Therefore disease resistance is most important when the disease is a limiting factor in production, and is especially important for many virus diseases. The high interest in, and the increasing present demand for breeding for disease and pest resistance is related to a generalized interest in releasing “environmentally friendly” vegetable cultivars requiring sparse or no use of pesticides. Breeding for post harvest traits, mainly transport quality, shelf life and cosmetic problems, is of increasing importance in vegetables. For example, in tomato, textural properties of fruits are important contributors to the overall quality for the fresh market and to the properties of products processed from tomatoes . Because cell wall disassembly in ripening fruit contributes to fruit texture, hydroponic channel modification of cell wall proteins and enzymatic activity during ripening can impact cell wall polysaccharide metabolism and influence texture.
Lettuce and other leafy vegetables used for salads deteriorate rapidly fol-lowing harvest, requiring a considerable investment of effort to maintain quality and shelf life of cut material. Harvesting increases respiration, stimulating deterioration, with increase in the synthesis of phenylalanine ammonia lyase and phenolic compounds, such as chlorogenic acid which cause tissue browning . Conse-quently, delaying leaf senescence is an important target for breeding of leafy vegetables. Also in lettuce, breeding efforts have targeted tipburn, marginal browning, and rib discoloration, which detract from overall appearance . Vegetable products with good transport quality, better shelf life and good appearance will be preferred by traders and also by consumers. Since vegetables are rich in vitamins, minerals and other micronutrients, and therefore vital for health, breeding objectives should include improving their nutritional value.
Historically vegetable breeders have applied selection pressure to traits related to agronomic performance, particularly yield and quality, because these are the traits important to the producer. Rarely have growers been paid for nutritional factors, so there have not been economic incentives to pay much attention to these traits. However,hydroponic dutch buckets consumers are becoming more aware of these traits . Vegetable breeding for nutritional quality was not mentioned as a primary goal in plant breeding text books through the mid-20th century.However vegetable breeding efforts targeting improved micronutrient content and composition had begun in the 1940s and 1950s with research describing the inheritance and development of tomato breeding stocks and lines high in provitamin A carotenoids and vitamin C . Lincoln et al. noted a fourfold variation in vitamin C among commercial cultivars and up to 1194 ppm in red-fruited tomato interspecific crosses with Solanum pimpinellifo-lium.
Similar research leading to the development of darker orange, and consequently high provitamin A, car-rots began in the 1970s . Yellow core color occurs only in older open-pollinated carrot cultivars since uni-form orange storage root color has been a trait of interest in carrot for over a century . Similar studies were made in squash where rapid gains in carotenoid content have been made with phenotypic selection for orange color versus green and cream . Genetic improvement to increase levels of specific micronutrients has been pursued in several other vegetables such as melon, spinach, sweet potato, potato, lettuce, broccoli, pepper, watermelon, collard, kale, peas, and bean. This field of study is relatively new, and also complex because of mineral interactions with each other, and numerous other com-pounds in the soil and in the plant . There is usually a large environmental effect, when the component is present in tiny amounts, such as for some micronutrients and phytochemicals.
Success in vegetable breeding for higher vitamin and mineral content must consider not only substance concentration but also organic components in plants that can be abundant and either reduce or increase bioavailability.With these numerous considerations, breeding vegetable plants for improved nutritional value is a complicated goal that needs expertise in many disciplines such as plant breeding, nutrition, and soil science. When a vegetable compound is found to be important for human health, and growers, vegetable markets and seed companies can capitalize on the value of the compound, there may be an opportunity for vegetable breeders to increase the amount of this compound. Breeders can be successful in reaching this goal, if the vegetable crop contains genetic variability for the com-pound, if selection is effective without detrimental pleiotropic effects, and if there is an easy method to measure the compound. Enhanced nutritional content would add value for poor, malnourished populations.