Abstract
The growth and quality of the fruit from adult ‘Fuyu’ and ‘Hiratanenashi’ Japanese persimmon (Diospyros kaki Thunb.) trees that were grafted onto D. kaki seedlings (S), Rootstock-a (R-a), ‘MKR1’, and own-rooted (O-R) trees planted in February 2003 were observed from 2015 to 2021, when almost all of the trees entered stable, high-productive phases and the fruit quality was stable. The trees that were grafted onto ‘MKR1’ were still dwarfed but produced fruit efficiently. After disbudding took place, in the first half of stage I of the double sigmoid growth curve there was a certain period of time when the fruit of the ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ was lighter than the trees that had been grafted onto S. However, in the second half of stage I, the fruit of the former was heavier than that of the latter, and the flesh cells of the former were larger than that of the latter. In stage II and III, the fruit of the latter grew faster and became significantly heavier just before the harvest. The fruit growth of the ‘Fuyu’ trees that were grafted onto R-a was retarded compared to the growth of trees that were grafted onto ‘MKR1’ throughout most of the growing period. Heavy disbudding (HD) effectively increased the fruit weight of the ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ in a certain time period in stage I and in stage III compared to normal disbudding. HD also improved the fruit growth of the ‘Fuyu’ trees that were grafted onto ‘MKR1’, although this was not as effective as the ‘Hiratanenashi’ fruit growth. The weight of the harvested ‘Fuyu’ trees’ fruit increased in the following order: R-a, ‘MKR1’, O-R, and S. The ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ had the lightest fruit among the rootstocks, but the HD treatment for the trees that had been grafted onto ‘MKR1’ increased the fruit weight, and the difference disappeared. The soluble solids concentration of ‘Fuyu’ fruit was the highest in ‘MKR1’ and the lowest in R-a. ‘Hiratanenashi’ fruit had the highest concentration of soluble solids in S and the lowest concentration in O-R. The level of firmness of ‘Fuyu’ fruit flesh was the hardest in ‘MKR1’ and the softest in R-a, whereas the level of firmness of ‘Hiratanenashi’ fruit was the same among the rootstocks. Although the a* value of ‘Fuyu’ fruit skin color was the same among the rootstocks, the a* value skin color of ‘Hiratanenashi’ fruit was the highest in ‘MKR1’ and R-a and the lowest in S. These results suggest that the persimmon rootstock partly affects growth and quality of the fruit, although there are differences in the reactions between scion cultivars.
Introduction
The persimmon (Diospyros kaki Thunb.) tends to grow to a large size, and growers often encounter difficulties in orchard management (Tao and Sugiura, 1992). Therefore, candidate dwarfing rootstocks for the Japanese persimmon have been sought out and selected (Hattori et al., 2015; Kimura et al., 1985). At the same time, methods for efficient clonal propagation of these rootstocks have been developed (Hejazi et al., 2018; Kagami, 1999; Tetsumura et al., 2000, 2003, 2009, 2017). As a result, several studies on the orchard growth of trees that have been grafted onto clonally propagated candidate rootstocks have been conducted, and the ability of some rootstocks to effectively dwarf the scions has been demonstrated (Hattori et al., 2015; Ohata and Kurahashi, 2022; Tetsumura et al., 2010, 2015, 2019; Yakushiji et al., 2008, 2021). Moreover, some of the rootstocks gave the scions useful characteristic features, such as an increase in flowers, a decrease in early fruit drop, and improvement in the yield efficiency (Hattori et al., 2015; Ohata and Kurahashi, 2022; Tetsumura et al., 2010, 2013, 2015, 2019; Yakushiji et al., 2021). An adaptability test of some registered persimmon cultivars as dwarfing rootstocks was initiated in 17 public research organizations in Japan in 2016. The scion cultivars used in the tests varied with the organizations, and fruit quality and tree growth continue to be evaluated.
‘Fuyu’ and ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’, a dwarfing rootstock, and were planted in 2003 in the orchard of the Field Science Center, University of Miyazaki, Japan, were long evaluated for their field performance and showed some characteristic features, including dwarfism (Tetsumura et al., 2010, 2013, 2015). One of the features was a significantly higher yield efficiency. The yield efficiencies of the ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ were higher than those of trees that were grafted onto D. kaki seedlings (S), Rootstock-a (R-a) (Tetsumura et al., 2003), and own-rooted (O-R) trees that had been produced by micropropagation, even when the tree was over 10 years old (Tetsumura et al., 2015). Although there was no consistent trend for the effect of rootstocks on ‘Hiratanenashi’ fruit quality, the fruit weight of the ‘Hiratanenashi’ trees that had been grafted onto ‘MKR1’ was the lowest in 2013 (Tetsumura et al., 2015). In 2014, we continued the investigation and found that the fruit weight of the ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ was still the lowest, although the yield efficiencies were still the highest. Specifically, there was a significant difference between the 183 g per fruit of ‘Hiratanenashi’ trees that had been grafted onto ‘MKR1’ and the 238 g of trees that were grafted onto S. There was also a significant difference between the 3.78 kg·m−3 of yield efficiency per canopy volume of the former and the 0.27 kg·m−3 of yield efficiency of the latter. We presumed that these results were due to overcropping of the trees that had been grafted onto ‘MKR1’ because they showed heavier flower bearing and less early fruit dropping, maintaining a high yield efficiency even when the trees reached the adult phase. In fact, the height of the trees that were grafted onto ‘MKR1’ did not increase after 2010, whereas the trees that were grafted onto S and R-a, along with the O-R trees, grew upward like young trees between 2010 and 2014 (Tetsumura et al., 2015).
Persimmon growers in Japan are recommended to use the process of disbudding and fruit thinning to help trees produce a marketable fruit size (Kitagawa, 1970). In this process, practically all of the flower buds but one per shoot are thinned, and all flower buds on shoots with five or fewer leaves are thinned before blossoms appear. After the completion of early fruit drop, the leaf-fruit ratio per fruiting mother shoot is adjusted to 25 through the process of fruit thinning. In the university orchard, we applied these cultural practices to all of the ‘Hiratanenashi’ and ‘Fuyu’ trees. Disbudding took place between mid-April and early May, and fruit thinning occurred between mid- and late July.
As for the ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’, Ishimura et al. (2015) found that there were fewer leaves per shoot due to the shorter shoots, and that the individual leaves were smaller, although the photosynthetic rate of the leaves was the same as those of the other trees. Additionally, the inhibition of an early fruit drop that occurs between the disbudding and fruit thinning phases was observed in ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ (Tetsumura et al., 2013). These results suggest that the over-fruiting of the ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ certainly occurred until the fruit thinning took place in July, resulting in smaller fruit at the time of harvest.
There are many reports showing that rootstocks affected fruit weight (Dubey and Sharma, 2016; Dubey et al., 2021; Giorgi, et al., 2005; Iglesias et al., 2019; Jiménez et al., 2011; López-Ortega et al., 2016; Lordan et al., 2017; Sharma et al., 2016; Webster and Wertheim, 1993). However, all these reports indicated the weight at the time of harvest, but not the change in weight during fruit development. Therefore, we investigated the effects of rootstocks on the fruit growth of the ‘Fuyu’ and ‘Hiratanenashi’ trees during their growing period, not only by measuring fruit weight and dimensions, but also by observing the development of flesh cells microscopically. Moreover, we found an improvement in disbudding and fruit thinning resulted from taking advantage of the characteristic feature of the trees that had been grafted onto ‘MKR1’, that is, the inhibition of early fruit drop, and investigated the time-course changes in fruit weight, dimensions, and flesh cells. In addition, the long-term effects of the rootstocks on tree growth, productivity, and fruit quality of both cultivars were evaluated.
Materials and Methods
Plant materials, cultivation management, and experimental design
The trees used in this study were those that were previously described (Tetsumura et al., 2010, 2015). In February 2003, the nursery stocks were planted in Andosol (kuroboku soil) in the orchard of the Field Science Center, Faculty of Agriculture, University of Miyazaki, Japan. Two rows were planted: one row of ‘Fuyu’ and one row of ‘Hiratanenashi’ trees, with each cultivar having a combination of three different types of rootstocks, S, R-a, and ‘MKR1’, and O-R. Spacing within the rows was 4 m, and the spacing between rows was 3 m. In the beginning, the experimental design was a randomized complete block with five replications comprising one tree each. However, except for the trees that were grafted onto ‘MKR1’, the number of trees decreased due to typhoon damage and tree thinning to alleviate overcrowding. As a result, from 2015 to 2017, there were four ‘Hiratanenashi’ trees that were grafted onto S, which was commercially available and therefore a control, and one tree in 2018, while there was one tree that was grafted onto the R-a rootstock and one O-R tree during the study. There were four ‘Fuyu’ trees that were grafted onto R-a trees from 2015 to 2017 and one tree in 2018, while one tree was grafted onto S and O-R trees during the study. Trees were pruned and trained to a modified central leader. Pest and fertility management were conducted as recommended.
On April 20, 2015 and April 28, 2016 before any blossoms appeared, all but one flower bud per shoot was thinned, and all flower buds on shoots with five or fewer leaves were thinned. In mid-July, after completion of early fruit drop, the leaf-fruit ratio per fruiting mother shoot was adjusted to 25 with the process of fruit thinning done by hand. After 2017, the three ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ were subjected to heavy disbudding (HD) around April 20 before any blossoms appeared. Flower buds were thinned to be adjusted to 25 for the leaf-bud ratio per fruiting mother shoot, and no fruit thinning was done. The other two ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ were subjected to the same normal disbudding occurring around April 20 and fruit thinning occurring in mid-July (ND), just like the trees that were grafted onto the other rootstocks. The three ‘Fuyu’ trees that were grafted onto ‘MKR1’ were subjected to HD in 2019 and 2020.
Evaluation of tree growth, productivity, and fruit quality
The tree height, trunk cross-sectional area (TCSA) at 40 cm above the soil, ground area covered by the tree canopy (canopy area), and the canopy volume were measured in February of each year before pruning. The canopy area and volume were estimated according to the research conducted by Kimura et al. (1985) to calculate an annual yield efficiency, which was also estimated using TCSA.
Every 10–14 days between April and October, five average samples of flower buds or flowers were collected, and the subsequent fruit was also collected from the ‘Hiratanenashi’ trees that had been grafted onto S and ‘MKR1’ along with the ‘Fuyu’ trees that were grafted onto R-a and ‘MKR1’. The samples from the ‘Fuyu’ trees grafted onto S were added in 2019, and those from the ‘Hiratanenashi’ trees that were grafted onto R-a were added in 2021. After the weight, longitudinal, and transverse diameters were measured, the samples were cut transversely at the equatorial position. A 5 mm cube of the mesocarp cut off from the transverse section was frozen in liquid nitrogen and embedded in a cryo-embedding medium (SCEM; Section-Lab, Hiroshima, Japan). Depending on the size of the fruit flesh cells, sections that were 5–40 μm thick were produced using a cryostat (Leica CM1850; Leica Microsystems GmbH, Wetzlar, Germany) at −20°C using Kawamoto’s film method (Kawamoto, 2003) with an adhesive film (Cryofilm Type 2C(10); Section-Lab), followed by staining with hematoxylin-eosin, and sections were observed under a light microscope (BX51; Olympus Corporation, Tokyo, Japan). After June, it was difficult to prepare sections because the cell wall was soft, so each sample was immersed in a 10% sucrose solution (w/v) for one full day to harden the cell wall before embedding took place. The size of the fruit flesh cells was evaluated by the cell- and space-size index (CSSI), which provides a rapid assessment of mean cell size (Sugiura et al., 1995). The means of CSSI were derived from each of four fruits, and the CSSI was investigated between 2015 and 2018.
All mature fruit with a skin color score of five or more (Yamazaki and Suzuki, 1980) were harvested and weighed every seven days. Each tree’s fruit was harvested between two and four times. The annual yield efficiency was calculated from the tree yield (kg) per TCSA (cm2) that was measured in the following year, per canopy area (m2) or per canopy volume (m3). During each harvest, the fruit qualities described below were evaluated for an average of five fruit per tree. The soluble solids concentration (SSC) was measured with a digital refractometer (PAL-100; Atago Co., Tokyo, Japan) after juice extraction using a grater. The SSC for ‘Hiratanenashi’ fruit was measured after performing a deastringency treatment at approximately 95% CO2 and 27°C for one week. The firmness of the fruit was measured on two paired surfaces at the equatorial region using a fruit hardness tester (KM-5; Fujiwara Scientific Company Co., Tokyo, Japan) fitted with a cone tip plunger. The color of the fruit’s skin was measured at two different points at the equatorial and apical regions as an a* value, which is highly correlated (R2 = 0.98) with the color score (Yamazaki and Suzuki, 1980) and was recorded using a colorimeter (CR-400; Konica Minolta, Inc., Tokyo, Japan). The number of seeds per fruit were counted, and the degree of physiological disorders observed on ‘Fuyu’ fruit, namely calyx-end cracking and stylar-end cracking, were evaluated. The degrees of all of the disorders were defined as zero (none), one (slight), two (moderate), and three (severe), which were the same degrees that were used in a previous study (Tetsumura et al., 2019). The means of the data on fruit quality were derived from each of the 10 replications obtained from the two sampling dates with the highest and second highest number of harvests.
The data on the trees that were grafted onto R-a and S along with the O-R tree, none of which were thinned, and the data derived from the average tree that had been grafted onto ‘MKR1’ in each of the disbudding treatments were used for the analysis.
Data analysis
The t-test (P < 0.05) was used to assess the differences between the two datasets such as were seen in CSSI in 2015 and 2016. Except for the degrees of physiological disorders, the other data were subjected to a one-way analysis of variance to determine the significance of differences between rootstocks. The means were evaluated using Tukey’s honestly significant difference test (P < 0.05). The degrees of all physiological disorders were subjected to the Kruskal-Wallis test, and the means were evaluated using the Steel-Dwass test (P < 0.05). The overall averages of fruit qualities and yield efficiencies were derived from the yearly data, except for the trees that were grafted onto ‘MKR1’ with HD treatment, and the year served as the block (replication) only in the randomized complete block experimental designs. All statistical analysis were performed using Ekuseru-Toukei 2015 (Social Survey Research Information Co., Ltd., Tokyo, Japan).
Results
Tree growth and fruit productivity
All ‘Fuyu’ trees stopped growing upward after 2014. The differences in all tree heights between 2014 and 2021 were approximately 100% (Table S1). However, they continued to grow, as evidenced by the fact that the increase in TCSA in 2021 was more than double that recorded in 2014, while the canopy area was approximately 2–3.5 times larger. The canopy volume also increased, and the percentages were similar to those of the canopy area because the tree heights did not change. All ‘Hiratanenashi’ trees showed the same growth as ‘Fuyu’ trees with the exception of the O-R trees, which still grew vigorously. These results indicated that almost all the trees grew laterally after 2014.
The trees that were grafted onto ‘MKR1’ showed superiority in terms of fruit productivity (Tables S2 and S3) just as they had in previous studies (Tetsumura et al., 2010, 2015), although the yield per tree was low because of the small size of the trees. The superiority of the yield efficiency per canopy area was less apparent than those per TCSA and canopy volume because the trees grew laterally rather than vertically after being established for 10 years (Tetsumura et al., 2015).
Fruit growth
Persimmon fruits have three growth stages, showing a double sigmoid growth curve in which two rapid-growth stages are separated by a slow-growth stage known as stage II (Kitagawa, 1970). The duration of stage II may vary from year to year because slow fruit growth is likely caused by high temperatures in the summer (Chujo et al., 1973). Thus, in this study, instead of explaining the significant differences that were seen using dates, we showed the state of each stage in which the differences occurred.
After disbudding took place, there was a certain period of time during the first half of stage I in which the fruit, which was actually in bud, flower, or small fruit, of the ‘Hiratanenashi’ trees that were grafted onto S was heavier than that of the trees that were grafted onto ‘MKR1’ (Fig. 1). However, in the second half of stage I, the former was lighter than the latter. In stage III, the ‘Hiratanenashi’ trees that were grafted onto S showed faster growth and became significantly heavier just before harvest. The growth curves of the longitudinal and transverse diameters had almost the same pattern as the fruit weight (Fig. S1). The flesh cell size of the ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ appeared to be larger than that of S in the second half of stage I, although no significant difference was observed in the CSSI between the rootstocks due to the large variation in cell size in each of the sections (Fig. 1).
The ‘Fuyu’ fruit of R-a was heavier than that of ‘MKR1’ during the first half of stage I, although this phase was short (Fig. 2). However, the fruit weight became lighter near the end of stage I, similar to the relationship between S and ‘MKR1’ for the fruit of ‘Hiratanenashi’ trees. The heavier fruit weight of the ‘Fuyu’ trees that had been grafted onto ‘MKR1’ rootstock continued until harvest. As the harvest approached, the transverse growth of the R-a fruit was significantly inferior to that of ‘MKR1’ with respect to fruit size, whereas the longitudinal growth did not differ significantly (Fig. S2). Although the difference in CSSI was not significant, the cell size of the fruit of ‘Fuyu’ trees that had been grafted onto ‘MKR1’ rootstock appeared to be larger than that of R-a for almost all periods after the middle of stage I (Fig. 2).
Compared to ND, HD effectively increased in terms of weight and the transverse diameter of the fruit from ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ during the first half of stage I in 2017 (Figs. 3 and S3). Specifically, after disbudding on April 20, the fruit of ‘MKR1’ with HD grew better than the fruit of ‘MKR1’ with ND, and became the same weight as S, which was heavier than that of ‘MKR1’ with ND in the first half of stage I. The change in fruit weight in the second half of stage I was the same as it was in 2015, whereas ‘MKR1’ with HD was heavier than that with ND. In stage III, the increase in the fruit weight of ‘MKR1’ with HD was not inferior to that of S. In the CSSI, there was a significant change in the middle of stage I resulting in a difference between S and ‘MKR1’. After that, an insignificant difference appeared in the second half of stage I (Fig. 3), similar to the results recorded in 2015 (Fig. 1).
The HD treatment for the ‘Fuyu’ trees in 2019 slightly improved the fruit growth (Fig. 4). The fruit weight of ‘MKR1’ with HD was significantly heavier than that of ‘MKR1’ with ND in the first half of stage II and S in stage II, but there were no significant differences among them in the first half of stage I and in stage III. The fruit weight of R-a was the lightest throughout the growing season in 2019.
The HD treatment for ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ given in 2020 improved the fruit growth as seen in 2017 (Fig. 5). Interestingly, R-a reduced the fruit growth of the ‘Fuyu’ trees, but not of the ‘Hiratanenashi’ trees. Conversely, the fruit of ‘Hiratanenashi’ trees grafted onto R-a was heavier than that of S in the second half of stage I.
Fruiting and fruit quality
Although it was slightly different depending on the year, the fruit weight increased for the ‘Fuyu’ tress in the following order: R-a, ‘MKR1’, O-R, and S (Table 1). The ‘Hiratanenashi’ trees that had been grafted onto ‘MKR1’ produced the lightest fruit among the rootstocks (Table 2). However, the HD treatment for the trees grafted onto ‘MKR1’ resulted in increased fruit weight, especially in the ‘Hiratanenashi’ trees. The SSC of the ‘Fuyu’ trees’ fruit was the highest in ‘MKR1’ and the lowest in R-a. The fruit of the ‘Hiratanenashi’ trees was the highest in S and the lowest in O-R. The HD treatment for the trees that were grafted onto ‘MKR1’ did not appear to change the SSC. The firmness of the ‘Fuyu’ fruit was the hardest in the ‘MKR1’ rootstock and the softest in R-a, whereas the firmness of the ‘Hiratanenashi’ fruit was the same among all of the rootstocks. Although the a* value of the skin color of ‘Fuyu’ fruit was the same among the rootstocks, the a* value of the skin color of ‘Hiratanenashi’ was the highest in the ‘MKR1’ and R-a rootstocks and the lowest in S. In both cultivars, there were no significant differences in the number of seeds among rootstocks in all of the years and means (data not presented). The ‘Fuyu’ fruit of the O-R tree produced the highest degree of calyx-end cracking, and the trees that were grafted onto S and ‘MKR1’ had the lowest degree of calyx-end cracking. The rootstock did not affect the degree of stylar-end cracking (Table 3).
Discussion
Our previous study (Tetsumura et al., 2015) showed that trees that were grafted onto ‘MKR1’ stopped growing upward after 2010, whereas the other trees grew upward vigorously between 2010 and 2014. After 2014, almost all of the trees stopped growing upward, but continued to grow laterally, in this study. In general, persimmon shoots that grow laterally produce fruit effectively, whereas shoots that grow upward, especially succulent shoots, promote vegetative growth. Therefore, all of the trees used in this study except for the ‘Hiratanenashi’ O-R trees entered stable, high-productive phases, and the fruit quality was also stable.
The annual yield efficiencies of the ‘Hiratanenashi’ trees that were grafted onto S, R-a, and O-R between 2015 and 2020 (Table S2) were higher than those recorded in 2013 (Tetsumura et al., 2015) with few exceptions, and the trees that were grafted onto ‘MKR1’ had a lower yield efficiency. However, the trees that were grafted onto ‘MKR1’ still produced fruit the most efficiently among the rootstocks. As for the ‘Fuyu’ trees, the annual yield efficiencies of ‘MKR1’ in this study were lower than those recorded in 2013 (Tetsumura et al., 2015), but were still significantly higher than those of R-a (Table S1). The mean shoot length of a persimmon tree is thought to be an index of fruit productivity (Kurahashi, 1998), and the mean shoot length of the ‘Taishuu’ tree that was grafted onto ‘MKR1’ was ideal for efficient fruit production (Tetsumura et al., 2019). Although the shoot length was not measured in this study, the appearance of shoot growth of the trees that were grafted onto ‘MKR1’ was unchanged from the previous study, in which the shoot lengths of both cultivars were significantly shorter than those of the other rootstocks (Tetsumura et al., 2015). Therefore, these facts demonstrate the high productivity of the trees that were grafted onto ‘MKR1’, as they continued producing fruit effectively for 18 years after being planted.
In this study, the change in fruit weight was used as an index of fruit growth, although the size of the fruit usually indicates this index (Kitagawa, 1970). As shown in Figs. 1–5 and Figs. S1–S5, all of the double sigmoid growth curves of fruit weight roughly coincided with those of longitudinal and transverse diameters. Interestingly, in stage III, differences in ‘Hiratanenashi’ trees between the different rootstocks were significant both longitudinally and transversely, whereas differences in ‘Fuyu’ trees only significant transversely. Hence, we regarded the increase in fruit weight as a change in fruit growth. The CSSI was devised as an index that can be quickly measured to indicate the size of apple and pear fruit flesh cells (Sugiura et al., 1995). In this study, it could be also used as an index to indicate the size of persimmon fruit flesh cells. ‘Hiratanenashi’ fruit flesh cells are known to be larger than other cultivars (Kitagawa, 1970), and in this study, the CSSI was about twice the size of ‘Fuyu’ fruit flesh cells (Figs. 1 and 2).
Most cell division in persimmon fruit is completed during the flowering period; thus, disbudding is recommended to increase the number of cells and produce larger fruit (Kitagawa, 1970). During the first half of stage I in 2015, the heavier weight of the flower bud, flower, and small fruit of ‘Hiratanenashi’ trees that were grafted onto S was presumably due to more active cell division that was induced by reserve nutrients that translocated from the source in the previous year (Kitagawa, 1970). The fruit size also showed significant differences between rootstocks (Fig. S1), whereas no difference occurred in the CSSI (Fig. 1). In stage III, when persimmon fruit grows by cell enlargement instead of cell division (Kitagawa, 1970), the fruit weight of the S was higher than that of the ‘MKR1’ due to the increased number of fruit fresh cells during the first half of stage I.
Cell enlargement of persimmon fruit begins at the end of the process of cell division in stage I (Kitagawa, 1970). The fruit weight and fruit size of ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ were significantly heavier and larger toward the end of stage I than those of trees that had been grafted onto S (Figs. 1, 3, S1, and S3). The CSSI showed that the superior fruit growth of the ‘MKR1’ rootstock was due to cell enlargement that was most likely caused by the active translocation of photosynthates from the leaves. Although the fruit of the trees that had been grafted onto S were larger and heavier during the first half of stage I, there were no significant differences in the weight and size of fruit in the middle of stage I, so active translocation of the trees that were grafted onto ‘MKR1’ may have started in the middle of stage I.
The harvested ‘Fuyu’ fruit of R-a were smaller than those of the other rootstocks, including those of ‘MKR1’ recorded in previous studies (Tetsumura et al., 2010, 2013, 2015). In this study, the ‘Fuyu’ fruit of R-a was apparently retarded in terms of weight, size, cell size, and SSC. R-a presumably suppressed the active translocation of nutrient reserves from leaves to the ‘Fuyu’ fruit because the fruit growth was weaker from the middle of stage I to the point of harvest (Fig. 2). However, during the first half of stage I when cell division occurred actively, the flower growth of R-a was better than that of ‘MKR1’, suggesting that active translocation from reserve organs, such as roots and limbs, to flowers occurred relatively smoothly. Therefore, there were no fewer cells in the ‘Fuyu’ fruit of R-a, but the fruit did not grow well up to the point of harvest. Moreover, the SSC was the lowest of the harvested fruit (Table 1), and the fruit productivity was the lowest (Table S2). These facts support the above-mentioned assumption that the photosynthates from the source were not actively translocated to the fruit. On the other hand, retardation was not observed in ‘Hiratanenashi’ fruit at all. Effects of persimmon rootstock on the scion were definitely different between the cultivars, as shown in other fruit trees (Dubey et al., 2021; Sharma et al., 2016).
The weight and transverse diameter of the ‘Hiratanenashi’ fruit from trees that were grafted onto ‘MKR1’ with HD during the first half of stage I were superior to those of ‘MKR1’ with ND and equal to S (Figs. 3 and S3). Hence, the results suggest the HD treatment for ‘MKR1’ that induced efficient translocation of the reserve nutrient to the bud, flower, and fruit, promoted cell division, and resulted in higher fruit flesh cell numbers than ‘MKR1’ with ND. However, regardless of the disbudding treatment, the flesh cells of the ‘MKR1’ fruit were significantly larger than those of S in the middle of stage I. Due to the difference in the number of fruit flesh cells, the fruit of the ‘Hiratanenashi’ trees that were grafted onto ‘MKR1’ with HD near the end of stage I were significantly heavier than fruit from the trees that were grafted onto ‘MKR1’ with ND (Fig. 3). During the process of fruit thinning by hand between mid- and late July, we were not aware of the difference in fruit size between the ‘Hiratanenashi’ trees and sometimes considered that the fruit of ‘MKR1’ were larger. The growth curves show that the size of the fruit from the trees that were grafted onto ‘MKR1’ was equal to or larger than the fruit from trees that had been grafted onto S in the second half of stage I (Figs. S1, S3, and S5). The main reason that the fruit weight of ‘MKR1’ with HD increased more than it did with ND in stage III was thought to be the result of a greater increase in cell numbers in the fruit during the first half of stage I. On the other hand, the HD treatment for ‘Fuyu’ trees appeared to be less effective than that for ‘Hiratanenashi’ trees since there was no significant difference in the fruit weight in the first half of stage I and in stage III in 2019 among ‘MKR1’ with ND, ‘MKR1’ with HD, and S (Fig. 4). However, in 2020, the harvested fruit of ‘MKR1’ with HD was the heaviest by far (Table 2).
Overall, the HD treatment for the trees that were grafted onto ‘MKR1’ was found to be the most useful cultural practice that could be practiced to produce larger fruit that could be highly valuable in the Japanese market. Compared to ND, the HD treatment may reduce the yield efficiency, but the reduction is likely minimal (Tables S2 and S3) because the trees that were grafted onto ‘MKR1’ scarcely showed any early fruit drop (Tetsumura et al., 2013, 2019). For persimmons, the amount of early fruit drop varies from year to year. Therefore, excessive disbudding to produce large fruit should be avoided even though it is more effective at increasing fruit size than fruit thinning (Kitagawa, 1970). The HD treatment for conventional trees, such as those grafted onto seedling rootstocks, may drastically reduce the production of fruit if a lot of early fruit drop occurs.
The trees that had been grafted onto ‘MKR1’ with ND produced the lightest fruit harvest (Tables 1 and 2). Interestingly, SSC was the highest in ‘Fuyu’ and did not differ in ‘Hiratanenashi’ (Tables 1 and 2). Presumably, the ‘MKR1’ trees with ND increased the low numbers of fruit flesh cells in the first half of stage I, but during stage III, the photosynthates were actively translocated to the fruit well enough for cell enlargement and for enrichment in SSC to occur. The rootstock also affected the firmness of ‘Fuyu’ fruit, as the fruit of the ‘Fuyu’ trees that were grafted onto ‘MKR1’ were significantly harder than those of the trees that were grafted onto R-a (Table 1). The shelf life of ‘Fuyu’ fruit harvested from the trees that were grafted onto ‘MKR1’ appeared to be longer than the fruit of trees were grafted onto R-a, although there was no significant difference for the 2-year period between the fruit harvested from ‘MKR1’ and R-a in a previous study (Tetsumura et al., 2015). The values for the ‘Hiratanenashi’ fruit skin color showed that fruit coloration had accelerated in ‘MKR1’, although all fruit were harvested based on the skin color score. In the previous study, such a trend was not observed in any cultivar (Tetsumura et al., 2010, 2015, 2019). Long-term studies with trees that were in a stable, highly productive phase may have revealed that the ‘MKR1’ rootstocks accelerate the coloration of the fruit at harvest. With regard to tree height, fruit productivity, and fruit quality, ‘MKR1’ was the most favorable rootstock for ‘Fuyu’ and ‘Hiratanenashi’ trees if HD was applied. On the other hand, R-a is not a favorable rootstock for ‘Fuyu’ trees. The ‘Taishuu’ tree that was grafted onto ‘MKR1’ was precocious and dwarfing, reduced early fruit drop, and produced fruit efficiently (Tetsumura et al., 2019). These characteristics are the opposite of those of the trees that were grafted onto seedling stocks, which are commonly used in Japan. Taking advantage of these characteristics including the active lateral growth, which the trees that were grafted onto ‘MKR1’ showed 10 years after planting (Tetsumura et al., 2015), the Tall Spindle apple planting system that has become the preferred planting system in several parts of the world by using dwarfing rootstock (Reig et al., 2019) may be applied to the Japanese persimmon planting system.
Including our previous studies (Tetsumura et al., 2010, 2015, 2019), most reports showed that the persimmon rootstocks had little or no effect on fruit quality (Hattori et al., 2015; Yakushiji et al., 2021; Yamada et al., 1997). In fact, all of the data were collected from the trees within 10 years of their first crop, and the trees did not appear to enter a stable, highly productive phase. Additionally, annual fluctuations in fruit quality were observed (Tetsumura et al., 2010; Yakushiji et al., 2021). In evaluating the fruit weight of the Calrico late peach cultivar that was grafted onto 15 rootstocks, the mean for five years was investigated and led to the discovery that the rootstock produced the heaviest fruit, although there was no significant difference in the weight of the final year’s harvest (Jiménez et al., 2011). In this study, the fruit quality data showed a stable tendency every year with the same trend for tree growth. Hence, we considered that the fruit qualities shown in this study precisely represented the effects of the rootstock on scions. It is important to note that some fruit qualities can be affected by cultural practices, as well as environmental changes. Growers need to know and understand the growth characteristics of trees that have been grafted onto clonally propagated rootstocks to select the ideal cultural practices that will produce excellent fruit in the most efficient way. Recently, another persimmon dwarfing rootstock, FDR-1, was found to effectively inhibit the early fruit drop of ‘Aikou’ trees that usually have a heavy early fruit drop (Okumura et al., 2018). ‘Aikou’ trees that were grafted onto FDR-1 produced fruit with a higher level of SSC, similar to fruit produced by the ‘MKR1’ rootstock (Asakuma and Takemura, 2021).
In conclusion, the trees that had been grafted onto ‘MKR1’ were still dwarfed even 18 years after being planted and produced fruit effectively. The size of the fruit and the pattern of the fruit growth curve are different between persimmon rootstocks even within the same cultivar. The HD treatment for adult trees that were grafted onto ‘MKR1’ was shown to be one of the best cultural practices that could be used to improve fruit weight. Regarding the R-a rootstock, the ‘Fuyu’ trees produced small, low-quality fruit and showed inefficient fruit production overall.
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