Yi-Chich Chiu1*, Suming Chen2, Yung-Chiung Chang3, Li-John Chou4
1Department of Biomechatronic Engineering, National Ilan University,
1, Sec.1, Shen-Lung Rd., Yi-Lan, 26047, Taiwan
2Department of Bio-Industrial Mechatronics Engineering,National Taiwan University,
1, Sec. 4, Roosevelt Rd., Taipei, 10617, Taiwan
3Department of Horticulture, National Ilan University,
4Department of Biomechatronic Engineering, National Ilan University,
The objectives of this research are to develop the grafting robots and an acclimatization chamber for watermelon, and tomato seedlings. For watermelon seedlings, a top plug-in robotic grafting system has been developed, which is applicable for grafting a scion into a mature rootstock before the scion’s cotyledons spread. It thus makes the scion to be very tender and greatly different from the rootstock at the seedling stage. Experimental results show that the grafting robotic system can accurately finish all grafting procedures and operations with an average success rate of 95%, and a working capability of 480 seedlings per hour.
For tomato seedlings, a tubing-type grafting robot has been developed by using soft rubber tubes as the grafting material. This robotic grafting system is suitable for rootstock and scion of fruit-bearing vegetables that have the similar stem diameter. The experimental results indicate that the average grafting success rate is 95.5% with working capability of 327 seedlings per hour. To provide a suitable curing environment for seedlings after grafting, an acclimatization chamber, with a dimension of 540 x 265 x 210 cm, has also been developed. The chamber accommodates 12,960 seedlings if 72-cell trays were used. The experimental results indicate that the survival rate of grafted seedlings kept in the chamber is more than 96%. Seedlings which were acclimatized in the chamber after grafting showed a higher survival rate than those acclimatized under protected cover in the open field.
Keywords: Grafting, Seedlings, Robot, Acclimatization
Grafting means that the crops (i.e. the scions) to be cultivated are grafted onto strong vitality plants (i.e. the rootstocks), where scions can grow, blossom and bear fruit due to soil-based nutrients. As a well-proven seedling technology, grafting features strong resistance to diseases from soil and good adaptive capability to unfavorable growing environment, as well as improved product quality and higher output. Therefore, grafted seedlings have become a popular trend in Taiwan for cultivation of fruits and vegetables in greenhouses or in the fields. The crops using grafted seedlings include watermelon, cucumber, muskmelon, tomato, bitter gourd, sweet pepper, papaya, and passion fruit. The number and categories keep rising. For this reason, many grafting farms are set up and dedicated to offer professional grafting and seedling services to the farmers. However, since grafting is a time-consuming job even for skilled workers, there is an important trend that is slowly started to gain popularity. As an alternative to manual grafting, grafting robots are now being developed and seen as having good and growing market potentials.
Post-grafting curing operations significantly affect the survival rate of grafted seedlings. An improper curing environment will cause the seedlings to languish, over grow, or lead to their eventual death. In most post-grafting curing operations, however, the seedlings are moved to the under protected cover in the field, while the climate affects the inner environment violently. The worker must regulate the inner environment by any probability method such as venting, covering, shading, shielding, etc. when the weather changes. Consequently, this work is time intensive, and requires careful and skillful attention. Therefore, it is necessary to develop an acclimatization chamber to provide a suitable curing environment for seedlings after grafting.
As grafting needs skillful, intensive and seasonal labor, developing an automatic grafting machine to produce high quality grafted seedlings at a lower cost is necessary. Many types of grafting machines were developed in Japan and Korea, and some of them would be available for commercial use very soon (Hwang et al., 1997; Ken et al., 1996; Ken and Masato, 1996; Nishiura et al., 1995; Kurata, 1994).
Japan has been actively engaged in the development of grafting techniques because of the wide use of grafted seedlings. Grafting robotic systems developed by the Biology Research Organization (Kobayashi and Suzuki, 1996; Kobayashi et al. 1996) can do 1,200 grafts per hour and obtain the success rate of 98% for grafting. Nishiura et al. (1995) developed a system of robotic grafting systems using the plug-in method. The system can reduce the mismatch of vascular bundles during grafting, and thus can fasten the healing of grafted seedlings and make them more vigorous. In Korea, Hwang et al. (1997) developed a grafting robotics for fruit-bearing vegetables based on inarching grafting. In Taiwan, Lee et al. (2001) developed automatic graft robotic systems for grafting Passion Melon. It can graft 114 seedlings per hour, and has a grafting success rate of 70% and survival rate of 95%. As for the commercialization of grafting robotic systems, three companies including YANMAR, ISEKI and MITSUBISHI, assisted by the Biology Research Organization, have developed a grafting robotic system respectively in Japan, which can do automatic grafting for cucurbits and solanaceous plants, with operation capability of 550~1000 seedlings per hour and a success rate of 95%. Most robotic graft systems mentioned above employ graft clips to join and fix rootstocks and scions. This will cause a high cost and require extra labor to recycle the graft clips for reuse before the grafted seedlings grow into stable plants.
The objectives of this study are to develop two types of grafting robots for watermelon and tomato seedlings, respectively. Also the acclimatization chambers are aimed to develop and provide the curing environments for grafted seedlings.
1. Top plug-in grafting robotic system for watermelon seedlings
In Taiwan, the scions of the watermelon seedlings are immediately grafted before their cotyledons spreads, so there is a great difference between the seedling age of rootstocks and scions. In summer, the proper time for the rootstocks to graft is about 12 days after sowing; it takes 18-20 days to the same during winter. On the other hand, in summer, scion seedlings need only four days after sowing to graft; in winter, it takes about seven days. Thus, this research aims to develop an automatic grafting robotic system applicable for seedlings of watermelon. It adopts the top plug-in grafting and is characterized by the nonuse of graft clips, so it is applicable for grafting scions immediately onto mature rootstocks before the cotyledons of scions spread. It thus allows very tender scions, (which is greatly different from the rootstocks in the seedling age) to be grafted onto rootstocks.
(2). Design of the top plug-in grafting robotic system
The top plug-in grafting robotic system for watermelon seedlings consists of a rootstock and scion processing units. The rootstock processing unit is in charge of removing/pinching the leaf bud from the rootstock and drilling a tiny hole in the center of the rootstock’s stem, so as to facilitate the plug-in of the scion. Both the rootstock and scion processing units have a rotating disk, while the disk of the rootstock processing unit is lower than that of the scion processing unit with two disks rotating oppositely. The Geneva Wheel intermittent motion mechanism is adopted to drive the disks to do simultaneous and opposite movements. While the scion processing unit cuts off the bottom of the scion, leaving the scion an oblique cut, and by plugging the beveling hypocotyl into the hole of the rootstock, the grafting operation is finished. The robot is designed to be semiautomatic, which utilizes manual work to place a rootstock and a scion separately into the positioning grippers. After the robot automatically grafts (the scion into the rootstock), it again relies on manual work to take out the grafted seedlings. The operation procedures are shown in Fig. 1.
Fig. 1. The operation flowchart of the robotic grafting system.
(3). Improvement on the strength and erectness of scions
Farmers in Taiwan cultivate watermelon scion seedlings using the following steps: The seeds are sown onto the soil of fluvial sand and covered with black shading net. The scions are pulled up for collection 4~7 days after sowing, and are grafted after washing with water. By the time, the scion seedlings turn slender and tender like bean sprouts (Fig. 2), which, during the mechanical cutting operation, are always tender and curved, making it difficult to insert into the open of the rootstock. To overcome the problem, some experiments have been conducted to improve the strength and erectness of scions through the lighting method. During the germination period of scion seeds, the improvement of strength and erectness of scions is within 6 days of lighting time in growing chamber of day/night temperature 30/28℃, through which scions with short and strong hypocotyledonary axis and of excellent erectness can be obtained for mechanical grafting. When compared to scions which are prepared by traditional cultivation, lighting treated scion seedlings are shorter and are more erect for mechanical grafting (Fig. 2 and Fig. 3). .
2. Tubing-grafting robotic system for tomato seedlings
Most robotic grafting systems employ graft clips to join and fix rootstocks and scions. They are high cost and require extra labor to recycle the graft clips before the grafted seedlings grow into stable plants. The developed top plug-in robotic grafting system for watermelon seedlings here requires lower cost for it has no need of clips. However, this system is not suitable for crops whose rootstock and scion seedlings have a small variation in ages, such as tomatoes, sweet pepper, bitter gourd and muskmelon. Thus, another research objective aims to develop a new type of grafting robotic system using rubber soft tube as the grafting materials and it is suitable for fruit-bearing vegetables. Those rootstock and scion seedlings have a small variation in stem diameters, such as tomato, sweet pepper, bitter gourd and muskmelon. This research used tomato as the grafted crop and eggplant as the rootstock. The rubber soft tubes, with elasticity and expansibility, can be used to enclose and fix the grafting union of rootstocks and scions. Compared with the grafting approach with grafting clips, soft rubber tubes are easy to obtain, complete with specification and are characterized by low price and the excellent quality of preserving water of grafting wounds. Not only can it reduce the production cost of grafted seedlings, it can also improve their survival rate. Moreover, with the growth of grafted seedlings, soft rubber tubes can automatically peel off from seedlings due to embrittlement, which saves the recycling process. Figure 4 illustrates the grafting method using graft clips and soft rubber tubes to fix the rootstock and the scion.
Fig. 4. Sketch of grafting operation.
(2). Design of the robotic tubing-grafting system
The robotic tubing-grafting system is composed of six major units: the chucking and fetching rootstocks, the cutting rootstocks, the chucking and fetching scions, the cutting scions, the supplying and cutting rubber tubes, and the guiding tubing. It is programmatically controlled by programmable logic controllers cooperated with pneumatic driving mechanisms. The whole tube grafting process can be divided into four stages: the cutting of rubber tubes, the cutting of rootstocks, the cutting of scions, and the tube grafting. The overall flowchart is depicted in Fig. 5. The rootstocks and the scions processing are controlled by the switch of the pedals respectively. Both the rootstocks and the scions processing can work independently. When the operator finishes placing seedlings and pads the pedal, the robotic system will cut rootstock and scion respectively, and will then insert the scion and the rootstock into the rubber tube sequentially. When the tube grafting is finished, the grafted seedling is then manually moved out, and the next rootstock and scion are put in.
Fig. 5. The flowchart of robotic tubing-grafting grafting process.
3. Acclimatization chamber for grafted seedlings
Present post-grafting curing operations: Seedlings are moved to the under protected cover in the field, where climate violently affects the curing environment; And it depends on the weather conditions, farmers must regulate the indoor environment by venting, covering, shading, and etc.
(2). Design of acclimatization chamber
The dimensions of the acclimatization chamber are cm with a 5 cm-thick insulation and 185 cm inner height. The chamber accommodates nine culture-carts. Each cart consists of five layers with 4 trays in each layer. The chamber has a capacity of 12,960 seedlings if 72-cell trays were used. A 40 W straight plant-lamp is mounted on the top of each layer in the cart to maintain constant spatial illumination in the chamber. The uniform air circulation and distribution in the chamber is regulated by three rows of 10-cm fans (5 fans each row), and a stainless steel net is used to form a laminar flow. The plant-lamps in the chamber were controlled by a 24-hourtimer. The heat produced by plant-lamps was followed by the assigned temperature-humidity control strategies. Ventilation openings installed on both sides of the chamber were controlled by timer to periodically accommodate CO2 concentration. If the outside temperature and relative humidity (RH) were suitable for curing of grafted seedlings, ambient air was introduced to regulate the chamber environment to save energy. We used ultrasonic humidifiers with the total capacity of 9 L h-1 to humidify the incoming air to prevent ulceration of the coalescent part of the grafted seedlings from fog. A 2 KW heater was used to warm or dehumidify indoor environment. An air conditioner (20.0 MJ h-1) was equipped to provide a cooler environment. Figure 6 and Figure 7 show the outward and cross section of designed acclimatization chamber.
The acclimatization chamber was equipped with an environmental controller deployed by linking Boolean functions and a psychrometric chart to regulate indoor environment. The Boolean functions were transformed to logical circuits and subsequently transformed to a programmable logical controller (PLC) program systematically.
Fig. 6. Exterior view of acclimatization chamber.
Fig. 7. Cross section of an acclimatization chamber for grafted seedlings.
(1). Development of the top plug-in robotic grafting system
A prototype of the circular robotic grafting system for melon seedlings with a dimension of 120 x 105 x 130 cm is shown in Fig. 8. The grafting robot has two operators: one is responsible for supplying rootstock and scion seedlings, and for the other placing the grafted seedling back to the tray after the grafting is complete. Figure 9 demonstrates a close-up grafting using the robotic system.
This research employs bottle gourd “Chiang-Li #1” as rootstock and watermelon “Fu-Bao #2” as scion to conduct mechanical grafting experiments. Experimental results show that the robotic grafting system can accurately finish all grafting procedures and operations, with an average success rate of 95% and a working capability of 480 seedlings per hour, which is not significantly different from the survival rate with manual grafting of about 95%. However, based on observations, the tightness between the rootstocks and the scions was better after manual grafting than after mechanical grafting, and the healing time of the manually grafted seedlings was shorter than that of the mechanically grafted seedlings. This is because, with manual grafting, the plug-in grafting can be adjusted according to the thickness of the rootstock and the scion, which creates a close fit. However, during mechanical grafting, the drilled hole is always the same size and is not adjusted to the thickness of the rootstock; this can cause a poor fit between the rootstock and the scion. In this situation, if the grafted seedling is good and the healing environment is also good, then the grafted seedling will require a long healing period to obtain a good overall survival rate. Demonstrations were held and farmers were highly satisfied with the system's functions and its performance.
(2). Development of the robotic tubing-grafting system
A prototype of the tubing-grafting robotic system is shown in Fig. 10. The whole system, 70 x 50 x 114 cm, is made of stainless steel with an aluminum frame. There are seedling plates that can swing on the two sides of the main body of the robot: the left side has two stories, with the plate area of 60 x 30 cm, which can be used for placing rootstock and grafted seedlings; the right side has a seedling plate of one story, on which scion seedlings can be placed. The whole grafting process can be operated by just one single operator.
This research used tomato as the grafted crop, and eggplant as the rootstock. As discovered through the measured experimental data, the time to manually place and put grafted seedlings was about 3.5 s, and hereby 327 seedlings can be grafted per hour by the grafting robotic system, which is slightly higher than the skilled worker’s doing. As indicated by managers of nurseries, it takes a long time to train a skilled worker, and there is risk cost of failed grafting at the early stage of workers’ training. However, by referring to the instructions of the robotic grafting system, it only takes about minutes for operators to master the whole operation, as long as they are familiar with the key issues and techniques of operating the robotic system. Therefore, it is of commercial value to develop such automatic grafting robotic system. In order to apply to more crops, the module units have also been designed to be applied to sweet melon, bitter gourd, and sweet pepper. A demonstration exhibition has been held and the farmers said highly of its operational refinement and performance. Furthermore, this robotic tubing-grafting system, in an operation manner, is similar to the manual grafting adopted by most farmers nowadays. Therefore, it will be highly acceptable to promote and apply this robotic system in the future. Figure 11 presents the grafted seedlings by mechanical grafting.
(3). Development of the acclimatization chamber
The set points for temperature ranges of free cooling in summer were 26 – 34°C, whereas the set points for RH ranges of free cooling were 80 – 90% in rainy days. The control target range of the temperature-humidity environment in the chamber was set at 26 – 28°C and 80 – 90 %. Figure 12 illustrates the indoor temperature and RH environments were regulated at 27.3 – 27.6°C and 83.1 – 87.2%, respectively. When the initial temperature and RH in the chamber were 29.1°C and 79.5%, the air conditioner associated with the humidifiers was automatically operated to regulate the indoor temperature and RH until the indoor temperature was lower than the upper-limit temperature of 28°C. The plant-lamps were switched on after 19:00 hr, the free cooling associated with the humidifiers was firstly performed to decrease the indoor surplus heat and to keep the indoor temperature and RH at 27.3 – 27.6°C and 83.1 – 87.2%, respectively. Since the time period of without illumination in the chamber was regulated from 07:00 to 19:00 hr, if the outdoor temperature and RH were suitable for modulating the indoor temperature and RH of the control target, the free cooling associated with the humidifiers was firstly engaged. Otherwise, the controller would close the vents to form a closed system, and the other suitable operations were performed in accordance with indoor states and psychrometric processes settings.
The acclimatization chamber developed reach to the goal of providing suitable curing conditions for grafted seedlings. The temperature variations within the chamber is about 1℃, RH variations is about 3.6%, and wind speed is lower than 0.21m/s. Regarding environment control, if the outside weather conditions reach the setting range, both the ventilation windows and circulation fans are activated and the ambient air is directed to regulate the chamber environment. Meanwhile, the air conditioner and humidifier are forced to stop if they are in operation. The outside weather regulation method can effectively save the operating electricity. The acclimatization experiments compared survival rates of grafted seedlings kept in the acclimatization chamber and protected field post grafting. Experimental results indicated the survival rate is 97.5% for seedlings kept in the chamber for three days after grafting, which is higher than that of the controlled seedlings, 95.7%, kept under protected cover after grafting. A higher survival rate is shown for the seedlings acclimatized in the chamber after grafting as compared to those grafted seedlings which were acclimatized under the protected cover in the field. Figure 13 shows the grafted seedlings on the culture-carts in the acclimatization.
Fig. 12. The indoor temperature and RH environments were regulated at 27.3 – 27.6°C and 83.1 – 87.2%.
Fig. 13. Grafted seedlings on the culture-carts of the acclimatization chamber.
The top plug-in grafting robot is applicable for grafting a scion onto a mature rootstock before the scion’s cotyledons spread. It thus makes the scion to be very tender and greatly different from the rootstock at seeding stage. The developed top plug-in grafting does not require the use of grafting clips, which becomes an advantage of this machine.
The robotic tubing-grafting system uses rubber tubes as grafting material, and achieve good grafting joint between the cutting surfaces of rootstocks and scions. The rubber tubes, also acting as protective tubes for cutting surfaces, can effectively protect the wounds and avoid the loss of water at the grafting joint. Therefore, it can effectively facilitate the healing of grafting wounds.
The acclimatization chamber developed can satisfactorily achieve the expected goal of providing suitable curing conditions for grafted seedlings. The temperature variations within the chamber is about 1℃, RH variations is about 3.6%, and wind speed is lower than 0.21m/s. The survival ratio for seedlings kept in the acclimatization chamber for three days after grafting are better than those seedlings for conventional culture in the under protected cover in the field. The operating cost due to electricity is US$0.27 per day per 1,000 seedlings. It has the potential to scale up a large acclimatization chamber for commercial applications.
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