200, PCE Instruments, UK) equipped with an 8 mm probe.

2.4.4. Colorimetric Analysis

Pulp color was assessed in the CIELab color space (CIE, 1976) using a tristimulus colorimeter (Minolta CM-700d) with an 8 mm aperture. The instrument was calibrated with a white reference plate. Measurements were taken at two orthogonal points in the equatorial region under illuminant D65 (6504 K) and a 10° observation angle. Chroma (C) and hue angle (h°) were calculated as:

$$\begin{gathered} {\mathit{C}}^{\mathit{*}}=\sqrt{{\mathit{a}}^{\mathit{*}2}+{\mathit{b}}^{\mathit{*}2}} \\ {\mathit{h}}^{\circ }=\mathit{a}\mathit{r}\mathit{c}\mathit{t}\mathit{a}\mathit{n}\left({\displaystyle \frac{{\mathit{b}}^{\mathit{*}}}{{\mathit{a}}^{\mathit{*}}}}\right) \end{gathered}$$

2.4.5. Total Soluble Solids (TSS)

TSS (°Brix) was measured using a temperature-compensated digital refractometer (Atago PAL-1, Tokyo, Japan) on juice extracted from opposite calyx ends of 30 fruits per treatment.

2.4.6. Titratable Acidity and pH

Titratable acidity (TA) and pH were determined using a potentiometric titrator (Titralab AT1000, HACH, Colorado, USA). A 10 mL aliquot of pulp was titrated with 0.5 N NaOH to the endpoint. Potassium hydrogen phthalate was used as the primary standard. TA was expressed as % citric acid. The TSS/TA ratio was calculated accordingly.

2.4.7. Dry Matter Content

Dry matter content (DMC) was determined by oven-drying samples at 105 °C (Binder EED240, Tuttlingen, Germany) until constant weight. DMC (%) was calculated as:

$$\mathrm{DMC}={\displaystyle \frac{\mathrm{dry}\mathrm{weight}}{\mathrm{fresh}\mathrm{weight}}}\times 100$$

2.5. Total Polyphenols and Antioxidant Capacity

Pulp samples were homogenized using an Ultraturrax blender (20,000 rpm; T25 Basic, IKA Werke, Germany). Total polyphenol content (TPC) and total antioxidant capacity (TAC) were quantified using a Lambda 35 spectrophotometer (PerkinElmer, Waltham, MA, USA). TPC (mg gallic acid equivalents g⁻¹ FW) was determined using the Folin–Ciocalteu method [16]. TAC was measured using the modified TEAC assay and expressed as mmol Trolox equivalents g⁻¹ FW [17]. Both hydrophilic and lipophilic fractions were included [18].

2.6. Statistical Analysis

Statistical analyses were performed using SPSS v.22.0 (IBM Corp., Armonk, NY). A two-way ANOVA was applied to evaluate the effects of treatment (T), year (Y), and their interaction (T × Y). Tukey’s HSD test was used for post-hoc comparisons. Principal Component Analysis (PCA) was conducted using XLSTAT 14.0 (Addinsoft, Paris, France).

3. Results

The data reported in Table 1 show that irrigation regime did not significantly affect fresh weight or the main morphometric traits of cold-stored Actinidia chinensis cv. Gold3 fruits.

Although a slight reduction in all variables was observed with decreasing irrigation volume (from treatment A to C), these differences were not statistically significant. At harvest, fresh weight ranged from 102.0 g in treatment A to 95.0 g in treatment C, indicating that the imposed water deficit did not substantially limit fruit growth. After storage, fruit weight decreased slightly across all treatments, and weight loss remained low and comparable (approximately 2–3 g), suggesting that preharvest irrigation level had minimal influence on postharvest water loss. Morphometric parameters followed similar trends. Polar diameter decreased from 62.40 mm (A) to 58.20 mm (C), and equatorial diameter from 58.10 mm to 54.20 mm, without significant differences. Relative length (1.05–1.06) and equatorial ratio (0.94–0.96) also remained stable, confirming that fruit shape and symmetry were unaffected by irrigation regime. No significant effects of year (Y) or treatment × year interaction (T × Y) were detected for any morphometric variable. In contrast, irrigation regime significantly influenced several postharvest quality attributes (Table 2).

Flesh firmness was lower in treatment A (1.50 kg cm⁻²) than in treatments B and C (2.00 and 2.05 kg cm⁻²). Total soluble solids increased from 12.50 °Brix (A) to 13.20 °Brix (C), while titratable acidity rose from 13.5 to 14.5 g L⁻¹. The TSS/TA ratio was similar in treatments A and B (0.93) but slightly lower in treatment C (0.91). Dry matter content increased significantly under reduced irrigation, ranging from 18.31% (A) to 19.88% (C), with intermediate values in treatment B (19.12%). Colorimetric parameters were also affected by irrigation level (Table 3).

Lightness (L) decreased from 73.5 (A) to 71.8 (C), while a increased from −5.94 to −3.74. The b* coordinate declined from 33.69 to 31.08, and chroma decreased from 34.2 to 31.3. Hue angle (h°) decreased from 100.11° to 96.88°, indicating a shift toward slightly darker and less yellow pulp. No significant effects of year or T × Y interaction were observed. Total antioxidant capacity (TAC) increased significantly under reduced irrigation (Table 4), with values of 47.31, 53.78, and 55.36 μmol Trolox g⁻¹ FW in treatments A, B, and C, respectively. Total phenolic content (TPC) did not differ significantly among treatments (96.53–99.31 mg GAE 100 g⁻¹ FW), and no year or interaction effects were detected.

Principal Component Analysis revealed a coherent multivariate structure (Figure 1). The first principal component captured the main covariation pattern, with strong positive loadings for fruit size (FW, ED, PD), color attributes (L, chroma), compositional traits (TSS, TSS/TA, DMC), and bioactive compounds (TPC, TAC). This component represented a major gradient associated with fruit maturity and internal quality. The second component was primarily driven by titratable acidity and specific color variables (h°, b), indicating a secondary source of variation. The spatial distribution of treatments A, B, and C reflected these patterns, with separation along PC1 corresponding to differences in maturity-related traits and separation along PC2 reflecting subtler differences in acidity and chromatic characteristics.

4. Discussion

The results of this study demonstrate that irrigation availability selectively influenced the postharvest quality traits of Actinidia chinensis cv. Gold3, whereas fruit morphometric characteristics remained largely unaffected. No significant differences were observed in fresh weight, weight loss, or fruit dimensions across irrigation treatments, indicating that fruit growth was not impaired by reduced water supply. This stability likely reflects the timing of irrigation differentiation, which was applied when fruits had already reached approximately 80% of their final volume. Similar responses have been reported in kiwifruit and other fleshy fruits, where moderate late-season water deficits do not significantly alter fruit size [9]. In contrast, several qualitative attributes were strongly influenced by irrigation regime, particularly after cold storage. Fruits from deficit-irrigated treatments (B and C) exhibited higher flesh firmness than those from fully irrigated vines, suggesting slower softening and greater structural integrity during storage. These responses may be associated with reduced activity of cell wall–degrading enzymes and delayed pectin solubilization, processes well documented in kiwifruit softening. Ripening in Actinidia spp. involves coordinated modifications of cell wall polysaccharides, including pectin and hemicellulose, and differential activities of polygalacturonase, pectin methylesterase, and β-galactosidase, all of which contribute to firmness loss [19,20,21,22]. Reduced irrigation may therefore attenuate these enzymatic processes, resulting in firmer fruit after storage. These structural changes were accompanied by higher total soluble solids and dry matter content under reduced irrigation, indicating a concentration effect due to lower water availability rather than increased absolute accumulation of solutes. Such concentration effects are widely reported under moderate water deficits, where reduced water supply decreases dilution of soluble sugars and other metabolites in fruit tissues [23,24,25,26,27]. Titratable acidity was maintained or slightly increased under reduced irrigation, suggesting that organic acid metabolism was not negatively affected. This is consistent with previous findings showing that deficit irrigation applied during the maturation stage can lead to higher TA levels in kiwifruit without detrimental effects on fruit yield or size [12]. Colorimetric parameters also responded to irrigation level. Fruits from deficit treatments showed lower lightness (L), chroma, and hue angle, together with higher (less negative) a values, indicating darker pulp with reduced green and yellow components and a slight shift toward warmer tones. These changes may reflect modulation of pigment metabolism, particularly carotenoids, which are sensitive to environmental conditions and developmental cues [28,29,30,31]. Although these variations did not compromise commercial acceptability, they demonstrate that irrigation management can modulate fruit visual quality independently of size. From a nutraceutical perspective, total antioxidant capacity (TAC) increased under reduced irrigation, whereas total phenolic content (TPC) remained largely unchanged. The enhancement of TAC after cold storage, despite similar levels at harvest, suggests that postharvest metabolic activity plays a key role. Fruits with higher dry matter and soluble solids likely contained a more concentrated pool of antioxidant-related compounds, and storage conditions may have further promoted their activation or transformation, resulting in increased antioxidant activity [3,32]. Principal Component Analysis (PCA) further supported these findings, revealing a coherent multivariate structure in which key quality traits responded consistently to irrigation regime. The first principal component was associated with fruit composition and postharvest quality traits, including firmness, soluble solids, dry matter content, color parameters, and antioxidant capacity, reflecting a coordinated physiological response. The second component was primarily associated with titratable acidity and hue angle, indicating a secondary source of variability linked to metabolic and pigment-related processes. Multivariate analyses such as PCA have been successfully applied to integrate multiple quality traits and identify major sources of variability in response to irrigation and management practices [13,33].

5. Conclusions

This study demonstrates that moderate reductions in irrigation supply can effectively enhance multiple postharvest quality attributes of Actinidia chinensis cv. Gold3 without compromising fruit morphometry. Across two seasons, fruit size, shape, and weight remained stable under all irrigation regimes, indicating that the species is capable of buffering moderate fluctuations in water availability through physiological regulation. In contrast, key qualitative and nutraceutical traits were highly responsive to irrigation level. Fruits from deficit-irrigated treatments exhibited higher flesh firmness, elevated soluble solids, greater titratable acidity, and increased dry matter content after storage, reflecting the physiological effects of mild water limitation, including reduced dilution, enhanced solute concentration, and improved tissue structural integrity. Colorimetric changes, characterized by lower lightness and reduced yellow intensity, further suggest that water availability can modulate pigment dynamics during storage. Total antioxidant capacity increased significantly under reduced irrigation, whereas total phenolic content remained largely unchanged, indicating that moderate water limitation can stimulate antioxidant potential without affecting phenolic accumulation. Multivariate analysis confirmed these patterns, revealing a dominant quality-related gradient integrating compositional, structural, and colorimetric traits, with acidity and hue contributing only marginally. This coherence highlights the coordinated physiological response of fruit to irrigation availability and underscores the central role of maturity-related attributes in defining postharvest quality. Overall, controlled irrigation reduction represents a viable and sustainable strategy to improve postharvest quality in yellow-fleshed kiwifruit. Moderate deficit irrigation enhances firmness, compositional richness, and antioxidant capacity while maintaining desirable fruit morphometry and reducing water use, making it particularly suitable for Mediterranean environments where water scarcity is a critical constraint.