The pattern and control of reproductive development in non-astringent persimmon (Diospyros kaki L.): A review

SClENTIA HORTICULTUR~E ELSEVIER

Scientia Horticulturae 70 (1997) 93-122

The pattern and control of reproductive development in non-astringent persimmon (Diospyros kaki L.)" a review A.P. George a,,, A.D. Mowat b, R.J. Collins c, M. Morley-Bunker d a Maroochy Horticultural Research Station, Queensland Departmem of Primary Industries, P.O. Box 5083, Sunshine Coast Mail Centre, Nambot4r, Queenshmd 4560, Australia b HortResearch, Ruakura Research Centre, Pricate Bag, Hamilton, New Zealand Department of Management Studies, Unicersity of Queensland Gamin College, Lawes, Queensland 4343, Australia d Department of Plant Science, Lincoln UnicersiO' College, Canterbury, New Zealand

Accepted 18 April 1997

Abstract Non-astringent persimmon is rapidly expanding as a new fruit crop in warm subtropical regions of the world. Most research and development of this fruit crop has occurred in Japan, where there is a considerable amount of published literature: on its performance. Much of this information is not readily accessible to other countries and needs to be interpreted and modified for other climatic regions. This paper reviews reproductive events from floral initiation to the completion of fruit growth. The timing and significance of these events is described in relation to the phenological cycle. Method of improving flowering, reducing fruit drop and altering the fruit maturity period are discussed. © 1997 Elsevier Science B.V. Keywords: Persimmon; Review; Flowering; Floral initiation; Fruit growth; Growth regulators

1. Introduction The p e r s i m m o n ( D i o s p y r o s kaki L.) is a native of China. The primary centre of diversity is m o u n t a i n o u s Central China with a secondary centre of diversity in Japan ( Z h e v e n and Zhukovsky, 1975). In Japan two distinct forms have been selected, astringent and non-astringent, the latter having a much lower concentration of water-

* Corresponding author. 0304-4238/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0304-423 8(97)00043-5

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soluble tannins (Rehder and Wilson, 1916; ltoo, 1980). Persimmon can be further subdivided into pollination constant and pollination variant types (Hume, 1914; Itoo, 1980). The pollination constant astringent (PCA) form is the oldest and comprises almost all the Chinese cultivars and several Japanese cultivars. Pollination variant astringent (PVA) and non-astringent (PVNA) forms originated in Japan about 800 years ago and exhibit flesh darkening when pollinated. The pollination constant non-astringent (PCNA) form is the most recent form. originating approximately 500 years ago in Japan (Yamada, pers. comm.). The PCNA form was originally derived from the PCA form, but has a different tannin composition (Yonemori et al., 1993). The PCNA form is normally free of astringency at harvest and astringency levels are independent of pollination effects. Residual astringency may' occur i~ PCNA cultivars grown under cool climate conditions (Zheng et al., 1990; Mowat and George, 1994; Mowat et al., 1995). Most commercial development of the crop has occurred in Japan where it is regarded as the national fruit (Kajiura, 1980: George and Nissen, 1984, 1985). Smaller but expanding persimmon industries are being established in Brazil, Italy, California, Israel, New Zealand and Australia (Collins et al., 1993). Renewed interest in this crop has occurred as a result of the availability of new non-astringent cultivars (e.g., 'Fuyu', 'Jiro', 'Izu', 'Suruga') and increasing awareness of the wide environmental adaptability, high yield potential, and excellent post-harvest storage life of this fruit (George and Nissen, 1985; George et al., 1994a; Mowat and George, 1994). In the cultivation of Japanese persimmons, physiological fruit drop and biennial bearing are the main factors causing unstable fruit production (Hodgson and Schroeder, 1947; Yamamura et al., 1989). Many factors are implicated in causing fruit drop including genetic, environmental and physiological. Management techniques to control persimmon yield productivity, primarily' by controlling fruit drop and preventing biennial bearing, are presently poorly understood. This paper reviews reproductive events from floral initiation to the completion of fruit growth. The timing and significance of these events is described in relation to the phenological cycle. The aim of the review is to provide a basis for developing phenological and productivity models which can be used for future research investigations and for evaluating management techniques.

2. Flowering and floral initiation 2.1. Floral m o r p h o l o g y

Persimmon flowers are borne laterally on current season growth arising from mixed buds (Hasegawa et al., 1991a). Pistillate flowers are generally solitary and located in the axils of leaves. Staminate flowers are produced in cymous clusters of 3-5 flowers together (or less through abortion), while the carpellate or female flowers are generally solitary (Spongberg, 1979). The flowering habit is complex, ranging fl'om dioecious to monoecious (Namikawa et al., 1932; Hodgson, 193%; Spongberg, 1977, 1979). Based on histological sections of floral bud development, Yasui (1915) describes D. kaki as monoecious, rather than dioecious, whose staminate flowers are disappearing under cultivation. Hybrid seedlings derived from a monoecious parent have a high proportion

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of staminate bearing plants in comparison to those produced from a pistillate constant parent (Oohata e t a l . , 1964). Most commercial cultivars are pistillate constant or pistillate sporadically monoecious, though several androdioecious and tri-monoecious cultivars exist (Iikubo et al., 1961; Rigitano etal., 1984: ltoo, 1986; Yamane etal., 1991a). Many of the staminate bearing cultivars are important as pollinizers. Staminate flowering PCNA cultivars are used in cross-breeding programmes (Yamada and Yamane, 1994). Hermaphroditic flowers are derived from staminate flowers and if fruit set occurs tend to produce smaller fruit than those originating from pistillate flowers (Namikawa et al., 1932). Sex expression of monoecious cultivars can be influenced by tree age, previous crop load, nutrient status, phytohormones, bud position and shoot type (Hodgson, 1939a; Minina, 1949; Nishida and Ikeda, 1961; Zaktreger, 1962; Yonemori etal., 1992, 1993). On rare occasions pistillate constant cultivars have been reported to bear male flowers (staminate-sporadic) (Kajiura and Blumenfeld, 1989). Stable staminate bearing bud mutations of severztl important pistillate constant cultivars ('Jiro', 'Fuyu') have now been identified and propagated for cultivation or use in breeding programs (Yamane et al., 1991b; Bellini and Giordani, 1994; Yakushiji et al., 1995). The stability of these bud mutations could be related to genetic changes in the regulation of endogenous phytohormones. A dwarf bud mutation of the monoecious cultivar 'Nishimura wase' has been shown to have significantly higher rates of staminate flower formation, and increased cytokinin and gibberellin activity in comparison to the original form (Fumuro and Murata, 1989, 1990).

2.2. Flowering patterns In warm subtropical regions, flowering normally occurs about 35 days after budbreak (George et al., 1994b, George e t a l . , 1994c). In New Zealand, due to cool spring temperatures, flowering does not commence until 70 days after budbreak (Mowat et al., 1995). Flowering is usually concentrated and occurs over a 7- to lO-day period but under cooler conditions may be more protracted. Field observations on flowering times and duration are in agreement with controlled temperature studies by George et ah (1994b) who found that the time period between budbreak and flowering and the length of the flowering period were increased three fold with de,creasing day/night temperature (range 32/27°C to 17/12°C). Chujo (1982) also reports that the cumulative temperature above 10°C required for budbreak, and from budbreak to flowering, to be 90 and 300°C, respectively. Flowering dates can differ by 1-2 weeks between cultivars (Watanabe et al., 1987). Early flowering cultivars, such as 'Ro19' and 'lzu', may require inter-planting of early flowering pollinizer cultivars to ensure adequate fruit set.

2.3. Timing of.floral initiation Persimmon flowers are normally initiated the year prior to anthesis. Secondary flowering can develop from primordia initiated in the current season under certain environmental conditions, such as those found in protected culture. A scarcity of information exists on the timing of and the nature of the stimulus leading to the stages of

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floral initiation and differentiation in persimmon. Few studies of persimmon flower formation identify and distinguish all the physiological stages of floral induction and evocation, and the morphological stages of floral initiation, differentiation and development. Allowing for environmental differences between different production regions, the time of floral initiation in persimmon appears to occur during a well-defined period (40-60 days after anthesis) (Harada, 1984; Glucina and Toye, 1985; Yamamura et al., ! 989) and the duration, although varying with temperature, occurs over a 6-week period (Harada, 1984; Yamamura et al., 1989). Time of floral initiation would appear to be a result of shoot tip termination, ambient temperature and carbohydrate status (Harada, 1984; Nii, 1989). In non-bearing shoots insoluble nitrogen, total carbohydrate, starch and total sugar content and the C - N ratio incre~tse prior to floral bud formation (Sobajima et al., 1968). Water stress together with high temperatures during floral differentiation can induce the formation of double or malformed pistils within floral primordia (Watanabe, 1985). Abnormal flowers that are less than half the size of normal flowers can develop in cultivars such as 'Fuyu' and 'Jiro' (Inoue, 1934). These abnormal flowers tend to be located on the apical region of floral laterals and their occurrence may be due to late floral initiation. Although floral initiation occurs in mid-summer, differentiation of the floral structure is not completed until shortly before flowering in the following season. Differentiation of each organs is initiated from the outer organ, namely in order of calyx, petal, staminodium and carpel (Sobajima et al., 1974a,b). Microscopic examination of the floral buds in the winter shows the presence of petal and sepal initials (Baldini, 1952; Moncur, 1988). Androecium and gynoecium tissue initials become evident shortly after shoot emergence. The ovule primordia are initiated on the infolded carpel edges near the base of the ovarian cavity and the formation of the ovule is complete about 2 days before full-bloom (Sobajima et al., 1974a). The fruit contains four carpels, each with two ovules. Irregular fruit shape can occur when abnormally high or low numbers of carpels form (Iwagaki, 1951). Differentiation of wascular tissue in the pedicel begins one month prior to bloom and ceases 1 month after bloom (Nii, 1980).

2.4. Intensity of floral initiation Many factors may affect the intensity of floral initiation including shoot type, vigour, crop loading, low irradiance, carbohydrate status, nutrition and water stress (Tanaka and Aoki, 1971; Kaneko, 1977; Glucina and Toye, 1985). Normal flower buds are formed from the distal five or six mixed buds on current season growth derived from the previous season's summer terminated shoots (Hasegawa et al., 1991b). Harada (1984) showed that floral bud primordia numbers were greater on shoots 20-30 cm in length compared with shorter shools and that primordia number were reduced by increasing fruit numbers set per lateral. In New Zealand, Glucina and Toye (1985) report that only the terminal and succeeding two or three buds on well matured shoots normally contain floral buds. In contrast, in Australia, George and Ni,;sen (1990, unpublished data) have shown that floral bud primordia are produced along the complete length of laterals except for the proximal two to three buds. George and Nissen (1990, unpublished data)

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also found new shoots of the cultivar 'Fuyu' had more floral buds per lateral (2.0 cf,, 1.6) than reported by Japanese researchers (Yamamura et al., 1989). Genetic factors may also influence floral intensity as the pistillate flower numbers per unit canopy volume and per unit leaf area were higher in a dwarf bud-mutation of cv. 'Nishimura wase' than the original form (Fumuro and Murata, 1990). The genetic differences in flowering intensity could be related to changes in the activity of endogenous growth regulators during shoot growth and flower initiation (Fumuro and Murata, 1989).

3. Methods of improving flowering Aspect and location of persimmon orchards are important factors used to moderate or avoid adverse climatic conditions that may interfere with normal floral development and fruit set. Environmental factors can also influence bearing by affecting carbon assimilation or through physical damage of reproductive organs (Mowat and George, 1994). Flower evocation and fruit drop are particularly sensitive to environmental stress (Kajiura, 1943; Nagasawa and Hirai, 1959). Floral buds and fruitlets can be prone to damage from cold, hail and wind (Chujo et al., 1972; Kim et al., 1988; Fumuro et al., 1991). Techniques such as shelter belt design, hail nets and frost control can be used in orchard layout to reduce the risk of environmental damage. Though D. kaki is cold tolerant, floral and vegetative bud are damaged by exposure to low winter temperatures (Yoshimura, 1961). In Korea, meteorological information has been used to define growing regions with winter conditions that are suitabJle for commercial cultivation of non-astringent persimmon. In the sub-tropics and tropical, persimmon has been suggested as a crop suited to cold locations. However, observations by Sharpe (1966) in Florida have shown that persimmon grown in mild regions can be more prone to low winter temperature injury in comparison with trees grown in temperate regions. After bud-burst, floral buds are killed by spring temperatures below --3.0°C (Nakagawa and Sumita, 1969). Choice of rootstock can influence cold tolerance, precocity of bearing and fruit set of persimmon trees (Schroeder, 1947). In temperate regions, persimmon is cultivated on D. lotus and D. ~,,irginiana rootstock, while the less hardy D. kaki and D. oleifera rootstocks are generally restricted to warm temperate and subtropical regions (Nuttonson, 1951; Pieniazek, 1967; Mowat and George, 1994). Graft union incompatibility between 'Fuyu' and D. lotus stock can cause dwarf growth and early bearing of 'Fuyu', but tree life is significantly reduced (Hodgson, 1939b). In Japan, dwarf clones of D. kaki have been selected with improved precocity and yield characteristics (Kimura et al., 1985). Pruning and shoot selection can affect the number and quality of flower buds. The number and quality of flower buds formed on cun'ent seasons growth is greater on shoots derived from previous seasons summer branches in comparison to shoots produced from previous seasons spring branches (Hasegawa et al., 1991a). On large older trees summer topping in the previous season can increase floral shoots and flower numbers in the lower parts of the tree and reduce floral deformities (Murata et al., 1982; Hasegawa and Nakajima, 1984).

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Japanese studies have shown that if fruit thinning is carried out within 20 days after full bloom, then flowering the following season is normal (Matsumoto and Kuroda, 1982). In Japan and the USA, cincturing has been shown to double flower numbers and increase yield by as much as 34% (Hodgson, 1938; Aoki et al., 1977; Hasegawa and Nakajima, 1991a). Girdling can also be used to induce early bearing in young trees (Naito et al., 1981). Branch bending and wire girdling of secondary scaffolds can also be used to promote flower formation in young persimmon trees (Hasegawa, 1981 ; Hasegawa and Sobajima, 1991). Training of trees on trellising systems, such as the Y-trellis, can increase floral initiation by improving light distribution within the bearing canopy (Himeno et al., 1991). In New Zealand, the use of reflective mulches on Y-trellis trained 'Fuyu' can significantly increase floral intensity of the lower canopy in the following season (Thorp, pers. comm.).

4. Pollination

As most commercial persimmon cultivars are pistillate constant, pollen producing cultivars may need to be inter planted with the main cultivar (Lunati, 1961). Many of the important pollinizer cultivars are monoecious, bearing inferior fruit. Pollinizers have been selected providing pollen for specific pistillate cultivars or for their adaptation to local conditions. Pollination is insect mediated with European honey bee (Apis m e l l i f e r a Lin.) or other bee species such as native Australian bees ( T r i g o n a spp.) being the main pollen carriers (Yokozawa, 1!)51, 1952; McGregor, 1976; George et al., 1993). George et al. (1993) have observed that under Australian conditions honey bees may be more attracted to flowers of the pollinizer trees perhaps due to the greater abundance of flowers a n d / o r pollen and nectar in the flowers. With plums, this problem may be overcome by spraying bee attractant chemicals along the tree rows. In Japan, Fukae et al. (1987) reports that for satisfactory pollination (;> 3 seeds per fruit), a persimmon flower requires at least 20 honeybee visits. Earlier res.earchers have suggested that wind pollination may also occur in persimmon (Kikuchi, 1933), but later studies by Asami and Chow (1941) have shown that wind pollination was not effective. The number of seeds produced in hand pollinated fruit is influenced by the stage of flower development when pollination occurs, pollen quality and environmental conditions (Tombolato, 1989). Fertilisation rate is improved when pollen is applied to receptive stigmas of open flowers rather than those in unopened flowers with petals removed (Tombolato, 1989). In vitro studies by Tsay et al. (1994) indicate that the optimum temperature for germination is 25°C. Low temperatures and rain at flowering can inhibit pollination as has been shown under New Zealand conditions (1-2 seed per fruit) In contrast, under plastic house conditions, seed numbers of hand and insect pollinated 'Fuyu' can exceed five per fruit (Mowat, pers. comm., 1988). In Australia, George et al. (1994b) also found that fruit set of hand-pollinated flowers was poor at day/night temperatures of 17/12°C compared with higher temperatures. Similarly, in Japan, Fukui et al. (1990a) found fruit set at temperatures of 15°C was poor due to low pollen germination and poor pollen tube growth.

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5. Fruit set

Fruit can be set parthenocarpically, but this is dependent on cultivar and environment (Sharma, 1961; Sharpe, 1966; Yamada et al., 1987). Cultivars vary in their seed setting and their parthenocarpic ability, characteristics which are expressed independently, with cultivars varying from high fertility plus high potential for parthenocarpy to those with low fertility and no parthenocarpy (Kajiura, 1980). Cultivars which have a strong ability to set fruit parthenocarpically include 'Hiratanenashi' and 'Suruga', those with moderate ability, 'Jiro', 'Nishimura wase', and those with low ability, 'Fuyu', 'Izu,' 'II-i-Q12' and 'Gosho'. (Yasunobu and Akiyama, 1979; Kajiura, 11980; Itoo, 1980, 1986). Histological studies have shown that fruits of some cultivars produce a low percentage of viable ovules (Yamamura and Osaki, 1982: Fukui et al., 1990b). For example, 'Hiratanenashi', a highly parthenocarpic variety, contained 50% abnormal ovules with undifferentiated egg cells and polar nuclei, compared with 20% for 'Fuyu'. With some cultivars such as 'Hiratanenashi' some of the few ovules that are fertilised contain incompletely differentiated endosperm and the cessation of embryo growth may be due to endosperm degeneration (Sobajima et al., 1975a; Ishida et al., 1990). The main factor disrupting seed development in 'Hiratanenashi' appears to be the abnormal embryo sac and aneuploid gametes caused by irregular division at meiosis, and the abnormal division and degradation of endosperm and embryos due to their unbalanced chromosome number (Zhuang et al., 1990a,b). The chromosome number of 'Hiratanenashi' is 2n = 135, in contrast to the typical chromosome number of D. kaki is 2n = 90 (Zhuang et al., 1990b). Zhuang et al. (1990b) has found that cultivars characterised by seed degeneration during early development ('Hiratanenashi', 'Miyazakitanenashi' and 'Watarizawa') tend to be polyploids where 2n > 90. Seed abortion can also occur in cultivars, such as 'Nishimura wase' and 'Maekawa Jiro', with a chromosome number of 2n = 90 and this process appears to be due to a different mechanism (Zhuang et al., 1992). The presence of seed is important for the natural loss of astringency in pollination variant, non-astringent cultivars such as "Nishimura wase'. In 'Nishimura wase' flowers, only six of the embryo sacs within the eight ovules at anthesis have the ability to set seed (Fukui et al., 1989). Degeneration of the embryo-sac mother cell is the main cause of seedlessness (Fukui et al., 1993a). Low night temperatures have also been shown to increase the level of different types of embryo and endosperm abnormalities (Fukui et al., 1990b). This may explain the seasonal variation that can occur in seed numbers within well pollinated fruit of 'Nishimura wase'. Variations in seed formation between trees appear to be related to differences in the frequency of abnormal embryo-sacs, but these differences do not appeal" to be related to shoot vigour (Kitajima et al., 1993a). Abnormal seeds, characterised by small size or lack of ,endosperm, can occur to wlrying degrees in cultivars with a chromosome number of 2n = 90 (Hasegawa and Nagata, 1993). After pollination, the failure of the endosperm to undergo free nuclear division or endosperm abortion are the main cause for abnormal seed development that occurs in cultivars such as 'Nishimura wase' and 'Fuyu' (Fukui et al., 1991, 1993b). Lu et al. (1982) have suggested that an accumulation o1' ABA and salicylic acid during and after

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anthesis prevents the ovary fi'om developing if pollination and fertilization do not occur in normal embryos.

6. Biennial bearing In common with many fruiting plants, the persimmon tends to irregular bearing, especially biennial bearing (Ojima et al., 1985; Anonymous, 1989). Most cultivars are prone to irregular bearing and can be forced into annual bearing by using management techniques such as thinning, pollination, or providing irrigation (Miller, 1984). Bearing behaviour in tree crops appears to be influenced by both internal growth regulators and carbohydrate stress (Sedgley, 1990). The involvemen! of both influences can be noted in persimmon. Several researchers suggest that extreme biennial bearing of Japanese persimmons is caused by heavy cropping, particularly of seeded cultivars (Hodgson, 1939a; Hodgson and Schroeder, 1947; Hirose et al.. 1971; George and Nissen, 1984; Ojima et al., 1985; Yamamura et al., 1989). It is generally understood that the internal carbohydrate status of a tree can be depleted by carrying a heavy crop load. If crop adjustment (bud or fruit thinning) is done before 30-40 days after bloom regular flowering and cropping occurs in the following season (Matsumoto and Kuroda, 1982; Yamamura et al., 1989). We suggest that floral initiation may be adversely affected by the carbohydrate stress induced by too many developing fruit, by changes in internal growth regulators originating from the fruit, or both. This hypothesis is supported by girdling studies by Kitajima et al. (1992) who found that bark ringing of fruiting shoots delayed fruit drop until after the cincture had healed. The application of GA 3 at flowering time (to enhance parthenocarpic fruit production) (Hirata et al., 1967; Yamamura et al., 1989) has been shown to severely reduce flower numbers ( > 50%) in the following season (Hasegawa, 1983; Yamamura et al., 1989). The concentration of GA 3 found to reduce return bloom is low (25 mg 1- l ) (Yamamura et al., 1989). Cytokinin activity in pollinated and non-pollinated fruit can also vary. Pollinated 'Fuyu' fruit has been shown to increase cytokinin activity markedly until 30 days after flowering, whereas the activity in non-pollinated fruit remains tow (Sobajima et al., 1974b). Early leaf fall caused by leaf diseases or by frost can affect both current season development of fruit and also flowering intensity and fruit set in the subsequent season. This response appears to be due to a failure of the tree to build up sufficient starch reserves in the autumn (Hirata and Kurooka, 1974).

7. Fruit drop

7.1. Pattern of fruit drop Persimmons have been shown to exhibit one to three waves in fruit drop, with two being the most common (Fujimura, 1935; Kajiura, 1941 a, 1943; Otani, 1961 ; Yamamura and Naito, 1975; Bargioni et al., 1979; Kitajima et al., 1990). Fruit associated with the first two waves of drop are normally seedless (Otani, 1961). The first wave of fruit drop

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normally occurs within 20-30 days after anthesis (Yamamura et al., 1976) at the time of maximum shoot growth and starch depletion. The third wave of fruit drop coincides with root growth and the fruit associated with the third wave of drop may be partially pollinated (Otani, 1961).

7.2. Pollination effects In the cultivation of Japanese persimmon, physiological fruit drop is regarded as one of the main factors causing unstable fruit production (Sobajima et al., 1969; Yamamura et al., 1989) with fruit drop affected by both genetic and environmental factors and their interaction. Fruit drop can also be a primary cause of low bearing in young orchards (Blumenfeld, 1981). Many studies have confirmed that pollinated and seeded fruit set more heavily (Kajiura, 1941b; Bargioni et al., 1976; George et al., 1993) and that cultivars with low parthenocarpic ability are more susceptible to fruit drop (Kajiura, 1941a). For example, studies by Yamada et al. (1987) and George et al. (1993, 1995) have shown that the seasonal variation in the fruit set of hand-pollinated flowers of 'Fuyu' is low (range 84-98%). In contrast, the fruit setting ability of non-pollinated fruit is highly variable (range 0-58%). Yamada et al. (1987) found that in seasons when conditions were not favourable for parthenocarpic fruit set, there was a high correlation (r = 0.87, P < 0.05) between fruit set and seed number of 'Fuyu', a cultivar which has low parthenocarpic fruit set. In the early stages of fruit development fruit which are smaller are more susceptible to fruit drop as compared with larger size fruit (60% cf., 10%) (Yamamura and Naito, 1975). On naturally pollinated trees the dropped fruit are predominantly seedless (Jordan, 1985). Within a shoot, seedless fruit drop is induced by both competition among fruit and also between vegetative sinks when seedless fruit develop on long shoots (Kitajima et al., 1993a). Location of seedless fruit within a shoot is also important as basal floral locations are more prone to fruit drop than distal locations (Jordan, 1985; Kitajima et al., 1993b). Kitajima et al. (1993b) have also shown that seeded fruit exert a stronger sink for nutrients than ]?arthenocarpic fruits on the same tree and exhibited less fruit drop.

7.3. Growth regulators Internal tissue levels of growth regulators may play a role in fruit drop. Different auxin type promoters were observed in persimmon depending on fruit size (Yamamura and Naito, 1975). Sobajima et al. (1974b) has shown that cytokinin activity of 'Fuyu' increases markedly for up to 30 days after flowering and appears to be closely related to the early stages of fruit growth, whereas in non-pollinated fruits it remains at a very low level. Seedless fruit have also been shown to have a total lack of gibberellin (Bargioni et al., 1979). An application of GA 3-paste to the flowers or fruit peduncle of unpollinated fruit 5 days after full-bloom can prevent fruit drop (Hasegawa and Nakajima, 1990b,c; Hasegawa et al., 1991b). Suzuki et al. (1989a) suggest that fruit abscission is induced by a reduction in endogenous fruit IAA and imbalances between fruit and sepal auxin levels 4 - 5 days before abscission. High concentrations of ABA-like substances occur in the fruit during the physiological stage of development that corresponds with the first wave

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of fruit drop (Hirata et al., 1978). Smaller fruit with low seed number have also been shown to have higher level of ABA inhibitors (Yamamura and Naito, 1975). Ethylene evolution and internal ethylene levels may also be implicated in fruit drop as exogenous applications of ethephon have been shown to increase abscission in fruits with low seed number (Yamamura and Naito, 1980; Suzuki et al., 1987; Yakushiji and Hase, 1991). With persimmon, the abscission layer develops between the junction of the calyx and peduncle (Nakamura and Wakasugi, 1978). The calyces may also be important sources of growth regulators (Nakamura, 1967; Maeda, 19681. Studies by Morley-Bunker et al. (1992) have shown that the application of GA 3 to the calyces alone increases fruit drop while, in contrast, the application of kinetin to the calyces has been shown to reduce fruit drop by as much as 40% (Nakamura and Ryugo, 1974).

7.4. Fruit dropibioclimatic, soil and nutritional effe'cts Environmental influences (:an affect a wide range of physiological processes that can affect fruit drop. Several Japanese studies have shown that fruit set is higher in seasons when sunshine duration during and after flowering was higher (Nagasawa et al., 1968; Takahashi et al., 1971; Kaneko, 1977; Yamada et al., 1987). In subtropical Australia, George et al. (1993, 1996a) found that both whole tree and calyx shading severely reduced fruit set of non-pollinated or poorly pollinated flowers. Fruit drop is also reported to be heavier on heavy alluvial soils (Kaneko et al., 1979). Increased fruit drop on these soil types may be due: to waterlogging which has been shown to increase fruit drop (Suzuki et al., 1988a). Water stress has also been shown to increase the level of fruit drop (Kajiura, 1942a; Bargandzhiya, 1977) with studies in Japan showing, that at leaf water potentials > - 1.8 MPa, fruit drop of poorly pollinated or non-pollinated fruit increases 5-15 times compared with well-pollinated fruit (Suzuki et al., 1988b). George et al. (1996b) also found that non-pollinated fruit were highly susceptible to water stress but fruit drop of well pollinated fruit was low even at moderate leaf water potentials ( - 1.8 MPa). Fruit abscission may be increased by excessive N (Suzuki et al., 1989b) but low levels may be equally detrimental to fruit set (Kajiura, 1942b; Gasanov, 1984). Late summer and autumn N application have been shown to beneficial to maintaining leaf health and in the accumulation of starch reserves for the following season's flowering and fruit set (Hirata and Kurooka, 1974).

7.5. Fruit drop--vegetatice growth Fruit drop is increased by excessive shoot growth during stage I of fruit development (Kajiura, 1942c; Kitajima et al., 1987, 1990). Vegetative part of the fruit bearing shoot was the main sink for dry matter accumulation until three weeks after pollination. After this period the fruit became more important. Kitajima et al. (1990) suggested that shoot growth competes with fruit and vegetative organs for assimilates. Excessive vigour may also be stimulated by low crop loads, excessive N application in early fruit development period and by severe winter pruning (George et al., unpublished data). Secondary growth has also been reported to promote a third wave of drop (Otani, 1961).

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8. Methods of reducing fruit drop

8.1. Methods of reducing fruit drop--growth regulators Many studies have shown that the application of GA_~ (10 mg 1-~-500 mg 1 ~) at anthesis or within 10-15 days after anthesis can increase fruit set of seedless fruit however the response may be seasonally dependent (Hiretta et al., 1967; Nagasawa et al., 1968; Bargioni et al., 1976, 1979; Blumenfeld, 1986; Yamamura et al., 1989). The application of other growth regulators (auxins and cytokinins) appear to be less effective (Yakushiji and Hase, 1991). However, combinations of GA 3 and either auxins or cytokinins can improve fruit set and be independent of seasonal influences (Cristoferi et al., 1981). In Japan, the beneficial effects of GA 3 on increasing fruit set may be outweighed by its possible adverse effects, even when applied at low' concentration (25 mg 1-l ), on reducing flowering potential in the following year (Yamamura et al., 1989). Further long term studies are needed in regions which have higher irradiance and floral initiation to see if there are similar detrimental effects from GA 3 applications under these conditions. The application of the growth regulators aminoethoxyvinylglycine (AVG) at 50 mg 1 -~ applied as a pre-bloom spray and naphthylacetic acid (NAA) at 500 nag 1 ~ in a lanolin paste to the apex of the fruit soon after flowering has been shown to reduce fruit drop in Japan (Suzuki et al., 1988b). A new growth regulator CPPU (KT30) which has cytokinin activity has been shown to be effective in reducing fruit drop by spraying the trees at full bloom and again 10-15 days later at a concentration of 3-5 mg 1-~ (Hasegawa et al., 1991b; Sugiyama and Yamaki, 1'995). This growth regulator has been shown to reduce fruit maturity and would not be suitable to use with early maturing varieties. Tape containing 1000 mg I ~ IBA applied to the fruit apices has also been shown to reduce fruit abscission (Maotani et al., 1989).

8.2. Methods of reducing fruit drop-- environmental factors Of the environmental factors affecting fruit drop, irradiance, water stress and excessive vegetative growth appear to be the most important. Kaneko et al. (1979) observed that heavy alluvial soil types were most likely to experience post-bloom drop. Such soils are typically poorly aerated and oxygen and water availability to the root system is reduced. Irrigation studies conducted in JapaJa have shown that soil moisture stress levels should be maintained at < 1.3 pF for 50% of the fruit development period to prevent a reduction in fruit development (t-[ase et al., 1988). In Australia, container studies have indicated that soil moisture leYels should be maintained > 50% field capacity to prevent fruit drop of pollinated fi'uit and near field capacity for non-pollinated fruit (George et al., 1996b). Shading tends to increase fruit drop and can adversely affect fruit quality (Mowat and Ah Chee, 1992; George et al., 1996a). Irradiance in the tree canopy may be increased using trellising systems such as Tatura, Y or palmette. Further control of shading and light levels within the tree canopy can be obtained by pruning (Okishima et al., 1983; Mowat and Ah Chee, 1990).

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8.3. Methods of reducing fruit drop--pollination factors A lack of suitable pollen sources was a major cause of crop failure in the initial persimmon orchards planted in the United States (Hume, 1909). Suitable pollen producing cultivars such as 'Gailey' were identified and interplanted in existing and new orchards to reduce fruit drop (Popenoe, 1920). Various reports have suggested that between 8-15 pollinizer trees be planted in commercial blocks (George and Nissen, 1984; Kitagawa and Glucina, 1984). Specific pollinizer varieties may be needed with different cultivars to obtain overlap of flowering. Although it has been observed that hand pollination is effective in preventing fruit drop, it requires a great deal of labour to gather pollen grains (Yamamura, 1982). Techniques for hand pollination have been described by previous researchers (Mori and Hamaguchi, 1950; Yasunobu and Akiyama, 1980). Artificial pollination media and carriers have been developed to maintain pollen viability in spray pollination systems (Ohno and Aoki, 1959; Ohno, 1962a,b; Ohno et al., 1964). The viability of persimmon pollen grains can be maintained for several weeks at room temperature by storing in polythethylene film pouches with a silica desiccant (Hirose et al., 1963). Temperatures below - 8 0 ° C are suitable for long term pollen storage (Wakisaka, 1964). In Japan, Fukae et al. (1987) reports that for satisfactory fruit production a persimmon flower requires at least 20 honeybee visitations. In many orchards the placement of bee hives would improve fruit set. In New Zealand it is recommended that at least three hives per hectare should be placed in the orchard (Kitagawa and Glucina, 1984) whereas, in Japan, it is suggested that three hive p e r 4 ha is satisfactory (Fukae et al., 1987).

8.4. Methods of reducing fruit drop-- management factors Choice of rootstock can influence flower initiation, fruit set and fruit drop. Schroeder (1947) showed that low yields of 'Hachiya' on D. l~irginiana and D lotus were due to lower floral initiation and higher fruit drop respectively in comparison to trees grown on D. kaki stock. 'Hiratanenashi' trees grown on D. lotus stock also appear to be more prone to fruit drop than those on D. kaki (Sobajima et al., 1975b). In drought prone regions of Southern China persimmon is grown on D. oleifera to improve growth and productivity (Mowat et al., 1995). The physiological basis of differences in fruit drop susceptibility between rootstock species is not clear and warrants closer investigation. Fruit thinning of shoots to one fruit per lateral can reduce fruit drop and stabilise annual bearing in heavy bearing cultivars such as 'Izu' (Horie et al., 1988). In Israel, a combination of girdling about 50-60% of limbs, soon after budbreak, and the application of GA 3 at 15-30 mg 1-t during full bloom was shown to increase yield by from 50-400% (Blumenfeld, 1981). Cincturing at anthesis and for up to 1 month after flowering has also been shown to reduce fruit drop in the current season and increase flowering in subsequent seasons presumably as a consequence of greater floral bud initiation (Hodgson, 1938). This technique is used mainly on young trees when vigour is excessive. A complete strip of bark about 5 mm wide is removed around the circumfer-

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ence of the tree. Wire strapping of secondary scaffold branches 20 days after full-bloom is effective in increasing fruit set (Hasegawa and Nakajima, 1991b). This technique appears to increase fruit set by reducing fruit drop in poorly pollinated fruitlets. Preliminary studies indicate that excessive vegetative growth, including secondary growth can be controlled using the growth retardant, paclobutrazol (Barkay and Tzamir, 1986; Mowat and Ah Chee, 1990; George et al.., 1995). Damage to the leaf canopy by wind, hail, drought, a~,rochemicals, pests and disease can increase fruit drop (Kajiura, 1942a,d). Avoiding damage to the leaf canopy during the sensitive stages of fruit drop by wind and hail protection and careful use of agrochemicals can provide important control measure for reducing the risk of fruit drop.

9. Fruit growth pattern Persimmon fruit exhibit a double sigmoidal growth pattern (Kitagawa, 1970; Nii, 1980; Kitagawa and Glucina, 1984). The pattern consisls of three growth stages and is comparable to growth models established for peaches (DeJong and Goudriaan, 1989). In 'Fuyu' cell division in the fruit pericarp tissue normally ceases 30 days after anthesis (Hirata et al., 1974). With 'Fuyu' (a mid-season cultivar), relative growth appears to be highest during stage I of growth which is completed about 70 days after anthesis (Nii, 1980, see Table 1). In contrast, under the cooler growing conditions of New Zealand, stage I of fruit growth of 'Fuyu' is not completed until 112-140 days after full bloom (Mowat and Ah Chee, 1990). Fruit which abscise in this period exhibit slower dry matter accumulation than normally developing fruit (Kitajima et al., 1990). Nii (1980) observed a marked increase in seed dry matter during stage II. However, seedless fruit yielding cultivars and seeded cultivars both exhibit the same pattern of three stages in fruit development (Otani, 1961; Hirata and Hayashi, 1978; Sugiura and Tomana, 1983; Yonemori and Matsushima, 1987). Higher fruit temperatures ( > 20°C) can extend the duration of stage II (Chujo, 1982). Greatest dry matter accumulation occurs in stage III which lasts between 100-150 days after anthesis (Nii, 1980). Fruit weight up to the end of stage I of growth has been correlated with seed number (r = 0.54, P < 0.01), but there is no correlation with fruit weight at maturity (Nii, 1980; George and Nissen, 1988). Seed size declines with seed number (Hasegawa et al., 1993). Fruit weight is also influenced by fruit cell number and size (Hirata et al., 1974). Fruit cell number is controlled by the status assimilate reserves within the tree during the previous autumn and cell size is determined by source availability during fruit growth (Hirata et al., 1975). The effect of poor autumn reserves on reduced cell division appears to be related to a lowering of the biological activity of growth hormones and nitrogen metabolites during flower development (Hirata et al., 1975). In regions with high irradiance and cool maritime temperatures during the fruit development period, positional effects in the tree canopy may have more important effects on final fruit size with fruit exposed to light being up to 35% bigger than non-exposed fruit (Mowat and Ah Chee, 1990, 1992). Differences in flesh temperature differences between exposed and shaded fruit can be as great as 0.5°C (Mowat and Ah

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Chee, 1994). On days of high irradiance fruit exposed to the light can have a fruit temperature of 5°C above air temperature in comparison with shaded fruits which can be 2°C below (Chujo et al., 1971). In warm environments location of fruit within the canopy may have no significant effect on fruit weight, but the lower light penetration in shaded fruit positions can reduce fruit colour and ,.~oluble solids (Hasegawa and Nakajima, 1991b). The optimum temperature for 'Fuyu' growth is between 20-25°C, at temperatures < 15°C and > 30°C, fruit size is reduced lChujo et al., 1972, 1973). Fruit shape may also be influenced by climatic conditions as, fruit of some cultivars have a greater length-diameter ratio when grown under hot dry climates in comparison to those grown under cool wet climates (Hodgson and Schroeder, 1946). An interesting feature of persimmon fruit is their unusually large calyx: at flowering the calyx may occupy more than 50% of the fi'uit weight. The calyx has many stomata and appears to be an important assimilation, respiration and transpiration organ (Nakamura, 1967). The phytohormone content of the calyx may have an important influence on fruit drop. Cultivars with high cytokinin levels in the calyx tend to be less prone to fruit drop than cultivars with lower levels (Nakamura and Ryugo, 1974). The calyx also contains gibberellins with similar activity to GA 3 (Yamamura and Naito, 1973). Recent studies in Japan have shown that calyx lobe removal at stage I and stage II, but not at stage III, inhibited fruit development by between 50-75% (Atsumi and Nakamura, 1959). Damage by agrochemicals, such as copper sulphate, insect damage and disease could result in a similar effect (Nakamura, 1967). Yonemori et al. (1995) has shown that calyx removal inhibits sucrose hydrolysis at unloading sites in the fruit and this may create unfavourable gradients for further allocation (Yonemori et al., 1995). Disruption to sucrose hydrolysis by calyx lobe removal appears to be due to inhibition of acid invertase activity (Hirano et al., 1995). Patterns of growth within a fruit can differ between the basal, median and distal regions (Fujimura, 1935). The growth rate of the basal region of 'Fuyu' is greater than median and distal regions (Nakamura, 1967). Clark and Smith (1990) have reported gradients in the mineral composition between regions within the fruit that are present throughout growth and development.

10. Methods of manipulating fruit size and fruit maturity 10. I. Pollination effects The presence of seed in fruit can influence fruit weight, shape, skin and flesh colour, soluble solids, astringency, texture and fruit cracking (Hume, 1913; Hasegawa and Nakajima, 1992). Differences in internal flesh colour, texture and astringency are more pronounced in pollination variant forms of persimmon (Ryugo, 1964). Seeded fruit have a higher respiration rate than unseeded fruit (Matsurnoto, 1932). Metaxenia has also been found to effect fruit size, shape and sweetness (Kajiura, 1934; Noguchi, 1934). Apical cracking and calyx cavity are disorders that cause significant marketable fruit loss in non-astringent cultivars such as 'Jiro' and 'Fuyu'. In susceptible cultivars the presence, number and location of the seed influences', the severity of both disorders. Seedless fruit are less prone to apex and calyx-end cracks (Yamada et al., 1986, 1991).

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Under good management conditions, pollinated fruit can be up to 25% bigger than seedless fruit (George et al., 1993, 1995). The determination of the maximum level of fruit size response to pollination is important as it will influence the decision on whether to include pollinizers in commercial plantings. Since pollinizers occupy about 10% of the orchard tree numbers, minimum fruit size increases of 10% must be achieved if yields per hectare are to be maintained. George et al. (1995) concluded in their studies on fruit thinning that, compared with non-pollinated fruit, pollinated fruit were more efficient in competing for dry matter, carbohydrates and nutrients indicating that they exert a greater sink force for these nutrients presumably through growth regulator mediated pathways. Recent studies in Japan (Hasegawa and Nakajima, 1990a,b; Kitajima et al., 1993b) have shown that the ratio of seeded:seedless fruit on trees may influence fruit size, fruit colour and soluble solids. On trees where the seeded:seedless fi'uit ratio is high, seedless fruit were small, slow to develop colour and had low soluble solids contents. In contrast, on trees where the seeded:seedless fruit ratio is low, seeded and seedless fruit were similar in size, colour and soluble solids. The conclusion that can be drawn from these studies is that it is better to produce all well-pollinated or all non- pollinated fruit on the same tree, but not both (Kitajima et al., 1993b,c).

10.2. Thinning effects-- timing Thinning can be undertaken to increase fruit size and can be carried out before, during or after flowering (Shigeta and Yasunobu, 1957). Disbudding involves removing unopened flower buds 10-14 days before full bloom. Early fruit thinning within 20 days of anthesis has been shown to increase fruit size and to reduce the percentage of fruit less than 200 g by ca. 70% (Matsumoto and Kuroda, 1982). In Japan the main commercial non-astringent cultivar 'Fuyu' is thinned in two stages: first flowers are thinned to twice the target crop and a second thin is made on fruitlets after the first drop period to reach a target of one fruit per shoot for shoots with more than five leaves (Yamada, pers. comm.). With some early maturing cultivars such as 'Izu' disbudding 25 days before anthesis has been shown to increase fruit size by about 10% (Yasunobu and Akiyama, 1980).

10.3. Thinning leuel Ideally fruit thinning recommendations should be varied to reflect the crop carrying capacity of each tree in conjunction with the local growing conditions. In New Zealand, Mowat and Ah Chee (1990) reported that fruit yield and size was greater in the upper part of the tree canopy where fruit number:unit leaf area was highest. This is contrary to other studies that show declining fruit weight with increasing values for fruit number:unit leaf area (Kishimoto, 1975). Mowat and All Chee (1990) ascribed the fruit size/fruit number contradiction to the fruit needing a thermal requirement for growth that was only fulfilled in the upper tree canopy. In Japan, Yamamura et al. (1989) reports that a leaf:fruit ratio of 20:1 is necessary to achieve maximum fruit size. In New Zealand, Jordan (1985) recommends that one fruit

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be left on shoots less than 200 mm and that longer shoots be thinned to two fruits, well spaced along the shoot. However several studies conducted in Australia have shown no correlation between final fruit weight and fruiting lateral parameters such as leaf areas or numbers (George et al., 1993, George et al., 1995). They concluded that leaf number or area and shoot length are not reliable criteria for determining thinning levels on an individual shoot basis. More recently, studies have shown that average fruit weight to be highly correlated with crop load per tree or per square metre of canopy surface area (Kishimoto, 1963, Kishimoto, 1964, Kishimoto, 1975; Collins et al., 1995, unpublished data). Whole-tree crop loading indices may prove to be more useful indices than individual shoot thinning indices for determining sustainable long-term yields and for maintaining a desired fruit size range. Mowat (pers. comm.) has suggested leaving 15 fruit per square metre of canopy when persimmon are grown on canopy structures and Collins et al., (1995, unpublished data) recommend leaving 10-12 fruit per square metre for non-trellised canopies.

10.4. Chemical thinning Currently in most persimmon producing countries, trees are hand thinned. The response to chemical thinning has been variable and dependent on season, tree vigour and weather (Sugimoto and Fukunaga, 1972). The main thinning chemicals tested have been ethephon and NAA (Yamamura and Naito, 1975, 1980; Nakamura and Wakasugi, 1978; Yamamura et al., 1980). The control of fruit abscission is probably complex with several factors and the degree of influence val~'ing at any one time. The application of AVG (aminoethoxyvinylglycine), an antimetabolite of ethylene, 3-5 weeks just before and after full bloom, did reduce abscission (Suzuki et al., 1988b). Fruit size on trees chemical thinned by NAA tends to be smaller than fruit on hand disbudded trees (Sugimoto and Fukunaga, 1972). The difference in response to fruit growth may be related to a lowering of assimilation during the early stages of fruit growth.

10.5. Growth retardants The application of the growth retardant paclobutrazol has been shown to increase fruit size of persimmon by 7-20% (George et al., 1995). Paclobutrazol applications can also enhance fruit maturity, colour and soluble solids (Mural, 1992). Both soil (1-2 g a.i. per tree) and foliar (2 g a.i. 1- i ) application have been effective in controlling shoot and tree vigour (George et al., 1995, unpublished data). Non-pollinated fruit exhibit more than double the fruit growth response to paclobutrazol indicating that control of shoot vigour may be more important in orchards without pollinizers.

10.6. Canopy manipulation Fruit quality in persimmon can be enhanced by the development of thin open canopies that improve light exposure to fruit (Iimuro el al., 1981; Takano et al., 1991; Hasegawa and Nakajima, 1991b; Yamamoto et al., 1993; Mowat and George, 1994). In

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New Zealand, a reduction in smaller size fruit < 200 g has been attributed to better light interception on the Y-trellis, compared with other systems of training (Mowat and Ah Chee, 1990). The upper levels in Y-trellis canopies exhibit higher levels of photosynthetically-active radiation, little renewal growth shading, low leaf area indices, and high specific leaf weights (Mowat and Ah Chee, 1990). A reduction in light reaching the fruiting zone can have two effects on fruit quality. The first is a reduction in the photosynthesis of leaves associated with the fruit and subsequent reduction in the transport of sugars to the fruit (Mowat and Ah Chee, 1990). Persimmon fruit are sensitive to shading as they have a high light saturation point for maximum photosynthetic rate (Amano et al., 1972). The second effect is to reduce thermal conditions around the fruit (Mowat and Ah Chee, 1990, 1994) Japanese studies have shown that fruit size is increased by optimising leaf area available to the fruit and reducing the partitioning of dry matter into trunk and major branches by trellising and pruning: (Kishimoto and Seike, 1972; Kishimoto, 1975). The calyx has stomata and appears to be an important gas exchange organ of the fruit. Stomatal conductance of the calyx has been shown to increase with increasing solar radiation (Suzuki et al., 1987). I0.7. Fruit maturity

Comparative studies on the rates of fruit development between Australia and New Zealand indicate similar rates of development after flowering but in New Zealand, budbreak and flowering are delayed by 4 - 6 weeks (Mowat et al., 1995). Time of budbreak is important because it affects the length of the fruit development period which, if shortened, may result in fruit not completely losing astringency by the time of harvest. Timing and the uniformity of budbreak can be manipulated by applying rest-release chemical such as hydrogen cyanamide (Sampaio et al., 1978; Shulman et al., 1986; Behnke, 1992). The timing of maturity can be influenced by tree training and planting density (limuro et al., 1981). The main effect of training and density appears to be related to their influence on fruit shading. Orchards systems that increase the proportion of fruit exposed to light tend to advance maturity. Fruit maturity has been shown to be advanced by the application of the growth retardant, paclobutrazol (Barkay and Tzamir, 1986) and ethephon (30 mg 1 l ) (Yasunobu, 1976, Zheng et al., 1991). In contrast, the application of GA 3 has been shown to retard fruit ripening by 2 - 3 weeks and delayed leaf abscission by about 3 weeks (Ben-Arie et al., 1986). The application of GA 3 (100 mg l - j ) during stage III of fruit growth has been found to delay fruit maturity (Zheng et al., 1991). Applications of low rates of CPPU (3-5 mg 1 ~) about 10 days after full-bloom can delay fruit coloration and harvest maturity (Hasegawa et al., 1991b; Sugiyama and Yamaki, 1995).

11. Conclusions

Research in tree fruits has emphasised two potential mechanisms viz. plant growth regulators and energy reserves in the regulation of flowering and fruit set (Wolstenholme and Whiley, 1989; Bower et al., 1990). With persimmon considerable research

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11 l

has been conducted to evaluate the role of both endogenous and externally applied growth regulators on flowering, fruit set and fruit drop. The tissue and morphological basis for fruit abscission has been described (Sobajima and Takagi, 1968). Fruit is retained when the completion of an abscission layer between the peduncle and calyx is delayed. Fruit retention appears to be favoured by high internal levels of GA 3, and cytokinin, and low levels of ABA produced in seeded fruit. Internal growth regulators released by the calyx may also have an important influence on fruit retention but this has been studied in much less detail. External applications of ethylene and ethylene antimetabolites promote or retard fruit drop respectively. There is evidence to show that internal levels of ethylene are variable. Ethylene may also play a part in the endogenous complex of factors determining fruit retention. Seedless and partially seeded fruit are more sensitive to environmental and internal physiological factors affecting fruit drop. Wolstenholme (1990) suggests that the order of priority among sinks is a function of both growth rate (sink activity) and the size of sinks. The order is usually seeds > fleshy fruit parls = shoot apices and leaves > cambium > root > storage (Cannell, 1985). The higher sensitivity of seedless fruit to fruit drop is probably related to that fruit's weaker ability to maintain an adequate balance in endogenous growth regulators and to the lower sink force exerted by these fruit. Compared with internal growth regulators, a scarcity of information exists on the role of energy reserves in the fruit set and retention of persimmon. Low irradiance and excessive vegetative growth in the period from anthesis to 40 days after, adversely affects fruit retention. These responses coupled with the spectacular yield increases recorded from cincturing implicate carbohydrate stress playing a major role in increasing fruit drop. Few studies (Archer, 1941; George et al., 1994c) have been conducted to examine the seasonal carbohydrate reserve patterns in persimmon and how these are affected by crop load, and vegetative vigour. The persimmon has a tendency to alternate bearing but there is no information on carbohydrate levels with the on-off phases of cropping. There is a general lack of information about net carbon dioxide assimilation rates in persimmon. Currently other subtropical and temperate fruits are being investigated to determine the relative importance and contributory value of the various photosynthetically active organs (Blanke, 1990). There appears to be no parallel study to determine the photosynthetic importance of leaves, calyces and fruit in providing photosynthates for the developing fruit and ['or quantifying the actual carbohydrate requirements for growth and respiration of persimmon fruit. Fruit set and retention in persimmons should be improved considerably by improving the level of pollination of flowers. This may be achieved by the placement of adequate number of bee hives in the orchard, proper selection of pollinizer variety and proper placement of pollinizers in the orchard. The apparent predominant attraction of honey bees to flowers of pollinizer trees may warrant grafting sections of pollinizer variety into trees of the commercial varieties. This might be an alternative option to planting separate pollinizer tree. Breeding could also be used to produce cultivars with less fluctuations in bearing between years (Yamada and Yamane, 1994). Light penetration into the canopy may be improved by training and trellising trees onto systems such as palmette, Tatura and Y-trellis. Preliminary research indicates that

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e x c e s s i v e vegetative growth may be controlled by the application of growth retardants such as paclobutrazol. In the longer term n o n - c h e m i c a l alternatives for growth control c o u l d be based on the use o f clonal d w a r f rootstock or root restriction techniques ( K i m u r a et al., 1985; Richards and R o w e , 1977). The application o f growth regulators such as G A 3 and cytokinins to increase fruit retention may be beneficial p r o v i d e d there is no adverse affects on subsequent flowering. The growth of tree fruit species, whether they be e v e r g r e e n or deciduous, follows a cyclic seasonal pattern which is repeated each year, 'though not necessarily on the same time scale or with the same intensity o f growth for each stage (Cull, 1986; W h i l e y et al., 1988). By r e c o g n i s i n g the stages o f growth and understanding their requirements and the interactions within the tree, m a n a g e m e n t practises can be m o d i f i e d and p r o g r a m m e d to d e v e l o p strategies which lead to productivity gains ( W h i l e y et al., 1988). In New' Zealand, productivity and m a n a g e m e n t models for p e r s i m m o n h a v e been d e v e l o p e d based on the currently available information ( M o o r e and M o w a t , 1988). T h e s e models which provide a wholistic v i e w o f tree p e r f o r m a n c e can be refined as further research findings b e c o m e available.

References Amano, S., Hino, A., Daito, H., Kuraoka, T., 1972. Studies on the photosynthetic activity in several kinds of fruit trees: I. Effects of some environmental factors on rate of photosynthesis. J. Jpn. Soc. Hort. Sci. 414 144-150, (In Japanese with English summary). Anonymous, 1989. Fuyu seminar research show benefit of thinning. California Grower 11, 26-28. Aoki, M., Tanaka, K., Okada, N., t 977. Effects of nitrogen fertilization and ringing treatment on initial yield of persimmon (Diospyros kaki), cv. Wasejiro. Res. Bull. Aichi-Ken Agric. Res. Centre, B Horticulture 9~ 119-130, (In Japanese with English summary). Archer, C.J., 1941. The starch cycle in the Hachiya persimmon. J. Am. Soc. Hort. Sci. 38, 187-190. Asami, Y., Chow, C.T., 1941. Is the pollen of Japanese persimmons carried by wind?. J. Hort. Ass. Jpn. 7~ 247-251. Atsumi, K., Nakamura, M., 1959. Physiological and ecological studies on the calyx of Japanese persimmon fruit: 1. Development of fruit affected by removal of the calyx lobes. J. Hort. Ass. Jpn. 28, 170-176, (In Japanese with English summaryl. Baldini, E., 1952. Studies in bud differentiation in kaki. Ann. Sper. Agar. 7, 675-685, (In Italian with English summary). Bargandzhiya, A.G., 1977. Biological characteristics of flowering and fruiting of oriental persimmon. Byulleten Vsesoyuznogo Ordena Lenia Instituta Rastenievodstva Imeni N.I. Vavilova 68, 36-38, (In Russian with English summary). Bargioni, G., Pisani, P.L., Ponchia, G., 1976. Observations on fruit set and growth in persimmons. Agricoltura Italiana 104, 53-63, (In Italian with English summary). Bargioni, G., Pisani, P.L., Ramina, A., Castelli, F., 1979. Physiological aspects of fruit set, drop, and growth of parthenocarpic and normal t?uits in Diospyros kaki. Rivista della Ortoflorofrutticoltura Italiana 63, 81-92, (In Italian with English summary). Barkay, Z., Tzamir, G., 1986. Cultar in Persimmons--Preliminary Result Report, Makhteshim, Israel, 2 pp. Behnke, H., 1992. Experiencias con la aplicacion de cianamida hidrogenada a frutales en latinoamerica. Acta Hortic. 310, 97. Bellini, E., Giordani, E., 1994. Monoic 'Hana Fuyu' (Diospyros kaki L.) fertilizing capability of pollen. Proc. XXIVth Int. Hort. Congress. Abstracts, 48. Ben-Arie, R., Bazak, H., Blumenteld, A., 1986. Gibberellin delays harvest and prolongs storage life of persimmon fruits. Acta Hortic., 807-813. Blanke, M.M., 1990. Photosynthesis in subtropical fruits. Acta Hortic. 275, 435-439.

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