INTRODUCTION

Vegetables are some of the important crops, which are rich sources of nutrients such as potassium, dietary fiber, and vitamin A and C. The vegetable production in the world have continuously increased and there are growing interest in traditional vegetables among farmers, researchers, policy makers and the public (Halewood et al., 2013;Hughes et al., 2014). However, vegetable production is affected not only by agricultural factors including climate change, extreme weathers, pests and diseases, but by social and economic factors including urbanization, aging of rural population, and policies (Liu and Dong, 1998).

Present production of vegetables in Southeast Asia is not sufficient to provide enough vegetables to its people. Vegetable production in the Association of Southeast Asian Nations (ASEAN) in 2017 was only 27.1 million metric tons, considering postharvest losses and international trade, whereas the number of its people was 648 million (FAOSTAT, 2017). In addition, annual vegetable consumption per capita varies significantly among countries, from 154.7 kg in Lao PDR to 33.7 kg in Cambodia (FAOSTAT, 2014).

Thailand is an agricultural country; however, annual vegetable consumption per capita in Thailand was 49.4 kg, which is lower than average of the world (FAOSTAT, 2014). Of the total land, 46% is used for agricultural purposes, 31% for forest and 21% for non-agricultural purposes (Table 1). From the 24 million hectares (ha) of agricultural land, almost half of it is used for the production of rice and only approximately 2% is for vegetables and ornamentals (Table 2). Despite of its small portion, vegetables are regarded as high value crops that are vital for improving the income of the farmers in Thailand.

The development of vegetable breeding and cultivation technologies is still in its early stage although the climate of Thailand is appropriate for growing vegetables. Information on the major crops and their leading cultivars or varieties is prerequisite for establishing successful breeding strategies and developing appropriate cultivation technologies for vegetables. Based on the study of major vegetables and their leading cultivars or varieties, breeding programs can figure out target crops, breeding objectives and traits that consumers prefer. It is also helpful for Official Development Assistance (ODA) programs to set priorities for which technology can be developed first and determine which technologies are adequate to improve the production of a given vegetable.

2. CURRENT STATUS OF AGRICULTURE AND VEGETABLE INDUSTRY IN THAILAND

Thailand is located between 5°37’-20°27’N and 97°22’- 105°37’E with a total area of 51.3 million ha (UNFPA, 2012). It has tropical climate with three seasons; summer (February-May), rainy (June-September) and winter (October-January). The agro-climatic conditions in Thailand make it possible to produce various crops including vegetables throughout the year.

Agriculture is one of the most important industries in Thailand. About 49% of its people are employed in the agricultural industry. In 2016, the gross domestic product (GDP) of Thailand amounted to around 14.4 trillion baht (approximately $436 million USD) of which, about 8% is derived from the agricultural sector. Despite of its importance in Thailand’s economy, most of the farms are relatively small and family-run. Agricultural land for a farm is about 4.4 ha, on average, although there are significant size differences among regions and farmers in the country.

The total area for vegetable production in Thailand is approximately 292,000 ha with a total production of about 2.6 million tons (Table 3). Top 10 vegetables of Thailand in terms of yield are listed in Table 3. Vegetables are planted in smaller areas compared to other crops since its production usually requires more intensive work. Vegetables are grown both in upland and in lowland. Thailand's variable rainfall has discouraged farmers with upland fields from using fertilizers which is essential for increasing yield.

Thailand is one of the important exporters of fresh and processed vegetables in Southeast Asia. Fresh vegetables of Thailand are exported to neighboring countries including Malaysia and Singapore, while processed ones are exported to the United States, Japan, Hong Kong, and European countries. In 2016, the exports of vegetables and by-products amounted to 67,502 million baht (approximately 2,046,000 USD), which was at the 11th place in the export of agricultural products in Thailand.

MAJOR VEGETABLES IN THAILAND AND TRAITS OF THEIR LEADING VARIETIES OR CULTIVARS

1. Chili

Chili or chili pepper is one of the economically important vegetables in Thailand. The total planting area and yield of chili in 2016 were 40,409 ha and 282,047 tons, respectively (Table 3). It is grown throughout the country and the Northern and Northeastern regions are its main producers. The major provinces for chili cultivation are Nakhonsawan, Sukhothai, Tak, Nakhonratchasima, Ubonratchathani and Nakhonphanom (Department of Agricultural Extension in Thailand, DOAE, 2016). Chili is used not only as fresh vegetable but processed ones such as chili sauces, pickles and powders.

Chili belongs to the genus Capsicum and Solanaceae family. Two chili species, C. annuum L. and C. frutescens L. are grown in Thailand. Chili in Thailand varies in sizes, shapes and Scoville scales, and its name also varies among different regions. There are four chili types such as Thai chili, Bird pepper, Bell pepper, and Thai sweet pepper with different harvesting areas and production (Fig. 1, 2). The ratio of F₁ hybrid cultivars and open-pollinated varieties (OPV) is about 20:80. For OPV, farmers usually reproduce chili seeds for sowing in their own fields. Cultivars and varieties in Capsicum species can be identified by morphological characteristics and hybridization studies (Ince et al., 2010).

1) ‘Huay Si Thon Sisaket 1’

It belongs to Bird’s eye chili type and is most popular in Thailand among the chilies grown for fresh fruits as well as dried ones. The plant height of ‘Huay Si Thon Sisaket 1’ ranges from 100 to 150 cm. Its canopy width on average is 80 cm. About 3-5 branches are diverged from the base of the main stem. The fruit is of middle size (about 5-8 cm in length) and has slender tips with relatively large bases (Fig. 3). The color of fruits is green and turns dark red when they ripen. Fruits can be harvested 120 days after planting. The yield of fresh fruits of ‘Huay Si Thon Sisaket 1’, with proper management, is 0.16-0.24 tons/ha and 1 kg of fresh chili can produce 0.43 kg of dried ones. The plant is tolerant to drought. Green and red fruits can be used in many ways. Commonly, they are used for cooking as spices. Both fresh and dried fruits have piquant taste.

2) ‘Hua Rua Sisaket 13’

‘Hua Rua Sisaket 13’ is an F₁ hybrid cultivar with plant height of 80-90 cm and an average canopy width of 80 cm. About 3-5 branches are diverged from the base of the main stem. The average length and width of the fruits are 7-8 cm and 1.0-1.1 cm respectively. The colors of unripe and ripe fruits are green and dark red, respectively (Fig. 4.). Harvesting can be done at 90 days after planting and the yield of fresh fruits is 0.27-0.29 tons/ha, which can make 76.3 kg/ha of dried chili. ‘Hua Rua Sisaket 13’ is resistant to Anthracnose. Both green and dark red fruits can be used for cooking and have spicy taste regardless of fresh and dried ones.

3) ‘Yellow Chili Phichit 3’

It belongs to C. annuum L. and Yuak chili type, whose fruits are yellow-green, waxy and mild tasting. ‘Yellow Chili Phichit 3’ is an F₁ hybrid cultivar with the plant height of 85- 90 cm and its canopy width ranges from 71 to 75 cm. The fruits are semi-elongated with the length of 14-15 cm and the width of 1.5-1.7 cm. The color of unripe fruits is green and unlike the other varieties, that of ripe fruits is yellow-orange (Fig. 5). Fruits can be harvested 90 days after planting and the yield of fresh fruits is 0.46-0.48 tons/ha. Fresh fruits, whether they are ripe or unripe, can be used for cooking. ‘Yellow Chili Phichit 3’ is resistant to Anthracnose.

2. Cabbage

Cabbage (Brassica oleracea var. capitata L.) is one of the important leaf vegetables with high economic value. It can be cultivated all year round. In areas of Northern and Northeastern Thailand, cabbage has become increasingly popular and can be harvested 3 to 4 times a year. It can be categorized into three major groups: green, red and savoy.

1) Green type

This cabbage has smooth outer leaves with pale to dark green colors. The inner leaves are pale green or white. Sometimes, the outer leaves are tied around the head so that the interior of the cabbage remains white. In addition to its good vigor and attractive round head shape in the early growth stages, it also has a dense head filled with leaves and good lodging resistance, and, therefore, it is suited for over-wintering. There are many varieties and cultivars of green cabbage grown in Thailand.

(1) ‘Copenhagen Market’ cabbage

This OPV can be harvested 63-100 days after transplanting. The heads rarely burst, and reach 15.2-20.3 cm in diameter and 1.4-1.8 kg in weight. It is great for slow cooking, for salads like coleslaw, and for fermented pickles like sauerkraut.

(2) ‘Golden Acre’ cabbage

It is an OPV and can be harvested as early as 65 days after transplanting and has a high quality. The heads are denseleaved and spherical in shape with 12.7 cm of average diameter and 1.4-1.8 kg of weight range. The plants require small space for its head size, which allows high density planting compared to other varieties. This cabbage is good for stir fries, coleslaw salad, and other fresh uses. This variety is not usually good in terms of storage length or shelf life.

(3) ‘F₁ Hybrid’ cabbage

It is an F₁ cultivar and the heads can be harvested 55-60 days after transplanting. The plant grows very vigorously and its outer leaves cover the head very well (Fig. 6). It has medium resistance to cabbage pests such as cut worms, diamondback moth larvae, and black rot (Xanthomonas campestris).

2) Red type

This type is also known as purple cabbage. Similar in flavor to green cabbage, red cabbage has deep ruby-red to purple outer leaves, with white veins inside. Recommended harvest time is during winter. Most of the red cabbages are popular in fresh market and good for salad. There are two cultivars of red cabbage; ‘Ruby Ball’ and ‘Ruby Perfection’ which all account for most commercially marketed cabbage in Thailand.

(1) ‘Ruby Bell’ cabbage

This F₁ hybrid cultivar can be harvested 70-80 days after transplanting. It has dense-leaved heads that burst late. The diameter and weight of its heads are 15.2-20.3 cm and 1.4- 1.8 kg, respectively. They have mild and sweet taste.

(2) ‘Ruby Perfection’ cabbage

It is an F₁ cultivar and is usually harvested 75-85 days after transplanting. This cultivar shows good head uniformity in terms of size and shape and the diameter of its head is usually 17.8-20.3 cm. The heads are globe-shaped and have mild and flavorful taste (Fig. 7). Each head weighs 1.4-1.8 kg. ‘Ruby Perfection’ shows low resistance to Black rot. It is one of the best cultivars in terms of storage life.

3) Savoy type

This cabbage has wrinkled leaves that form a less compact head than other types. It is also known as the wrinkled cabbage. Savoy cabbage has a more delicate texture and milder flavor than other cultivars. The leaves of this vegetable have a pale to dark green color and are covered with a waxen substance. The leaf is characterized by an irregular surface. Savoy cabbage is mainly produced for salads and coleslaw.

(1) ‘Savoy Ace’

It is an F₁ hybrid cultivar that can be harvested 80 days after transplanting. This cultivar features early maturity compared to others. The heads have deep green color with fine wrinkled leaves. It is globe-shaped and weighs approximately 1.59 kg. Production of ‘Savoy Ace’ is recommended during winter in Thailand. This cultivar has desirable market characteristics.

(2) ‘Savoy King’

This F₁ hybrid cultivar can be harvested 80-85 days after transplanting. Its semi-flat heads are larger and heavier than others, showing 20.3-27.9 cm in diameter. The heads are also short-cored and finely-textured (Fig. 8.). It is also an attractive fresh-market savor type.

3. Shallot

Shallot or Thai red onion (Allium ascalonicum L.) is a very important crop and common to all cuisines in Thailand. A number of varieties and cultivars are increasingly grown in Thailand for domestic consumption and export. In addition, it is an economically important vegetable due to its health promoting compounds and has a potential to be processed for medicinal purposes. Shallot is grown mainly in the North and Northeast of Thailand and Si Sa Ket province is famous for the best quality of shallot. Shallots are grown from October to late February (late winter in Thailand). Shallot is under the onion family and is generally smaller than its European counterpart. Shallot with purplish-red skin has stronger smell and more pungent than that with yellowish-orange, which is milder and sweeter in taste.

Shallot plants have a bunch of bulbs attached together and each pear-shaped bulb consists of a flattened conical stem at its base, with concentrically overlapping leaves which narrow at the tip of the bulb. There are three to eight cylindrical, hollow leaves which grow in two rows, extending as a sheath from the base of the bulb. Fibrous roots arise from the base of each bulb. The distal portions of the leaves are light to dark green and have a waxy bloom. The height from the bottom of the bulbs to the top is about 50 cm. Shallot is a biennial crop. Plants in the first year store its nutrients in the bulbs and die down. In the following year, they grow again from bulbs, produce flowers and fruits, and finally set seeds. The seeds allow them to reproduce again. There are three major shallot varieties in Thailand.

Shallots have several uses. Its fresh bulb can be eaten. Because of its pungency, it can be used as seasoning and spice for meat and seafood. It can be sliced and mixed with soy sauce or fish sauce and other ingredients. Shallots can also be pickled or fried. The young inflorescence (flowering shoot) can be eaten as a vegetable. Shallots can be used for medicinal purposes to reduce fever or heal wounds due to its antibacterial property. When eaten either fresh or cooked, it can lower blood sugar levels and inhibit platelet aggregation. Some farmers plant shallots for crop rotation and pathogen prevention.

1) ‘Si Sa Ket’

‘Si Sa Ket’ is a landrace and generally harvested 90 days after planting. The bulbs are oval-shaped with purple-red wet shell (Fig. 9). It has drooped neck with green leaves and yellow buds. The average yield of ‘Si Sa Ket’ is 15 tons/ha. The bulbs have pungent taste.

2) ‘Bang Chang’

It is also a shallot landrace of Thailand and is usually harvested 100 days after planting. It has global-shaped bulbs with purple or reddish wet shell (Fig. 9). It has dark green leaves and yellow buds. Compared to the other varieties in Thailand, ‘Bang Chang’ shows the highest yield of 25-30 tons/ha.

3) ‘Chiang Mai’

It is also one of the landraces of Thailand and that is harvested 100-120 days after planting. It produces ovalshaped bulbs which do not have wet shell (Fig. 9). Its leaves and buds are green and yellow, respectively. The taste of ‘Chiang Mai’ is not less pungent than other landraces.

4. yard long Bean

yard long bean (Vigna Unguiculate L. sesquipedalis L.) is subtropical and tropical plants, and widely grown in Southeastern Asia and Southern China. In Thailand, its high productivity has given way for export in both fresh and frozen forms. The planting area and production of yard long bean in a growing season from 2015 to 2016 were 12,930 ha and 99,874 tons, respectively (DOAE, 2016). There are two types of yard long bean: bush and vine (pole) types. Vine type needs more time to produce seedpods which are used for green vegetables, but it produces seedpods more in longer time than bush type. Farmers grow vine type more than bush type due to high productivity.

1) ‘Pichit 84-3’

It is one of the yard long bean varieties in Thailand, grown for domestic consumption and export. This variety has been recommended by the Department of Agriculture (DOA) since 2013. It has uniform characteristics in either pod size or total yield per area. Varietal characteristics include pod length of 45.3 cm and width of 0.98 cm. The color of the pods is green (Fig. 10). Harvesting of pods can be done 43 days after planting. The yield ranges from 0.56 to 0.62 tons/ha.

2) ‘Pichit 2’

It is also a recommended variety of yard long bean by the DOA for domestic consumption and export purposes. It is an OPV that produces green pods that have width and length of 0.6 cm and 43-46 cm, respectively (Fig. 11). They can be harvested 46 days after planting and the yield ranges from 0.43 to 0.48 tons/ha.

3) ‘Nan 1’

This OPV is also one of the recommended varieties of yard long bean by DOA, which is produced for domestic consumption. Its seedpods have good qualities in terms of taste, crispiness and strength, with charming red color, representing high anthocyanin content (Fig. 12). Its seedpods are 50-55 cm and 0.6 cm in length and width, respectively. One plant can produce 43 seedpods, on average, which can be harvested 46 days after planting, with a total crop yield of 0.51-0.53 tons/ha during dry season. ‘Nan1’ has high anthocyanin content (10.33 mg/g), which is a water-soluble vacuolar pigment.

5. Lettuce

Lettuce (Lactuca sativa L.) is a popular vegetable and considered as one of the most important crops, harvested all year round in Thailand. Generally, fresh lettuce is used for fast foods as salads. In addition, it contains significant amounts of health promoting compounds, including dietary antioxidants, which are known to have a protective effect against various forms of cancer and, cardiovascular and cerebrovascular diseases. Therefore, it is not surprising that Thai people consider lettuce as a good healthy food. There are several types of lettuce, but three (leaf, head and romaine) are popular in Thailand.

1) Leaf Lettuce

Leaf lettuce, also known as loose leaf, cutting or bunching lettuce, has loosely bunched leaves and is the most widely planted lettuce type in Thailand. It is used mainly for salads. Leaf lettuce is a promising and attractive cultivar for fresh-cut market. It is also mixed with other lettuce leaves for salads and garnish.

(1) ‘Green Oak’

This variety can be harvested 40-50 days after transplanting. It can be cultivated in open field hydroponics facilities. Weighing 200-280 g, it has beautiful, bright, fresh green colored and sturdy leaves (Fig. 13). The plant grows best in full sun and under cool temperature (18-20°C). It can be grown in hot weather provided with partial shades during the day to prevent or delay bolting. Temperature above 27°C will generally result in poor or zero germination of seeds.

(2) ‘Red Oak’

The variety ‘Red Oak’ can be harvested 40-50 days after transplanting. It has dark red colored and sturdy leaves (Fig. 13). Each bunch weighs 200-250 g. It is best grown under full sunlight and cool temperature (10-25°C).

2) Iceberg/Crisphead Lettuce

It is very heat-sensitive and believed to be low in flavor and nutritional content. It has more water content compared to other lettuce types. This lettuce type can be harvested at 40-50 days after transplanting. It can be grown in both open field and hydroponics. The leaves are fresh green in color and sturdy, with curled edges (Fig. 14). Every head weighs 200-280g. It is generally grown between October to January. The best growing season is from November to December. From June to September, this lettuce type can be planted in the highlands of Northern Thailand with temperatures ranging from 15.5 to 21°C.

3) Cos or Romaine Lettuce

Used mainly for salads and sandwiches; this type forms long, upright heads. This is mainly used for Caesar salads. This is one of the most common types of lettuce in Thailand. It is usually harvested at 45-60 days after transplanting. Cultivation methods for this variety also include open field production and through hydroponics. This variety has beautiful closed heads with glossy dark green leaves with the weight of 300 g, on average (Fig. 14). Optimal growth of Cos or romaine lettuce can be obtained if grown under full sunlight with cool temperature ranging from 10 to 25°C. The leaves of this lettuce type are crispy and crunchy, with sweet taste.

BREEDING PERSPECTIVE OF MAJOR VEGETABLES IN THAILAND

Vegetable production for the next decades should be focused on increasing quality, health promoting compounds and nutritional value of fresh, frozen, processed and ready to eat vegetable products. Also, the climate change, that is due to global warming, is the main driving force for breeding in various vegetables. Farmers expect new varieties and cultivars with tolerance to abiotic stresses such as excess water and drought. These have to be done without changing the traits or characteristics of existing leading varieties or cultivars.

UN-FAO reported that Thai people’s health was in serious situation due to low consumption of vegetables. Fortunately, the demand for organic vegetables has recently increased as a result of Thai consumers’ growing interest in health consciousness (Kitinoja and Kader, 2015). Therefore, Thailand, like the other ASEAN countries, should now find the way to maximize crop productivity and profitability to ensure sufficient supply of health-promoting vegetables while minimizing losses, wastes and other activities that have negative impacts on the sustainability and resilience of the environment (Holmer, 2011).

Recently, most vegetable breeding programs heavily depend on plant germplasm in search of traits related to food processing and sustainable agriculture (Halewood et al., 2013;Ker et al., 2013). It is needless to say that the exploitation, screening and utilization of elite germplasm are the keys to success of breeding programs. Vegetable breeding programs in Thailand should start with the evaluation and screening of vegetable germplasm for food processing traits such as quality and quantity of fresh, frozen, and processed vegetable products and for agricultural traits such as yield, quality, resistance to pests and diseases, and biotic or abiotic stresses.

Seed industry in Asia accounts for 37% of the global seed market and China, India and Thailand are the emerging market showing meteoric rises (ISAAA, 2016). However, most of the vegetable seeds distributed in these countries are OPVs which are cheap but generally produce low quality, productivity and uniformity of vegetables. The growing number of farmers in this region has been exposed to F₁ hybrid and high quality seeds that has become a breakthrough for increasing the quality and productivity of many tropical crops such as tomato, cucumber, watermelon and pumpkin.

The use of F₁ hybrid seeds can improve productivity and uniformity of vegetables, and farmers get more benefits from using F₁ hybrid cultivars. Although the use of F₁ hybrid seeds becomes popular in these regions, the ratio of farmers using F₁ hybrid seeds is still low and most farmers still can not afford to buy them. In this situation, it is a good alternative to use seeds from double crosses such as three-way or four-way crosses which have relatively good productivity and uniformity as well as reasonable price for farmers in ASEAN countries.

CONCLUDING REMARKS

Vegetable breeding programs for Thailand with changing climate should focus on developing new varieties and cultivars with tolerance to abiotic stresses such as excess water and drought, and resistance to various diseases and pests. The program also considers health promoting compounds, nutritional value and the food processing traits for fresh, frozen, processed and ready to eat vegetable products. These traits have to be incorporated into the existing leading varieties, not changing its original characteristics that consumers in Thailand prefer. Finally, these leading varieties should be distributed to farmers in the form of seeds from double crosses, which will be ultimately replaced by F₁ hybrid seeds.

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