INTRODUCTION
The kiwifruit is a woody plant belonging to the Actinidia genus of the Actinidiaceae family (Ferguson, 1984). The Actinidia contains more than 76 species and 125 taxa. The kiwifruit vine is a deciduous and dioecious, which originated in temperate forests in the mountains and hills of southwestern China. The kiwifruit has strong solid antioxidant properties due to its high content of vitamin C, carotenoids, and flavonoids. Kiwifruits also contain sugars, organic acids, amino acids, proteins, and minerals, which benefit human health (Xu and Zhang, 2003). These properties have led to an increase in the kiwifruit consumption and the expansion of its agricultural production (Poudel et al., 2019). There are three major types of kiwifruits consisting of green-fleshed, yellow-fleshed, and red-fleshed ones. Of these, green-fleshed and yellow-fleshed cultivars are the most common cultivated kiwifruits in the world (Ferguson and Huang, 2007;Montefiori et al., 2011). The yellowfleshed kiwifruits are more tasteful and aromatic than greenfleshed ones (Testolin and Ferguson, 2009). Their cultivation has recently increased in Korea and other countries.
‘Sweet Gold’ is a new promising kiwifruit cultivar with high sweetness (Kim et al., 2018) and has replaced existing yellow-fleshed cultivars such as ‘Jecy Gold’ and ‘Halla Gold’ and green-fleshed cultivars such as ‘Hayward’. However, despite its high sweetness, this cultivar exhibits a broad range of fruit quality variations, especially in flesh coloration. Maintaining high fruit quality and uniformity within a vine is essential for ensuring successful promotion in the highly competitive fruit market because kiwifruits are harvested at one time regardless of some variations in fruit quality parameters. Several factors, including canopy density, cane thickness, fruit-bearing shoot type, and the number of fruits may affect a range of parameters related to fruit quality. The responses to these factors may differ among plant genotypes. In ‘Hayward’ kiwifruit, it was reported that fruits close to the leader on canes emerged far away from the trunk showed higher soluble solid content (SSC) than those of other fruits (Hopkirk et al., 1986). Moreover, fruit size and SSC are also affected by fruit set position and fruit-bearing shoot diameter and length (Kulczewski, 2003;Sagredo et al., 2014;Pyke et al., 1996). However, to date, there have yet been no studies reporting the range of variation in fruit quality of yellow-fleshed kiwifruits including ‘Sweet Gold’.
Therefore, this study aimed to evaluate the variation levels of various fruit quality parameters within vines and their association with different cane-based fruiting positions at harvest in the yellow-fleshed ‘Sweet Gold’ kiwifruit.
MATERIALS AND METHODS
Plant Materials
The five-year-old yellow-fleshed ‘Sweet Gold’ (A. chinensis var. chinensis) grafted on the seedling of the ‘Hayward’ (A. chinensis var. deliciosa) were used for this study. These vines at a distance of 6 × 4.8 m were trained with the pergola system in a plastic house located at Jeju-si, Korea. Five vines with similar canopy sizes and vine vigor were randomly selected as biological replicates. General cultural management was performed according to conventional practices. including artificial pollination, flower, and fruit thinning, summer pruning, nutrition management, and pest control.
Classification of Cane-based Fruiting Position
The vine canopy was horizontally divided into four quadrants. Each quadrant was classified into six canebased fruit positions (sectors 1–6) (Fig. 1). Sectors 1–3 indicated fruiting positions in the proximal part of the canes from the trunk to the leader and sectors 4–6 indicated fruiting positions in the distal part of the canes from the trunk to the leader. The size of each quadrant was approximately 3×2.4 m2 and each cane-based fruiting position consisted of 3 or 4 canes.
Evaluation of Fruit Quality
Fruit quality parameters including fruit weight, percent of dry matter (DM), SSC, titratable acidity (TA), firmness, and flesh color were evaluated at harvest. Twenty-six fruits were collected from each cane-based fruiting position of a vine in 2019 and 2020, respectively. Fruit weight was measured using a digital balance (EL2000S, USA). DM was determined using a 2–3 mm thick slice obtained from the middle part of the fruit according to the method described by Burdon et al. (2016). TSS and TA were measured using a Brix acidity meter (GMK-707R, G-won Hitech, Korea). Fruit firmness was measured using a fruit hardness tester (FHM-5, Takemura, Japan) equipped with a plunger (f5 mm). Flesh color was determined using a chroma meter (CR-400, Konica Minota, Japan).
Analysis of Non-structural Carbohydrates
Starch and soluble sugar contents were measured according to the methods described by Wichaya et al. (2015). The fruits harvested were freeze-dried at -70 °C, homogenized using a blender and stored at -70 °C in the powdered form before analysis. One gram of the powder was mixed with 10 mL of 80% ethanol for 30 min at room temperature and then centrifuged at 2,000 gn for 10 min. The residue used for starch extraction and quantitation colorimetric according to the method described by Magel (1991). The supernatant was filtered through a filter paper (F1002, Chmlap, Spain), evaporated using a rotary evapo-rator (Laborota 4000, Heidolph, Germany), re-suspended with DDW, and then filtered through a C18 Sep-Pak cartridge (Sep-Pak, Waters, USA) and a 0.2 μm syringe filter. Soluble sugar contents were measured using a high-performance liquid chromatography (HPLC) instrument equipped with the RID10A detector (Shimadzu, Japan) and a Shim-pak GIS NH2 column (5 μm, 250 × 4.6 mm, Shimadzu, Japan). The mobile phase was an 80% (v/v) acetonitrile/DDW solution with a flow rate of 1.5 mL·min-1 and the column temperature was set to 35 °C.
Statistical Analysis
Our experimental design was completely randomized and statistical analysis was performed at the 95% confidence level using the SPSS software (version 22.0; SPSS, Armonk, NY). Significant differences among group means were analyzed with Duncan's multiple range test.
RESULTS AND DISCUSSION
Variation in Fruit Quality Associated with Canebased Fruiting Positions
The range of fruit quality variations in the different cane-based fruiting positions of ‘Sweet Gold’ kiwifruit vines was evaluated during two crop seasons 2019 and 2020 (Table 1). There were no significant differences in fruit weight among the different cane-based positions at harvest in 2019 and 2020. However, fruit weights at different cane-based fruiting positions were a little different between 2019 and 2020. Fruits located in sector 3 and sec tor 6 far away from the trunk, were generally heavier. There were also no significant differences in DM among the different cane-based positions at harvest in 2019 and 2020. However, as in fruit weight, the DM at different cane-based fruiting positions differed slightly between the years 2019 and 2020. Fruits with the highest DM were consistently obtained in sector 1. The mean DM for the two years indicated that kiwifruits positioned on canes closer to the trunk had higher DM than those on canes distal to the trunk. Fruits with the highest SSC were obtained at sector 1 in 2019 and in 4 in 2020, respectively. Like the patterns observed in DM, the mean SSC during the two years was higher in fruits positioned on canes proximal to the trunk. However, SSC and DM showed different overall tendencies among cane-based fruiting positions. TA at different cane-based fruiting positions showed minor variation among the six fruit quality parameters and did not show any appreciable pattern of variation. Fruit firmness showed a wide range of variation among cane-based fruiting positions. There were no significant differences in 2019 and the mean values of 2019 and 2020. However, it was significantly different in 2020 and fruits located at sector 6 were most hard. Flesh color was not significantly different among the cane-based positions in both years. The lowest flesh color value was obtained in sector 4 in 2019, whereas in sector 5 in 2020.
Shoot growth and type, the number of fruit per shoot and fruiting position on the fruit-bearing shoot affect fruit quality parameters (Kulezewski, 2003). Sagredo et al. (2014) reported that fruits originating near base shoots on canes had better quality and higher firmness, and fruits harvested from the shoots at the distal position on the cane had higher fruit mass. This study was conducted to be minimized the possibility of association with these two factors as fruit sampling was conducted only at basal positions of fruit-bearing shoots with similar shoot vigor. Fruits developed from early opened flowers tend to be heavier and to have a higher maturity index with higher SSC and lower acidity than those developed from late-opening flowers (Richardson et al., 2019). Richardson et al. (2001) reported that flowers on canes near the distal end of a leader and on shoots at the distal end of canes tended to open earlier than those on canes near the trunk and shoots at the base of the cane. However, the flowering order of flowers in a shoot could be different depending on flowering type with no, one or double lateral flowers (Thorp et al., 2015). Boyd et al. (2008) reported small differences in fruit quality parameters among fruits at different fruiting positions within a vine and the differences were inconsistent across three crop seasons. Also, the fruits were heavier at positions far away from the trunk and leader and DM was higher at positions close to the trunk and leader. Richardson et al. (2001) reported that flowers on canes near the distal end of a leader and on shoots at the distal end of canes tended to open earlier than those on canes near the trunk and shoots at the base of the cane. Moreover, several factors, including the cultivar, training system, cane vigor such as cane length and thickness, and mineral and carbohydrate supply, might also affect the quality parameters of fruits at different fruiting positions within a vine (Smith et al., 1997;Thorp et al., 2003;Richardson et al., 2019;Lievre et al., 2021). Therefore, the results indicated that these various factors disturbed compositely typical variations of fruit quality parameter to be not significant even though fruit quality parameters had a definite tendency. Further detailed studies are also required to determine the typically spatial patterns of fruit quality parameters at different fruiting positions within a vine in ‘Sweet Gold’ and the other yellowfleshed cultivars.
Variation in Starch and Soluble Sugar Contents Related to Cane-based Fruiting Positions
The starch and soluble sugar contents with the composition were compared in fruits at different cane-based fruiting positions (Table 2 and 3). The fruits located at sector 6 showed the highest starch content, whereas those in sector 2 had the lowest starch content although there was no statistical significance. Starch content mostly consisted of amylopectin in kiwifruits at harvest (Table 2). Unlike the starch contents, the soluble sugar contents were highest in sector 1 and the pattern of variation was not similar to starch contents. The major soluble sugars were fructose and glucose in kiwifruits at harvest (Table 3). These starch and soluble sugar contents were not matched well with any fruit quality parameters, including fresh weight, DM, and SSC (Table 1, 2, and 3).
Kang et al. (2021) reported that the starch content of ‘Sweet Gold’ kiwifruits decreased rapidly from 170 days after anthesis, whereas the soluble sugar contents increased from 120 days after anthesis, which was accompanied by an increase in the levels of fructose and glucose. These contrasted changing patterns in starch and soluble sugar contents may also differ depending on the cultural environment, such as the temperature (Costa et al., 1997;Boldingh et al., 2000;Kang et al., 2021;Li et al., 2021). In fruits such as apples, starch content and composition are changed with developing stages and most starch consists of amylopectin at maturity (Fan et al., 1995). This study showed that amylopectin was major component of starch in kiwifruits at harvest, which was in accord with that reported previously in apples. The starch content and SSC of fruits have been used as harvest indices and fruits with high starch content and low SSC are not considered to be harvestable. In this study, the starch content of fruits was not distinctly related to the total SSC among cane-based fruiting positions. Fruits located at sector 6 had the highest starch content (Table 2), the heaviest fruit weight and the highest firmness (Table 1).
In contrast, fruits positioned in sector 1 exhibited the highest DM and SSC (Table 1). Thus, the result indicated that fruit quality parameters might control complex interaction with the other factors related to cane-based fruiting positions. To elucidate the effect of fruiting positions on fruit quality in kiwifruits based on complex interaction of various factors much clear, future studies of consecutive evaluation combined with other factors such as genotypes, canopy density, cane and shoot vigor and cultural environment are needed.
적 요
본 연구는 제주지역 무가온 플라스틱 하우스에서 덕식으로 재배되고 있는 키위 스위트골드’(Actinidia chinensis var. chinensis)의 수확기 착과 부위별 과실 품질에 미치는 영향을 알아보고자 6개의 구역으로 구분하여 수행하였다.
주간에서 멀리 위치한 구역(3구역 및 6구역)에 착과한 과실 들에서 과중이 높게 나타난 반면, 주간에서 가까운 구역(1구 역 및 3구역)에 착과한 과실들은 건물중(DM)과 당도(SSC)가 높고 과육 색도의 발현이 좋았다.
과육 경도는 주간과 주지에서 멀리 위치한 구역(6구역)에서 가장 높게 나타났고, 산도는 착과 부위와는 거의 관계없었다.
전분의 함량은 주간과 주지에서 멀리 위치한 6구역의 과실 에서 가장 높았으나, 가용성 당 함량은 주간과 주지에서 가깝 게 위치한 1구역의 과실에서 가장 높게 나타냈다.
본 연구 결과, 키위 수체 내 착과 부위별 수확기 과실 품질 특성에는 부분적으로 차이가 있었으나, 이들 양상은 해에 따 라 다르게 나타날 수 있으며 과실 품질 특성인자들 간에 명확 한 관계를 나타내지는 않았다
