2015 Volume 84 Issue 3 Pages 202-213
A series of investigations were conducted to identify an efficient cultivation method for producing uniform fruit in young ‘Hongro’ and ‘Fuji’ apple trees through vigor balance control and crop load adjustment at commercial orchards in Pocheon, South Korea, for two consecutive growth seasons (the third and fourth leafy years), namely, the years 2012 and 2013. There was a highly significant correlation between the cross-sectional area of the main stem (the trunk) and those of all the branches emerging from the main stem, with coefficients of 0.857** (P < 0.001) in ‘Hongro’ and 0.699** (P < 0.001) in ‘Fuji’. The vigor balance, defined as the ratio of branches to the main stem, was the most densely distributed in the range of 3.0 to 3.5 for diameter and around 1.2 for cross-sectional area in both varieties. The present study evaluated the level of crop load by measuring flower cluster density, crop density, and yield efficiency, and found the most densely distributed range was lower than the mean value. The apple trees showed higher productivity with a vigor balance of 1.1 to 1.3 in both varieties, but showed lower productivity with a vigor balance above 1.3 in ‘Hongro’ and below 1.1 in ‘Fuji’ trees in the third and fourth leafy years. Upon adjusting crop load by yield efficiency, the present study obtained higher productivity with a higher degree of yield efficiency in the two consecutive years. However, fruit size and other characteristics demonstrated little change by vigor balance control and crop load adjustment. The uniformity of fruit size and other characteristics was improved by the individual or combined application of vigor balance (from 1.1 to 1.3 in both varieties) and crop load adjustment (from 0.9 to 1.3 kg per cm2 TCA in ‘Hongro’ and from 0.7 to 1.1 kg per cm2 TCA in ‘Fuji’). Notably, the uniformity of fruit size and total soluble solids was efficiently improved by the application of crop load adjustment and vigor balance control, respectively.
When buying apples, consumers generally assess the market price by the size of each fruit (Naschitz and Naor, 2005). Therefore, apple orchard managers attempt to make trees produce large fruit for high profitability in spite of the resulting low productivity (Marini and Sowers, 1994). Agreeing with Westwood (1993) that fruit size is determined by environmental factors and cultural practices as well as hereditary properties, apple orchard managers apply certain cultivation practices such as removing excessive fruit in order to produce large ones (Dennis, 2000; Lauri et al., 2007; Tustin et al., 2012).
Park et al. (1998) indicated that the uniformity of external appearance and internal characteristics had become an additional criterion for determining the market price of apples. The researchers defined a uniform fruit as one having a symmetrical appearance with an L/D (fruit length/fruit diameter) ratio of more than 0.87 and found that the frequency of uniform fruit varied depending on endogenous and exogenous factors such as genetic properties and environmental conditions (Park et al., 1998). Marini and Sowers (1994) described the physiological basis of uniform fruit as that such fruit could be obtained through the even partition of mineral nutrient uptake and dry matter consumption within a tree. Then, researchers attempted to increase the frequency of uniform fruit using certain cultivation practices, such as vigor balance control and crop load adjustment, in apple trees (Kim and Seo, 2007; Nagano Prefecture, 1997) and pear trees (Kwon et al., 2007, 2011).
A group of researchers attempted to discover the physiological basis of the even partition of mineral nutrient uptake and dry matter consumption within a tree through the relationship of canopy volume to main stem (trunk) vigor. Some of them reported that the canopy volume was proportional to the trunk cross-sectional area (TCA) in apple trees (Ogata et al., 1986), and others similarly described that the total above-ground weight of a fruit tree showed a linear correlation with TCA (Kwon et al., 2007; Strong and Azarenko, 2000; Westwood and Roberts, 1970). In particular, Kwon et al. (2007) indicated that the ratio of the cross-sectional areas of total branches to the trunk could be used as a convenient index for measuring the vigor balance between branches and an entire tree, under the detected basis that a tree with optimal vigor balance showed appropriate vigor in the growth of each branch.
Another group of researchers applied TCA to assessing the productivity of fruit trees (Caruso et al., 1999; Strong and Azarenko, 2000). Productivity was expressed through the values of crop density and yield efficiency obtained by dividing the total number and total weight of fruit in a tree by TCA (Choi et al., 2009; Jiménez and Díaz, 2003; Wang et al., 2006). In apple trees, Cho and Yoon (2006) reported the appropriate crop density as six fruit per cm2 TCA in a five-year-old ‘Hongro’ apple tree. Choi et al. (2009) determined the appropriate crop density in a three-year-old ‘Fuji’ apple tree as four to five fruit per cm2 TCA (50 to 60 fruit per tree) and four fruit per cm2 TCA (70 to 75 fruit per tree) for trees older than four years. For other fruit trees, Marini and Sowers (1994) explained the appropriate range of crop density and yield efficiency as zero to 8.7 fruit and zero to 1.6 kg per cm2 TCA for peach trees, and Kwon et al. (2007) indicated that 2.0 fruit and 1.4 kg per cm2 TCA are suitable for ‘Niitaka’ pear trees.
As mentioned earlier, two important cultivation techniques determine the external appearance and internal characteristics of fruit: vigor balance control and crop load adjustment (Cho and Yoon, 2006; Inglese et al., 2002; Kwon et al., 2007; Park et al., 1998). However, earlier researchers focused only on crop load adjustment by regulating crop density and yield efficiency to obtain fruit of large size and good appearance (Marini and Sowers, 1994; Nagano Prefecture, 1997). For that reason, the authors of this paper examined crop load adjustment and vigor balance control as critical factors for successful high-frequency uniform fruit production in apple trees. Another aim of the present study is the establishment of a physiological basis for producing uniform fruit in apple trees.
The current study is composed of a preliminary examination and a supplementary investigation. The preliminary examination involved observation of vegetative growth and the fruiting process in order to identify the appropriate vigor distribution of canopy in a tree and the supplementary investigation assessed tree architecture using vigor balance and crop load adjustment that is appropriate for producing uniform fruit. Detailed descriptions of this work are provided below.
Observation of tree growth and fruiting process in apple varietiesA preliminary examination was conducted at three commercial apple orchards in Pocheon district, South Korea, for two consecutive growth seasons (the third and fourth leafy years), namely, the years 2012 and 2013. Table 1 provides the geographical information and meteorological conditions at the experimental fields for the two growth seasons.
Geographic information and meteorological conditions at the experimental fields in Pocheon district in 2011 and 2012.
The experimental fields were established in early April 2010 using one-year-old ‘Hongro’ and ‘Fuji’ apple trees on M.9 rootstocks with planting space of 4 × 2 m (1250 trees·ha−1). ‘Hongro’ apple is a mid-season variety from a cross between ‘Spur EarliBlaze’ and ‘Spur Golden Delicious’ in South Korea, in 1988, and ‘Fuji’ apple is a late-season variety from a cross between ‘Red Delicious’ and ‘Ralls Genet’ in Japan, in 1962 (Kim et al., 2007). After the establishment, soil conditions and tree growth were managed by fully following the recommended guidance of the Rural Development Administration in Korea. We kept the content of organic matter and mineral nutrients in soil within the allowable range and managed the training system of trees with a slender spindle (Rural Development Administration, 2013). After two growth seasons had passed, the three apple orchard provided three plots of 20 trees having moderate vigor for both varieties. Every plot was arranged as 20 trees in a block of five trees with four replications (Fig. 1).
Overviews of the experimental fields in the third leafy year of ‘Hongro’ (left) and ‘Fuji’ (right) trees on M.9 rootstocks in Pocheon district, established in 2010 and photographed in 2012.
The preliminary investigation involved the measurement of tree growth and evaluation of the fruiting process. The measurement of tree growth meant recording tree height, tree width, and TCA and observing the number and length of branches at the end of vegetative growth, namely, on August 10, in the third and fourth leafy years. For evaluation of the fruiting process, we counted the number of flower clusters at the time of full bloom every year. Fruit of a tree in every block were collected one week earlier than the commercial harvesting time for each variety, early September for ‘Hongro’ and late October for ‘Fuji’, in both growth seasons. Then, we calculated the number and total weight of fruit for each tree.
Measurement of vigor balance between branches and an entire treeThe preliminary examination involved a correlative analysis between the vigor of branches and an entire tree in order to identify the criteria affecting the vigor of each. The 60 individual trees were rearranged into three plots of 20 trees for each variety, which was replicated in the three orchards. In the analysis, the independent variable was the vigor of an entire tree, as defined by tree height, tree width, and TCA. The dependent variable was the vigor of branches, as defined by the number and total length of branches and the sum of branch cross-sectional areas (BCAs). On the basis of the results of the correlative analysis, we applied the ratio of BCAs to TCA to measure the vigor balance of a tree.
The vigor balance of a tree was measured following the method of Kwon et al. (2007): the diameters or the cross-sectional areas of all the branches directly emerging from the main stem were added up and then divided by those of the main stem (trunk) (Fig. 2). Then, TCA was applied to calculate flower cluster density, crop density, and yield efficiency as follows. Flower cluster density was obtained by dividing the number of flower clusters in a tree by TCA. Crop density and yield efficiency were calculated by dividing the number and total weight of fruit in a tree by TCA (Choi et al., 2009; Jiménez and Díaz, 2003; Wang et al., 2006).
Diagrammatic representation for measuring vigor balance between branches and an entire tree for the third leafy year of apple trees. Formula for vigor balance measurement: diameter or cross-sectional area of (a + b + c + d + e + f + g + h/A).
For the supplementary investigation, the present study arranged apple trees into two categories at one orchard among the three orchards in the preliminary examination. One category was for assessing optimal vigor balance control and the other was for measuring appropriate crop load adjustment (measured by yield efficiency). The experimental field was arranged as three plots of 20 trees for each category and variety. The levels of vigor balance were regulated to below 1.1, 1.1–1.3, and above 1.3 for both varieties by controlling the number of remaining branches. The authors attempted to preserve the branches with moderate vigor by thinning out the rest at the dormant pruning time. Crop load was adjusted in term of yield efficiency of below 0.9, 0.9–1.3, and above 1.3 kg per cm2 TCA for ‘Hongro’ trees and below 0.7, 0.7–1.1, and above 1.1 kg per cm2 TCA for ‘Fuji’ trees. The yield efficiency was adjusted by regulating the remaining amount of fruit on a tree. The authors thinned out flower clusters and fruitlets from the time of full bloom to the young fruit stage by accurately following the recommended culture practices (Rural Development Administration, 2013). Yield efficiency was calculated by predicting the mean fruit weight at harvest time for each variety.
Then, apple trees were managed by the individual or combined application of vigor balance control and crop load adjustment. A plot of 20 trees was controlled by setting the vigor balance of 1.1–1.3 for both varieties and another plot of 20 trees was adjusted by setting the crop load to yield efficiency of 0.9–1.3 kg per cm2 TCA for ‘Hongro’ trees and 0.7–1.1 kg per cm2 TCA for ‘Fuji’ trees. The third plot of 20 trees was regulated by the combined application of these two practices. A plot of 20 trees was managed without any regulation as a control. We replicated these processes three times.
The present study compared the productivity and fruit characteristics for each application. The productivity was calculated through recording the number and total weight of fruit; in addition, the characteristics were assessed by measuring fruit weight and size, total soluble solids (TSS), flesh firmness, and titratable acidity. We measured fruit weight with an electron balance (VOL 210; Ohaus, Pine Brook, NJ, USA) and fruit size with vernier calipers (CD-15CP; Mitutoyo, Kawasaki, Japan). The value of TSS was obtained using a hand refractometer (N-1E; Atago, Tokyo, Japan). Flesh firmness was measured using a digital hardness tester (DFT-01; Proem, Seoul, Korea) and calibrated in Newtons as force (N). Acidity in flesh juice was titrated for the amount of malic acid up to pH 8.5 with 0.1 N NaOH using a titrator (950 titrator; Thermo Fisher Scientific, Landsmeer, Netherlands). Later, we compared the distributions of fruit weight and TSS with the mean values for the applications.
Statistical analysisThe present study compared the variance in the obtained mean values by t-test between the varieties and by one-way analysis of variance (ANOVA) for Tukey’s honestly significant difference (HSD) test between the treatments within each variety, and by standard deviation between the varieties.
The present study evaluated the vegetative growth and fruiting process in two varieties of apple trees for two consecutive years, after two leafy years. In the third leafy year, the two varieties of apple trees demonstrated distinctive features in terms of their vegetative growth and fruiting process. ‘Hongro’ trees reached 312.2 cm high in tree height, 215.5 cm wide in tree width, and 1394 mm2 thick in TCA. The branch growth was 15.1 pieces in number of branches and 955 cm in total length. ‘Fuji’ trees showed more vigorous growth than ‘Hongro’ trees, although little difference was seen in tree width. ‘Hongro’ trees showed a higher level of fruiting process than ‘Fuji’ trees: 119.5 flower clusters and 49.4 fruit with a total weight of 12.073 kg in ‘Hongro’ trees compared with 106.3 flower clusters and 39.0 fruit with a total weight of 11.231 kg in ‘Fuji’ trees.
The two varieties of apple trees followed similar tendencies in the fourth leafy year. ‘Hongro’ trees exhibited well-restricted vegetative growth with high productivity compared with ‘Fuji’ trees, except for fewer flower clusters (Table 2).
General features of vegetative growth and fruiting process in the third and fourth leafy years of ‘Hongro’ and ‘Fuji’ apple trees on M.9 rootstocks investigated from 2012 to 2013 (60 variables by 3 replications).
We conducted a correlative analysis between the growth of branches and an entire tree to discover the chief criteria affecting the growth of each. In ‘Hongro’ trees, all of the independent variables showed highly significant correlation with all of the dependent variables, presenting coefficients from 0.398 (P = 0.0016) to 0.857 (P < 0.001). The highest probability was found for the correlation between TCA and total BCAs. ‘Fuji’ trees showed statistical significance in some correlations, but not in others. Tree height maintained statistically significant correlations with the number of branches and total BCAs, with coefficients of 0.528 (P < 0.001) and 0.479 (P = 0.0001), and tree width maintained significant correlations with total branch length and total BCAs, with coefficients of 0.557 (P < 0.001) and 0.397 (P = 0.0017). TCA showed a significant correlation only with total BCAs, with a coefficient of 0.699, showing the highest probability (P < 0.001) (Table 3).
Correlation coefficients of the factors affecting the vigor between branches and an entire tree in the third leafy year of ‘Hongro’ and ‘Fuji’ apple trees on M.9 rootstocks investigated in 2012 (60 variables by 3 replications).
The present study measured the vigor balance between branches and an entire tree using the method in Figure 2. The mean values of vigor balance by diameter were 3.30 ± 0.51 in ‘Hongro’ trees and 3.44 ± 0.54 in ‘Fuji’ trees. The mean values of vigor balance by cross-sectional area were 1.23 ± 0.30 and 1.22 ± 0.33 in ‘Hongro’ and ‘Fuji’ trees, respectively. The vigor balance of diameter exhibited the most densely distributed range at 3.0 to 3.5 in both varieties and that of cross-sectional area displayed the most densely distributed range at 1.2 to 1.4 in ‘Hongro’ trees and 1.0 to 1.2 in ‘Fuji’ trees. Interestingly, the distribution diagram shifted to the left in ‘Hongro’ trees but to the right in ‘Fuji’ trees for both ratios (Fig. 3).
Distribution chart of vigor balance between branches and an entire tree by diameter and cross-sectional area in the third leafy year of ‘Hongro’ (left) and ‘Fuji’ (right) trees on M.9 rootstocks investigated in 2012 (60 variables by 3 replications). CA: cross-sectional area. Vertical bars indicate standard errors. Values under each chart indicate mean ± SD (min to max).
The two varieties of apple trees’ productivity was measured using flower cluster density, crop density, and yield efficiency. Flower cluster density had mean values of 10.68 ± 7.16 clusters with the most densely distributed range of 5.0 to 7.5 clusters per cm2 TCA in ‘Hongro’ trees and 8.62 ± 7.62 clusters with the most densely distributed range of around 5.0 clusters per cm2 TCA in ‘Fuji’ trees. Crop density was 4.45 ± 2.98 fruit per cm2 TCA in ‘Hongro’ trees and 3.06 ± 2.43 fruit per cm2 TCA in ‘Fuji’ trees. In terms of crop density, ‘Hongro’ trees showed 1.11 ± 0.79 kg per cm2 TCA and ‘Fuji’ trees had 0.89 ± 0.73 kg per cm2 TCA. When observing crop load adjustment for both varieties by crop density and yield efficiency, the present study found the most densely distributed range to be lower than the mean value (Fig. 4).
Distribution chart of productivity indexes in the third leafy year of ‘Hongro’ (left) and ‘Fuji’ (right) trees on M.9 rootstocks investigated in 2012 (60 variables by 3 replications). Flower cluster density: number of flower clusters on cm2 TCA. Crop density: number of fruits per cm2 TCA. Yield efficiency: total fruit weight per cm2 TCA. Vertical bars indicate standard errors. Values under each chart indicate mean ± SD (min to max).
Two leafy years later, the present study assessed the productivity of apple trees for the next two consecutive years according to the two criteria obtained from the results of Figures 3 and 4: vigor balance and crop load (yield efficiency).
In the third leafy year, ‘Hongro’ trees exhibited higher productivity when vigor balance was below 1.1 (50.7 fruit with 12.673 kg per tree) or 1.1 to 1.3 (52.5 fruit with 12.848 kg per tree), but ‘Fuji’ trees showed better results when vigor balance was 1.1 to 1.3 (46.1 fruit with 13.196 kg per tree) or above 1.3 (46.8 fruit with 13.677 kg per tree). On the other hand, crop load adjustment by higher yield efficiency resulted in more productivity: from 35.9 fruit with 8.238 kg per tree to 64.4 fruit with 16.363 kg per tree in ‘Hongro’ trees and from 20.2 fruit with 5.756 kg per tree to 57.0 fruit with 16.594 kg per tree in ‘Fuji’ trees.
In the fourth leafy year, the two varieties of apple tree exhibited greater productivity to the same range for both indexes compared with the results in the third leafy year. Apple trees controlled by setting vigor balance showed more productivity in the ranges of lower than 1.1 (59.7 fruit with 14.025 kg per tree) and 1.1 to 1.3 (52.3 fruit with 15.026 kg per tree) in ‘Hongro’ trees and in the ranges of 1.1 to 1.3 (47.1 fruit with 13.325 kg per tree) and above 1.3 (45.3 fruit with 12.921 kg per tree) in ‘Fuji’ trees. Similarly, apple trees with more crop load produced more fruit in both varieties. However, the greater crop load did not account for higher fruit productivity, unlike the results of the previous year in both varieties (Table 4).
Fruit productivity regulated by vigor balance control and crop load adjustment (yield efficiency) in the third and fourth leafy years of ‘Hongro’ and ‘Fuji’ apple trees on M.9 rootstocks investigated from 2012 to 2013 (60 variables by 3 replications).
Controlling by setting different degree of vigor balance did not alter the fruit characteristics in both varieties, except for fruit weight in ‘Hongro’. ‘Hongro’ trees produced heavier fruit as vigor balance declined: the apple trees controlled by setting vigor balance above 1.3 produced light fruit than those controlled by setting vigor balance at 1.1 to 1.3 or below 1.1. Table 5 hardly shows a regular tendency in the variance of fruit characteristics by vigor balance with the results showing no significance: increase in vigor balance decreased fruit size and the level of other characteristics in ‘Hongro’ trees but brought about increases in ‘Fuji’ trees.
Fruit characteristics regulated by vigor balance control and crop load adjustment (yield efficiency) in the third leafy year of ‘Hongro’ and ‘Fuji’ apple trees on M.9 rootstocks investigated in 2012.
Crop load adjustment by using yield efficiency made little difference to fruit size and other characteristics in both varieties. However, some exceptions were observed, such as fruit weight and fruit diameter in ‘Hongro’ and TSS in ‘Fuji’, which did not show a regular tendency. With an increase in crop load, ‘Hongro’ trees produced fruit with increased weight and diameter and ‘Fuji’ trees produced fruit with increased TSS (Table 5).
Figures 5 and 6 show the distributions of fruit weight and TSS for individual fruit against the mean values. These trees were managed by individual or combined application of vigor balance control and crop load adjustment, based on the results shown in Figures 3 and 4.
Distribution chart of the uniformity in fruit weight regulated by vigor balance and/or yield efficiency in the third leafy year of ‘Hongro’ (left) and ‘Fuji’ (right) trees on M.9 rootstocks investigated in 2012 (60 variables by 3 replications). ○: non-regulation, ●: regulation by vigor balance and/or yield efficiency. Vertical bar indicates standard error. Values on top indicate ± SD (min to max) of the non-regulated trees. Values under each figure indicate ± SD (min to max) of the trees.
Distribution chart of the uniformity in total soluble solids content regulated by vigor balance and/or yield efficiency in the third leafy year of ‘Hongro’ (left) and ‘Fuji’ (right) trees on M.9 rootstocks investigated in 2012 (60 variables by 3 replications). ○: non-regulation, ●: regulation by vigor balance and/or yield efficiency. Vertical bar indicates standard error (n = 3). Values on top indicate ± SD (min to max) of the non-regulated trees. Values under each figure indicate ± SD (min to max) of the trees.
The non-regulated apple trees produced fruit with a standard deviation in fruit weight of ± 8.15% ranging from −18.86 to 12.93% in ‘Hongro’ trees and ± 5.95% ranging from −16.29 to 12.37% in ‘Fuji’ trees. The individual or combined application of vigor balance control and crop load adjustment reduced the sizes of the standard deviations and distribution ranges in both varieties. The standard deviations decreased to ± 6.10% ranging from −8.28 to 11.16% in ‘Hongro’ and ± 4.75% ranging from −6.03 to 10.22% in ‘Fuji’ by the individual application of vigor balance control, and decreased to ± 4.60% ranging from −7.55 to 9.55% in ‘Hongro’ and ± 4.98% ranging from −8.49 to 10.35% in ‘Fuji’ by the individual application of crop load adjustment. The combined application of these two practices failed to decrease the standard deviation noticeably compared with the individual applications, except the application of vigor balance control in ‘Hongro’ (Fig. 5).
The fruit of non-regulated apple trees exhibited standard deviations in TSS of ± 4.15% ranging from −8.41 to 9.68% in ‘Hongro’ and ± 5.59% ranging from −11.10 to 12.45% in ‘Fuji’. The application of vigor balance control decreased the standard deviations in TSS of 2.68% ranging from −3.77 to 5.68% in ‘Hongro’ and ± 4.16% ranging from −7.28 to 10.96% in ‘Fuji’. Crop load adjustment by yield efficiency did not bring about further improvement in the standard deviation in TSS with or without vigor balance control in both varieties (Fig. 6).
Robinson (2003) explained that apple trees in a dense planting system completed their canopy architecture at up to the third leafy year and then reached an age of high productivity. Therefore, in the present study, the results of Table 2 are regarded as an agreeable expression of the growth and fruiting process for both varieties. According to Table 2, ‘Hongro’ trees showed restricted vegetative growth and increased fruiting process compared with ‘Fuji’ trees for the two leafy years. Shin et al. (1989) reported that ‘Hongro’ apple trees showed well-controlled tree vigor and high productivity. Other researchers previously reported similar tendencies in tree growth and fruiting process after observing both varieties of apple trees (Choi et al., 2009; Paek et al., 2007). Therefore, we took the apple trees for the present study as appropriate criteria for their hereditary properties in tree growth and fruiting process (Table 2).
A correlative analysis demonstrated that TCA had a close relationship with the sum of total BCAs in both varieties. After Ogata et al. (1986) pointed out the linear correlation between TCA and canopy volume in apple trees, other researchers reported that TCA indicated the vigor of the main stem and BCA implied the vigor of the branches (Caruso et al., 1999; Strong and Azarenko, 2000). In particular, Kwon et al. (2007) assessed the vigor balance between branches and an entire tree by measuring the sum of total BCAs and TCA in a pear tree. The authors postulated that the vigor of an entire tree is composed of the vigor of all of its branches (Table 3).
According to a previous report by Kwon et al. (2007), pear trees having two to five branches exhibited vigor balance of diameter from 1.5 to 2.3. However, the apple trees in the present study showed a vigor balance of diameter in approximately 3.0. Kwon et al. (2007) obtained the vigor balance from pear trees having two to five branches, whereas the present study obtained the values from apple trees having more than 15 branches, as seen in Table 1. This difference in the number of branches between the two studies might suggest a significant difference in vigor balance of diameter. However, the results of the current research are similar to those of Kwon et al. (2007) in vigor balance of cross-sectional area as showing approximately 1.2 in spite of the difference in the number of branches. For that reason, it is reasonable to suppose that a fruit tree could express its vigor level using the vigor balance of cross-sectional area and the appropriate level of vigor balance was around 1.2 (Fig. 3).
In earlier studies, researchers used crop density and yield efficiency rather than flower density to determine the appropriate crop load in fruit trees (Cho and Yoon, 2006; Choi et al., 2009; Jiménez and Díaz, 2003; Paek et al., 2007; Wang et al., 2006). From the results of Figure 4, we could surmise that ‘Hongro’ trees produced about 4.5 fruit with individual weight of 250 g per cm2 TCA (yield efficiency of 1.1 kg) and ‘Fuji’ trees produced around 3.0 fruit with individual weight of 290 g per cm2 TCA (yield efficiency of 0.9 kg). The present study involved picking fruit one week earlier than the commercial harvesting time, which could account for the difference with some previous studies in terms of crop density and yield efficiency, as described above (Cho and Yoon, 2006; Choi et al., 2009). Meanwhile, the most common values of the two indexes were lower than the mean values for both varieties. This discrepancy will be discussed below (Fig. 4).
Comparing the productivity of apple trees for the two growth seasons with other previous results (Cho and Yoon, 2006; Choi et al., 2009; Paek et al., 2007), the present study showed similar level of production in spite of harvesting occurring one week earlier than in a commercial setting. A previous study pointed out that apple trees could show appropriate fruiting through adequate vegetative growth (Nagano Prefecture, 1997).
Surveying the results in the two consecutive years, the present study yielded agreeable results in terms of the productivity of the apple trees maintaining their vigor balance at 1.1 to 1.3 for both varieties. Apple trees decreased their productivity as vigor balance rose above 1.3 for ‘Hongro’ and fell below 1.1 for ‘Fuji’ in both growth seasons. Kwon et al. (2007) reported that pear trees with low vigor balance decreased fruit yield and later Park et al. (2008) showed that apple trees with strong vigor had low productivity. On the basis of results of the two consecutive years, the present study suggests that apple trees should not have their vigor balance controlled above 1.3 for ‘Hongro’ trees and below 1.1 for ‘Fuji’ trees.
Although many orchard managers in South Korea attempted to maintain their apple trees with low yield efficiency, as shown in Figure 4, the present study found that apple trees adjusted to high yield efficiency showed high productivity in the two consecutive cropping seasons. Previous researchers agreed that apple trees with increased yield efficiency exhibited high cropping (Cho and Yoon, 2006; Choi et al., 2009). On the other hand, the highest yield efficiency resulted in the highest productivity for the apple trees in the third leafy year, but did not for those in the fourth leafy year. Lauri et al. (2007) reported that crop regulation could be obtained through appropriate artificial removal of flower clusters or fruitlets, and Tustin et al. (2012) indicated that extremely high productivity may decrease flower bud load in the next growth season for apple trees. In the current study, we found that the apple trees with excessively high yield efficiency did not maintain high productivity in the following cropping season (Table 4).
Many researchers have attempted to identify the factor that is critical for success in producing large fruit with good characteristics. To obtain large fruit, some researchers have controlled vigor balance (Inglese et al., 2002) and others have adjusted crop load (Choi et al., 2009; Dennis, 2000; Johnson and Handley, 1989; Lauri et al., 2007; Marini and Sowers, 1994; Naor et al., 1999; Tustin et al., 2012; Yim, 2005). Researchers reported that fruit trees with appropriate crop load produced fruit with high TSS (Cho and Yoon, 2006; Fallahi and Simons, 1996; Mohamed et al., 2001; Wünsche et al., 2000). However, the present study did not find a consistent tendency to suggest that vigor balance control and crop load adjustment made fruit trees produce larger fruit with better characteristics. The two cultivation practices brought about significant differences in fruit weight by vigor balance and crop load in ‘Hongro’, fruit diameter by crop load in ‘Hongro’, and TSS by crop load in ‘Fuji’, but they failed to exhibit a consistent tendency. In previous studies, some researchers controlled fruit size using crop load adjustment (Lauri et al., 2007; Tustin et al., 2012; Yim, 2005), but others regulated fruit characteristics using vigor balance control for an even partition of dry matter (Costes et al., 2006; Koike et al., 1990). Koike and Ono (1998) stressed the leaf activity for fruit development because the composition of fruit flesh was largely affected by the assimilation of adjacent leaves. The current study postulated that the small differences in Table 5 were caused by the even distribution of leaves for fruit in a canopy (Table 5).
Considering the results above (as shown in Figures 3 and 4, and Tables 4 and 5), we could conclude that an increase in crop load brought about an increase in fruit yield without a change in fruit size only when leaves for a fruit were evenly distributed within a canopy. However, the uniformity of external appearance and internal characteristics has become an additional criterion for the market price of apples (Park et al., 1998). Previous researchers attempted to obtain uniform fruit using the individual application of vigor balance control or crop load adjustment (Costes et al., 2006; DeJong and Grossman, 1994; Inglese et al., 2002; Park et al., 1998).
In the present study on the uniformity of fruit in terms of fruit weight and TSS, the individual application of vigor balance control or crop load adjustment reduced the standard deviation significantly, but control by both of these two practices did not bring about a further decrease. The authors regard the decrease of standard deviation as representative of improvements of fruit uniformity in terms of fruit size and quality. In terms of the significant decrease of standard deviation, previous researchers attempted to identify the factors critical for the success of fruit uniformity from crop load adjustment (Choi et al., 2009; Fallahi and Simons, 1996; Wünsche et al., 2000; Yim, 2005) or vigor balance (Marini and Sower, 1994). However, more detailed surveying revealed that the most efficient cultivation practice to control the standard deviation was crop load adjustment for fruit weight and vigor balance control for TSS. On the basis of the results of previous studies (Kwon et al., 2007; Strong and Azarenko, 2000), we could postulated that fruit weight chiefly depends on the uptake of mineral nutrients into branches through the main stem, whereas TSS mainly relies on the partition of dry matter within the canopy. Therefore, the control of vigor balance might be an efficient means to ensure even uptake of mineral nutrients and to adjust crop load for even consumption of dry matter. After considering all of the results, the present study proposes the combined application of the two cultivation practices in order to obtain high productivity of uniform fruit without the restriction of tree vigor (Figs. 5 and 6).
