原著論文
Growth, Yield, and Fruit Quality of Own-rooted ‘Irwin’ Mango Trees Propagated by Air Layering in Pot Culture
2024 年 93 巻 1 号 p. 42-48
詳細
2024 年 93 巻 1 号 p. 42-48
To investigate the practicality of using own-rooted trees in pot culture, the growth, yield, and fruit quality of pot-planted ‘Irwin’ mango trees, propagated by air layering versus grafting, over an 11-year period after planting were measured. The trunk diameter of the own-rooted trees propagated by air layering was significantly smaller than that of the grafted trees propagated by conventional methods until six years of age, after which there was no significant difference between the two tree types. The trunk diameter of the own-rooted trees was significantly greater than that of the scion diameter of the grafted trees after seven years of age. The total length of green branches was significantly longer in the own-rooted trees than in the grafted trees after nine years, and the number of leaves was significantly greater on the own-rooted trees than the grafted trees after six years. The height of the own-rooted trees was 35 cm shorter than that of the grafted trees. The fresh and dry weights of the leaves, green branches, thick branches, scaffold branches, and fine roots of the own-rooted trees were significantly greater than those of the grafted trees. However, the fresh and dry weights of the trunks and thick roots of the own-rooted trees were significantly smaller than those of the grafted trees. The weights (fresh and dry) of the aboveground parts of whole own-rooted trees were significantly greater than those of the grafted trees, but the weights of the underground parts did not differ significantly between the two tree types. The T-R ratio (the weight of the aboveground part of the tree excluding the leaves/the weight of the underground part of the tree) of the own-rooted trees was significantly greater than that of the grafted trees. The yield per tree of the own-rooted trees was significantly lower than that of the grafted trees for two years after fruiting; however, this difference resolved after 10 years. The average yield per unit land area over nine years was 2.46 kg·m−2 for own-rooted trees and 2.32 kg·m−2 for grafted trees. The numbers of fruits per tree did not differ significantly between tree types, except during years 4 and 5. The average fruit weight did not differ significantly between the tree types up to nine years, with one exception in year 5. However, the average fruit weight of the own-rooted trees was significantly greater than that of the grafted trees after 10 years. Fruit quality did not differ significantly between the two tree types. Since the height of the own-rooted trees was shorter, and their growth and yield were equal or greater than those of the grafted trees, pot culture using own-rooted trees of ‘Irwin’ mango appears to be highly beneficial.
The root-zone restriction technique was introduced to cultivate several fruit-tree types to promote early crop harvesting and reduce labor requirements. Many farmers apply this technology to mango (Mangifera indica L.) cultivation (Yonemoto, 2008). Pot culture is among the root-zone restriction techniques, and it is also effective in mango in controlling tree vigor and promoting fruit set.
The influence of pot culture on mango-tree traits has been examined in several recent studies, including the effects of L-F ratios (ratio of the number of leaves to the number of fruits) on yields, fruit quality, and cluster formation in the cultivars ‘Irwin’ and ‘Aikou’ (Kindai University, 2008) in pot culture (Fumuro, 2011a); and of pot soil volume on tree growth, yield, and fruit quality in the cultivar ‘Aikou’ (Fumuro, 2017).
Growth, yield, and fruit quality have been compared between own-rooted and grafted trees in the cultivars ‘Spirit of ‘76’ (Fumuro, 2018) and ‘Aikou’ (Fumuro, 2019), but there are no equivalent comparisons for the cultivar ‘Irwin’, which is less vigorous than the other two cultivars. Thus, this study investigated the growth characteristics of own-rooted ‘Irwin’ trees.
‘Irwin’ is the predominant mango cultivar in Japan. It was introduced from Taiwan in the 1970s (Yonemoto, 2008) and currently accounts for over 95% of the mango cultivated area. ‘Irwin’ is an apple mango with a red skin, sweet aroma, juicy flesh and reduced fiber content, and is very popular with consumers. The fruit set of this cultivar is stable, and the ripe fruit has an excellent shelf life at room temperature (Fumuro et al., 2018).
The fruit-set position tends to be higher in pot culture than with conventional ground planting due to the added height of the pot (40–60 cm). Grafting is generally performed on rootstocks at 30–50 cm above ground level (Yonemoto, 2008), which results in longer rootstocks and higher fruiting positions. Working with both hands raised above the shoulders exerts a heavy load on the arms, neck, and shoulders of agricultural laborers (Kikuchi, 2010), reducing work efficiency and thereby increasing labor costs. Reducing tree height in pot culture is therefore an important issue.
The height of own-rooted ‘Aikou’ trees was shorter than that of grafted trees (Fumuro, 2019) because own-rooted trees do not have rootstocks, which permits arrangement of scaffold branches close to the ground. However, comparable information is not yet available for the cultivar ‘Irwin’. The yields and fruit qualities of the cultivars ‘Spirit of ‘76’ (Fumuro, 2018) and ‘Aikou’ (Fumuro, 2019) do not differ between own-rooted and grafted trees. The effects of own-rooting on fruit productivity have yet to be determined in the cultivar ‘Irwin’.
‘Irwin’ is a cultivar with weak tree vigor, so it is suitable for conventional ground planting culture in relatively small greenhouses. Since tree vigor in pot culture is more likely to decline than that in conventional ground planting, it is necessary to verify whether pot culture cultivars using own-rooted trees can be cultivated for many years. If own-rooted ‘Irwin’ trees can maintain superior tree vigor in pot culture for many years compared to grafted trees, the production costs due to seedling repotting can be reduced, contributing to stable production management.
In this study, the practicality of using own-rooted ‘Irwin’ trees in pot culture was examined. The growth, yield, and fruit quality of own-rooted trees propagated by air layering (hereafter, “own-rooted trees”) and of grafted trees propagated by conventional methods (hereafter, “grafted trees”) were compared for 11 years after planting.
‘Irwin’ trees were planted in a double-span greenhouse (width: 9 m; length: 54 m; height: 4.3 m; area: 972 m2) at the Kindai University Experimental Farm (Yuasa, Wakayama Prefecture, Japan). Air layer propagation was performed on May 20, 2008 following the procedures of Fumuro (2011b). On July 24, 2008, rooted branches were removed and planted in 1.3-L plastic pots (diameter: 13.5 cm; height: 11 cm). On July 6, 2009, own-rooted nursery trees previously subjected to air layer propagation were transferred to 25-L non-woven fabric pots (diameter: 32 cm; height: 35 cm) filled with a mixture of mountain soil, perlite, vermiculite, and compost (volume ratio: 1:1:1:1).
On the other hand, for the grafted trees, on June 15, 2009, ‘Irwin’ scions were grafted onto two-year-old rootstocks (Taiwan native strain seedlings) planted in 8.5-L plastic pots (diameter: 24 cm; height: 24 cm). The average grafting position was 21 cm above ground level. On October 22, 2009, these plants were transferred to non-woven fabric pots, as described above.
Based on the ages of the underground parts, both the own-rooted and grafted trees were two years old in December 2009. Ten own-rooted and ten grafted trees were used for this study.
2. Cultivation managementAll pots were managed in the double-span greenhouse through October 2012. The pots were arranged with 1.4-m plant spacing between rows and 1.5-m plant spacing within rows. In November 2012, all pots were transferred to a smaller plastic greenhouse (width: 6 m; length: 18 m; height: 4 m; area: 108 m2), where they were arranged with the same spacing.
To ensure a minimum temperature of 6°C, the greenhouse was heated from early December. The minimum temperature was increased gradually beginning mid-February and was maintained at 18–20°C during the flowering period. A fan was used for ventilation to ensure that the internal air temperature remained < 30°C until flowering and < 35°C after flowering. Irrigation was controlled using an automatic timer. Approximately 50 g slow-release fertilizer (N:P2O5:K = 10:10:10%) was applied to each tree five times a year. Approximately 50 g organic fertilizer (N:P2O5:K = 6:8:4%) was applied to each pot once a year. Assuming that the number of trees per 1,000 m2 was 497 from the planting density, 15.7, 16.2, and 15.2 kg nitrogen, phosphoric acid, and potassium, respectively, were applied each year. Pruning began when the harvest was close to completion and ended in mid-September. Two to three scaffold branches per tree and an appropriate number of fruit-bearing shoots were set within a crown diameter of 1.3–1.4 m. The pruning intensity in 2019 was reduced because a dissecting survey was scheduled for December of the same year. Diseases and pests were controlled following conventional procedures.
3. Fruit management and harvestingIn mid-May of each year, the fruits were thinned so that the L-F ratio was approximately 50–60. Each fruit was covered with a bag-shaped net before harvesting, and tree-ripe fruits that dropped naturally into the nets were harvested from mid-July until late September each year.
Measurements 1. Tree height and tree growthTree height from the bottom of the pot was measured in December 2019 at the time of the dissecting survey. The stem diameter, number of leaves per tree, and length of the green branches on each tree were measured in late December of each year from 2009 until 2019. Stem diameters were measured using calipers; the measurements were conducted at 3 cm above ground level on the own-rooted trees, and at 10 cm above ground level (trunk diameter) and 5 cm above the grafted part (scion diameter) on the grafted trees. The lengths of green branches with < 10% lignification were measured, and the total length of green branches was calculated for each tree.
2. Fresh and dry weights of the organsA dissecting survey was performed between December 25, 2019 and January 20, 2020. The trees were 12 years old at the time of dissection. The different parts of the tree were categorized as follows: leaf, green branch, thick branch, scaffold branch, trunk (including trunk derived from rootstock), fine roots (< 1 mm diameter), and thick roots (≥ 1 mm in diameter, including the shoot-derived wood subjected to air layer propagation). The fresh weight of each organ was measured. Each organ sample was dried to determine the proportion of dry matter. The total dry weight of each organ was calculated by multiplying the percentage dry matter by the total fresh weight of each organ.
The T-R ratio was calculated by dividing the weight of the aboveground parts of a tree (excluding the leaves) by the weight of its underground parts. Forty leaves were randomly sampled from each tree, and their areas were measured using an automatic leaf-area meter (AAM-9; Hayashi Denko Co., Ltd., Tokyo, Japan). The leaf area of each tree was calculated by multiplying the leaf area per unit leaf fresh weight by leaf fresh weight per tree.
The trunk of each tree was sawn through at the position where the diameter had been measured. The outlines were traced onto paper and the cross-sectional area of each trunk was measured using an automatic leaf area meter.
3. YieldThe weight of each fruit that dropped naturally into the net was measured each day, and the yield per tree and the average individual fruit weight were calculated.
Statistical analysisExcept for stem diameter measurements, all data were compared by t-test to identify statistically significant effects. A P-value < 0.05 was considered significant. Differences in stem diameter were assessed using Tukey-Kramer’s multiple range test.
The average height of the own-rooted trees (1.53 m) was significantly lower than that of the grafted trees (1.88 m) (Table 1).
Comparison of the total leaf area per tree, stem cross-sectional area, and tree height of own-rooted and grafted twelve-year-old mango cv. Irwin trees.
The trunk diameter of the own-rooted trees was significantly lower than that of the grafted trees up until six years of age, but thereafter the diameters did not differ significantly between the two tree types (Fig. 1A). The trunk diameter of the own-rooted trees was significantly greater than that of the scion diameter of the grafted trees after seven years of age. At 12 years of age, the mean trunk diameters of the own-rooted and grafted trees were 9.2 and 8.7 cm, respectively, and the mean scion diameter of the grafted trees was 6.4 cm. The mean stem cross-sectional areas of the own-rooted and grafted trees were 61.9 and 61.3 cm2, respectively (Table 1).
Annual changes in the (A) stem diameter, (B) total length of green branches, and (C) total number of leaves of own-rooted and grafted mango cv. Irwin trees cultivated in pots. Vertical bars represent the standard error (n = 10). Values followed by the same letter do not differ significantly (P < 0.05; Tukey-Kramer’s multiple range test). NS, *, **, and *** indicate non-significant difference and significant differences at P = 0.05, 0.01, and 0.001, respectively (t-test).
The total green branch length did not differ significantly between the two tree types up to eight years of age. Thereafter, the total length of green branches was significantly longer on the own-rooted trees than on the grafted trees (Fig. 1B).
The own-rooted trees had fewer leaves per tree at three and four years of age, but had more leaves per tree after six years of age, compared with the grafted trees (Fig. 1C). At 12 years of age, the average numbers of leaves were 1,587 and 1,257, and the leaf areas were 8.38 and 6.01 m2, in individual own-rooted and grafted trees, respectively (Fig. 1C; Table 1).
Figure 2A and 2D show the shapes of the two tree types prior to dissection.
Twelve-year-old (A–C) own-rooted and (D–F) grafted mango cv. Irwin trees cultivated in pots. A, D: Whole view of own-rooted and grafted trees. B, E: State of the underground parts of the own-rooted and grafted trees. White arrow indicates the grafted part. C, F: State of fruiting in the own-rooted and grafted trees.
The fresh and dry weights of the leaves, green branches, thick branches, scaffold branches, and fine roots of the own-rooted trees were significantly greater than those of the grafted trees. However, the fresh and dry weights of the trunks and thick roots of the own-rooted trees were significantly lower (Table 2). The fresh and dry weights of the aboveground parts of the own-rooted trees were significantly greater (11.21 and 4.48 kg, respectively) than those of the grafted trees (8.04 and 3.13 kg, respectively). The fresh and dry weights of the underground parts did not differ significantly between the own-rooted (3.34 and 1.28 kg, respectively) and grafted trees (3.63 and 1.43 kg, respectively). The fresh and dry weights were greater in whole own-rooted (14.55 and 5.76 kg, respectively) than those of the grafted trees (11.67 and 4.56 kg, respectively).
Comparison of the fresh and dry weights of own-rooted and grafted twelve-year-old mango cv. Irwin trees.
The fresh T-R ratio of the own-rooted trees (2.38) was 51% greater than that of the grafted trees (1.58) (Table 2). The dry T-R ratio of the own-rooted trees (2.40) was also 51% greater than that of the grafted trees (1.50).
Figure 2B and 2E show the morphologies of the underground parts of the own-rooted and grafted trees.
YieldThe harvest period for both tree types lasted from mid-July to late September. The yield per tree was significantly lower at four and five years of age, but significantly greater at ≥ 10 years of age in own-rooted trees compared with grafted trees (Fig. 3A). The respective yields per tree of own-rooted and grafted trees were: 3.9 and 3.6 kg at six years of age, 6.3 and 5.3 kg at eight years of age, 6.6 and 5.4 kg at 10 years of age, and 8.4 and 7.0 kg at 12 years of age.
Annual changes in (A) yield, (B) number of fruits, and (C) average fruit weight of own-rooted and grafted mango cv. Irwin trees cultivated in pots. Vertical bars represent the standard error (n = 10). NS, *, **, and *** indicate non-significant difference and significant differences at P = 0.05, 0.01, and 0.001, respectively (t-test).
The number of fruits per tree was significantly lower in own-rooted trees than that in the grafted trees at four and five years of age, but the difference between tree types was not significant after six years of age (Fig. 3B). Average fruit weights did not differ significantly between the two tree types up to nine years of age, except for individuals that were 5 years old. The average individual fruit weight of the own-rooted trees was significantly greater than that of the grafted trees at ≥ 10 years of age (Fig. 3C). The average fruit weight of the own-rooted trees (except four-year-old individuals) was 400–550 g, compared with 370–450 g in grafted trees.
Figure 2C and 2F show the fruiting morphologies of own-rooted and grafted trees at 11 years of age.
Fruit quality did not differ significantly between the two tree types (Table S1).
The average tree height of the own-rooted trees was 1.53 m, i.e., 35 cm lower than that of the grafted trees, and lower by 19% (Table 1). In Japan, most mangoes are cultivated in relatively small greenhouses with a frontage of 6–7 m and a ridge height of approximately 4 m. Since the roofs of the greenhouses are relatively low, reducing mango tree height is an essential objective. The scaffold branches are attached horizontally with string to the ground, and cutback pruning is performed each year to prevent increases in tree height (Yonemoto, 2008). When trees are potted, tree heights are increased by the depth of the pot and the grafting position on the rootstock. Hence, own-rooted trees have advantages in pot culture because they have no rootstock.
The fruit appearance of ‘Irwin’ mango is among the important factors that determine its commercial value. To increase the red coloring of fruits, farmers lift them individually with string to ensure full exposure to sunlight. Before harvesting, the naturally falling fruits are caught in nets. These activities are easier when tree height is reduced. An own-rooted tree height of 1.53 m is considered appropriate for mango cultivation. Fumuro (2019) reported that the height of own-rooted trees of ‘Aikou’ mango in pot culture was shorter than grafted trees and suggested that the own-rooted form is advantageous for this reason.
In this study, the growth of the own-rooted trees was equal to or greater than that of the grafted trees in terms of stem enlargement, total length of green branches, and total number of leaves at 12 years after planting (Fig. 1; Tables 1 and 2). The own-rooted trees exhibited inferior growth initially, but over time gradually outperformed the grafted trees.
The superior initial growth of the grafted trees can be explained by the vigorous performance of the native Taiwanese rootstock. The growth of the grafted trees planted in pots of non-woven fabric was initially superior. The growth of the scions of the grafted trees slowed down after the 7th year, and as a result, the total length of green branches and the number of leaves of the grafted trees were lower than those of the own-rooted trees after the 9th year. However, the growth potential of own-rooted ‘Irwin’ trees gradually became evident over time.
Fresh and dry weights of the organsThe weights of the underground parts of the two tree types were very similar, but the aboveground parts of own-rooted trees were significantly heavier (Table 2). Although the trunk weight of own-rooted trees was lower than that of the grafted trees, the leaves, green branches, thick branches, and scaffold branches were heavier, and as a result, the aboveground parts of the own-rooted trees were heavier than those of the grafted trees. Consequently, the mean T-R rate of the own-rooted trees was greater than that of the grafted trees. This indicates that own-rooted trees allocate significant resources to the growth of aboveground parts.
Fumuro (2018, 2019) examined pot-cultured own-rooted and grafted trees of the cultivars ‘Spirit of ‘76’ and ‘Aikou’ and found a greater T-R ratio in own-rooted than in grafted trees. Oya et al. (2015) reported a lower dry weight of the underground parts of own-rooted Japanese pear trees compared with grafted trees.
With respect to the fine root weight, own-rooted trees were significantly heavier than grafted trees, and showed similar results to ‘Spirit of ‘76’ (Fumuro, 2018). However, no significant difference was found between own-rooted and grafted trees among ‘Aikou’ (Fumuro, 2019), suggesting that there is a close relationship between the growth of aboveground parts and the growth of fine roots. Fumuro (1999) found a positive correlation between the scion and root weights (fine root and/or thick root) of Japanese persimmon. However, this relationship has not been examined in the two mango tree types.
YieldPrevious studies have compared the yields and fruit qualities of own-rooted and grafted trees obtained from vegetative propagation (Couvillon, 1985; Oya et al., 2015; Tetsumura et al., 2010; Tous et al., 1997), but few comparable reports have focused on mangoes (Fumuro, 2017, 2018, 2019).
In the present study, the yield per mango tree increased with age, but significantly decreased after 10 years in grafted trees compared with own-rooted trees (Fig. 3A). This is likely because the total length of the green branches and the number of leaves were lower. At the age of 12, the yield was 8.4 kg for the own-rooted trees and 7.0 kg for the grafted trees (Fig. 3A). The average yield per unit land area over nine years (age range 4–12 years), calculated from the planting density of the pots, was 2.5 kg·m−2 for own-rooted trees and 2.3 kg·m−2 for grafted trees. According to Fumuro (2019), the average yields per unit land area over six years (age range 4–9 years) of both own-rooted and grafted ‘Aikou’ trees were 3.0 kg·m−2. Pot (or container) culture of fruit trees has been widely used as an early high-yield technique to increase planting density, and the current findings in mango agree with previous reports.
The average weight per fruit was 400–500 g for own-rooted trees, which was slightly greater than that (370–450 g) of grafted trees up to nine years of age (Fig. 3C). Both tree types reached the standard fruit weight (400–500 g) of the cultivar ‘Irwin’ (Fumuro, 2011a). The greater leaf-to-fruit ratio in own-rooted trees may explain their greater fruit weights at 5 and 10–12 years of age. Fumuro (2011a) reported that the greater the leaf-fruit ratio, the greater the average fruit weight in the cultivar ‘Irwin’.
The fruit quality was approximately equal to the standard values for ‘Irwin’ (soluble solid content 15–16%, citric acid content 0.2–0.3%, and flesh firmness 10–12 N·cm−2) (Fumuro, 2011a).
Practical features regarding growing own-rooted trees in pot cultureCultivation in relatively small pots was maintained satisfactorily over the 11-year period. During this time, high-quality fruits were produced with no significant physiological deficiencies. No decline in tree vigor caused by root clogging in the pots was seen. The fresh weight of the underground parts of both tree types was only approximately 3.5 kg, and no root clogging due to root enlargement was observed; therefore, pot culture may be feasible over longer periods.
The pots generally used for mango culture have a capacity of 80–100 L, but in this study pots with one-third or one-quarter of this capacity were used. Smaller soil volumes require more precise watering and fertilization, but tree vigor is more manageable, and savings in terms of tree-management labor can be expected. Equivalent experiments should be conducted using larger pot volumes.
Vigor is suppressed in pot culture, but grafted trees tend to be taller than own-rooted trees due to the additional height of the pot and rootstock. ‘Irwin’ mango is a cultivar with moderate tree vigor and abundant shoot sprouting from thick branches low on the trunk; consequently, reducing tree height is relatively straightforward. Own-rooted trees in pot culture can be maintained more readily at reduced heights compared with grafted trees because they have no rootstock. This advantage in combination with their excellent growth and yield implies that the use of pot culture for ‘Irwin’ mango trees is highly desirable and relatively easy to implement.
