The effect of summer pruning time and severity on antioxidant capacity and storage quality of Hayward kiwifruit

2.1 Plant materials

To evaluate the effect of severity and timing of summer pruning on the quantitative and qualitative traits of fruits and storage viability of Hayward Kiwifruit (Actinidia deliciosa Liang and Ferguson cv. Hayward), a two-year research study was conducted in Astara County, Guilan, Iran (latitude 22°38’38” North and 48°51’ East longitude) during the years 2020 and 2021 on 10-year-old vines. This region has hot and humid summers and mild to cold winters, with the possibility of snowfall and temperatures dropping below zero in some years. The highest temperature recorded in this city in the last five years was 36.6°C (August) and the lowest temperature was -7.6°C (February). The research site is 22 + meters above sea level and the average annual rainfall is 1381 mm. Some weather data for each year are presented in Table 1. The experimental vines were planted in north-south rows with a distance of 4x4 meters and were trained by the T-bar system, and all of them had the same conditions in terms of growth and development and the implementation of horticultural operations (pruning, watering, feeding, and fighting against pests and weeds).

To homogenize the experimental material, it was necessary to use a single pattern for winter pruning of all vines in the winter of the year prior to the implementation of summer pruning methods. For each vine, 30 to 32 canes with a length of 18 to 20 buds were kept. All the vines and the soil were managed according to standard cultural practices. The vines were regularly drip irrigated during the season; water was supplied based on evaporation demand. Tilling and mowing kept the area mostly weed free.

In the current study, four levels of summer pruning treatment were applied: 1) no pruning (control), 2) shoots pruned after the third-fourth leaf after the last fruit (common pruning), 3) tip squeezing, and 4) zero leaf pruning in 20% of fruiting shoots with unlimited growth. The pruning time was also considered in two stages: 1) one week after fruit set (1WAFS) and 2) four weeks after fruit set (4WAFS). According to the growth of the shoot during the season, tip squeezing was done in several stages with a time interval of 10-15 days, starting one week and four weeks after the fruit set according to the pruning treatment. It should be noted that fan and flat flowers were thinned before the sepals began to separate. Each pruning treatment consisted of three repetitions, with one kiwifruit vine as the experimental unit, and a total of 24 kiwifruit vines were treated in the present experiment.

2.3 Measurement of traits

Fruits were harvested when TSS reached 6.2-6.5% (mid-November) [8]. Immediately, the harvested fruits were transferred to the laboratory and some of their physicochemical characteristics were evaluated. It should be noted that 25 fruits were randomly taken from each vine from different directions of the vine crown.

Fresh weight of fruits: A sensitive laboratory balance (Sartorius model, GM-6101) was used to measure the weight of each fruit with an accuracy of one-tenth.

Fruit dimensions and fruit size index: The length, diameter and thickness of the fruit were measured using a digital caliper (Guanglu model, Taiwan) with an accuracy of 100 mm, and the fruit size index (FSI) was obtained from the average length, diameter and thickness of the fruit [7].

The firmness of fruit flesh (kilogram of force per square centimeter): First, the skin of the equatorial part of the fruit was removed by a sharp blade, and then the 8 mm probe tip of the FT011 handheld pressure gauge was placed perpendicular to the surface of the fruit on the peeled part. With uniform and gentle pressure of the rod of the device into the fruit (to the extent specified by a groove), the amount of force required to pierce the flesh of the fruit and enter the rod into the fruit tissue in terms of kilograms of force per square centimeter, which is an expression of the hardness of the fruit tissue specified and recorded.

Fruit dry matter (percentage): three fruits were randomly selected from each vine and 9-10 mm thick slices were prepared from their middle part. The fruit slices were weighed quickly and then, in order to dry, they were arranged in Petri dishes and placed in an oven with a constant temperature of 75°C. After three days, when the weight loss of the fruit samples was zero, the weight of the samples of each vine was recorded as the dry weight of the fruit. In this way, with the fresh and dry weight of the fruits, the percentage of dry matter was calculated [8].

TSS (percentage): After cutting each fruit from the equatorial part, a drop of fruit extract was poured on the sensitive screen of an optical refractometer (model Atago-ATC-20, Japan) with a range of 0 to 20%, and the concentration of TSS was determined.

TA (percentage): five milliliters of each fruit extract prepared with a juicer, mixed with 25 milliliters of distilled water and two drops of phenolphthalein reagent, and the resulting mixture was mixed with 0.1 normal sodium hydroxide in a digital burette. (BRAND, Germany) was poured, and titrated until the appearance of light pink color. Finally, the total acidity was calculated according to the predominant acid (citric acid) [9].

Vitamin C content of the extract (mg per 100 grams of fresh weight of the fruit): the titration method of the extract with the combination of 2,6-dichlorophenol indophenol (DCIP) was used. For this purpose, one gram of fruit tissue was weighed with a digital scale and three milliliters of 3% metaphosphoric acid were added to extract vitamin C. After 30 minutes of mixing these two substances, one milliliter of the said compound was titrated with the dye 2,6-dichlorophenol indophenol until a light pink color appeared and the number read was used to calculate the amount of vitamin C [10].

Total phenol content: Total phenol content was measured according to Folin Ciocalteu’s method using a model spectrophotometer (NanoDrop® ND-1000 UV-Vis, USA) [11]. The absorbance of the extract was read at a wavelength of 765 nm using a model spectrophotometer. Finally, total phenol content was calculated from the absorption rate of the sample and standard samples in terms of a milligram of gallic acid equivalents (GAE) per 100 grams of fresh weight (FW).

Antioxidant capacity: the antioxidant capacity of the extracts was determined through the free radical scavenging activity property of 2,2-diphenyl-1-picrylhydrazyl (DPPH) according to the method of Du et al. (2009) at a wavelength of 515 nm. Finally, the antioxidant capacity of the extracts was determined as DPPH inhibition percentage [11]. The percentage of DPPH, which was scavenged (% DPPHsc), was calculated using the following formula: Antioxidant activity ( % DPPH SC ) = Acont - Asamp Acont × 100

Where A_cont is the absorbance of the control, and A_samp is the absorbance of the sample.

2.4 Statistical analysis

Data were analyzed as a combined analysis across the year based on a randomized complete block design with three replications. The effect of the experiment years was also analyzed due to the climate changes during the two years of the experiment and the difference in the performance of the vines during these two years (Table 2). Statistical analysis of the experiment was done using SAS software (Version 9.1 2002-2003, SAS Institute, Cary, NC). A comparison of average data was also done using Duncan’s multi-range test at a 5% probability level. It should be noted that before statistical analysis, the data were checked for normality.

3.1 Fruit weight and size

The results showed that the tip-squeezing treatment had a higher average fruit weight and size index (Table 2). The highest average fruit weight and size index were observed with 124.00 g and 61.18 in the tip-squeezing treatment at 4WAFS pruning time in the first year. The lowest fruit weight and size index were observed in the zero-leaf pruning treatment at 4WAFS pruning time in the second year, which was not statistically significant compared to other treatments in the second year (Table 2).

There are several reports of the effect of time and severity of summer pruning treatments on the average weight of kiwifruit [12, 13], and the findings of the present experiment are consistent with their results. In the present study, light pruning by tip squeezing at 4WAFS pruning resulted in the highest fruit weight and size. In contrast, zero-leaf pruning at both pruning times significantly reduced the weight of the fruits, which was in agreement with the results of the experiment by Adouli et al. (2020) [14]. In their experiment, among the three pruning treatments, including zero-leaf pruning, pruning after three leaves after the last fruit, and leader pruning, zero-leaf pruning in the second year had the smallest fruit.

The production of larger fruits in pruning treatments can be attributed not only to the effects of pruning on various physiological aspects such as hormonal balance, branch strength, and the competition between vegetative and reproductive growth but also to the negative correlation between yield and fruit size, which has been observed in previous studies, with increasing yield resulting in a decrease in fruit size [15, 16]. In the present experiment, the decrease in fruit size in different pruning treatments in the second year compared to the first year resulted from the high yield in the second year. Heavy pruning (zero leaf pruning) caused a further decrease in fruit weight in both years and both pruning times, although no statistically significant difference was observed between pruning times. A significant increase in fruit size has been reported in light kiwifruit pruning [17], which is consistent with the results of the present experiment. Common pruning in time of 1WAFS pruning increased the average fruit weight, but in 4WAFS, pruning by tip squeezing showed more efficiency in producing a high average fruit weight. Therefore, an increase in the size of the fruits in zero leaf pruning treatment in time of 4WAFS pruning may result from more light penetration [14] or an increase in the leaf-to-fruit ratio [14, 18].

3.2 Total soluble solids (TSS) content

The average TSS at the time of harvest was the same in different pruning severities and did not reveal significant differences in the results of the kiwifruit TSS in the pruning time and experimental year (Table 2). However, during the post-harvest period, the average TSS in the different pruning severities showed significant differences. In four pruning severities, with the increase of storage time, TSS increased and the highest TSS was obtained at the end of 90 days of storage in two pruning times (Tables 3–6). In the control treatment, its content increased from 7.91% and 8.01% at the harvest time to 15.22% and 15.35% at the end of storage in 1WAFS and 4WAFS pruning time in the second year, respectively (Table 3).

In the pruning after 3-4 leaves treatment, the lowest total soluble solids (TSS) with 7.21% and 7.35% were recorded at harvest time in 1WAFS and 4WAFS pruning time in the first year, respectively. The highest TSS, with 14.33% and 15.08%, was recorded at the end of storage in 1WAFS and 4WAFS pruning time in the second year, respectively (Table 4). In the tip squeezing treatment, the highest TSS of 14.91% and 14.97% were related to 4WAFS pruning time in the first year and 1WAFS pruning time in the second year, respectively, without any significant differences between them (Table 5). In the zero leaf pruning treatment, the highest TSS of 15.27% and 14.95% were related to 1WAFS pruning time in the first year and 4WAFS pruning time in the second year, respectively, without any significant differences between them (Table 6).

TSS is one of the important quality indicators that has a direct relationship with the edible quality of the fruit at the time of ripening, and consumers prefer ripe fruit with high TSS. Increasing TSS indicates the hydrolysis of starch to hexose sugars. TSS is an important and main indicator of kiwifruit harvesting. The lack of significance of this index at the time of harvest during the two years of the experiment showed that the fruits were harvested during the two years of the present experiment with similar TSS and the variation range in quantitative and qualitative traits was not affected by the time of harvest [9].

An increase of kiwifruit TSS in early and light pruning compared to later pruning has been reported in studies [2, 19]. Although, the simple effect of pruning time and severity had no significant effect on on fruit TSS content, but their interaction effects revealed that light pruning, tip squeezing, at the time of 4WAFS, and heavy pruning, zero leaf pruning at the time of 1WAFS, caused the most TSS at the end of the three-month storage. High TSS in light pruning treatments may be the result of a higher leaf-to-fruit ratio and an increasing rate of photosynthesis [20]. On the other hand, the high TSS of fruit in zero leaf pruning treatment may be the result of receiving more light (Abedi Gheshlaghi et al., unpublished data).

Furthermore, pruning may result in the change of reserve material and, consequently, it may negatively affect the subsequent vegetative growth, probably also altering the following sink– source relationship during fruit development [21, 22]. Therefore, pruning has an indirect effect on the enhancement of fruit TSS content.

The amount of TA at the time of harvest was not affected by the pruning severities. However, the interaction of the pruning severities with the timeof pruning and years of the experiment showed a significant difference in the TA content of kiwifruit. In the control treatments, the highest TA at the harvest time, at 1.81%, was related to 1WAFS pruning in the second year, with no significant differences with 4WAFS pruning in that year. In the tip squeezing treatments, the highest TA at the time of harvest, at 1.74%, was related to 4WAFS pruning time in the second year, and the lowest, at 1.56%, to 1WAFS pruning time in the first year (Table 2).

During the post-harvest period, the average TA in the different pruning severities revealed significant differences. In four pruning severities, the TA showed a decreasing trend during the first 30 days of storage, and with the increase of the storage time to 60 days, TA increased in two pruning times. However, the changes in TA in the third month of storage depended on pruning time and experiment year (Tables 3–6). In the control treatment, the highest TA content at 60 days of storage was related to 1WAFS pruning in the first year, without any significance with 4WAFS pruning time (Table 3). In the pruning after 3-4 leaves treatment, at the end of 30 days of storage, the amount of TA decreased and in the second month increased again, with the highest TA of 1.86% observed in 1WAFS pruning time in the first year (Table 4). In the tip squeezing treatment, the highest TA of 1.89% was related to 4WAFS pruning time in the first year, without any significant differences among treatments at the 60 days of storage (Table 5). In the zero-leaf pruning treatment, the amount of TA decreased at the end of the first month of storage and increased with the continuation of storage, but no significant difference was observed between the pruning treatments in the last two months of storage (Table 6).

A decrease in fruit TA content is suitable for fruit consumption in terms of taste and customer acceptance [10]. Contradictory reports have been reported about the effect of summer pruning on kiwifruit TA. In one research, severe pruning 30 days after petal abscission caused less TA and light pruning at the time of petal abscission caused more TA production in kiwifruit [20]. In another study, removing 60% of the crown leaf surface increased TA during the harvesting of Hayward kiwifruit [18]. In examining the severity and times of summer pruning, the lowest fruit TA was observed in one-fifth pinching of the shoot growth from the petal abscission until harvest with a monthly interval, and pruning after 6 leaves after the last fruit at the time of petal abscission (early and light pruning) [3]. Meanwhile, leaving 2, 4 and 8 leaves after the last fruit in Hayward kiwifruit [23], and squeezing and twisting reproductive shoots before flowering and removing the vegetative parts of reproductive shoots after fruit set in Jinyan golden kiwifruit did not have a significant effect on the TA content of kiwifruit [19]. In the present study, differences in the TA content at different pruning severities were related to pruning time, years of experiments during the harvest time, and storage.

The organic acid content during the fruit harvest period depends on the amount of total soluble solids (TSS) and the rate of decomposition of acids [9], which is influenced by the cultivation conditions of the region [16]. Other studies have reported a reduction of fruit acid during storage [3], which is consistent with the results of this study. The decrease in the concentration of TA during the storage time of fruits in cold storage conditions can be interpreted and explained by the organic acids being used in the processes of respiration and their conversion to sugar [24]. Obviously, the milder the reduction of this concentration is, the longer the shelf life of fruits in storage conditions. On the other hand, the lower the rate of respiration in fruits and the slower the fruits lose their water, the aging process will be delayed and fruits will last longer in storage [25].

TSS and TA are the most important factors determining fruit quality, especially in kiwifruit. The increased TSS and decreased TA during fruit ripening are due to an increase in respiration [26]. The change in sugar and acid content during storage time is related to an increase in the rate of cellular respiration, the production of ethylene, the concentration of the fruit extract, and the reduction of the fruit juice. In the present study, fruit TA decreased in the first month of storage, but its content increased and decreased in the following months of storage depending on the year of the experiment and pruning time. There is a report of a negative correlation between yield and TA fruit at the time of harvesting kiwifruit [16]. In the present study, despite the high yield in the second year compared to the first year, the fruits did not show a significant difference in the TA of the fruit 4WAFS pruning at the time of harvest (Table 2).

3.4 Vitamin C content

The average vitamin C content of the fruit at the time of harvest was not affected by the pruning severities. However, the interaction of the pruning severities with time since pruning and years of the experiment showed a significant difference in the vitamin C of kiwifruit in zero leaf pruning. The highest vitamin C content of 46.05 mg/100 g at the time of harvest was related to 1WAFS pruning in the second year, with no significant differences with 4WAFS pruning in two years. The lowest vitamin C content at the harvest time, with 35.65 mg/100 g, was related to 1WAFS pruning in the first year (Table 2).

During the post-harvest period, significant differences in the average vitamin C content were observed in the different pruning severities. In four pruning severities, the vitamin C showed a decreasing trend during storage in two pruning times (Tables 3–6). In the control treatment, the highest vitamin C was 48.14 mg/100 g, related to 4WAFS pruning time in the second year, without any significance with other treatments at the beginning of storage. The lowest vitamin C was 29.24 mg/100 g, related to 1WAFS pruning time in the first year, without any significance with other treatments at the end of storage (Table 3). In the pruning after 3-4 leaves treatment, the highest vitamin C was 41.84 mg/100 g, related to 4WAFS pruning time in the second year, without any significance with other treatments at the beginning of storage. The lowest vitamin C was 25.62 mg/100 g, related to 4WAFS pruning time in the first year, with significant differences with other treatments at the end of three months of storage (Table 4). In the tip-squeezing and zero-leaf pruning treatments, the highest vitamin C was 46.21 and 46.50 mg/100 g, respectively, related to 1WAFS pruning time in the first year without significance with 4WAFS pruning time at the beginning of storage. The lowest vitamin C were 27.75 and 26.53 mg/100 g, respectively, related to the tip-squeezing and zero-leaf pruning treatments obtained at 1WAFS pruning time in the first year at the end of 90 days of storage (Tables 5 and 6).

The average vitamin C of the fruit at harvest was higher in the second year than in the first year than the first year of the experiment in all four pruning severities at two pruning times. In all four pruning severities, the amount of vitamin C in the years of the experiment differed by approximately 3-5 units, and this annual difference was maintained until the end of 90 days of storage in both pruning times. It is likely due to the higher average temperature in the second year (Table 1), which can enhance the production of secondary compounds such as vitamin C.

There have been reports of the positive effect of summer pruning on vitamin C content in kiwifruit. In one report, the highest vitamin C content in Hayward kiwifruit was observed when pruning after 5 leaves in shoots with medium growth and pruning after 20 leaves in shoots with unlimited growth, two and fifteen days after the complete abscission of the petals [2]. In another study, among three different methods of summer pruning, the highest vitamin C content in Hayward kiwifruit was observed with zero leaf pruning [27]. Additionally, tip squeezing and twisting reproductive shoots before flowering, and removing the vegetative parts of reproductive shoots after fruit set, increased fruit vitamin C in Jinyan golden kiwifruit [19]. However, leaving 2, 4, and 8 leaves after the last fruit in Hayward kiwifruit [3], zero leaf pruning, pruning after 3 leaves after the last fruit, and leader pruning did not affect fruit vitamin C [14]. In the present study, the interaction effect of pruning time and year on vitamin C at the harvesting time of fruit was significant in zero leaf pruning. Moreover, the effect of four pruning severities on the variation range of vitamin C of fruit during storage was significant in two pruning times and two experiment years.

Kiwifruit is renowned for its high vitamin C content, even more so than citrus fruits. Vitamin C is known to be beneficial due to its links with the prevention of human illnesses, such as cancer, diabetes, cardiovascular diseases, and neurological disorders, in addition to its antiviral activity. Therefore, increasing the vitamin C content is beneficial for fruit consumption in terms of health. Vitamin C is an important nutrient quality parameter and is very sensitive to degradation due to its oxidation compared to other nutrients during food processing and storage [10]. It has been reported that the vitamin C content of kiwifruit may vary according to growth conditions and degree of ripeness. In another study, Tavarini et al. (2008) recorded that the concentration of vitamin C in the fruits of the Hayward cultivar decreased at the end of a long period of cold storage, which was consistent with the results of their experiment [28]. In kiwifruit, ascorbic acid content significantly decreases during cold storage and the time of harvest also significantly affects ascorbic acid content [28]. It seems that one of the reasons for the reduction of vitamin C in fruit during storage is the result of water evaporation and fruit weight loss because fresh weight loss (water loss) can also accelerate the degradation of ascorbic acid [28].

Decreasing the ascorbic acid content of fruits and vegetables during storage in cold storage has been reported due to their susceptibility to spoilage. Among vitamins, vitamin C has the least stability and is very sensitive to degradation due to oxidation [29]. Antioxidants donate electrons to the active species of oxidized oxygen, thus destroying their oxidizing and damaging power. During fruit storage, vitamin C content, which is one of the most important antioxidants, decreases due to its use as an electron donor to oxidants to neutralize free radicals. Ascorbic acid is decomposed by the ascorbate peroxidase enzyme during storage or fruit ripening, resulting in a decrease in its content [30]. In the present study, fruit vitamin C content decreased during storage.

3.5 Fruit firmness

The firmness of the fruit at the harvest time was affected by the pruning severities and the interaction of pruning severities with the time of pruning and years of the experiment. The highest firmness of the fruit at harvest, with 8.11 and 8.04 kg of force, was related to zero leaf pruning and tip squeezing at 4WAFS pruning time in the first year, with no significant differences with 1WAFS pruning in this year, respectively. The lowest fruit firmness at the harvest time, with 5.92 kg of force, was related to the control at 1WAFS pruning in the first year (Table 2).

During the post-harvest period, significant differences in average fruit firmness were observed in the different pruning severities. In four pruning severities, the fruit firmness decreased during storage in two pruning times (Tables 3–6). In the control treatment, the highest fruit firmness of 6.09 and 6.05 kg of force was observed in the 1WAFS pruning time in two years without any significance with other treatments at the beginning of storage. The lowest fruit firmness of 1.59 kg of force was related to 4WAFS pruning time in the first year at the end of storage (Table 3). In the pruning after 3-4 leaves, tip squeezing, and zero leaf pruning treatments, the highest fruit firmness was 8.04, 7.61, and 8.11 kg of force, respectively, observed in the 4WAFS pruning time in the first year at the beginning of storage. The lowest fruit firmness was 1.51 and 1.95 kg of force, observed in the pruning after 3-4 leaves and tip squeezing at the 4WAFS pruning time in the first year at the end of 90 days storage, respectively. In zero leaf pruning, the lowest fruit firmness was 1.65 kg of force, observed in the 1WAFS in the first year at the end of 90 days of storage (Tables 4–6).

The firmness of the fruit is an important quality index in kiwifruit and can be used as a key criterion and harvest indicator for consumption, storage, and export [28, 31]. The comparison of average fruit firmness shows that summer pruning on vines increases the firmness of the fruits. Moreover, fruit firmness in the first year was higher than in the second year in two pruning times, likely due to the better climatic conditions of the region (Table 1), which was in agreement with the results of Aduli et al. (2021) regarding the effect of the year on the firmness of the fruit [6]. There is a positive regression relationship between texture firmness and dry matter percentage of fruits [32]. Therefore, the high dry matter in the first year may have caused the high firmness of the fruit this year.

Contradictory reports of the effect of pruning on fruit firmness have been reported. Mohseni et al. (2015) reported the highest fruit firmness of Hayward kiwifruit in summer pruning treatment by leaving eight leaves from the last fruit [33]. In another experiment, the lowest firmness was observed in summer pruning, by squeezing and twisting reproductive shoots before flowering and removal of vegetative parts of reproductive shoots after fruit set, in Jinyan golden kiwifruit [19]. While leaving two, four, and eight leaves after the last fruit, pruning after five leaves in medium growth shoots and pruning after twenty leaves in unlimited growth shoots in two times two and fifteen days after the complete petals abscission [2], and also removing sixty percent of the crown leaf surface [18], did not affect the firmness of the fruit. At harvest time in the present study, the firmness of the fruit was higher on the pruning vines than on the control, and the highest one was on zero leaf pruning at the 4 WAFS pruning time.

It has been reported that light is very effective in increasing fruit calcium [34], and it seems that in the present experiment, different pruning treatments compared to the control maintained high photosynthetic capacity in shoots, increased the rate of photosynthesis and photosynthetic products (carbohydrates) in leaves and fruit tissue [35, 36], and increased the amount of light received by leaves and fruits, which increased calcium and firmness of fruits. On the other hand, the ratio of leaf to fruit was low in the pruning treatments compared to the control, and following our findings, an increase in flesh firmness at low leaf-to-fruit ratios has been reported [18, 37].

Softening is a phenomenon that occurs during fruit ripening, and one of the limiting factors for storage in kiwifruit is the reduction of firmness. Fruit texture is determined by cell wall composition, cell pressure, cell anatomy, and cell water content [38]. The stability of the cell walls and cell membranes is closely related to the firmness of the fruit flesh. The reduction in firmness is caused by the stimulation of pectin esterase enzyme capacity, which causes the loss of pectin in the cell wall as well as the loss of soluble pectin [9]. Increasing the storage time of Hayward kiwifruit had a significant effect on reducing the degree of firmness of fruits [32], and in the present study, the firmness of fruit during storage revealed a significant decrease. During storage in the first year, its reduction was more than in the second year. Therefore, in four pruning severities, fruit firmness at the end of 90 days of storage of the second year was higher than that of the first year in two pruning times. Pruning treatments effectively reduced the fruit’s firmness during storage time. At the end of 90 days of storage, the firmness of fruit from the tip squeezing and zero leaf pruning treatments was higher than that of the common pruning and control treatments in two pruning times.

3.6 Fruit dry matter

The pruning severities did not affect the fruit dry matter content at harvest time. However, the interaction of pruning severities with the time and years of the experiment showed a significant difference in fruit dry matter content in pruning after three to four leaves and zero leaf pruning. In these treatments, the highest fruit dry matter at the time of harvest was related to 1WAFS and 4WAFS pruning time in the first year. The lowest fruit dry matter at harvest, with 13.76%, was related to zero leaf pruning at 1WAFS pruning time in the second year (Table 2).

During the post-harvest period, significant differences in average fruit dry matter were revealed in the different pruning severities. In four pruning severities, the fruit dry matter showed an increasing trend during storage in two pruning times (Tables 3–6). The variation range of dry matter during storage was different in pruning treatments and years. In the control and zero leaf pruning treatments, the fruits in the first year of the experiment reached the maximum dry matter at the end of 30 days of storage and did not change significantly until the end of 90 days of storage. Whereas, in the second year, the highest fruit dry matter was obtained at the end of 60 days of storage. In the control treatment, the highest fruit dry matter was 18.12%, observed in the 4WAFS pruning time in two years at the end of storage, without any significance with other treatments in 30, 60, and 90 days of storage (Table 3). In zero-leaf pruning, the highest fruit dry matter was 18.6%, observed in the 4WAFS pruning time in the second year at the end of storage, without any significance with other treatments at 60 and 90 days of storage, and at 30 days storage in the first year (Table 6). In the pruning after 3-4 leaves and tip squeezing treatments, the highest fruit dry matter was 18.89 and 18.70%, observed in the 1WAFS and 4WAFS pruning time in the first year at the end of 60 storage, respectively. The lowest fruit dry matter was 14.62 and 15.24%, revealed in the pruning after 3-4 leaves and tip squeezing at the 1WAFS pruning time in the second year in the beginning storage, respectively (Tables 4 and 5).

The average dry matter of fruits in the first year, especially after pruning 3-4 leaves and zero leaf pruning, was higher due to the better climatic conditions of the region, which was consistent with the results of Aduli et al. (2021) regarding the effect of year on the dry matter of fruits [6]. Several factors affect the dry matter content of the fruit, such as warm springs, cool summers, and warm autumns, which increase dry matter at harvest time [39]. The high leaf-to-fruit ratio, the increase in the photosynthetic efficiency of the leaves, and the low yield in the first year may also contribute to the higher dry matter of the fruits this year.

The content of dry matter in kiwifruit, which is mainly composed of starch, has a strong correlation with TSS and soluble sugars after fruit ripening, and it is an important indicator of fruit taste and flavor in kiwifruit [40]. It has been reported that the training systems have a significant effect on the dry matter content of the fruits; fruits harvested from the T-bar system compared to the pergola [41] and the modified TY system and Y shape compared to the T-bar system had higher dry matter content [23].

There are significant reports on the effects of light and early summer pruning on the increase of fruit dry matter [2, 19], and heavy pruning on the reduction of kiwifruit dry matter [18]. Different crown management methods affect fruit dry matter. Inappropriate crown pruning in spring/summer stimulates significant regrowth and can result in reduced fruit size and dry matter [31], as competition for carbohydrates between vegetative regrowth and fruit growth can significantly limit the availability of high fruit dry matter [42]. In the present study, light late pruning compared to severe early pruning increased fruit dry matter content (Tables 3–6). Although nutritional, exogenous hormones, moderate drought stress and other methods can be used to improve the dry matter of fruit crops [43]. However, fruit growers prefer to use winter pruning, girdling, summer pruning, etc. to control shoot growth and vegetative growth to achieve optimal fruit dry matter, fruit size, and yield [15, 40]. Because such methods are safer, without potential chemical contamination, more effective and have fewer adverse effects on both the vine and the environment. Dry matter content is a reliable quality indicator for kiwifruit, which should be at least 16.1% for consumer acceptance [44]. In the present study, the fruit dry matter content at the end of storage obtained more than 17.00% in pruning treatments for two years.

3.7 Fruit weight loss

The variation in weight loss of stored fruits in the different pruning severities revealed significant differences during the post-harvest period. In four pruning severities, the weight loss of stored fruits showed a decreasing trend during storage in two pruning times for two years (Tables 3–6). The highest weight loss occurred in the first month of storage and with the continuation of storage, the amount of water loss decreased. In the control treatment, the lowest weight loss of stored fruits was 3.97%, observed in the 4WAFS pruning time in the second month of the second year storage, without any significance with other treatments in the second and third months of storage (Table 3). In the pruning after 3-4 leaves, the lowest weight loss of stored fruits was 3.85%, observed at the 1WAFS pruning time, and in tip squeezing and zero leaf pruning treatments were 3.73 and 4.20%, respectively, obtained at the 4WAFS pruning time in the third month of the first year storage (Tables 4–6). Considering the large size of the fruits in the first year compared to the second year (Table 2), the large decrease in fruit weight may be the result of the high level of evaporation compared to the fruit volume in large fruits.

Weight loss is one of the most important quality indicators, the lack of control of which after harvesting causes a decrease in the economic value and marketability of the product [45]. It not only leads to direct quantitative losses (reduction in marketable weight) but also causes loss of appearance (withering and wrinkling) and texture quality (softening, loosening, loss of juiciness) and food quality. Weight loss causes unfavorable changes in the taste and texture of the fruit and ultimately leads to deformity and a decrease in the appearance quality of the fruit [46]. The relative humidity is as important as temperature during storage, since water loss is rapid during the initial storage days and maintaining a high relative humidity can minimize this loss. Weight loss has been reported by many researchers [3, 28] to occur in kiwifruit during storage, which was consistent with the results of the present study.

3.8 Total phenol content

The results showed the average total phenol was the highest in the zero leaf pruning at harvest time (Table 2). In the control and zero leaf pruning treatments, the highest total phenols were 86.08 and 113.50 mg GAE 100g-1 FW, respectively, observed in the 4WAFS pruning time in the second year. In the pruning after 3-4 leaves and tip squeezing treatments, the highest total phenols were 104.57 and 98.94 mg GAE 100g-1 FW, respectively, observed in the 1WAFS pruning time in the second year. The lowest total phenol was 52.69 mg GAE 100g-1 FW, observed in pruning after 3-4 leaves in the 4WAFS pruning time in the first year, which was not statistically significant compared to the 1WAFS pruning time in this year (Table 2). The average total phenol content of the fruit was higher in the second year than in the first year at the time of harvest. It is likely due to the higher average temperature in the second year (Table 1), which can enhance the production of secondary metabolites such as phenolic compounds.

During the post-harvest period, significant differences were revealed in the average total phenol of the different pruning severities. In four pruning severities, the fruit total phenol showed an increasing trend during storage in two pruning times for two years (Tables 3–6). However, the variation range of total phenol during storage was different in pruning treatments and years. In the control treatment, the highest fruit total phenol was 98.50 mg GAE 100g-1 FW, observed in the 4WAFS pruning time in the second year at the end of storage, without any significance within the 4WAFS pruning time at 30 and 60 days storage in the second year. The lowest total phenol content was obtained in the 4WAFS pruning time in the first year (Table 3). In three pruning severities, the difference in total phenols between the treatments was high at the beginning of the storage period and decreased at the end of the storage period (Tables 4–6). In pruning after 3-4 leaves and tip squeezing treatments at the end of 90 days of storage, there was no significant difference in total phenol at different times of pruning in two years (Tables 4 and 5). However, in zero leaf pruning at the end of 90 days of storage, a significant difference was observed between the treatments, with the highest total phenol being 133.96 mg GAE 100g-1 FW, observed in the 4WAFS in the second year. The lowest total phenols, 58.10 mg GAE 100g-1 FW, were observed in the 1WAFS pruning time of the first year at the beginning of storage (Table 6).

A direct relationship was reported between fruit polyphenolic compounds and the amount of received light, and a reverse relationship with the amount of the product. Fruits exposed to more light produce more phenolic compounds [47]. The actual phenols content is the result of the balance between the speed of their synthesis and the speed of their reduction or consumption. If the amount of the crop is high, the shading of the fruits on each other increases, and as a result, the amount of phenolic compounds in the fruit may decrease [48].

Pruning helps to reduce vegetative growth, thus lessening carbohydrate losses and competition between vegetative and fruit growth. Additionally, pruning opens the canopy for natural pollinators (bees) and allows for more airflow within the canopy [21, 22], which enhances the production of fruit secondary compounds such as phenolic compounds. In the present study, although the yield in the second year was higher than the first year due to more chlorophyll of leaves and more light penetration from the crown in the second year compared to the first year (Abedi Gheshlaghi et al., unpublished data), the total phenol content was higher than the first year and the higher yield of the second year did not reduce the total phenol content of the fruit. Therefore, it seems that the effect of light on fruit phenol content is greater than on yield. On the other hand, the highest total phenol content in the treatment of zero leaf pruning in the present experiment may be the result of more light penetration in this treatment (Table 6).

3.9 Antioxidant capacity

The results showed that the antioxidant capacity at the time of harvest was highest in the zero leaf pruning (Table 2). In the control and zero leaf pruning treatments, the highest antioxidant capacities were 48.44 and 51.73%, respectively, observed in the 1WAFS pruning time in the first year. In the pruning after 3-4 leaves and tip squeezing treatments, the highest antioxidant capacities were 48.12 and 49.97%, respectively, observed in the 4WAFS pruning time in the first year. The lowest antioxidant capacity was 31.9%, observed in control in the 4WAFS pruning time in the second year, which was not statistically significant compared to the 1WAFS pruning time this year (Table 2).

During the post-harvest period, significant differences in the antioxidant capacity of the different pruning severities were revealed. In four pruning severities, the fruit antioxidant capacity showed an increasing trend during storage in two pruning times for two years (Tables 3–6). In the control treatment, the antioxidant capacity of the fruit reached its maximum value in the 1WAFS pruning time in the first year at the end of 30 days of storage and did not show significant differences with 60 and 90 days of storage. The lowest antioxidant capacity was 31.91%, obtained in the 4WAFS pruning time in the second year (Table 3).

In pruning after three to four leaves, the antioxidant capacity of two pruning times of the first year and 1WAFS pruning time of the second year showed a significant increase at the end of three months of storage, while the 1WAFS pruning time of the second year did not show a significant effect in increasing the antioxidant capacity of the fruit (Tables 4). In tip squeezing treatments, the highest and lowest antioxidant capacities were 72.91 and 26.78%, respectively, observed in the 1WAFS pruning time at the end of 90 days of storage and 4WAFS pruning time at the beginning of storage in the second year (Table 5). In zero leaf pruning, the highest and lowest antioxidant capacities were 86.75 and 26.78%, respectively, observed at the end of 90 days and the beginning of storage in the 4WAFS pruning time of the second year (Table 5).

The antioxidant capacity of fruits and vegetables is due to the presence of enzymatic compounds such as catalase, ascorbate peroxidase and superoxide dismutase, as well as non-enzymatic compounds including vitamin C, phenolic compounds and carotenoids [49]. The main antioxidant capacity of kiwifruit is due to its phenolic compounds and vitamin C [11], which are influenced by several genetic and environmental variables [10, 50]. Increased antioxidant capacity as a result of summer pruning and increased light penetration in kiwifruit [11, 18] and peach, apple and apricot [51] have been reported at the time of fruit harvesting, which is consistent with the results of the present experiment.

Studies showed a positive relationship between total phenol content and antioxidant capacity. In four pruning severities, the fruit’s antioxidant capacity at the time of harvest was higher in the second year than in the first year (Table 2). However, the increase in fruits’ antioxidant capacity in the second year’s storage period was more than that in the first year (Tables 3–6). This caused the significant difference in antioxidant capacity between treatments to decrease at the end of storage. Changes in antioxidant capacity during storage are different from the stage of fruit ripening at the time of harvest. They may be controlled by a combination of ascorbic acid and total fruit phenol content and substantial changes in polyphenol components [11, 52].

In the present study, the highest antioxidant capacity was observed in the zero leaf pruning treatment in 4WAFS pruning time at the end of storage of the second year, which may be due to the effect of pruning treatments on vitamin C and total phenol contents of the fruit. The high antioxidant capacity in this treatment may result from the high vitamin C and total phenol of fruit (Table 6). In general, antioxidant capacity and phenol content increase during storage, while vitamin C decreases [53]. A significant increase in phenol content after exposure to cold storage for one week was observed in the fruit of some fruit trees and Hayward kiwifruit [52]. In the present experiment, the antioxidant capacity increased at the end of three months of cold storage in the first year, which is consistent with their results. The increase in fruit antioxidant capacity in the first year occurred despite the significant decrease in vitamin C and no significant changes in total phenol content. This result may be related to the main role of compounds other than total phenol content and ascorbic acid in the antioxidant capacity of the fruit [54].

The antioxidant capacity of fruits and vegetables is related to their enzymatic compounds, e.g., CAT, APX, POD, and SOD, and their non-enzymatic compounds, e.g., ascorbic acid, phenol compounds, and carotenoids [55]. However, after three months of cold storage in the second year of the present study, there was a significant decrease in vitamin C and no significant changes in phenol, resulting in a decrease in the total antioxidant capacity of the fruit. Several different studies have reported that the antioxidant capacity of strawberry [56], mango [57], and Hayward kiwifruit [58] decreased during storage, and the results of this research were consistent with them.