In this article we will discuss about:- 1. Historical Background of Grafting 2. Status of Grafted Vegetable Cultivation 3. Purpose 4. Species and Varieties 5. Robotic Grafting 6. Healing and Acclimatization of Grafted Plants 7. Effects 8. Problems 9. Future Prospects.
Historical Background of Grafting:
Grafting involves the joining together of plant parts by means of tissue regeneration in which the resulting combination of plant parts achieves physical union and grows as an independent plant. Grafting was known to the Chinese at least since 1560 BC and is discussed in the agricultural writings of Aristole (384-322 BC) and Theophrastus (371-287 BC).
It is most studied in fruit and nut crops since the technique is commonly used as a mean of vegetative propagation. Typically, buds or stems of a desirable scion are inserted in a rootstock either produced by seed or by vegetative propagation so that the cambium tissues align and form a graft union.
Grafting of herbaceous vegetable crops are also an old practice. In cucurbits, it was briefly described in a 17th century book written by Hong (1643-1715) in Korea. He described methods of producing large gourd fruit by approach grafting of two plants and then thinning to a single shoot after the union.
Vegetable grafting, however, does not appear to have been a common practice until the 20th century in Asia. Today, it has become a common practice in Asian countries, especially with small farmers where successive cultivation is inevitable. The grafting technology of vegetable production expanded with the introduction and active use of plastic films in horticulture which also triggered explosive increases in protected cultivation in temperate Asian countries like Korea, Japan and China.
The production of grafted plants first began in Japan and Korea in the late 1920s where scions of watermelon were grafted to rootstocks of bottle gourd to overcome the yield decline problems associated with successive cropping mostly due to soil borne diseases. It had been practiced on small scale among farmers in Korea from the early 1950s.
In 1959, scions of brinjal were grafted on large scale onto rootstocks of Solanum integrifolium to avoid the injury caused by soil borne diseases such as Verticillium wilt, Fusarium wilt, bacterial wilt and nematodes. In the 1960s, grafting was introduced as a commercial practice in Japan and Korea for cucumber and tomato.
By 1990, the percentage of grafted plants for the production of fruit bearing vegetables (brinjal, tomato, cucumber and various melons) reached 59% of the area in Japan and 81 percent in Korea. At present, virtually all the cucurbits for greenhouse cultivation are being grafted in Korea and Japan.
Status of Grafted Vegetable Cultivation:
Grafted vegetables increased in plastic greenhouses and high tunnels because of stressful conditions from late fall to early spring such as low light intensity, high humidity and low temperature. These conditions cause various physiological as well as pathological disorders leading to severe crop loss. Other factors involved included successive cropping and the extended growing period of protected culture as compared to field planting.
Percent area under grafted vegetables in Korea and Japan is presented in Table 11.1. About 50% of the cucumber is grafter for field cultivation as compared to 95% under protected structures. In Japan on 8% of grafted tomato is field-grown as compared to 48% in greenhouses; a similar difference occurs with brinjal.
In Korea melon is commercially grafted regardless of growing conditions and grafted pepper is rapidly increasing under greenhouse conditions to avoid severe crop loss due to soil borne disease such as Phytophthora. About 10% of the green peppers are grafted in Korea (about 12 million plants), but the cultivation area of grafted peppers under protected houses will rapidly increase.
The micro-environments in most winter greenhouses are humid and cool with short day and low light intensity conditions prevailing from December to February thus providing favourable environments for disease infection and rapid spread. In nutrient culture, using grafted seedlings is a basic technology for successful growing since an extended harvesting period is regarded as major advantage for hydroponic culture.
Purpose of Grafting in Vegetable Cultivation:
In greenhouse vegetable cultivation, most damage due to continuous cropping and monoculture comes from soil borne diseases and nematodes. Since soil solarization is not a complete measure in destroying soil borne pathogens, therefore grafting of desired scions on to resistant root stocks is the only alternative for taking repeated crops of vegetables in greenhouses. Grafting gives increased disease tolerance and vigour to crops, therefore, it will be useful in the low-input sustainable production.
Its basic purpose in vegetable crops is explained here under:
(a) Disease Tolerance:
The foremost advantage of using grafted plants is to utilize the strong tolerance or resistance of rootstocks to certain soil borne diseases. These include fusarium wilt in cucurbits and tomato, phytophthora disease in pepper, and virus in tomato.
Since the spores of many of the soil borne pathogens penetrate plants via the root, it is natural that selection of resistant root stocks for the specific kind of soil borne disease could easily and efficiently minimize the infection as well as the spread of certain diseases.
Even viral diseases of tomato can be significantly reduced by proper use of root stocks. The most widely used root stock for cucurbits is ‘Shintozwa’, an interspecific hybrid between Cucurbita maxima and Cucurbitamoschata. The hybrid exerts strong resistance to all four races of fusarium and good graft compatibility with watermelon, melon and cucumber.
In addition, it also possesses strong tolerance to low soil temperature and high salt conditions. Bottle gourd is exclusively used for watermelons and is susceptible to fusarium race iv (Fusarium osysporum f. sp laginarae) and does not have good compatibility with melons.
Cucurbita ficifolia possessing excellent tolerance to low soil temperature is the preferred stock for greenhouse cucumbers and is used as a root stock for summer squash production in winter greenhouses. Other rootstocks such as ash gourd, bur cucumber, and African horned cucumber are being tested and used by a limited number of growers for nematodes resistance and other purposes.
Since watermelon is affected when grafted on to squash or gourd, therefore, resistant sources were sought in watermelon genotypes that could minimally influence fruit quality. Promising root stocks have been developed in Taiwan. The wide range of disease resistance in tomato indicates availability of promising new root stock cultivars in near future.
(b) Yield Increase:
In oriental melon, fresh fruit weight increases of 25-55% are obtained compared to own rooted plants. These yield increases were closely correlated with the maintenance of good plant vigour until late in the growing season rather than disease resistance itself.
Of course, virtually no yield is obtained from plants heavily infected with fusarium as in tomato. Up to 54% increase in marketable yield is expected with ‘Kagemusia’ and 51% with ‘Hilper’ rootstocks. Tomato plants grafted on to most rootstocks also reduces abnormal fruit bearing as compared to the own rooted plants.
(c) Low Temperature Tolerance:
The transplanting of seedlings for protected cultivation is usually done in early to mid-winter and fruit harvesting is usually finished by spring to early summer. Even though many growers heat their greenhouses during the winter. There are many growers who do not have electric or gas generated heating systems and depends solely on preservation of solar energy captured during the daytime.
These growers find it difficult to maintain optimum temperatures in winter greenhouses, especially soil temperatures which are far below the optimum causing transplanted seedlings to suffer during the first two months of cultivation. This is, especially, true with crops that require high temperatures for optimum performance such as watermelon and oriental melon.
Grafting watermelon, melon, cucumber, and even summer squash on to low temperature tolerant rootstocks such as the interspecific hybrid between Cucurbita maxima and Cucurbita moschata can greatly reduce the risk of severe growth inhibition caused by low soil temperatures in winter greenhouses.
Cucumber grafted on to figleaf gourd (Cucurbita ficifolia), an excellent grower even at low soil temperature, grows much faster than own rooted cucumber because of their ability to absorb water and nutrient at low temperature. Many physiological disorders can be effectively minimized by using grafted seedlings.
(d) Reduced Fertilizer and Agrochemical Application:
Most rootstocks for cucurbits have larger and stronger root systems as compared to the scion varieties. Thus, to avoid excessive leaf and stem growth and poor fruit growth with inferior quality, it is routinely recommended that fertilizers for grafted plants of cucurbits be reduced to about one-half to two-thirds of the recommended rate for own-rooted plants.
The frequency of agrochemical application also can be significantly reduced by using vigorous rootstocks. Expression of deficiency systems may be minimized with proper rootstocks. In the recent years, there has been a marked increase in the use of appropriate rootstocks in response to the demand for environment friendly produce. Wise selection of rootstocks can also be effectively replace methyl bromide.
Species and Varieties for Grafting:
Grafting is a commercial practice in herbaceous plants principally cucurbitaceae (watermelon, muskmelon, cucumber, etc.) and the solanaceous crops (tomato, brinjal and bell pepper). Inter-generic grafting is generally used in vegetable crops like cucumber grafted on pumpkin, watermelon on bottle gourd and muskmelon on ash gourd, whereas interspecific grafting is applied to brinjal. Scarlet eggplant (Solanum integrifolium) and Solanum toruum are popular rootstocks used for the grafting of brinjal.
Robotic Grafting:
The first robot developed was the “Cutting-off Cotyledon Grafting” (CCG) system developed by IAM BRAIN of Japan to graft cucurbit vegetables. The robot took into account variation of seedling shape, location of cutting and gripping, cutting, and attachment.
Seedlings are cut at the point of attachment of the cotyledon to the hypocotyl at an angle of 10° for the scion and 30° for the rootstock. The prototype grafting robot was constructed in 1987 and the second in 1989. It take three seconds to make a grafted plant with 95 percent survival rate.
Tools, Clips and Grafting Aids:
A number of grafting tools to perform grafting and clips to hold the graft union together have been developed by various agricultural companies. Unfortunately, however, most of them have not been widely used by the growers. Simple grafting aids, such as grafting clips, tubes, tapes and pins have been selectively but widely used for grafting.
The ordinary grafting clips consisting of a round spring made out of plastic, have been more extensively used for tongue approach grafting in cucurbits and other crops. These clips are inexpensive, easy to operate and handle for various stem sizes and can be used for many times.
There are other clips, especially elastic tube shaped clips with or without attachment of supporting pole for the grafted seedlings. These are also widely used by many commercial growers for manual grafting as well as for machine or robot grafting. Much smaller elastic slit tubes are being used in the Netherlands for tomato and pepper grafting.
A ceramic pin is very handy and efficient aid to fox a grafted interface, and highly suitable for machine or robot grafting. It can be used manually, with a simple pencil shape device, or with machine or robots. Adhesive tape or glue is another means of holding the grafted counterparts in place.
Especially designed knives and gimlets for grafting have been manufactured and are used by growers in different parts of the world. A special knife with a self-feeding connection of skimmed milk to inactivate some potent virus has been developed in Netherlands and Korea. Rapid changes have been taken place recently and it is evident that marked progress will be made on these devices with the improvement of grafting technology and introduction of new and efficient grafting robots.
Monitoring Grafting Success:
Grafting success can be judged subjectively by observation of experienced growers and researchers. A transverse cut across the graft union is still the major way of judging in most nurseries. Recently, objective determination of grafts have been developed for mass production of grafted seedlings using measurement of electrical resistance across the graft interface, by thermal imaging of leaf temperature, assessment of hydraulic connection by displacement transducer and electrical wave transmission.
In tomato grafts, electrical resistance for the first two to three days increased rapidly in step with the formation and thickening of the isolation layer. In the next three to four days electrical resistance decreased steadily, as the isolation layer observed and disappears during callus proliferation inter-digitations.
Then, resistance began to drop to the level of intact stem, which seemed to indicate that symplastic connection and vascular unification had been completed. In Amaranthus tricolor or Lycopersicon esculentum grafts, resistance increased steadily with the establishment of an isolation layer, which remained un-ruptured.
Thermal cameras with an image processor have been used to evaluate the quality of graft take based on leaf temperature. With the successful grafting, water moves smoothly from the root to leaf of the scion, decreasing in temperature due to transpiration. Leaf temperature of successfully grated plants was 2-3°C lower than the poor grafts. Seedlings with thicker leaves are better for grafting because thicker leaves can maintain the higher moisture content facilitating faster graft union.
Measurement of electrical wave transmission from the scion to root stock across the grafting interface may be another technique to evaluate grafting.
Healing and Acclimatization of Grafted Plants:
Grafting should be carried out in a shade place sheltered from the wind, to avoid wilting of the grafted plants. Grafted plants usually heal and are acclimatized in plastic tunnels. 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 3-5 klx.
The following precautions must be taken before grafting for proper union of stalk and scion:
(i) Expose the scion and root stock to sunshine for 2-3 days.
(ii) Withhold water from the plants to avoid spindly growth.
(iii) Make sure that the scion and rootstock has stems of similar diameter.
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 root stock to come into contact, by maximizing the area of cut surface that are spliced together, and by pressing the spliced cut surface together.
The cut surfaces should not be allowed to dry out. After grafting, keeping the grafted plants at about 30°C and with more than 95 percent 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 daily weather, with more shade on a sunny day.
Effects of Grafting:
(i) Vigour:
Rootstocks affect the growth and yield of scion plants and are often performed to provide vigour. However some rootstocks may also depress the growth and yield of scion plants.
(ii) Physiological Disorders:
Fruit physiological disorders often appear after grafting depending on the root stock. Such abnormalities include abnormal fruit fermentation in muskmelon and reduction in total soluble solids, persistent green colour in the suture stripe and appearance of non-viral yellow mottle symptoms on the leaf of tomato.
(iii) Stress Tolerance:
Tolerance to temperatures, drought, flooding and salt stress may be influenced by the rootstock. The increased performance at low soil temperature with certain rootstocks in cucurbits is one of the main benefits of the grafting.
Problems Associated with Grafted Plants:
In addition to labour and skilled persons required for grafting and post-graft management growers may encounter unexpected problems associated with use of grafted seedlings as compared with self-fruited seedlings.
These problems are as follows:
(i) Incidence of unexpected diseases such as virus infection in rootstock seeds.
(ii) Occurrence of secondary diseases such as anthracnose in bottle gourd, internal fruit decay or premature fermentation in melon and tobacco mosaic virus and others in tomato.
(iii) Poor fusion of vascular bundles in grafted unions, changes graft compatibility with the growing season, environmental stresses and inferior quality of fruits harvested from the grafted plants.
(iv) Quality deterioration of fruits occurs such as decrease in firmness and shelf-life.
There are additional costs and investments that must be considered such as the cost of rootstock seeds, the expenses needed for the grafting operation and the capital for the purchase of machine and robots.
Future Prospects of Grafting Vegetables:
The technique of using grafted vegetables developed in Asian countries is spreading rapidly worldwide. The seeds of rootstocks for vegetable have been introduced by the Rijk Zwaan seed company. These are ferroRZOR for melon and watermelon; 64-05 RZ® for cucumber, melon and watermelon, 64-13® for watermelon, and 61-53® for TMV resistance in tomato. Watermelon and tomato are the two major vegetables for grafting and worldwide distribution.
Grafting is routinely practiced in several other vegetables such as cucumbers, melons, oriental melons, greenhouse squash, eggplant and capsicum. Introduction of excellent rootstock possessing multiple disease resistance and efficient grafting machines including grafting robots will greatly encourage the extended use of grafted vegetables the world over.
There are many problems commonly associated with grafting and cultivating grafted seedling. These include the additional cost for rootstock seeds, labour required for grafting and raising of grafted seedlings, lack of technical know-how and technique of grafting and cultivation of grafted plants and incidence of possible physiological disorders associated with grafting. However, there are enormous benefits from using grafted seedlings.
These include increase in income through high yield and half season growing, lower input of fertilizer and irrigation water due to the wide root system of the rootstock, extension of the harvest period, efficient maintenance of popular cultivars against diseases and other physiological disorders, no need for long term crop rotations, overcoming problems due to saline soils, reduced expense needed for soil fumigation, ease of producing organically grown vegetables, and reduced use of agro-chemicals.
Partial or full utilization of these benefits will depend on various factors such as farm size and degree of mechanization, cultivation practices such as crop rotation and transplanting, technology level, understanding the full benefits and risks of grafted seedlings, and the use of protected cultivation and hydroponics. Use of grafted seedlings in tomato, pepper, eggplant and cucumber is strongly recommended for hydroponics culture.
Growers can now purchase grafted seedlings of any specific combination from many commercial plug seedling growers rather than doing the tedious grafting themselves. In this case, the grower should make an order for their seedlings in advance. This is, especially, true in Japan, Korea, and the Netherlands and perhaps in many other countries. With the invention of more efficient grafting robots and acclimatization facilities, the price of grafted seedlings could be considerably reduced in future to meet the grower expectations.
Grafting is extremely laborious and time consuming and growers are trying to reduce the labour input required. Attempts have been made to mechanize grafting operation since 1987. Tube grafting was developed as a manual operation for small plugs which reduced the time required for manual grafting by at least one half.
An adhesive and a hardener have been used to support the graft union in several crops. Five tomato grafted at two leaf stage simultaneously are plugged with adhesive, using grafted plates. Grafting robots or plugs have also been developed, by combining the adhesive and grafting plates.
This robot makes it possible for 8 plugs of tomato, eggplant or pepper to be grafted simultaneously. Robotic grafting is about 10 times faster than conventional hand grafting. Tomato and eggplant grafted by robot produce 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 environment is artificially controlled are now being used by many nurseries which produced grafted plugs.
As grafting operations and the healing of grafted plants become easier, grafted vegetables may become popular all over the world. Plants gain disease tolerance and vigour by grafting. Grafting of vegetables may be useful in the low input sustainable horticulture of the future.