LETTUCE
Lactuca Sativa L., family CompositaeLettuce is a major U.S. vegetable crop grown on 234,440 acres in 1970 and valued at about $223 million. California produced more than half of the crop (146,00O acres) with Arizona second (50,900 acres). About 2,300 acres were devoted to lettuce seed production, mostly in California. About 2 million pounds of seed were imported.
Plant:
Lettuce is an annual, grown from seed for its succulent leaves, which form a head that is harvested a few months after the seed is planted. About a month after the head forms, if lettuce is not harvested, the stem within the head elongates and branches to produce the inflorescence, which is 2 to 4 feet high. The seed is produced by the flowers of the inflorescence. From 1/4 to 11/2 pounds of seed are planted per acre. An acre yields 300 to 800 pounds of seed (Foster and Van Horn 1957, Griffiths et al. 1946*, Hawthorn and Pollard 1954 *), depending on the cultivar and method of harvest. The seeds are planted in rows 18 to 22 inches apart and thinned to 12 to 14 inches in the row. The heads are sometimes mutilated to permit the flowering stem to extrude and elongate.
Inflorescence:
This many-branched plant, with numerous leaves near its base, is relatively leafless toward the terminal. The terminal of the inflorescence is primarily a panicle or cluster of yellow flowering heads. Each head is about one-half inch long and is surrounded by a series of overlapping bracts called the involucre. A head contains 10 to 25 florets (fig. 123) that develop simultaneously. The floret ovary is one celled and produces only one seed (actually a fruit called achene), thus a head may produce 10 to 25 seeds (Hawthorn and Pollard 1954*). All of the florets in a head open on the same day, early in the morning, and close shortly afterwards, never to reopen. In some instances, they are only open one-half hour (Purseglove 1968*, Jones and Rosa 1928*, Thompson 1933), but remain open longer on cool cloudy days, sometimes until 2 p.m.
Flowering on a plant may continue for 2 months or longer. A seed ripens 11 to 13 days after the flower opens (Jones and Rosa 1928*). Seeds left too long on the plant may shatter and be lost. Therefore, if all of the seeds are to be saved, the heads must be shaken over a bag at intervals. Usually, the plant is cut at the peak of seed setting, and the bulk of the ripe seeds are salvaged.
The lettuce flower is usually considered to be self-pollinated (Watts 1958, Thompson et al. 1958, Oliver 1910, Jones 1927, Jones and Rosa 1928 *, Hawthorn and Pollard 1954*). The method of self-pollination was described by Knuth (1908*, p. 690), who stated that the style emerges through the anther tube and branches when it is about 2 mm above the tube. These two branches curl back upon themselves, usually make contact with pollen grains on the sides of the style, and self-pollination results. The pollen is pushed out of the anther tube by the brushes on the style and is easily available to bees. There is no evidence in the literature that lettuce secretes much, if any, nectar, although Jones and Rosa (1928*) indicated in a sketch that a nectary exists at the base of the style. Also, Jones (1927) and Thompson (1933) stated that the bees Agapostemon texanus, Californicus crawford, and Halictus spp. collect "mostly" pollen, indicating that some nectar may be collected also.
Besides honey bees and the above-mentioned wild bees, various other insects have also been reported on the lettuce flowers. Knuth (1908*, p. 690) reported "various flies." Watts (1958) reported various species of hover-fly and a few butterflies, although he was unsuccessful in getting hover-flies to pollinate heads enclosed in muslin bags. Jones and Rosa (1928*) mentioned flies and several species of short-tongued bees. Hawthorn and Pollard (1954*) stated that the flowers are frequently visited by wild bees and other insects. Honey bees have been observed by the author collecting pollen from lettuce flowers in southwestern Arizona.
[gfx] FIGURE 123. - Lettuce flower. A. Longitudinal section, x 10; B, longitudinal section of one floret, x 30.
Pollination Requirements:
The structure of the lettuce flower encourages self-pollination and the plants are self-compatible; therefore, seeds can be produced on plants bagged to exclude insects. The pollen is not windblown. However, cross- pollination has been observed (Thompson 1933, Thompson et al. 1958, Watts 1958). To determine if insects affected the transfer of pollen, Jones (1927) compared stigmas of flowers exposed to open pollination with those bagged to exclude pollinating insects. He observed 70 bagged flowers, of which 58 had no pollen grains on their stigmatic surfaces, and the other 12 flowers bore only one to seven grains. However, of 70 flowers exposed to pollinating insects, all stigmas had from 4 to 51 grains of pollen present. This showed that pollinating insects contribute to the effective transfer of pollen to the stigma, within the flower and likely between flowers. As a result, Jones and Rosa (1928*) concluded that cross-pollination between plants may be much more frequent than was formerly supposed. When Flemion and Henrickson (1949) bagged dill plants with insect pollinators present, they obtained 1,000 seeds per umber compared with only 59 per umber on plants caged without insects present. If the authors had performed a similar test on lettuce, the test by Jones (1927) indicates that they might have obtained similar results.
Furthermore, the discovery of male sterility in lettuce (Ryder 1963, 1967) opens the way for production of hybrid lettuce seed, if means can be found to effectively transfer the pollen from male-fertile to the male- sterile plants. So far, the only conceivable way is to have insects transfer the pollen. Without the presence of pollen on the male-sterile plants, the insects must be enticed there by the presence of nectar. Because the flower is only open briefly, the concentration of insects would need to be high for effective cross-pollination.
Pollinators:
Although flies, wild bees, and butterflies have been mentioned as visitors to lettuce flowers, none of them are present in commercial lettuce fields in a sufficient quantity when desired to cross-pollinate male-sterile lines necessary for hybrid seed production.
Honey bees can be supplied at any time by commercial beekeepers and honey bees are concerned with collecting nectar. Therefore, they would appear to be the only potential insect at present that would be suitable for pollinating the male-sterile plants.
Pollination Recommendations and Practices:
None.
LITERATURE CITED:
FLEMION, F., and HENRICKSON, E. T.
1949. FURTHER STUDIES ON THE OCCURRENCE OF EMBRYOLESS SEEDS AND IMMATURE EMBRYOS IN THE UMBELLIFERAE. Boyce Thompson Instit. Contrib. 15(6): 291-297.FOSTER, R. E., and VAN HORN, C. W.
1957. LETTUCE SEED PRODUCTION IN ARIZONA. Ariz. Agr. Expt. Sta. Bul. 282, 22 pp.JONES, H. A.
1927. POLLINATION AND LIFE HISTORY STUDIES OF LETTUCE (LACTUCA SATIVA L.). Hilgardia 2: 425-448.OLIVER, G. W.
1910. NEW METHODS OF PLANT BREEDING. U.S. Dept. Agr. Burl Plant Indus. Bul. 167, 39 pp.RYDER, E. J.
1963. AN EPISTATICALLY CONTROLLED POLLEN STERILE IN LETTUCE (LACTUCA SATIVA L.). Amer. Soc. Hort. Sci. Proc. 83: 585 - 595.______ 1967. A RECESSIVE MALE STERILITY GENE IN LETTUCE (LACTUCA SATIVA L.). Amer. Soc. Hort. Sci. Proc. 91: 366-368.
THOMPSON, R. C.
1933. NATURAL CROSS-POLLINATION IN LETTUCE. Amer. Soc. Hort. Sci. Proc. 30: 545 - 547.______WHITAKER, T. W., BOHN, G. W., and VAN HORN, C. W.
1958. NATURAL CROSS POLLINATION IN LETTUCE. Amer. Soc. Hort. Sci. Proc. 72: 403 - 409.WATTS, L. E.
1958. NATURAL CROSS-POLLINATION IN LETTUCE, LACTUCA SATIVA L. Nature 181(4615): 1084.
