Damage from Continuous Cropping
Continuous cropping is inevitable in greenhouses, but this reduces the yield and quality of produce. Takahashi (1984) reported that 68% of cases of failure in Japanese vegetable production under continuous cropping were caused by soil-borne diseases and nematodes ( Table 1(0)). Since soil sterilization can never be complete, grafting has become an essential technique for the production of repeated crops of fruit-bearing vegetables grown in greenhouses.
Application of Grafting to Vegetable Production
The production of grafted plants first began in Japan and Korea in the late 1920s with watermelon ( Citrullus lanatus Matsum. et Nakai) grafted onto gourd rootstock (Lee 1994). Eggplant was grafted onto scarlet eggplant ( Solanum integrifolium Poir.) in the 1950s. Since then, the area of fruit-bearing vegetables based on grafted plants has increased. The proportion of the area in Japan producing grafted watermelon, cucumber, melon, tomato and eggplant reached 57% of the total production area in 1980, and 59% in 1990 (Oda 1993) ( Table 2(0)).
Objectives of Vegetable Grafting
The main objective of grafting is to avoid soil-borne diseases such as Fusarium wilt in Cucurbitaceae (Cucumber, melon etc.) and bacterial wilt in Solanaceae (tomato, pepper etc.). Details of the objectives of grafting for each vegetable crop are shown in Table 3(0).
Species and Varieties for Grafting
Inter-generic grafting is used in the production of many fruit-bearing vegetables, i.e. cucumber ( Cucumis sativus L.) grafted on pumpkin ( Cucurbita spp.), watermelon ( Citrullus lanatus Matsum. et Nakai), on bottle gourd ( Lagenaria siceraria Standl.), melon ( Cucumis melo L.) on white gourd (also known as wax gourd) ( Benicasa hispida Cogn.). Inter-specific grafting is generally applied to eggplant ( Solanum melongena L.). Scarlet eggplant ( S. integrifolium Poir.) and S. torvum Swartz are popular rootstock for eggplant production. A large number of varieties for rootstock ( Table 4(0)) have been bred and released for use by growers in Japan.
Grafting Methods for Different Types of Fruit-Bearing Vegetable
Tomato plants are mainly grafted by conventional cleft grafting. Tube grafting has recently been developed for vegetable seedlings grown by plug culture.
The seeds of the rootstock are sown five to seven days earlier than those of the scion ( Fig. 1(0)). The stem of the scion (at the fair-leaf stage), and the rootstock (at the four to five-leaf stage) are cut at right angles, each with 2-3 leaves remaining on the stem ( Fig. 2(0)). The stem of the scion is cut in a wedge, and the tapered end fitted into a cleft cut in the end of the rootstock. The graft is then held firm with a plastic clip.
Tube grafting makes it possible to graft small plants grown in plug trays two or three times faster than the conventional method. The smaller the plants, the more plants can be fitted into healing chambers or acclimation rooms. For this reason, tube grafting is popular among Japanese seedling producers.
The time schedule for tube grafting of tomato plugs is shown in Fig. 3(0). The optimum growth stage for grafting varies according to the kind of plug tray used. Plants in small cells must be grafted at an earlier growth stage, and require tubes with a smaller inside diameter.
First, the rootstock is cut at a slant. The scion is cut in the same way. Elastic tubes with a side-slit are placed onto the cut end of the rootstock. The cut ends of the scions are then inserted into the tube, splicing the cut surfaces of the scions and rootstock together ( Fig. 4(0)).
Eggplant is grafted mainly by cleft or tube grafting. The growth rate differs according to the species of rootstock used. The number of days from sowing to grafting varies accordingly.
Cleft grafting of eggplant is a similar process to that done for tomato. The time schedule for cleft grafting of eggplant is shown in Fig. 5(0).
The time schedule and grafting methods for tube grafting of cucumber are similar to those used for tomato plants. However, the seeds of S. torvum must be sown a few days earlier than those of the other rootstock species.
Tongue Approach Grafting
The survival ratio of grafted Cucurbitaceae plants is higher if tongue approach grafting is used. This is because the root of the scion remains until the formation of the graft union. In this method, seeds of cucumber are sown 10 - 13 days before grafting, and pumpkin seeds 7 - 10 days before grafting, to ensure uniformity in the diameter of the hypocotyls of the scion and rootstock ( Fig. 6(0)). The shoot apex of the rootstock is removed so that the shoot cannot grow. The hypocotyls of the scion and rootstock are cut in such a way that they tongue into each other ( Fig. 7(0)), and the graft is secured with a plastic clip. The hypocotyl of the scion is left to heal for 3 - 4 days and then crushed between the fingers. The hypocotyl is cut off with a razor blade three or four days after being crushed ( Fig. 7(0)).
Slant-cut grafting is easy to do, and has recently become popular. Stages at which the scions and rootstock should be grafted are the same as those shown in Fig. 6(0). This grafting method was developed for robotic grafting. It is important to remove the 1st leaf and lateral buds when a cotyledon of rootstock is cut on a slant ( Fig. 8(0)).
Melon plants are mainly grafted by tongue approach grafting. The time schedule for tongue approach grafting of melon plants is shown in Fig. 11(0). Tongue approach grafting for melon is similar to that used for cucumber plants, shown in Fig. 7(0).
Healing and Acclimatization
Grafting should be carried out in a shady place sheltered from the wind, to avoid wilting of the grafted plants.
Grafted plants are usually healed and acclimated in a plastic tunnel ( Fig. 12(0)). The healing and acclimatization are very important for grafted plants to survive. The tunnel is covered with materials which provide shade and maintain inside humidity: silver/white cheese-cloth (outside) and transparent film (inside). During acclimatization, it is recommended to keep light levels at about 3 to 5 klx.
- Expose the scion and rootstock to sunshine for two to three days;
- Withhold water from the plants to avoid spindly growth, and
- Make sure that the scions and rootstock have stems of a similar diameter (Oda et al. 1993).
All these will improve the survival rate of grafted plants. When grafting is performed, it is important to increase the chances for vascular bundles of the scion and rootstock to come into contact (Oda et al. 1994b), by maximizing the area of the cut surfaces that are spliced together, and by pressing the spliced cut surfaces together. The cut surfaces should not be allowed to dry out. After grafting, keeping the grafted plants at about 30Â°C and with more than 95% relative humidity for three days of healing promotes the survival ratio. Gradually, the relative humidity is then lowered and the light intensity increased. During healing and acclimatization, it is important to keep a constant air temperature in the tunnel, in order to maintain high humidity. If wilting is observed, foliar spraying of grafted plants with water is effective in helping them survive. The shading materials and films should be adjusted according to the daily weather, with more shade on a fine day.
Grafting is extremely laborious and time-consuming, and growers are trying to reduce the labor input required. Attempts have been made to mechanize grafting operations since 1987. Tube grafting was developed as a manual operation for small plugs by Itagi et al. (1990), and reduced the time required for manual grafting by at least one-half. Morita (1988) and Oda and Nakajima (1992) have applied an adhesive and a hardener to support the graft union in several crops. With the adhesive, five tomato plugs at a two-leaf stage were grafted at the same time, using grafting plates (Oda et al 1994a). Grafting robots for plugs have also been developed, by combining the adhesive and grafting plates (Kurata 1994, Oda 1995). This robot makes it possible for eight plugs of tomato, eggplant, or pepper to be grafted simultaneously. Robotic grafting is about ten times faster than conventional hand grafting. Tomato (Oda et al. 1995) and eggplant (Oda et al. 1997) grafted by robot produced a yield of fruit similar to that of plants grafted by conventional methods.
Healing has also been mechanized. The survival ratio is consistently high when the newly developed healing chambers are used. Healing chambers in which the environment is artificially controlled are now being used by many nurseries which produce grafted plugs.
As grafting operations and the healing of grafted plants become easier, grafted vegetable crops may become popular all over the world. Since plants gain disease tolerance and vigor by grafting, grafting of vegetables may be useful in the low-input, sustainable horticulture of the future.
- Itagi, T., K. Nakanisi and S. Nagashima. 1990. Studies on the production system of the grafted seedlings in fruit vegetables. 1. Methods of grafting, the kind of plug tray, conditions of acclimatization and the process during raising tomato plugs. Jour. Japan. Soc. Hort. Sci. 59, 1: 294-295. (In Japanese).
- Kurata, K. 1994. Cultivation of grafted vegetables. 2. Development of grafting robots in Japan. HortScience 29: 240-244.
- Lee, J.M. 1994. Cultivation of grafted vegetables. 1. Current status, grafting methods, and benefits. HortScience 29: 235-239.
- Morita, S. 1988. A new grafting method for fruit-bearing vegetables by the application of adhesives. Agriculture and Horticulture 63: 1190-1196. (In Japanese).
- Oda, M. 1993. Present state of vegetable production using grafted plants in Japan. Agr. Hort. 68:442-446. (In Japanese).
- Oda, M. 1995. New grafting method for fruit-bearing vegetables in Japan. Japan Agricultural Research Quarterly 29: 187-194.
- Oda, M, S. Akazawa, T. Mori and M. Sei. 1995. Growth and yield of tomato plants grafted using a grafting instrument. Bull. National Research Institute for Vegetables, Ornamental Plants and Tea A10: 33-38.
- Oda, M., M. Nagaoka, T. Mori and M. Sei. 1994. Simultaneous grafting of young tomato plants using grafting plates. Scientia Horticulture 58: 259-264.
- Oda, M. and T. Nakajima. 1992. Adhesive grafting of Chinese cabbage on turnip. HortScience 27: 1136.
- Oda, M., K. Okada, K. Sasaki, S. Akazawa and M. Sei. 1997. Growth and yield of eggplants grafted by a newly developed robot. HortScience 32: 848-849.
- Oda, M., K. Tsuji, K. Ichimura and H. Sasaki. 1994. Factors affecting the survival of cucumber plants grafted on pumpkin plants by horizontal grafting at the hypocotyl level. Bull. national Research Institute for Vegetables, Ornamental Plants and Tea 9: 51-60.
- Oda, M., K. Tsuji and H. Sasaki. 1993. Effects of hypocotyl morphology on survival rate and growth of cucumber seedlings grafted on Cucurbita spp. Japan Agricultural Research Quarterly 26: 259-263.
- Takahashi, K. 1984. Injury by continuous cropping in vegetables: various problems in the cultivation using grafted plants. Yasaishikenjo Kenkyu Shiryo 18: 87-89. (In Japanese).
(By K. Ito)
Fig. 7(0). Schematic diagram of tongue approach grafting for cucumber plants.
The scion hypocotyl is cut after a healing period. (By K. Ito).
Index of Images
Figure 1 Time Schedule of Cleft Grafting for Tomato Plants
Figure 2 Schematic Diagram of Cleft Grafting
Figure 3 Time Schedule for Tube Grafting of Tomato Plugs (128-Cell Tray).
Figure 4 Schematic Diagram of Tube Grafting for Tomato Plugs on Plug Tray
Figure 5 Time Schedule of Cleft Grafting for Tomato Plants
Figure 6 Time Schedule of Tongue Approach Grafting for Cucumber Plants
Figure 8 Slant-Cut Grafting for Cucurbit Plants (by K. Kobayashi)
Figure 9 Time Schedule of Cut Grafting for Watermelon
Figure 10 Schematic Diagram of Cut Grafting for Watermelon Plants.
Figure 11 Time Schedule for Tongue Approach Grafting of Melon.
Figure 12 Structure of Acclimatization Tunnel and How to Acclimatize Grafted Plants. (by K. Ito)
Table 1 Cases of Transplant Failure in Vegetable Production
Table 3 Objectives of Grafting Fruit-Bearing Vegetables
Table 4 Major Varieties of Rootstock Used for Fruiting Crops in Japan
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