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Article

Agronomic Strategies to Manipulate Kiwifruit Calcium Content to Understand Its Role in Fruit Physiology

by
Elena Baldi
1,*,
Moreno Toselli
1,
Alessandro Bonora
2,
Alexandra Boini
1,
Maurizio Quartieri
1,
Margherita Germani
3,
Greta Polidori
4 and
Luca Corelli Grappadelli
1
1
Department of Agricultural and Food Sciences, University of Bologna, 40127 Bologna, Italy
2
Ri.Nova Soc. Coop., Post-Harvest Sector, 47522 Cesena, Italy
3
Department of Plant Sciences, University of California, Davis, CA 95616, USA
4
Horta s.r.l., 29122 Piacenza, Italy
*
Author to whom correspondence should be addressed.
Horticulturae 2025, 11(3), 237; https://doi.org/10.3390/horticulturae11030237
Submission received: 27 January 2025 / Revised: 21 February 2025 / Accepted: 22 February 2025 / Published: 23 February 2025
(This article belongs to the Section Plant Nutrition)
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Abstract

:
Calcium (Ca) is one of the most important nutrients involved in fruit quality and storability; therefore, its application in fruit trees is often used in pre- and post-harvest. The aims of this study were to manipulate soil Ca, K, and N availability, photosynthetic active radiation, and fruit transpiration rate to understand their implication on fruit Ca accumulation on green-flesh kiwifruit grown in calcareous soil. Our results show that Ca partitioning into the fruit is not affected by the applications of Ca, K, and N, as well as the increase of photosynthetic active radiation. However, the presence of reflective films reduced fruit firmness and increased soluble solid content at harvest and during cold storage, thus enhancing fruit quality. Fruit calcium accumulation is decreased by the reduction of fruit transpiration rate; however, it has the possibility to recover, even close to fruit harvest, when the fruit transpiration is restored. The presence of bags reduced fruit weight from 84 to 63 g even though bags were removed. Our data provide evidence of the inefficiency of calcium fertilization in kiwifruit in calcareous soils and demonstrate the extension of calcium transportation into the fruit, which seems to occur during the entire growing season.

1. Introduction

According to the Food and Agriculture Organization, the worldwide annual production rate of kiwifruit in 2023 was nearly 2.36 million metric tons, with China being the main world producer, followed by New Zealand, Italy, and Greece [1]. Green-fleshed kiwifruit is still dominating the global market, even though yellow- and red-fleshed fruits are gaining more importance.
Calcium (Ca) is a macronutrient essential for plants, serving both physiological and structural functions. Calcium serves as a second messenger in a variety of processes ranging from root or pollen tube growth and fertilization to responses to abiotic as well as biotic stress [2,3,4]. Besides acting as an intracellular messenger in the cytosol and as a counter-cation for inorganic and organic anions in the vacuole, the ion Ca2+ is required for structural functions in the cell wall and organization of plasma membranes [5]. Optimal Ca2+ concentrations in harvested fruits improve their storability and reduce economic losses due to its role in maintaining tissue mechanical strength [6]. In particular, high Ca2+ concentration is also associated with reductions in the incidence of pre- and post-harvest physiological disorders in fruits such as apple [7], table grape [8], and kiwifruit [9]. Calcium is a low-mobile nutrient that is mainly accumulated in leaves, where it accounts for approximately 85% [10] to 93% [11] of total Ca2+ uptake. As an indirect consequence, fruit is a weaker sink compared to leaf, because of its low transpiration rate during its growth [12]. According to Montanaro at al. [10], maximum Ca2+ accumulation in kiwifruit occurs at the beginning of stage II (approximately 55 days after full bloom) in addition it was reported that 61–75% of Ca2+ fruit content at harvest is accumulated 8 weeks after fruit set [13]. During the rest of the growing season, Ca2+ enters the fruit at a low rate, with the last 6 weeks of fruit growth resulting in a minimal gain in Ca2+ [10]. On the other hand, the amount of Ca2+ removed yearly by fruits of Actinidia chinensis is limited compared to leaves [14], meaning that Ca requirements of the fruit are smaller than those of the leaves.
Calcium movement in the plant is almost exclusively via xylem, mainly driven by transpiration [15]. It was demonstrated [10] that the highest fruit water loss occurs after fruit set, and thereafter it declines towards a minimum value at the end of stage II; these values are then maintained until the end of the season [10]. Higher transpiration rates promote higher Ca2+ accumulation in the fruit [10], even if, at low transpiration rates, the proportionality is not respected [16]. Other factors responsible for Ca2+ accumulation in fruits are photosynthetic active radiation (PAR), vapor pressure deficit (VPD), and wind speed. The increase of PAR enhances total fruit Ca2+ content up to 80% during the first 40 days after flowering [17]. VPD seems to be the main driving force of fruit transpiration [18], thus influencing Ca2+ accumulation in fruits. The transpiration rate of fruits is affected by the boundary layer conductance, which is influenced by wind speed [19]. However, the proportional relationship between Ca2+ delivery and transpiration is weak, also when its concentration in the xylem sap is low [15], supporting the idea that transpiration is not the only factor governing the movement of Ca. High Ca concentration in kiwifruit fruit is a pre-requisite for lower incidence of diseases and high fruit quality in pre- and post-harvest.
Balanced fertilization ensures application of the appropriate amount and proportions of macro- and micronutrients. Nitrogen (N) as nitrate or ammonium affects absorption of other nutrients, such as potassium (K) and Ca [20]; consequently, increasing N application could negatively affect plant nutrition. In an experiment on apples, a decrease in Ca and K content in fruits was observed as a consequence of increasing N in soil solution [21].
The aim of the present research was to clarify aspects of Ca accumulation into green-fleshed kiwifruit fruit through three different experiments. To define the best strategy to enhance Ca content in fruit, we aimed specifically at evaluating the effect of: (1) application of Ca, K, and N (experiment 1); (2) modification of photosynthetic active radiation (experiment 2); and (3) fruit transpiration rate on Ca partitioning into the fruit (Experiment 3).

2. Materials and Methods

2.1. Ca, K, and N Application—Experiment 1

2.1.1. Orchard Description and Treatments

The trial was conducted in 2020 in Italy in a kiwifruit orchard located in the Emilia-Romagna region (44°26′65″ N–11°93′21″ E) on a Hypocalcic Haplic Calcisols soil [22]. The main soil properties are described in Table 1. The climate in the area is temperate, with an average annual precipitation of 674 mm (in the period 2015–2019) and an average temperature of 14.9 °C; in 2020 the average temperature was 14.6 °C, precipitation 562 mm [23].
The experiment was conducted on self-rooted vines of the cultivar Hayward (Actinidia chinensis var. deliciosa) planted in 2017, with a distance of 5 m between rows and 2 m between two plants on the row, trained to a pergola system. Water was provided through drip irrigation and sprinkler emitters, according to daily evapotranspiration.
The study compared, in a split-split-plot experimental design with 4 replications (consisting of 5 plants each), two levels of N application (0 and 200 kg ha−1) as a main plot, two levels of K (0 and 200 kg ha−1) as a sub-plot, and two levels of Ca (0 and 200 kg ha−1) as a sub-sub-plot, for a total of 32 plots. During the growing season, starting from 5 May, the fertilizers were manually distributed on the vine row every 10 days. Nitrogen (as ammonium nitrate; N = 34.5%), was applied from flowering (5 May) until 3 August (fruit at 70% of final weight); application of K (as potassium sulphate; K = 44%) started 10 days after N (15 May) and stopped on 13 August; Ca (as calcium chloride; 27.8%) was applied from the beginning of June (4 June) until the beginning of August (3 August).
During winter, older fruiting canes were removed to leave, for each plant, 12 younger shoots growing from the base of the previous year’s canes, spaced every 20–30 cm along the permanent main branch.

2.1.2. Sampling and Analysis

From May until harvest, every 21 days, six leaves and six fruits from each plot were collected: fresh and dry weight were measured, and samples were analyzed to quantify the total N, K, and Ca concentration. Leaf and fruit N concentration were determined with the Kjeldahl method [24], while K and Ca were determined by spectrometry emission (ICP-OES, Ametek Spectro, Arcos, Kleve, Germany) after mineralization [25] in an Ethos TC microwave lab station (Mile-stone, Bergamo, Italy). Fruit N, K, and Ca content was calculated by multiplying fruit nutrient concentration by fruit dry mass. At harvest (21 October), vine yield was assessed; from each plot, 15 fruits were sampled and used for quality parameter determination. Fruit firmness was measured using a penetrometer with an 8 mm diameter tip (FTA53220 Güss, Strand, South Africa). Fruits were then cut into slices; part was squeezed to obtain juice for determination of soluble solid content (SSC) with the employment of a digital refractometer PR-1 (Atago Tokyo, Japan). One slice per fruit was placed in the oven for dry matter content; another slice was frozen at −20 °C for mineral analysis. Two more samples of 15 fruits each were stored in a cold storage room at 0–2 °C (without moisture control) and analyzed after 107 (5 February) and 154 (24 March) days of storage; on each date, firmness, SSC, and dry matter were measured, as described above. Frozen fruits were lyophilized and analyzed, as described above.

2.1.3. Physiological Measurements

Water potentials (Ψω) of stem (ΨωS), leaf (ΨωL), and fruit (ΨωF) were measured at solar noon, following methods and procedures from [26], using a Scholander pressure chamber (Soilmosture Equipment Corp., Santa Barbara, CA, USA). Stem Ψω was taken after isolating a leaf, distant from the fruit, close to the trunk, in ad-hoc envelopes (aluminum foil on the outside and black on the inside). Leaf Ψω was estimated from well-illuminated leaves, distant from fruit. Fruit Ψω was taken from representative fruit, not exposed to light. Leaf gas exchanges were measured with a Li-COR 6400 (Li-COR 6400, LI-COR Inc., Lincoln, NE, USA), integrated with a fluorimeter chamber that allowed the setting of the actual radiation in the orchard. Well-illuminated leaves were chosen on one-year shoots, distant from both fruit and apical leaves.

2.1.4. Statistical Analysis

Data were analyzed as a split-split plot experimental design, with N rate (2 levels: 0 and 200) as the main plot; K rate (2 levels: 0 and 200) as the sub-plot; and Ca rate (2 levels: 0 and 200) as the sub-sub-plot. Physiological parameters were also analyzed with the software R (version 4.3.3) according to a discriminant canonical analysis (DCA). DCA is a dimension-reduction technique related to principal component analysis and canonical correlation, and it finds linear combinations of the quantitative variables that provide maximal separation between classes or groups.

2.2. Solar Radiation—Experiment 2

2.2.1. Orchard Description and Treatments

The trial was conducted in 2021, in Italy, in a kiwifruit orchard located in the Emilia-Romagna region (44°19′36.1″ N–11°56′02.5″ E) on a Fluvic Cambisol soil [22]. The main soil properties are described in Table 1 and the climate in the area is the same as described for experiment 1, with an average temperature of 14.3 °C and average rainfall of 473 mm in 2021. The experiment was conducted on self-rooted vines of the cultivar Hayward (Actinidia chinensis var. deliciosa) planted with a distance of 4.7 m between rows and 2 m between two plants on the row (1064 plants ha−1), trained to a pergola system. Water was provided through drip irrigation and sprinkler emitters and nutrients were supplied according to the Integrated Crop Management guideline of the Emilia-Romagna region [27].
To modify orchard microclimate and PAR, on 5 May (petal fall stage) reflective white films (Extenday®, Extenday New Zealand, Auckland, New Zealand) were positioned on the soil, between two tree rows (interrow), and kept onsite until harvest (Figure 1); this treatment (FILM) was compared to a control (NO FILM) without reflecting films. The trial was conducted in a single row, divided in half; in one part, on both the left and right sides, reflective sheets were positioned in the interrow; the rest of the row was the untreated control. Fifteen border plants were left between the two treatments to avoid the influence of the reflective sheet on the control plants. In each treatment, 4 plots (replicates) of three plants each were defined, for a total of 12 plants monitored per treatment. The experiment was conducted as a complete randomized design with 4 plots (replicates) of three plants each, for a total of 12 plants per treatment.

2.2.2. Sampling, Analysis, and Physiological Measurements

Starting from 16 June, 5 fruits from each plot were periodically sampled to measure DM. On 13 July, fully expanded leaves were sampled from each plot, washed, oven-dried and analyzed for all macro- and micronutrient concentration (as described above). At harvest (25 October), vine yield was assessed; in addition, 15 fruits were sampled from each plot and used for quality parameter determination (as described above). Thirty more fruits were set aside and stored at 0–2 °C for 3 and 5 months before quality analysis. On 16 June, 13 July, 25 August, and 5 October, 8 vines were selected for measurements of ΨwS, ΨwL, and ΨwF plus leaf gas exchanges.

2.2.3. Statistical Analysis

The data were analyzed as a complete, randomized design. Physiological parameters were also analyzed with the software R (version 4.3.3) according to a discriminant canonical analysis (DCA).

2.3. Fruit Transpiration—Experiment 3

2.3.1. Experimental Conditions and Measurements

This experiment was conducted in the same orchard as experiment 2. On 11 May, three plants were randomly chosen, and on each vine, 70 fruits were enclosed in a previously perforated plastic bag (BAG) (Figure 2); plastic bags were kept on fruits until 13 July. The period of closure was defined in relation to xylem functionality [6]. Three more plants were used as un-bagged controls (Control). From 23 June, 5 fruits inside the bag and five control ones were periodically sampled and measured for DM, as described before. At harvest, after 3 and 5 months of cold storage, the main quality parameters (see above) were determined. Fruit Ψω was monitored during the season, in both fruit enclosed in bags and controls.

2.3.2. Statistical Analysis

Data were analyzed as a complete, randomized design. Physiological parameters were also analyzed with the software R (version 4.3.3) according to a discriminant canonical analysis (DCA).

3. Results

3.1. Ca, K, and N Application—Experiment 1

The supply of Ca, N, and K did not significantly influence nutrient concentrations in leaves and contents in fruits during the vegetative season.
No interaction between N, K, and Ca application was observed; consequently, in Table 2 and Table 3, only the effects of principal factors were reported. Yield was higher in plots not fertilized with Ca and was not influenced by N and K supply, while fruit weight at harvest was not influenced by N, K, and Ca supply (Table 2).
Fruit firmness was lower in Ca200 compared to Ca0 at harvest, while no differences were observed after storage (Table 3). The applications of Ca induced an increase in SSC at harvest, while no significant differences were observed between the two treatments during cold storage (Table 3). Fruit dry matter was not influenced by treatments (Table 3).
The discriminant canonical analysis performed on the main physiological parameters (ΨωS, ΨωL,ΨωF, Pn, gs, and E) showed that 100% of the variability of this data set was represented by the first canonical variable; as a consequence, the distribution of variables was represented only for this axis (Figure 3). The box plot clearly separates Ca0 from Ca200; leaf Ψω was not influenced by Ca supply, while fruit and stem Ψω showed a positive effect of Ca application (Figure 3). Plant photosynthesis, as well as stomatal conductance and leaf transpiration, were enhanced by the supply of Ca (Figure 3) with significant differences in June and August (Table S1 in Supplementary Materials). The supply of N and K did not influence the physiological parameters.

3.2. Solar Radiation—Experiment 2

Reflective films induced an increase in photon flux density of direct light beneath the row (Figure 4a); in the interrow, both direct and reflected light was higher as a consequence of reflective film positioning, in comparison to the control (Figure 4b).
The use of reflective films induced an increase in fruit weight at the beginning of September, while non-significant differences were observed on other sampling days (Figure 5a). Fruit Ca content was not influenced by the treatment (Figure 5b).
Fruit dry matter was enhanced by reflective film application in all sampling dates, with the exception of mid-July and harvest (Figure 6). Yield was not influenced by the treatments.
Film application increased fruit firmness at harvest and after 3 months of cold storage, while no differences were observed at 5 months (Table 4). Soluble solid content and DM (only after cold storage) were higher in plots with the reflective films than in the control ones (Table 4).
The discriminant canonical analysis performed on physiological parameters (ΨωS, ΨωL, ΨωF, Pn, gs, and E) showed that 100% of variability of this data set was represented by the first canonical variable; as a consequence, the distribution of variables was represented only for this axis (Figure 7). The box plot separated film from control; ΨωL was influenced by the absence of the reflective film, while all other parameters, although with varying magnitude, were more influenced by the film positioning (Figure 7), even if no significant differences between treatments were observed (Tables S2 and S3).

3.3. Fruit Transpiration—Experiment 3

Fruit weight was higher in control fruits than in bagged ones during the entire season (Figure 8a). In addition, fruit Ca content was higher in control fruit in July and August, while no differences between treatments were observed in other sampling dates (Figure 8b). Fruit dry matter accumulation during the season was not influenced by the presence of plastic bags (Figure S1).
At harvest, fruit firmness was higher in control fruits than in those closed into a plastic bag, while no effect was observed during cold storage (Table 5). Soluble solid concentration was higher in bagged fruits only after 5 months of cold storage, while no differences were observed at harvest and after 3 months (Table 5). Fruit DM was higher in bagged fruits than in control after 3 and 5 months of cold storage, while no differences were observed at harvest (Table 5). No effect of the bag was observed on ΨwS, ΨwL, and ΨwF (Table S4).

4. Discussion

Calcium application to kiwifruit has received considerable attention since maintaining relatively high Ca concentrations in fruit tissues is believed to promote nutrient balance, stabilize and maintain cell wall stabilization and integrity, and improve fruit quality and storability [28,29]. However, as for all the other nutrients, Ca application should also be planned according to environmental availability. Calcium excesses are detrimental to plant physiology and negative from an economic point of view. In our experimental conditions, Ca applications were probably not necessary, since soil and irrigation water are rich in Ca as a consequence of the high level of limestone in the soil. In the present experiment, soil analyses showed considerable native soil fertility that has reduced or masked the effects of Ca, K, and N fertilization on fruit weight, quality, and storability. The lack of effectiveness of soil-applied Ca solution in increasing fruit Ca concentration was also found in other investigations [30,31], and it could be due to the soil type. As a result, leaf Ca concentration was also not modified by Ca fertilization.
The application of additional Ca, in some circumstances, showed a negative effect on yield and firmness, bringing evidence to the hypothesis that the high Ca availability can become toxic for the cultivar Hayward. In broad bean, extracellular Ca2+ at high concentrations (i.e., >200 mM) was found to block water channels and reduce water flow into the guard cells with stomatal closure [32]. On the other side, in experiment 1, Ca was found to improve the C assimilation rate, and, at harvest, a positive effect was evidenced on soluble solid concentration. The latter result, together with the reduction in fruit firmness, could lead to the hypothesis of a possible early ripening effect, the opposite of what was expected. In fact, auxin and abscisic acid pathways involve Ca as a protein-binding secondary messenger and in membrane transport mechanisms, thus modifying turgor and solute accumulation [33]. An increase in Ca availability, by acting on hormonal pathways, has the potential of altering expression levels of enzymes that modify cell turgor pressure, apoplast solute accumulation, and cell wall modification, thus influencing fruit softening.
The application of Ca at a rate of 200 kg ha−1 decreased the stem Ψω; similarly, in grape, pedicel hydraulic conductivity of xylem vessels decreased throughout ripening, potentially due to occlusion formed by pectin deposition [34]. Likewise, the formation of physical barriers linked to increased Ca2+ availability could be the cause of lower values of fruit water potential; in confirmation of this, a study on kiwifruit showed a general increase in xylem hydraulic resistance during the second half of fruit development [35].
The light-dependent import of Ca2+ into the chloroplast and dark-stimulated stromal Ca2+ flux, shows that Ca2+ is closely linked to photosynthesis. Indeed, Ca mainly acts as a structural component of photosystem II, having a regulatory function in the Calvin–Benson cycle, thus playing a pivotal role in cyclic and linear electron flow and minimizing photo-oxidative damage [36]. Consequently, the increased availability of Ca could have positively affected physiological patterns of plants.
The properties of reflective films are able to modify the orchard microclimate by reflecting solar radiation into the tree canopy [37,38]. Several experiments evidenced an increase in photosynthetically active radiation (PAR) absorption of the apple tree canopy, varying from 40% [39] to 68% [40]. Kiwifruit berries growing in light-exposed positions in the canopy were of better quality and could be stored for longer periods [41]. Similarly, it was evidenced [42] that a high level of light availability determines an increase in xylem stream, leading to higher Ca accumulation in different organs and improvement of fruit storage quality. This seems linked to the positive effect of light on vascular development of the fruit and its pedicel [43]; in our experiment, indeed, the use of reflective film increased light in the row and interrow by around 23% and 60%, respectively. The enhancement of light availability was able to increase canopy photosynthesis and assimilate supply to the fruit, which led to higher dry matter in fruits during the season. Films also led to increased transpiration and, thus, a more negative leaf water potential; this could affect the irrigation strategy, which should probably be modified with a higher supply of water when reflective films are used. No effect of reflective film coverage was observed on fruit Ca accumulation; however, fruits from covered plots evidenced higher fruit quality, both at harvest and after cold storage.
The use of plastic bags to reduce the fruit transpiration rate had the objective of clarifying Ca accumulation in fruits. The reduction of fruit transpiration during the first phase of fruit growth was expected to impair Ca fruit partitioning. In fact, it’s widely demonstrated that early fruit development is a pivotal period for Ca accumulation in kiwifruit. Predominantly, this happens since Ca is xylem-mobile but phloem-immobile [44]. In our experiment, as expected, we observed a reduction of fruit size in fruit wrapped into bags with a constant difference during the entire vegetative season, although bags were removed after 63 days. On the other hand, the trend of Ca partitioning into the fruit showed an increase after bag removal, indicating the capability of fruit to also accumulate Ca in the last part of the season. There may be 2 explanations: (1) Ca does not only move through the xylem, or (2) Ca moves with the xylem flow during the whole season according to fruit request. These hypotheses open a new perspective on the Ca nutrition of kiwifruit. In addition to fruit transpiration, its requirement for Ca and the physicochemical features of its conducting tissues (e.g., ion adsorption and desorption occurring at exchange sites across the walls of the xylem pathway) could influence the movement of Ca in plant organs [45]. In apple, for example, Ca fruit accumulation seems variety-dependent [46] and lasts for a longer period than previously suggested, reaching harvest time.

5. Conclusions

From our data, we can conclude that in limestone soils, the application of soil Ca is not useful for improving fruit quality, whereas a possible alternative could be the use of reflective films, which showed encouraging results on fruit quality. Finally, our results open questions on the timing of Ca accumulation in the fruit, which seems longer than that reported in the literature, with effective calcium partitioning into the fruit up to 60 days before harvest, laying the basis for possible modification of kiwifruit orchard management strategies, with the goal of increasing the calcium supply in the fruit.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/horticulturae11030237/s1, Table S1: Effect of sheet application on total photosynthesis (Pn), stomatal conductance (Gs), and transpiration (E) during the season—experiment 1; Table S2: Effect of sheet application on stem, fruit, and leaf water potential (bar) during the season—experiment 2; Table S3: Effect of film application on total photosynthesis (Pn), stomatal conductance (Gs), and transpiration (E) during the season—experiment 2; Figure S1: Effect of plastic bag on fruit dry matter accumulation during the season—experiment 3; Table S4: Effect of sheet application on stem, fruit, and leaf water potential (bar) during the season—experiment 3.

Author Contributions

Conceptualization, L.C.G. and M.T.; methodology, M.Q., E.B. and M.T.; validation, M.T., E.B., A.B. (Alessandro Bonora), and A.B. (Alexandra Boini); formal analysis, A.B. (Alessandro Bonora), A.B. (Alexandra Boini), M.G., G.P. and E.B.; investigation, A.B. (Alessandro Bonora), A.B. (Alexandra Boini), M.G. and G.P.; resources, L.C.G. and M.T.; data curation, E.B., A.B. (Alessandro Bonora), and A.B. (Alexandra Boini); writing—original draft preparation, E.B.; writing—review and editing, E.B., M.T., A.B. (Alessandro Bonora), and A.B. (Alexandra Boini); supervision, L.C.G. and M.T.; funding acquisition, L.C.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was part of the project S4Post.Frut supported by the Region Emilia-Romagna (Focus Area 3°A, protocol # 5111526). Additional support was provided by the Regional Rural Development Program 2014–2020—Type of operation: Support for demonstration activities and information actions—Focus Area 3A—Project: “Communicating innovation in agriculture in the Emilia-Romagna Region: from the field to the smartphone—AGRISOCIAL.

Data Availability Statement

Dataset will be open access after 16 June 2025 at https://doi.org/10.6092/unibo/amsacta/8049.

Conflicts of Interest

Alessandro Bonora was employed by the Ri.Nova Soc. Coop. and Greta Polidori was employed by the Horta s.r.l. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
CaCalcium
NNitrogen
KPotassium
PARPhotosynthetic active radiation
VPDVapor pressure deficit
SSCSoluble solid content
DMDry matter
DCADiscriminant canonical analysis
ΨωWater potentials
ΨωSStem water potential
ΨwLLeaf water potential
ΨwFFruit water potential
PnTotal photosynthesis
GsStomatal conductance
ETranspiration

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Horticulturae 11 00237 g001
Figure 1. Reflective white films positioned on the soil of the kiwifruit orchard.
Figure 1. Reflective white films positioned on the soil of the kiwifruit orchard.
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Figure 2. Bagged fruits.
Figure 2. Bagged fruits.
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Figure 3. Discriminant canonical analysis (DCA) of the effect of Ca applications on physiological traits (a) and the corresponding weights (b) of stem Ψω (stem), leaf Ψω (leaf), and fruit Ψω (fruit), as well as on total photosynthesis (Pn), stomatal conductance (Gs), and transpiration rate (E) during the entire growth season—experiment 1.
Figure 3. Discriminant canonical analysis (DCA) of the effect of Ca applications on physiological traits (a) and the corresponding weights (b) of stem Ψω (stem), leaf Ψω (leaf), and fruit Ψω (fruit), as well as on total photosynthesis (Pn), stomatal conductance (Gs), and transpiration rate (E) during the entire growth season—experiment 1.
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Horticulturae 11 00237 g004aHorticulturae 11 00237 g004b
Figure 4. Effect of reflecting film on photon flux density beneath the row (a) and in the interrow (b)—experiment 2.
Figure 4. Effect of reflecting film on photon flux density beneath the row (a) and in the interrow (b)—experiment 2.
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Figure 5. Effect of reflecting film on fruit fresh weight (a) and fruit Ca content (b)—experiment 2. ns, ***: effect not significant or significant at p ≤ 0.001, respectively.
Figure 5. Effect of reflecting film on fruit fresh weight (a) and fruit Ca content (b)—experiment 2. ns, ***: effect not significant or significant at p ≤ 0.001, respectively.
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Figure 6. Effect of film application on fruit dry matter during the season—experiment 2. ns, *: effect not significant or significant at p ≤ 0.05, respectively.
Figure 6. Effect of film application on fruit dry matter during the season—experiment 2. ns, *: effect not significant or significant at p ≤ 0.05, respectively.
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Figure 7. Discriminant canonical analysis (DCA) of the effect of soil white cover/reflective film on physiological traits on the left side of the figure and the corresponding weights (on the right) of stem Ψω (stem), leaf Ψω (leaf), and fruit Ψω (fruit), as well as on total photosynthesis (Pn), stomatal conductance (Gs), and transpiration (E) during the entire growth season.
Figure 7. Discriminant canonical analysis (DCA) of the effect of soil white cover/reflective film on physiological traits on the left side of the figure and the corresponding weights (on the right) of stem Ψω (stem), leaf Ψω (leaf), and fruit Ψω (fruit), as well as on total photosynthesis (Pn), stomatal conductance (Gs), and transpiration (E) during the entire growth season.
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Figure 8. Effect of bag application on fruit fresh weight (a) and Ca content (b)—experiment 3. ns, *, **, ***: effect not significant or significant at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001, respectively.
Figure 8. Effect of bag application on fruit fresh weight (a) and Ca content (b)—experiment 3. ns, *, **, ***: effect not significant or significant at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001, respectively.
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Table 1. Main soil properties of the two fields where the study was conducted.
Table 1. Main soil properties of the two fields where the study was conducted.
Parameter Experiment 1Experiments 2 and 3
pH7.568.16
Total lime (% CaCO3)313
Active carbonate (% CaCO3)12.711.6
Organic matter (%)3.321.97
Total N (%)2.041.30
Available P (mg kg−1)9829
Exchangeable K (mg kg−1)322324
Exchangeable Ca (mg kg−1)55124141
Exchangeable Mg (mg kg−1)183408
Salinity (mS cm−1)2.75-
CEC 1 (meq 100 g−1)30.625.0
Sand (%)3510
Loam (%)4949
Clay (%)1641
1 CEC = cation exchange capacity. Analyses were performed according to official Italian methods for soil analysis by an external laboratory. In detail, pH was determined with a potentiometric method; total lime with the De Astis calcimeter; active carbonate with the method of Drouineau; organic matter with the Walkley–Black methods; total N with the Kjeldhal method; available P with the Olsen method; exchangeable K, Ca, and Mg and CEC with extraction in DTPA. Soil texture was determined according to USDA classification.
Table 2. Effect of N, K, and Ca supply on fruit size and yield—experiment 1.
Table 2. Effect of N, K, and Ca supply on fruit size and yield—experiment 1.
TreatmentFruit Average Weight
(g)		Total Yield
(kg pt−1)
N075.121.0
N20074.119.4
Significancensns
K074.920.4
K20074.420.0
Significancensns
Ca076.121.7
Ca20073.118.7
Significancens 1*
N × Knsns
N × Cansns
K × Cansns
N × K × Cansns
1 ns, *: effect not significant or significant at p ≤ 0.05, respectively.
Table 3. Effect of application of N, K, and Ca on fruit dry matter soluble solid concentration (SSC) and firmness at harvest and after storage.
Table 3. Effect of application of N, K, and Ca on fruit dry matter soluble solid concentration (SSC) and firmness at harvest and after storage.
TreatmentHarvest107 Days of Storage154 Days of Storage
Dry Matter (%)SSC (°Brix)Firmness (kg)Dry Matter (%)SSC (°Brix)Firmness (kg)Dry Matter (%)SSC (°Brix)Firmness (kg)
N019.3 8.0 7.63 19.816.21.9219.316.01.04
N200 19.2 8.1 7.53 19.716.91.6719.216.20.99
Significancens 1nsnsnsns*nsnsns
K0 19.3 8.1 7.54 19.616.21.7319.016.11.04
K200 19.1 8.0 7.63 19.816.91.8619.616.11.00
Significancensnsnsnsns**nsns
Ca0 19.1 7.9 7.72 19.616.61.8619.116.01.05
Ca20019.4 8.2 7.45 19.916.51.7319.516.20.99
Significancens***nsnsnsnsnsns
N × Knsnsnsnsnsnsnsnsns
N × Ca nsnsnsnsnsnsnsnsns
K × Ca nsnsnsnsnsnsnsnsns
N × K × a nsnsnsnsnsnsnsnsns
1 ns, *, **: effect not significant or significant at p ≤ 0.05 or p ≤ 0.01, respectively.
Table 4. Effect of reflecting film on fruit firmness, soluble solid concentration (SSC), and dry matter (DM) at harvest and after 3 and 5 months of cold storage—experiment 2.
Table 4. Effect of reflecting film on fruit firmness, soluble solid concentration (SSC), and dry matter (DM) at harvest and after 3 and 5 months of cold storage—experiment 2.
Treatment Firmness
(kg)		SSC
(%)		DM
(%)
HARVEST
NO FILM6.976.2016.2
FILM7.536.9216.1
Significance* 1***ns
3-MONTH COLD STORAGE
NO FILM1.7611.813.5
FILM2.3612.815.7
Significance*********
5-MONTH COLD STORAGE
NO FILM1.1812.214.7
FILM1.2514.517.1
Significancens******
1 ns, *, ***: effect not significant or significant at p ≤ 0.05, p ≤ 0.001, respectively.
Table 5. Effect of film application on fruit firmness, soluble solid concentration (SSC), and dry matter (DM) at harvest and after 3 and 5 months of cold storage—experiment 3.
Table 5. Effect of film application on fruit firmness, soluble solid concentration (SSC), and dry matter (DM) at harvest and after 3 and 5 months of cold storage—experiment 3.
Treatment Firmness
(kg)		SSC
(%)		DM
(%)
HARVEST
NO BAG6.976.2016.2
BAG5.826.0316.2
Significance*** 1nsns
3-MONTH COLD STORAGE
NO BAG1.7611.813.5
BAG1.7412.214.7
Significancensns**
5-MONTH COLD STORAGE
NO BAG1.1812.114.8
BAG1.1513.218.8
Significancens**
1 ns, *, **, ***: effect not significant or significant at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001, respectively.
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Baldi, E.; Toselli, M.; Bonora, A.; Boini, A.; Quartieri, M.; Germani, M.; Polidori, G.; Corelli Grappadelli, L. Agronomic Strategies to Manipulate Kiwifruit Calcium Content to Understand Its Role in Fruit Physiology. Horticulturae 2025, 11, 237. https://doi.org/10.3390/horticulturae11030237

AMA Style

Baldi E, Toselli M, Bonora A, Boini A, Quartieri M, Germani M, Polidori G, Corelli Grappadelli L. Agronomic Strategies to Manipulate Kiwifruit Calcium Content to Understand Its Role in Fruit Physiology. Horticulturae. 2025; 11(3):237. https://doi.org/10.3390/horticulturae11030237

Chicago/Turabian Style

Baldi, Elena, Moreno Toselli, Alessandro Bonora, Alexandra Boini, Maurizio Quartieri, Margherita Germani, Greta Polidori, and Luca Corelli Grappadelli. 2025. "Agronomic Strategies to Manipulate Kiwifruit Calcium Content to Understand Its Role in Fruit Physiology" Horticulturae 11, no. 3: 237. https://doi.org/10.3390/horticulturae11030237

APA Style

Baldi, E., Toselli, M., Bonora, A., Boini, A., Quartieri, M., Germani, M., Polidori, G., & Corelli Grappadelli, L. (2025). Agronomic Strategies to Manipulate Kiwifruit Calcium Content to Understand Its Role in Fruit Physiology. Horticulturae, 11(3), 237. https://doi.org/10.3390/horticulturae11030237

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