WO2005077147A1

WO2005077147A1 - Method for induction of compact shoots as propagule of potato

Info

Publication number: WO2005077147A1
Application number: PCT/KR2004/000775
Authority: WO
Inventors: Sun Lee; Jae-Geun Yoo; Suxo Chang
Original assignee: Nexgen Biotechnologies Inc.
Priority date: 2004-02-18
Filing date: 2004-04-02
Publication date: 2005-08-25
Also published as: KR20050082341A; KR100614533B1

Abstract

The present invention relates to a method for inducing a compact shoot, which comprises cultivating a potato plant concomitantly with treating said potato plant with a plant growth inhibitor to stimulate lateral buds or axillary buds of said potato, whereby said compact shoot as a propagule is generated and serves as a vegetative tissue or organ. Unlike conventional propagation methods using tuber-shaped seed potatoes through in vitro culture, soil culture and hydroponic culture, the present invention can provide compact shoots being subject to suitable inhibition of internodal growth for application purpose. The compact shoots of this invention can be served as a propagule for potato propagation, replacing conventional seed potatoes or being employed as a propagule for production of conventional seed potatoes. According to the present invention, the improved substitutes of virus-free potatoes can be rapidly produced in a massive manner.

Description

METHOD FOR INDUCTION OF COMPACT SHOOTS AS PROPAGULE OF POTATO

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a method for induction of compact shoots as a propagule of potato, in particular, to a novel method for inducing compact shoots of potato serving as a substitute for tuber-shaped seed potatoes for efficient propagation of various potato cultivars .

DESCRIPTION OF THE RELATED ART According to production size of seed potatoes, the conventional production methods are classified into: (a) microtuber production; (b) minituber propagation through planting of microtuber; (c) seed potato production with conventional size through planting of minituber or microtuber; and (d) seed potato production with conventional size through separate planting seed potato production with conventional size. Furthermore, according to cultivation method, the production of seed potatoes is classified into: (a) in vi tro culture; (b) hydroponic culture; and (c) soil culture including culture bed. Such cultivation methods are finally directed to the most effective propagation and therefore other factors such as pathogen-susceptibility of seed potatoes appears a second consideration. The most effective approach in the mass production of seed potatoes and their substitutes is defined as the most abundant production of seed potatoes and their substitutes in unit area using minimal capital and labor. The employment of virus-free potato through tissue culture and the establishment of cultivation facilities to prevent infection of various pathogens such as virus are considered fundamental matters in the production of all seed potatoes, so that they do not play pivotal role in improving the production efficiency of a seed potato and its substitute.

SUMMARY OF THE INVENTION Endeavoring to develop a novel method to substitute tuber- shaped conventional seed potatoes for the efficient propagation of various potato cultivars, the present inventors have discovered that compact shoots could be induced by inhibiting extremely the stalk extension from lateral or axillary buds of aerial stalks and served as a propagule of potato. Accordingly, it is an object of this invention to provide a method for inducing a compact shoot . It is another object of this invention to provide a method for preparing a compact shoot . It is still another object of this invention to provide a compact shoot of potato. It is further object of this invention to provide a method for propagating potatoes using a compact shoot. It is still further object of this invention to provide a potato prepared by the method using a compact shoot.

Other objects and advantages of the present invention will become apparent from the detailed description to follow taken in conjugation with the appended claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 represents a conventional seed potato for potato propagation (left side) and a compact shoot of this invention (right side) . Fig. 2 shows a potato plant having compact shoots induced by the present method. Fig. 3 shows compact shoots generated by the present method. Fig. 4 represents the dense cultivation for the mass production of compact shoots of this invention. Fig. 5 is a photograph showing the rooting and growth pattern of seed potatoes produced using compact shoots of this invention. Fig. 6 represents underground tubers of seed potatoes produced using compact shoots of this invention. Figs. 7-9 are photographs showing marketable potatoes cultivated using compact shoots of this invention.

DETAILED DESCRIPTION OF THIS INVETNION In one aspect of this invention, there is provided a method for inducing a compact shoot, which comprises cultivating a potato plant concomitantly with treating said potato plant with a plant growth inhibitor to stimulate lateral buds or axillary buds of said potato, whereby said compact shoot as a propagule is generated and serves as a vegetative tissue or organ. In another aspect of this invention, there is provided a method for preparing a compact shoot, comprising harvesting the compact shoot induced by the present method for inducing a compact shoot . In still another aspect of this invention, there is provided a compact shoot of potato prepared by the present method for preparing a compact shoo . The term used herein, "compact shoot" is a coined word introduced firstly by the present inventors, referring to an aerial stalk of which internodal growth is extremely inhibited. For adopting the novel term, the present inventors have assumed a compressed tuber prepared by extremely inhibiting the internodal growth of tubers. In the present invention, the term, "compact shoot" is exclusively intended to apply to plants belonging to Solanum, meaning an aerial stalk with extremely inhibited internodal growth including apical bud and lateral bud serving as vegetatives . The plant growth inhibitor useful in this invention may be any conventional one for inhibiting the growth of plants, for example, including ethephon, ethrel, abscisic acid, phosphon D, Amo-1618, B-905 (N-dimethyl-amino succinamic acid), ancymidol, carbaryl, chlormequat, chloro IPC, daminozide, flurprimidol, hydrogen cyanamide, maleic hydrazide, mefluidide, mepiquat chloride, paclobutrozol, prohexadione calcium and uniconazole. Preferably, the plant growth inhibitor is ethephon, ethrel, abscisic acid, phosphon D, Amo-1618 or B-905, more preferably, ethephon, ethrel and abscisic acid, and most preferably, ethephon and ethrel . The treatment of plant growth inhibitor to potato plants showing vigorous growth patter before or after blooming may be performed in the manner of spraying at an interval of 2 days-2 weeks, preferably 5 days-8 days, and more preferably about 1 week. The foliar spray is the most preferable for the treatment of plant growth inhibitor. Preferably, the plant growth inhibitor is a solution of 1-1,000 ppm. For example, ethephon and ethrel preferably are 10-1,000 ppm solution and abscisic acid preferably is 1-100 ppm solution. More preferably, ethephon and ethrel are 100 ppm solution and abscisic acid is 10 ppm solution. Meanwhile, the spraying interval and concentration of the regulator could be controlled according to the response of potato plants treated (abnormal growth pattern of leaf and stalk, etiolation of leaves and the falling of leaves) . According to a preferred embodiment, the induction of compact shoots can be more promoted by adjusting a photoperiod together with the treatment of plant growth inhibitor. Although the photoperiod is generally maintained to above 12 hr for vigorous growth of potatoes, the photoperiod shorter than 8 hr is preferably to promote the induction of compact shoots according to the present method. According to a more preferred embodiment, the present method is performed under a photoperiod of 0-8 hr. The photoperiod of 12-16 hr is maintained before and after blooming stage and then the photoperiod shorter than 8 hr or 7-day dark condition is adopted concomitantly with the treatment of plant growth inhibitor, so that the formation of compact shoots is maximally induced. According to a preferred embodiment, the induction of compact shoots can be more promoted by treating potato plants with trehalose together with the treatment of plant growth inhibitor. Trehalose, one of storage carbohydrates in potatoes is a disaccharide accumulated in plants when growth conditions become worse. Preferably, trehalose is 10-100 mM, more preferably, 30-70 mM and most preferably about 50 mM solution. It is preferred that the treatment of trehalose is performed to potato plants exhibiting vigorous growth pattern before or after blooming stage at an interval of 2-5 days. The foliar spray is the most preferable for the treatment of trehalose.

Meanwhile, the spraying interval and concentration of trehalose could be controlled according to the response of potato plants treated (e.g., the response showing expansion of the basal part of apical buds and lateral buds under inhibited internodal growth) . To promote the induction of compact shoots, the irrigation is preferably performed. The preferable irrigation for induction of compact shoots along with the treatment of plant growth inhibitor is performed to reach toward wilting point as close as possible unless the growth of compact shoots is inhibited. In hydroponic culture, the control of irrigation is accomplished by adjusting the osmotic pressure of nutrient solution with salt such as sodium chloride. It is preferred that sodium chloride is added in the concentration of 0.3-3%, most preferably about 1.5%. According to a preferred embodiment, the increase in mean weight and production efficiency of compact shoots per plant is accomplished by preventing the formation of underground tubers through inhibiting the transfer of carbon assimilates in the potato to underground tubers . Such manipulation may be in various manners, preferably, physical manner in view of operation convenience. According to one example, potato plants are planted on solid material through which their roots cannot pass, for soil culture or culture using a culture bed. After roots show a taking root appearance, surface soil is removed to inhibit the formation of stolon and to make it easy to remove stolon generated. In hydroponic culture, after roots was taken, the plant is taken up to an extent that the upper part of roots is exposed to the outside of hydroponic culture bed, so that the formation of stolon is inhibited and stolons generated are rendered to remove . Meanwhile, the planting density for efficient production of compact shoots is above 5-fold higher than that of conventional soil culture or hydroponic culture for tuberization. Specifically, 20-50 plants/1 m² are planted. The plant planted include any vegetative of potato such as tuber, seedling for tissue culture, seedling for cutting and compact shoot. It could be realized that the dense cultivation leads to the prevention of tuberization and the dense contact results in the competition between plants, so that the compact shoot production is promoted. In summary, the present method for inducing compact shoots comprises inevitably the treatment of the plant growth inhibitor and alternatively, at least one of (i) the treatment of trehalose; (ii) the adjustment of photoperiod, temperature and irrigation; and (iii) the inhibition of formation of underground tubers. More preferably, 4 manipulations described above are taken. The harvesting in the preparation of compact shoots is preferably performed at the time that a lignification of potato stalks and etiolation of leaves occurs. At this time, the more accumulation of carbon assimilates does not occur. However, the harvesting time can be controlled, if necessary. The earlier harvest of compact shoots not completely hardened results in the decrease in the activity of compact shoots due to water loss after harvest. The present invention is directed to the effective propagation of various potato cultivars . Unlike conventional propagation methods using tuber-shaped seed potatoes through in vitro culture, soil culture and hydroponic culture, the present invention can provide compact shoots being subject to suitable inhibition of internodal growth for application purpose. The compact shoots of this invention can be served as a propagule for potato propagation, replacing conventional seed potatoes or being employed as a propagule for production of conventional seed potatoes. According to the present invention, the improved substitutes of virus-free potatoes can be rapidly produced in massive manner.

In further aspect of this invention, there is provided a method for propagating potatoes, which comprises the steps of: (a) planting the compact shoot of potato; and (b) cultivating said compact shoot planted. In still further aspect of this invention, there is provided a potato prepared by the present method for propagating potatoes. The planting of compact shoots and cultivation of compact shoots planted may be performed in accordance with the conventional planting and cultivation methods of plants belonging to Solanum (JANG, Byeong-Ho, Potato Encyclopedia, Seo jinMoonhwa, Inc. pp. 286(1997)). According to a preferred embodiment, the planting of compact shoots may be performed in such a manner that the circular or conical processed forms containing compact shoots surrounded by peat moss, antibiotics, insecticide, fertilizer and useful microbes are seeded. Such planting is advantageous in the sense that it may be done mechanically. The peat moss includes any conventional one used in the art. The antibiotic includes any conventional one use in this art, for example,

Captan™, Tanzestar™, and Daconil™. As an insecticide, the any conventional one available to one skilled in the art may be employed, for example, Conido™, Dagal™ and Desis™. The fertilizer may include single or complex ingredients of urea, ammonium sulfate, nitrate, potassium chloride and ammonium phosphate. The useful microbes may include Trichoderma,

Pseudo onas, Tricoderma, Bacillus, Brevibacillus , Streptomyces, Azotobacter, Tricothecium and VA mycorrhizae. Since the compact shoots of this invention has a dormancy period shorter than that of conventional seed potatoes and can be grown to plug seedlings, the requirement for initial weeding to largely demand man power in the conventional cultivation becomes significantly lowered and the injury owing to the first frost of the season can be avoided. According to the present method for propagating potatoes, the seed potatoes as well as marketable potatoes can be produced. The following specific examples are intended to be illustrative of the invention and should not be construed as limiting the scope of the invention as defined by appended claims .

EXAMPLE I: Induction of Compact Shoots A lateral bud and axillary bud of potato stalk normally grown maintain generally dormancy with no exterior stimuli such as removal of terminal bud. However, exterior stimuli including treatment of plant growth regulators, adjustment of photoperiod and infection of pathogens sometimes generate aerial tuber with restricted internodal growth. According the present invention, various treatments were individually or concurrently performed to induce aerial tubers artificially and homogeneously, so that the internodal growth is artificially controlled to induce compact shoots. The present invention is represented as follows : In the following Examples, the potatoes used are Atlantic and Superior cultivars widely cultivated over the world. (a) Treatment of Plant Growth Regulators Potato plants prosperously grown before or after a blooming stage were treated for 2-4 weeks at an interval of 0.5-1 week with a plant growth regulator including Ethephon, Ethrel, Abscisic acid, Phosphon D, Amo-1618 and B-905 (B-905 (N- dimethyl-amino succinamic acid) in the manner of foliar spray, so that the induction of compact shoots was promoted. In this experiment, Ethephon and Ethrel were 100 ppm solution, Abscisic acid was 10 ppm solution, Phosphon-D was 100 ppm solution, Amo- 1618 was 100 ppm solution and B-905 was 50 ppm solution. At this time, the spraying interval and concentration of the regulator were controlled according to the response of plants . For example, if the response of plants is shown to exhibit a significant growth inhibition, the spraying interval and concentration of the regulator are adjusted to 1 week and 1/2- 1/5 of standard value described above, respectively. TABLE I Production Efficiency of Compact Shoot by Treatment of Plant Growth Regulator Plant growth Production efficiency of compact shoot depending regulator on treatment concentration

The results indicated in Table I were obtained from 30 plants treated with each plant growth regulator. The treatment was carried out for 4 weeks at an interval of 1 week and the harvest was performed 6 weeks after the final treatment . All procedures of cultivation were performed in a greenhouse to take refuge from the rain. As shown in Table I, the production efficiency of compact shoots in 100 ppm Ethephon treatment group was revealed 131+21.5 compact shoots/potato, which is the largest value in all treatment groups .

(b) Treatment of Trehalose Trehalose, one of storage carbohydrates in potatoes is a disaccharide accumulated in plant when growth conditions become worsen. Potatoes were treated for 2-4 weeks at an interval of 0.5-1 week with 50 mM trehalose in the manner of foliar spray, so that the generation of compact shoots was promoted. At this time, the spraying interval and concentration of trehalose were controlled according to the response of plants. For example, if the response of plants is delayed, the spraying interval and concentration of the regulator are adjusted to 0.5 week and 2-5 fold higher than that of standard value . TABLE II Production Efficiency of Compact Shoot by Cotreatment of Plant Growth Regulator and 50 M Trehalose

The results indicated in Table II were obtained from 30 plants treated with each plant growth regulator. The treatment was carried out for 4 weeks at an interval of 1 week and the harvest was performed 6 weeks after the final treatment . All procedures of cultivation were performed in a greenhouse to take refuge from the rain. As understood in Table II, the production efficiency of compact shoots in 100 ppm Ethephon and 50 mM treahalose cotreatment group was revealed 164±27.8 compact shoots/potato, showing elevated pattern compared to the group with single treatment of plant growth regulator (production increase rate 25%) .

(c) Adjustment of Photoperiod and Irrigation Water Although the photoperiod longer than 12 hr is preferred for prosperous growth of potatoes, it was revealed that the photoperiod shorter than 8 hr could promote the induction of compact shoots. The photoperiod of 12-16 hr was maintained before and after a blooming stage and then the photoperiod shorter than 8 hr or 7-day dark condition was adopted concomitantly with the treatment of plant growth regulator, so that the formation of compact shoots was maximally induced. TABLE III Production Efficiency of Compact Shoot Depending on Photoperiod/Treat ent Period

The results indicated in Table III were obtained from 30 plants treated with each plant growth regulator. The treatment was carried out for 4 weeks at an interval of 1 week and the harvest was performed 6 weeks after the final treatment. All procedures of cultivation were performed in a greenhouse to take refuge from the rain and the shading was performed using silver foil. As indicated in Table III, when the photoperiod of 8 hr was maintained for 4 weeks, the production efficiency of compact shoots in 100 ppm Ethephon treatment group exhibited 143±20.1 compact shoots/potato, showing elevated pattern compared to the group with single treatment of plant growth regulator (production increase rate 9%) . When the dark condition was maintained for 1 week, 139±29.6 compact shoots/potato were produced, which is similar production efficiency to that of the photoperiod of 8 hr for 4 weeks. Therefore, it could be appreciated that the method to maintain dark condition for 1 week is preferred in view of work convenience . The suitable irrigation for induction of compact shoots during the treatment of plant growth regulator was revealed that the irrigation was performed to reach toward wilting point as close as possible unless the growth of compact shoots is inhibited. It could be understood that these results are due to increase of stress associated with water shortage. After the formation of compact shoots, wetting condition was maintained to meet the requirement, of water for growth of compact shoots, so that compact shoots with larger size and improved nature could be harvested. In hydroponic culture, osmotic pressure of culture solution was controlled using sodium chloride. Sodium chloride was added in the concentration of 0.3-3%, preferably about 1.5%. TABLE IV Production Efficiency of Compact Shoot Depending on Irrigation Method During Treatment of Plant Growth Regulator

The results indicated in Table IV were obtained from 30 plants treated with each plant growth regulator. The treatment was carried out for 4 weeks at an interval of 1 week and the harvest was performed 6 weeks after the final treatment . All procedures of cultivation were performed in a greenhouse to take refuge from the rain and the shading was performed using silver foil. The conventional hydroponic culture was carried out using lOOOx solution of Multifeed-69™ available from Korea Horticulture Materials Co., Ltd. As indicated in Table IV, when the restricted irrigation was performed, the production of compact shoots in 100 ppm Ethephon treatment group exhibited 147±29.1 compact shoots/potato, showing improved production pattern compared to the group with single treatment of plant growth regulator in soil culture and the hydroponic culture group (production increase rate 12%) .

(d) Inhibition of Tuberization It was revealed that the prevention of transfer of carbon assimilates to underground tuber in potatoes could increase the mean weight and harvest quantity of compact shoots per plant . To this end, potato plants were planted on solid material through which their roots cannot pass, for soil culture or culture using a culture bed. After roots showed a taking root appearance, surface soil was removed to inhibit the formation of stolon and to make it easy to remove stolon generated. In hydroponic culture, after roots were taken, the plant was taken up to an extent that the upper part of roots was exposed to the outside of hydroponic culture bed, so that the formation of stolon was inhibited and stolons generated were rendered to remove . While the compact shoots induced according to Examples described above showed appearance very similar to underground tuber, leaves were attached to most of the compact shoots and their terminal buds are under non-dormancy or slight dormancy (see Figs. 1-3) . The left side of Fig. 1 represents conventional tuber-shaped seed potato and the right side shows the compact shoot of the present invention. Interior tissues of compact shoots show less accumulated starch than those of underground tubers. Since interior tissues of compact shoots exhibit dark green, it could be realized that they have significantly high level of alkaloid; therefore the compact shoots exhibit much higher long-term storage stability.

EXAMPLE II: Mass Production of Compact Shoots For the mass production of compact shoots, the method of this invention for inducing compact shoot was applied to a multitude of conventional methods for producing seed potato. However, it was elucidated that the planting density for efficient production of compact shoots was above 5-fold higher than that of conventional soil culture or hydroponic culture for tuberization (see Fig. 4) . This result is ascribed to the promotion of compact shoot production owing to (i) the prevention of tuberization by dense cultivation and (ii) the competition between plants by dense contact .

(a) Mass production of Compact Shoot in Soil Culture The field for seeding of compact shoots was plowed in a depth of about 20 cm and the furrows were immediately made followed by seeding, so that drought injury was prevented. The spraying of compost was performed in the manner of broadcast application before or after plowing. The spraying of fertilizer was carried out in the manner of broadcast or furrow application, preferably, furrow application. For the spring cultivation, a single and double row planting may be adopted and a double row planting was advantageous in view of soil temperature and water content management . The width of furrows generally was 60-75 cm. When a double row planting was performed, the width of furrows was preferably 40-50 cm and the length of planting was preferably 20-30 cm. For the summer cultivation, the single row planting was preferable and the width of furrows and length of planting were the same as those of the spring cultivation. Seeding was shallowly performed when the temperature was low and humidity was high, but seeding was deeply carried out when the temperature was high and humidity was low. In the single row planting, the heaping soil around a plant was performed 2-3 times. The heaping soil of the first time was done when potato eye was grown to 15-20 cm and the second time heaping was done 15-20 days after the first time heapin . The selection of harvest time was determined with reference to maturation time of potatoes . The suitable harvest time varies depending on variety, cultivation environment and seeding time. The early harvest results in low level of starch, high level of reducing sugar and weaker peel . The late harvest is responsible for corruption susceptibility. Therefore, the suitable harvest time is important. When potatoes are maturated, tubers are firmly ripened and cauline leaves undergo withering.

This time when such phenomena were shown was selected as harvest time. Where aerial parts remained, they were subject to withering by use of withering agent for cauline leaves and then harvest was carried out . Such operation contributed to decrease in pathogenesis at harvest time. The temperature on the harvest day should be above 10°C. If the temperature becomes lower, the sugar content becomes higher and the cold injury becomes more dangerous . In summer, potatoes harvested were stacked for storage in a place to be free of the admission of wind or putted into vessels for storage. In winter, potatoes were stored with the prevention of lowering the temperature to below 0^°C. During storage, the weight of potatoes becomes decreased because of respiration, transpiration and use of carbohydrates. In particular, the storage temperature is compelled to increase for maintaining the level of chemical components in stored potatoes. For potatoes for process, since the storage temperature is generally 10^°C and therefore the weight loss of potatoes should be carefully prevented. In addition, spontaneous budding becomes problematic during storage. For spontaneous budding may be inhibited by use of budding inhibitor. In the case of low-temperature storage, potatoes are also susceptible to the infection of pathogens and therefore the careful management is required. In storage, pathogens invading potatoes are likely to rapidly spread out other normal potatoes. Therefore, the cares to prevent cold injury and rapidly remove decayed potatoes should be taken. (b) Mass production of Compact Shoot in Hydroponic Culture Seedlings for tissue culture, microtubers or seedlings for shoot cutting of potatoes were planted into pores with 2.5 cm radius over a planting plate of styrene foam. The interior temperature of greenhouse was managed to become below 28°C in the daytime and above 13 °C in the nighttime. As a nutrient solution for culture, 100Ox solution of Multifeed-69™ available from Korea Horticulture Materials Co., Ltd was employed. The concentration of nutrient solution (EC) refers to the total concentration of various mineral salts contained in ^"nutrient solution, affecting absorption of mineral ingredients. In particular, the concentration is related to the absorption ratio of water and mineral ions dissolved in water and therefore the suitable concentration should be selected for accomplishing a continual absorption of water and mineral ions at a constant ratio. Generally, in the high-temperature incubator, transpiration vigorously occurs to increase water requirement; therefore it is preferred that the EC is maintained to have relative low value for sufficient water absorption. In the low-temperature incubator, the water requirement is relatively low; therefore it is preferred that the EC is maintained to have relative high value for 'inhibiting excess flourish of plants due to water integration into plant. The EC was adjusted to 1-2 mS/cm during cultivation. If the initial EC is controlled to below 1 mS/cm or above 2 mS/cm, the fluctuation of concentration is excessive. The suitable pH of nutrient solution ranges from 5.5-6.5. If pH becomes above 7, iron, manganese and phosphorous become insoluble and then unavailable to plants. If pH becomes below 4, the deficiency with regard to calcium, potassium and magnesium may occur. In practical cultivation, pH of nutrient solution initially becomes decreased due to absorption of NH₄-N and then slowly increased due to absorption of N0₃-N after depletion of NH₄-N. In the case that pH is increased, acid such as sulfuric acid, phosphoric acid and nitric acid, or NH₄-N fertilizer such as ammonium nitrate (NH₄N0₃) ammonium phosphate (NH₄H₂P0₄) and ammonium sulfate ((NH₄)₂S0₄) is employed to decrease pH. The temperature of nutrient solution was adjusted to 15- 25°C with heating or cooling. In winter cultivation, the electric heating bar was used for heating, and the pipeline circulating cool water was used for cooling in summer cultivation. In winter cultivation, the change of temperature of nutrient solution due to the alteration between day and night in surrounding temperature is responsible largely for plant growth. The potato plantlet cultured was taken up for inhibiting tuberization inside hydroponic culture bed and facilitating removal of tubers formed during cultivation. Where the temperature inside greenhouse is above 25°C or the photoperiod is longer than 14 hr, the stolons was developed not to tubers but to stalk, so that the loss of carbon assimilates associated with the formation of tubers may be prevented to increase the production quantity of compact shoots . The treatments of plant growth regulator and trehalose and adjustment of photoperiod for inducing compact shoots are substantially same as those of soil cultivation. For imposing water stress, the restricted irrigation was performed using 0.3-3%, preferably 1.5% sodium chloride (NaCl) in nutrient solution. EXAMPLE III: Storage and Processing of Compact Shoots Harvested It was revealed that the harvest of compact shoots was preferably performed at the time that lignification of potato stalk is in progress and leaves undergo etiolation, so that the accumulation of carbon assimilates does not occur. If necessary, the harvest time may be adjusted. The earlier harvest of compact shoots not completely hardened resulted in the decrease in the activity of compact shoots due to water loss after harvest . The weight of compact shoots harvested ranges from 0.5-50 g. The compact shoots were classified to 5 grades and stored. The compact shoots harvested were treated with Disen hydrating agent or captane powder and stored at 8-10°C. As a result, the compact shoots of this invention could be stored for above 6 months with no lowering of activity.

EXAMPLE IV: Production of Seed Potatoes and Marketable Potatoes Using Compact Shoots The seeding of compact shoots and primary cultivation management were performed according to the conventional potato cultivation method. Since the compact shoots of which dormancy period are shorter than that of conventional seed potatoes could be grown to plug seedlings, the requirement for initial weeding to largely demand man power in the conventional cultivation became significantly lowered and the injury owing to the first frost of the season could be avoided. Using compact shoots, the seed potatoes as well as marketable potatoes could be produced.

(a) Production of Next-Generation Seed Potatoes Using Compact Shoots The compact shoots were compactly planted at an interval of 10-25 cm by use of seedling plate of 90-120 cm width with no furrows (see Fig. 4). Seeding and planting of compact shoots were performed in accordance with conventional potato cultivation. The field for seeding of compact shoots was plowed in a depth of about 20 cm and the furrows were immediately made followed by seeding. If the temperature is low and the humidity is high, the seeding was shallowly performed; however, if the temperature is high and the humidity is low, the seedling was deeply performed. The compact shoots planted initiated to take roots and growth (see Fig. 5) . From the initial stage of growth, the compact shoots generated a multitude of tubers (see Fig. 6) . A nitrogenous fertilizer was applied as preplane fertilization with no exceeding 30 kg/lOa. If necessary, the fertilizer was additionally supplied in the manner of foliar spray. For harvesting more seed potatoes, the heaping soil around a plant was performed 2-3 times after seeding. The heaping soil of the first time was done when potato eye was grown to 15-20 cm and the second time heaping was done 15-20 days after the first time heaping. Where the heaping soil is troublesome in dense cultivation, light surface soil may be applied using fertilizer spray for heaping soil . (b) Production of Marketable Potatoes Using Compact Shoots The planting of compact shoots was performed with the planting density of conventional cultivation. For early harvest, the dense cultivation of 125% was possible (see Figs. 7-9). The application of fertilizer and management of cultivation were performed in accordance with those of conventional method. After harvest, the mini-sized potatoes without marketability were reused as a seed potato for next cultivation unless they were infected with pathogens . Having described a preferred embodiment of the present invention, it is to be understood that variants and modifications thereof falling within the spirit of the invention may become apparent to those skilled in this art, and the scope of this invention is to be determined by appended claims and their equivalents .

Claims

What is claimed is:

1. A method for inducing a compact shoot, which comprises cultivating a potato plant concomitantly with treating said potato plant with a plant growth inhibitor to stimulate lateral buds or axillary buds of said potato, whereby said compact shoot as a propagule is generated and serves as a vegetative tissue or organ.

2. The method according to claim 1, wherein said plant growth inhibitor is at least one selected from the group consisting of ethephon, ethrel, abscisic acid, phosphon D, Amo-1618, B-905 (N-dimethyl-amino succinamic acid), ancymidol, carbaryl, chlormequat, chloro IPC, daminozide, flurprimidol, hydrogen cyanamide, maleic hydrazide, mefluidide, mepiquat chloride, paclobutrozol, prohexadione calcium and uniconazole.

3. The method according to claim 1, wherein said cultivating is performed under a photoperiod of 0-8 hr.

4. The method according to claim 1, wherein said method further comprises the step of treating said potato plant with trehalose .

5. The method according to claim 1, wherein said cultivating is performed with preventing the formation of underground tubers by inhibiting the transfer of carbon assimilates in said potato to underground tubers .

6. The method according to claim 1, wherein said plant growth inhibitor is a solution of 1-1,000 ppm.

7. The method according to claim 4, wherein said trehalose is a 10-100 mM solution.

8. A method for preparing a compact shoot, comprising harvesting the compact shoot induced by the method according to any one of claims 1-7.

9. The method according to claim 8, wherein said harvesting is performed at the time that a lignification of potato stalks and etiolation of leaves occurs.

10. A compact shoot of potato prepared by the method according to claim 8 or 9.

11. A method for propagating potatoes, which comprises the steps of : (a) planting the compact shoot of potato of claim 10; and (b) cultivating said compact shoot planted.

12. A potato prepared by the method according to claim 11.