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Ryder, E.J. and W. Waycott. 1993. New directions in salad crops: New forms, new tools, and old philosophy. p. 528-532. In: J. Janick and J.E. Simon (eds.), New crops. Wiley, New York.

New Directions in Salad Crops: New Forms, New Tools, and Old Philosophy

Edward J. Ryder and William Waycott


  1. NEW DIRECTIONS IN USE AND FORM
    1. Usage in the United States and Europe
    2. Variation in Form and Appearance
    3. Roots and Seeds
  2. NEW DIRECTIONS IN TECHNIQUE AND TECHNOLOGY
    1. Genetic Engineering
    2. Backcross Procedures
    3. Hybrids
    4. Whither Plant Breeding?
  3. REFERENCES
  4. Table 1
  5. Table 2

Salad crops as such are not new. We are, however, finding new ways of using salad vegetables and they are turning up in new forms and colors. Familiar salad vegetables for one group of people may be quite new to another. There are even a few salad vegetables that would qualify as new in the narrowest sense, such as wild species and non-domestic plants that are now consumed as salad greens. The objective of this paper is to discuss new directions and trends in salad crops, as well as some new approaches in our research. We will also briefly discuss trends in plant breeding research that we consider to be unfortunate for the development on few and improved cultivars.

NEW DIRECTIONS IN USE AND FORM

Usage in the United States and Europe

Profound changes in the use of lettuce and other saladcrops have been taking place in the United States, western Europe, and several other parts of the world. The most surprising change has been the adoption of crisphead or iceberg lettuce in countries where it was hardly used before. In England, for example, people consumed mostly butterhead lettuce until the late 1970s. At that time, only about 3% of the lettuce was of the iceberg type. Then, the British discovered iceberg lettuce and it now comprises about 80% of the lettuce con-sumed in Britain. Similar changes have occurred in the Scandinavian countries and are beginning in other countries as well.

On the other hand, we in the United States have rediscovered that not all lettuce heads are round, crisp, and hard and once again are eating romaine, butterhead, and leaf lettuces, not to mention endive and escarole (Cichorium endivia), and spinach (Spinacia oleracea). In addition, we have discovered Little Gem, a Latin type lettuce, part cos and part butterhead, which is small, crisp, and sweet. We have discovered radicchio, a red Italian chicory (C. intybus L.); mizuna, a leafy vegetable from Japan (Brassica japonica L.); and rocket (Eruca sativa Mill.), formerly found in the wild, but now known in the cultivated form as arugula. Mizuna, arugula, and spinach are often found in a salad mixture called mezclun, which is made up of tiny lettuce leaves of various shapes and colors and other salad greens.

Variation in Form and Appearance

The proliferation of form and color existing in lettuce germplasm is quite astounding. Some relatively new variations in form and color can be seen now in commercial lettuce fields. In a recent planting of more than 400 lettuce Plant Introduction (PI) accessions, we found lines with leaves that ranged from dark green to light green, from deep crimson to light pink, as well as yellows and golds, and even blue teal. One line had plants with red rhubarb-like stems, another had plants that resembled balls of frilly green lace. (Table 1).

There are other crops we may see in the near future. Stem lettuce is a type found in both China and Egypt. The Chinese peel and cook the stems; the Egyptians eat them raw like a stalk of celery. This form probably originated in Egypt and was carried overland to eastern Asia many years ago. There are similar forms from Central Asia as well. Zuckerhut, also known as Pan di Zucchero or Sugarloaf, is an elongated, green chicory which heads nicely in late summer and has a milder flavor than radicchio.

Salad vegetables may be lightly processed. These are chopped or shredded prewashed leaves that are packaged for the consumer. About 12% of the harvested crop of iceberg lettuce is prepared in this way. MacDonalds alone purchases 2% of the entire United States iceberg lettuce crop in the chopped form.

Several kinds of miniaturized vegetables are appearing in market produce bins. These include carrots, watermelons, squashes, pumpkins, bok choi, and tomatoes. Soon mini-iceberg lettuce will join this group on the market. Mini-iceberg is an intriguing byproduct from one of our gibberellin biosynthetic studies. Germinating seeds of a genetic early flowering line were treated with ethyl methanesulfonate. Among the segregating progeny were several dwarf forms, which flowered early, but had smaller leaves and considerably shortened stems. One dwarf was crossed with a standard iceberg lettuce cultivar and among the progeny was an iceberg lettuce about one-fourth the size of a normal head, but otherwise normal in appearance. A press release about this material has generated considerable publicity in media as diverse as Wall Street Journal and National Enquirer. We will release this material early in 1992 and it should be available on the market by early 1993. We assume it will appeal mostly to single people or others, who, for some reason, cannot seem to get through a whole head of regular lettuce with dispatch.

Roots and Seeds

Most crops are produced for their above-ground parts and researchers often have neglected the contribution of roots to yield and quality. This has been true in lettuce research, at least until recent years. The modern butterhead and crisphead types of the United States and Western Europe generally have long narrow taproots and a relatively sparse secondary system. Recently, we examined roots of over 400 accessions of L. sativa and found substantial variation in root structure. Of specific interest were a number of cos (Romaine) types from Turkey and a stem lettuce from China. These root systems were characterized by more extensive branching off the tap root and may have longer tap roots. Two wild species were also studied, L. serriola and L. saligna. These species had similar root designs with several stout secondary roots and finer tertiary roots. However, they did not closely resemble the cos or stem lettuce root systems. We believe that selection on the basis of root structure may affect shoot development and reaction to injurious root colonizing organisms. Understanding the nature of root structures may also help in water and nutrient uptake studies.

Lettuce seed was probably used by the ancient Egyptians as a source of edible oil. One type of lettuce in our collection has a primitive, almost wild, shoot architecture. Although these plants resemble wild species, they have features which indicate they were selected for seed characteristics. The seeds of these plants are unusually large and germinate at high temperatures, compared to most cultivated lettuces. The plants also flower rapidly on multiple stems, producing large quantities of seeds. Research on the chemistry and quality of this oil is in progress today in modern Egypt (Shoeb et al. 1969; Ramadan 1976) .

NEW DIRECTIONS IN TECHNIQUE AND TECHNOLOGY

Plant breeders frequently make the point that plant breeding is a long term proposition, requiring not only time, but perseverance and dedication. At the same time, we do look for ways to make the job easier and/or faster. In lettuce breeding, we have some tools that enable us to do that.

Genetic Engineering

Several new ways of altering plant genotypes have appeared over the past 15 years. Some of these are very promising and recent results indicate they will be useful additions to a breeder's set of tools. The potential of plant transformation is particularly exciting. In a sense, it is the ultimate backcross procedure. Ideally, one can transfer useful genes into a desired genotype, with little or no modification of the original phenotype. Results of studies with non-functional marker genes and genes for herbicide and pest resistance, hormone regulation, and nutritional enhancement, point towards phenomenal changes for plant genomic science. However, biotechnology is expensive and to do breeding work with an emphasis in this area may not be cost effective in the long term. Nevertheless, one can not afford to ignore the prospects.

We use transformation as a tool in lettuce breeding. Aside from the need to discover, clone, and incorporate needed genes for disease and insect resistance, we are particularly interested in genes which control lettuce growth and development. These include genes, such as those for early flowering and induction of stem elongation, which perceive day length and other environmental cues and thus, affect rate of growth. If these genes could be cloned and transferred to other species, they might be highly useful as backcross breeding aids.

Biotechnology is also making our job of detecting variation among lettuces much easier. We use RFLP and RAPD analysis to identify major differences between species as well as minor differences between and within lettuce cultivars. With this information, we are able to determine similarities, which should be useful for cultivar fingerprinting. It also may be useful in identifying duplicate accessions that we may wish to purge from our collection. These methods can also be used to map genes in preparation for cloning.

Backcross Procedures

One preferred breeding method in our lettuce program is the backcross procedure, for transferring single genes to a desirable recurrent parent. This procedure requires repeated crosses. Crossing can be difficult in lettuce, because of its flower structure. Each flower is a composite of several florets consisting of a single ligulate petal and a reproductive structure consisting of a tube formed by fused anthers, surrounding a single style with a bipartite stigma. The style elongates as the anthers shed pollen on their interior surfaces. The stigmas emerge from the top of the tube, covered with pollen. It is important to remove the pollen before it can germinate and replace it with pollen from the male parent. Some procedures have been developed by which one can maximize the proportion of hybrid to selfed seeds (Oliver 1910; Ryder and Johnson 1974).

Despite the difficulty of making crosses, backcrossing is an excellent means of conserving a valuable genotype. This is important in lettuce, as cultivar groups tend to last for many years. In this century, the Western iceberg lettuce industry has, in the main, been based upon four cultivar groups: New York types in the 1910s and 1920s, Imperial types in the 1930s and 1940s, Great Lakes types in the 1950s, 1960s, and early 1970s, and the Salinas types starting in the late 1970s and still in use. Historically, cultivar types last for many years. This longevity lends itself to a program of combining useful genes within manageable population sizes.

If more than one desirable gene is available for backcrossing, each can be backcrossed into the same desirable cultivar in parallel programs. At the completion of the backcross programs, the modified forms can then be crossed to each other and progeny selected with the contributed genes. The similarity of the respective populations insures that the number of other genes segregating is relatively less and the populations required will be smaller.

Lettuce breeders have a unique genetic tool that can reduce drastically the total time required to complete a backcross program. Several years ago, two partially dominant genes for flowering time, Ef-1 ef-1 and Ef-2 ef-2, were identified. In the double-dominant condition, these genes reduce flowering time in head lettuce from, 140 days to 45 days (Ryder 1983, 1988). These time periods are specific to summer conditions in the greenhouse at our location and may vary in other locations. Because of the partial dominance, flowering times can be accurately predicted for all nine genotypes (Table 2).

The partial dominance of the early flowering trait means that the desired heterozygous segregants are identifiable. They are early, but not as early as the double-dominant. And, the preserved heterozygosity means that the early allele can be eliminated in favor of the late one at the end of the program. A modified backcross procedure was developed, with these genes as a vehicle to transfer useful genes to a recurrent parent in half the time required with backcrossing of normal late flowering cultivars (Ryder 1985). This method is being used to transfer a lettuce mosaic resistance gene to the cultivars 'Prizehead' and 'Salinas'. Also, an early flowering line was used during the breeding of the mini-lettuce. Similar genes exist in and may be similarly useful in other species.

Hybrids

Fl hybrids have been important in many cross-pollinated species. In recent years, hybrids have also become increasingly important in some self-pollinated crops, such as tomatoes. We have observed apparent heterosis in lettuce, principally as increased seedling size, compared to the parents, but we have no data to support this observation. However, F1 hybrids will probably not be important in our lettuce work. The reason for this is primarily functional. Lettuce pollen is sticky and cannot be carried by the wind. And, there are no known insect species that work lettuce flowers effectively to transfer pollen. Therefore, hand pollination is necessary to make a cross. The lettuce flower is a composite of 12 to 20 florets. Each pollination will yield a maximum of 12 to 20 seeds, some of which may result from selfing. Modern planting methods require 250,000 seeds to sow one hectare of lettuce, which would require at least 12,500 pollinations to produce enough seed. It is clear, that with present technology, the task is quite daunting. There is, of course, the possibility that one can create a single hybrid plant and then multiply it in cell culture. However, one runs the risk of somaclonal variation induced in tissue culture which could quickly alter a pure genotype.

Heterosis exists in many crops, but a major factor in the interest in F1 hybrids among most seed companies is to conceal the identity of the hybrid parents, even though the biological and agricultural benefits of the hybrid may be limited.

Heterosis, as in the F1 hybrid, is probably due to the accumulation of favorable alleles (Jinks 1983). The prevailing view is that a cross between two diverse genotypes gives the benefit of one set of dominant alleles from one parent and another set from the other parent. It is important also to consider that there may be recessive genes that are beneficial. In lettuce, for example, two important disease resistance alleles are recessive; mo against lettuce mosaic and cor against corky root rot. At each locus in which one parent has the dominant allele and the other has the recessive, it is necessary to fix the recessive alleles in the inbred populations in order to save the recessive effect in the hybrid. Population improvement is also necessary to save both alleles at each additive locus, to prevent loss of about half the effect at the locus, if it remained in the heterozygous condition. Population improvement is also useful to preserve epistatic effects.

In self-pollinated species, hybrid combinations are less likely to be superior to their parents for the vast majority of traits. The primary reason for the use of hybrids in self pollinated species is therefore, protection of the parents' identity. Some authors have expressed doubts over the usefulness of hybrids for self-pollinated crops, such as wheat and barley (Simmonds 1979; Wilson and Driscoll 1983).

Whither Plant Breeding?

We are advocates of cultivar development without economic gain as the primary motivation. As private sector breeding increases its market share and as public breeding institutions are scaled down or closed altogether, emphasis on basic and fundamental research as a foundation for cultivar development is sure to fade. Just as the F1 hybrid has become a tool for profits, a similar phenomenon appears to be developing in the world of biotechnology. There are, as cited earlier, many important aspects of biotechnology that merit serious discussion. Yet, can we afford to limit our scope and purpose, continuing to sacrifice public plant breeding programs on the altar of financial gain. One of the tools of sacrifice is the dogma that the province of the public sector is basic research and that practical breeding, or the development of marketable products, is the province of private industry.

We believe that the current loss of public programs has lead to an ever narrowing set of breeding goals, whose principal criteria are to enable the development of profitable, protected products as fast as possible. Profit generation is a perfectly acceptable function of private industry. The problem is that it may not properly serve science, agriculture, humankind, or our society, particularly in the years to come.

Agriculture is biology in the environment; it is complex, ever shifting, and constantly interactive. Problems in agricultural research are often difficult, complex subjects, requiring intense study over an extended period, with limited concern for the financial gain. Private industry cannot and will not deal with many of these types of problems, e.g., the study of gibberellin biosynthesis and root branching in lettuce, which demand long-term commitments with uncertain conclusions. As public plant breeding programs disappear, or operate at the behest of the private sector, these difficult, lengthy, unpromising problems will simply continue to accumulate. Significant knowledge will remain buried and we will lose the commitment to probe important genetic potential in plants. Yet the probing of these concepts would permit us to answer important questions as well as to discover and develop new ideas essential for maintaining the high agricultural productivity we have known and come to rely on for the past several decades.

REFERENCES


Table 1. Partial list of lettuce USDA Plant Introductions (PI) with unusual color and form variants.

USDA PI number Origin Description
177418 Turkey Dark green leaves
169511 Turkey Pale green leaves
249536 Spain Crimson leaves
206965 Turkey Intense red leaves
271476 India Light red leaves
289042Cz Hungary Yellow leaves
220524 Afghanistan Blue-green leaves, red stem
178923 Turkey Tiny head, frilly green leaves
183234 Egypt Large green leaves
zSubsample of 289042.


Table 2. Days to first flower for all genotypes of two flowering time genes. Planted 13 July in greenhouse, Salinas, California.

Genotype Range (days)
Ef-1 Ef-1 Ef-2 Ef-2 43-49
Ef-1 Ef-1 Ef-2 ef-2 52-57
Ef-1 Ef-1 ef-2 ef-2 59-76
Ef-1 ef-1 Ef-2 Ef-2 49-54
Ef-1 ef-1 Ef-2 ef-2 60-73
Ef-1 ef-1 ef-2 ef-2 74-88
ef-1 ef-1 Ef-2 Ef-2 93-118
ef-1 ef-1 Ef-2 ef-2 119-134
ef-1 ef-1 ef-2 ef-2 137-143


Last update April 28, 1997 aw