시설재배 Atemoya(Annona cherimola x A . squamosa) Hillary White 품종의 개화·결실 및 과실발육 조절

Abstract

This paper presents the outcome of the study carried out for three years from 2007 to 2009 in order to establish more effective and systematic methods for better flowering and fruit-setting of the atemoya currently grown in the greenhouses in Korea. For this purpose, many aspects of atemoya 'Hillary White' such as flowering characteristics, pollen germination, artificial pollination, fruit-setting, harvesting, fruit growth, and fruit quality were examined and analyzed. The findings obtained through this study can be summarized as follows.

Pruning and defoliation were simultaneously executed in spring to promote even shoot germination and flowering because the leaves didn't spontaneously fell off when the temperature in the greenhouse had been maintained above 0?C during winter. Shoot germination started in 8 to 16 days, and flowering did in 50 to 55 days after pruning. For more efficient work in the greenhouse, pruning was applied to three different points in the 3-stem-trained tree. The measurement after 120 days from pruning ranked the shoot length in descending order of upper-, center-, and lower-part pruning. Other two characteristics, the number of nodes and the diameter at the base of the shoot, also showed the greatest growth in case of upper-part pruning. However, the opposite was observed in internode length. The number of flowers peaked in the period of from 10 to 25 days after the first flowering. It then gradually decreased, finally arriving at stability.

In greenhouse cultivation, it is important to determine a proper time for artificial pollination by scrutinizing the changes in the state of pistils and stamens happening over the flowering stages. In fine weather, little change was observed in the state of petals and stigmas in the evening two days before full-bloom (D-2P), small hiatuses began to appear on the petals in the morning one day before full-bloom (D-1A), and petals began to split open around 17 o'clock one day before full-bloom (D-1P), with vigorous secretion on stigma. Stigma remained moist until 9 o'clock next morning, and then secretion began to decline. The state of petals and stigmas stayed unchanged until the morning of the day when the flower attained to full bloom. In the evening of the same day, fluid on the stigma could be barely noticed and the petals dehisced more widely so that petals and anther scattered off at a slight touch. Next morning, it was observed that petals turned rather dry or fell off, and stamens turned brown, with germination rate staying quite low.

High temperature in summer and the likes cause a smaller number of flowers and an unstable fruit-set rate on the branches in hypertrophy . Summer-season pruning/defoliation was tried to verify if it would raise the exuberance of flowering and fruit-set rate. Shoot development and initial flowering, respectively, took about 10 and 30 days. Better outcomes in the number of flowers and that of fruits, were observed in descending order of upper-, center-, and lower-part pruning. No significant difference was found between the pruning alternatives applied to either a point above the 3rd node or the 6th. Growth regulator soaking treatment was applied to the fruits in 30 or 40 days after artificial pollination in order to improve fruit-set rate. Higher fruit-set rate was realized in the treated fruits than in the non-treated ones. But the difference between those treated in 30 days and those treated in 40 days turned out to be insignificant.

Atemoya gives out flowers before new leaves sprout up, which induces markedly low germination rate of the pollens in early flowers, and thus makes timely artificial pollination utterly important. When pruning/defoliation was carried out, the germination rate reached its peak 30 days after initial flowering, and began to drop in 50 days. Also, high proportion of tetrad pollens and low rate of pollen germination were seen among the pollens in early flowers. Examination on the growth of pollen tubes in pollen incubation uncovered that the pollen tubes barely grew at 10?C of temperature, and that they developed better at 25?C rather than at 15?C.

Atemoya is a plant whose pistils and stamens pass through different development stages. It also has a short flowering period and scanty anther dust, which makes artificial pollination quite difficult. To find out efficient pollination methods, pollen germination rate and viability of artificial pollination were studied for a set of pollen storage times. Pollen germination rate was highest for those pollinated on the collection day, but it showed a tendency to rapidly drop for those pollinated on the 3rd day of storage or later. Study on the fruit-set rate after artificial pollination, compared by the pollen storage time, disclosed that higher pollen germination rate tends to lead to the similar rate in fruit setting. Pollen germination rate showed a significant difference among those pollinated at different flowering stages such as D-1A, D-1P, D-0A, D-0P, and D+0A. Similar differences were also noticed among the pollens in different duration of incubation and those collected at different flowering time. Pollen germination rate was extremely low for those pollinated in early flowering stages. It also exerted significant effect on the fruit-set rate, but the effect on the fruit quality couldn't be concluded as significant. Fruit-set rate turned out to be the highest for the pollination done in the afternoon one day before full-bloom (D-1P), followed by the rates for the stages D-1A and D-0A. Conspicuous drop in the fruit-set rate was witnessed for the later flowering stages.

Artificial pollination is indispensable to the production of high-quality fruits. Right timing of artificial pollination is therefore extremely important in growing atemoya. Artificial pollination done 10 days after the initial flowering resulted in heavier weight but poorer shape of the fruits. Comparatively even distributions in shape index and fruit weigh were gained for the fruits produced by pollinating the flowers opened 30 to 50 days after pruning. Fruit set rate showed similar tendency as pollen germination rate did, which means that pollination done 20 or 30 days after flowering caused high fruit-set rate, while that done at 10-day timelag led to a low rate. Comparison of fruit-set rates by the pollination time showed that the pollination practiced around 18 o'clock led to the highest fruit-set rate, and the rate was kept relatively high if the pollination was executed by around 10 o'clock next morning. Fruit droppage happened mostly around 20 days after pollination. Pollination failure could be visually identified in 4 days of pollination. Fruit-set rate eventually approached 38.2% 70 days after pollination.

Flowering and fruit-set could be boosted by exercising pruning and defoliation on the plants having scanty flowers and poor fruit-setting. Late fruit-setting, however, led to short fruit growth period and thus to lean fruit flesh. To stimulate early fruit growth, fruits were treated by soaking them in growth regulator. Widthwise thickening could be promoted with the treatment using GA 500 or GA 1000 mg?l-1 regulators, compared to the results seen in the fruits, non-treated or GA 50 mg?l-1-treated. It was also noted that fruit thickening started to accelerate around 30 days after pollination. In widthwise diameter of the fruits measured 100 days after pollination, no significant difference was identified between the two groups, non-treated and GA 500 mg?l-1-treated. That diameter could be enhanced to 71.2mm with GA 500 mg?l-1 treatment, and to 74.5 mm with GA 1000 mg?l-1 treatment. Widthwise thickening continued to progress from the start right up to harvesttime.

It is beneficial for atemoya fruits to undergo a ripening treatment for a certain length of time after harvest. It is difficult to determine right time for harvest by visual inspection. In order to find out an appropriate time, four harvesttime alternatives, measured by the number of days after pollination, were tested. Harvested fruits were put on the after-ripening treatment for 8 to 10 days at 20?C of temperature. Comparing the fruit quality characteristics for the groups of different harvesttime revealed that the fruits in the groups with more than 140 days benefited stable result from after-ripening treatment and that incidents of skin browning were most frequently identified in the 120-day group. Level of sugar content turned out high in the groups of more than 140 days than that in the group of 120 days. It was observed that hexose content gradually increased in the days of after-ripening treatment and then began to decline around 9 days after the beginning of the treatment. Hexose gave out strong taste of sweetness, being made up mostly of fructose and glucose. Sucrose content was measured at a comparatively low level.