|Simon: Pubs: 93hb__0171|
Hort. Bras. 11(2), novembro 1993. 171-173.
Philipp W. Simon
Vegetable Crops Research Unit/USDA
Dept. of Horticulture, University of Wisconsin
Madison, Wisconsin 53706, EUA
To meet the needs of growers, plant breeding programs must focus on genetic improvement of factors imparting sustained or increased crop production with reduced input of capital, labor, fuel, fertilizer, pesticides, and/or water. Improvement of postharvest storage durability and processing quality are also key objectives for many crops. Beyond these goals, improvement of consumer quality is also important for many crops, including all horticultural crops. This paper deals with the genetic improvement of vegetable quality, especially carrot, cucumber, onion and garlic.
The improvement of vegetable quality requires knowledge of consumer needs and desires. Consumer quality can include such diverse attributes as vitamin content, absence of antimetabolic compounds, flavor, texture, color, appearance and convenience. Clearly non-genetic influences play a major role in the determination of vegetable quality, but substantial genetic variation frequently exists for many quality traits. To initiate genetic improvement of vegetable quality the breeder must determine which quality attributes are important to consumers and develop methods to rapiddly and accurately assess these attributes. Selection for improved quality can only be delivered to consumers if most or all of the production, storage, and processing requirements of the crop are met as well. Consequently, quality improvement must be performed concurrently with improvement of productivity.
The challenges of breeding for improved quality vary widely among carrot, cucumber, onion and garlic. Attributes of quality and nutritional value for each crop and genetic potential for improvement of these attributes will be addressed.
Carrot, Daucus carota L., is a diploid outcrossing species which is biennial but able to be handled as an annual in a breeding program. Cytoplasmic male sterility is widely used to develop hybrid cultivars which are replacing open-pollinated varieties. Carrot can be self-pollinated but generally suffers significantly from inbreeding depression. Rather extensive genetic variation exists in carrot gennplasm for many production and quality attributes useful for carrot improvement but historically relatively little effort has been expended on carrot breeding in comparison to many other crops. Consequently, the potential future progress in the development of improved carrot germplasm is great. Furthermore, carrot is easily manipulated in tissue culture, genetic transformation systems are well-developed, and diversity of molecular markers (e.g. RFLPs, RAPDs, isozymes) is extensive so that new technologies can be readily applied to carrot as they develop (Peterson and Simon, 1986; Simon, 1984).
Carrot is a significant source of vitamin A accounting for an estimated 30% of the dietary vitamin A in the U.S. diet (Simon, 1992). Carotenoids, including and carotene, are abundant in carrot and they account for both high provitarnin A content and familiar orange color.
In addition to the nutrients provided by carrots, flavor is also an important component of overall quality. The major flavor attributes of raw carrot include sweetness, harshness, and bitterness. Consumers generally prefer sweet carrots without harsh, turpentiney aftertaste or bitterness (Simon et al., 1980; Simon, 1985). Sugars and volatile terpenoids account for sweetness and harshness, respectively (Simon et al., 1982). In contrast to sweet and harsh flavor, bitterness is only found in stored carrots exposed to ethylene and is thought to be caused by isocoumarin, although this is not well-established. Sweetness and bitterness levels in raw carrots persists after cooking or heat processing (Simon, 1985; Simon and Lindsay, 1983).
Another important component of raw carrot quality is texture. Succulent juicy carrots are preferred by consumers. Texture is related to fiber and pectate levels but clear relationships have not been established (Martens et al., 1985; Simon, 1985).
Significant genetic variation occurs for carotenoid levels in adapted carrot germplasm. Carotene content in U.S. carrots has increased over 50% since 1970 due to new darker orange varieties developed from a broad germplasm base. U.S. carrots today contain approximately 150 ppm carotenoids so that one medium-sized (60g) carrot provides enough provitamin A carotene to fulfill adult vitamin A needs for one day. Darker orange carrot strains containing 300 ppm carotene have been evaluated in international trials and found to be suitable in temperature and highland tropical areas (Simon, 1990). One square of carrots which contain 300 ppm carotenes can fulfill the vitamin A needs of an adult for one year. Even higher carotene content is possible and carrot strains containing 400 to 600 ppm carotene are being tested ( Simon et al., 1989; Simon, 1987, 1990). Methods for selecting carotene content are well-established (Simon and Wolff, 1987). Visual selection is moderately successful for improving carrot carotene content up to 200 ppm but laboratory analysis is necessary for accurate selection at higher levels.
Genetic selection for carrot flavor using a trained evaluator is effective in a breeding programa (Simon et al., 1981). Harshness, sweetness, and texture can all be evaluated in this way. More accurate sugar can be gained by evaluating soluble solid levels.
The genetics of carrot sugars and volatile terpenoids important in raw carrot flavor has been studied. Over a broad range of germplasm, sugar content ranges from 3 to 7% for carrots grown in organic soil (Stommel and Simon, 1989). Production of carrots on mineral soils can yield carrots with 7 to 16% sugar. Realized heritability for sugar content is 40 to 45%. In addition to the quantitative variation for total sugar content, a single gene controls sugar type (sucrose vs. reducing sugar) in carrots (Freeman and Simon, 1983).
Harsh carrot flavor results from high levels of volatile terpenoid. Volatile terpenoid content varies from 5 to 330 ppm among inbreds and is quantitatively inherited with incomplete dominance for low terpenoid levels. Carrot roots containing 30 to 50 ppm volatile terpenoids are mild-flavored, unlike those with a higher content, yet they have adequate carrot flavor (Simon, 1981). Analysis of volatile terpenoids requires gas chromatographic evaluation (Senalik and Simon, 1987).
The genetic basis of bitterness in stored carrots has not been studied. There has been some indication of genetic variation for bitter flavor development in stored carrots, however no selection has been initiated (Simon, 1985). Genetic selection for improved carotene content, sweetness, and harshness levels has no deleterious effect on productivity of carrots (Simon et al., 1987). Selection for succulent texture, however, can yield roots which are prone to cracking. Consequently carrot texture improvement is limited by production and handling techniques.
Cucumber, Cucumis sativus L., is a diploid outcrossing species subject to little inbreeding depression. Hybrid cucumber varieties are developed with a system of gynoecy to control pollen production (gynoecious x monoecious). Commercial Fl hybrids are widely used. Cucumber germplasm has relatively limited genetic variation but the abundance of monogenic traits, the ease of inbreeding, rapid life cycle (34 generations per year), and intensive breeding effort has yielded a broad range of often useful traits which cucumber breeders can utilize to alter plant architecture, sex expression, and disease resistance (Fanourakis and Simon, 1987). In addition to classical plant improvement techniques, molecular marker systems are quite well-developed in cucumber (Knerr and Staub, 1992; Kennard and Havey, 1993) and genetic transformation is possible.
Cucumber, either fresh or pickled, are highly regarded as a crunchy, low-calorie food with little nutritional value. Cucumbers actually are a fair source of vitamin C (one-sixth that of oranges). Furthermore, cucumber peel (exocarp) does contain carotenes and oriental types with orange flesh (mesocarp) are known (Kooistra, 1971; Qi et al., 1983; Yang et al., 1991). Beta-carotene is the major carotene present in cucumber peel and flesh (Simon, 1991). Cucumber sugar content also varies developmentally and among genetic stocks (McCreigh et al., 1978).
Cucumber texture is an important component of consumer appeal. Exocarp and mesocarp structure both affect texture (Peterson et al., 1978). The chemical basis of texture has not been elucidated although stage of fruit development is known to be important.
Several opportunities exist for genetically improving cucumber nutritional quality. Significant genetic variation occurs for carotene content in cucumber peel. Dark green peel contains 25 to 45 ppm carotene whereas pale green peel contains 8 a 15 ppm carotene. This translates to 3 to 5 ppm carotene content on a whole fruit basis for dark green cucumbers. The increased consumer acceptability of dark green cucumbers will provide a means for plant breeding efforts to improve consumer nutrient availability (Simon, 1991).
Preliminary studies of carotene accumulation in cucumber flesh are also underway. The orange flesh characteristic has been transfeffed to U.S. pickling and slicing cucumber types to yield immature fruit with 25 ppm carotenoids and mature fruit with 15 ppm carotenoids (Simon, 1991). Pickling does not significantly reduce carotene content of mature orange types but immature orange fruit color fades. The unusual appearance of orange cucumber flesh color may limit its application although the consuming public is often willing to evaluate new foods.
Cucumber texture varies widely over genetic stocks. Rapid evaluation of texture is possible with several types of pressure testers. Research to date has indicated moderate heritability of texture and progress by breeding programs in conjunction with new processing methods had yielded improved crunchy texture in fresh and pickled cucumbers (Peterson et al., 1978).
Onion, Allium cepa L., and garlic, Allium sativum L., are both diploid outcrossing species which are strictly biennial. Beyond this, the breeding and genetics of onion differs greatly from that of garlic.
Onion is usually propagated from seed. Like carrot onion has a cytoplasmic male sterility system which is used widely for developing hybrid cultivars. It also suffers significantly from inbreding depression and the cumulative breeding effort is fairly small. Onion germplasm has a moderate level of diversity for improving production and quality attributes.
Garlic has been asexually propagated for all of its known history. Consequently garlic genetics and breeding has been unknown until very recently when the production of true seed was accomplished in Japan (Etoh et al., 1988), Germany (Konvicka, 1984) and the U.S. (Pooler and Simon, 1993). Considering the lack of sexual reproduction in garlic until now, a surprising level of genetic variation, as reflected by isozymes, occurs in garlic. Nevertheless, garlic genetic variation cannot be considered extensive. As true seed production becomes more widely used, the full potential for garlic breeding will be better understood.
Application of biotechnological tools for onion and garlic improvement is limited by a paucity of isozymes of RFLP markers and the lack of a developed genetic transformation system. Havey (1993) recently found onion cytoplasmic male sterility to be conditioned by an apparently alien cytoplasm and readily evaluated with molecular probes.
Onion and garlic flavor is an important component of quality. The pungent flavor typical of these crops is the result of an interaction between alk(en)yl cysteinc sulfoxides in the cytoplasm with the vacuolar enzyme alliinase when cells are ruptured. Pungency can be estimated accurately in the laboratory (Schwimmer and Guadagni, 1962). Sweetness, determined by free fructose and glucose, is another important component of onion and garlic flavor. The fructose polysaccharide fructans are the major storage carbohydrates of Alliums. The dynamic equilibrium between the nearly flavorless fructans and their very sweet component fructose monomers is not well-understood during bulb development and storage, so that sugar or soluble solids levels are not always well-correlated with sweetness. Furthermore, high pungency levels can mask sweetness.
Consumer health benefits from an array of onion and garlic compounds which are derived from the same chemicals responsible for their distinctive flavor. These sulfur containing cysteine derivatives are the breakdown products of more complex molecules found in intact plant tissue and they have been implicated in the reduction of blood cholesterol levels, hypertension, cancer, atherosclerosis, and a wide range of other health problems (e.g. Block, 1992; Dorant et al., 1993; Lawson and Hughes, 1991; Sendl and Wagner, 1991). Several specific compounds which apparently impart health benefits are ajoene, allicin, alliin, and diallyl sulfide. It is difficult to predict relative healthfulness from an analysis of plant tissue since a wide array of breakdown products are produced.
Onion and garlic contain moderate quantities of ascorbic acid. Their contribution to vitamin C status of consumers has not been evaluated.
Long-storage onions for temperate latitudes are typically more pungent than short-storage 'short-day' types. Preliminary evaluation of onion pungency genetics indicates incomplete dominance for low levels (Simon, 1984) and selection for low pungency has been exercised in varietal development (Peterson et al., 1986). Beyond these few generalizations, little is known about the potential for genetic improvement of onion flavor. Furthermore, the genetics of sweetness and ascorbic acid accumulation have not been examined. As the healthful derivatives of flavor may be expected to be correlated with intensity of pungency, the development of pungent onions may yield onions with greater health benefits to consumers than would mild onions. Improved, rapid methods for pungency evaluation and analytical methods to evaluate compounds beneficial to human health will improve our knowledge of onion quality genetics.
Potential for improvement of garlic quality is less understood than that for onion. Many of the same analytical methods will likely apply as for onions. Opportunities for garlic quality improvement will be greatly expanded as true seed production and its concomitant sexual recombination becomes routine.
Profitability and sustainability will continue to be the primary challenges for vegetable breeders. Opportunities for continued genetic improvement of carrot cucumber, onion, and garlic consumer quality and nutritional value are excellent as a broader base of knowledge for genetics, plant metabolism, genetic engineering, and naturally-occuring nutritional compounds becomes more well-developed. Furthermore, producers and consumers both are willing to evaluate new commodities unknown in the past. Farsighted plant breeding programs will exercise selection for both increased field productivity and improved quality.
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