CARROT
Daucus carota L., family UmbelliferaeCarrots were grown on 78,530 acres in the United States in 1969, with a farm value of $82,967,000. The seeds, about a million pounds, were produced on about 2,000 acres, primarily in California, Idaho, and Oregon (Whitaker et al. 1970).
Plant:
When grown for seed (fig. 62), two methods may be employed. If the seed-to-seed method is used, the seeds are planted in the late summer, the root overwinters in the soil, and the following year the growth produces a seed crop. In the root-to-seed method, the roots or stecklings are removed from the soil in the fall, stored at 33 deg F until the following spring, then transplanted, and the seed crop is harvested from the plant in the fall. In both instances, the crop is grown in rows and cultivation is necessary. Franklin (1948) concluded that proper storage of stecklings was the greatest single problem in carrot seed production.
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FIGURE 62.- Carrot seed field about ready to harvest.Inflorescence:
The inflorescence, typical of the umbelliferae, consists of a terminal or primary compound umbel of white flowers, 5 to 6 inches across, and a system of second-, third-, and fourth-order umbels, named in relation to their appearance on the plant below the primary umbel. The umbels decrease in size as the order number increases. The first and fourth order umbels are of little importance in seed production (Borthwick 1931). The individual flower is usually perfect (Knuth 1909*, p. 502), although Braack and Kho (1958) reported that a tendency to produce only male flowers occurs and with increasing frequency in the umbels of high orders. A flower normally has five functional stamens and two styles, which lead to the two locules of the ovary. Each locule contains a single ovule, thus two seeds per flower from flowers in a room free of harmful insects and supplied with flies to pollinate the flowers.
Nectar is secreted from a swollen disk on the upper surface of the ovary and is easily available to all types of insects. Pellett (1947*) reported that 100,000 to 150,000 pounds of honey is produced from carrots annually, but its quality is poor. Carrot pollen is attractive to numerous insects (Bohart and Nye 1960). Gary et al. (1972) showed that carrot blossoms were much more attractive to honey bee pollen collectors than onion blossoms, as only 7 percent of the visitors to onion flowers were collecting pollen compared with 66 percent of the visitors to carrot blossoms. Flowering extends over about a month, and dehiscence within an umbel covers about 7 days. Within a floret, the anthers dehisce over a 1-to 2-day period, the stigma receptivity begins on the third or fourth day. The stigma may remain receptive a week or possibly longer ( Hawthorn and Pollard 1954*, Hawthorn et al. 1960, Franklin 1953, Poole 1937).
Pollination Requirements:
Jones and Rosa (1928*) and Enzie (1943) stated, without supproting data, that carrots were "mostly insect- pollinated." Rather thorough studies of carrot pollination were made by Hawthorn et al. (1960) (fig. 63). By comparing production from open plots with that from plots caged (a) to exclude all insects, (b) to exclude all but tiny insects, or (c) to enclose a colony of honey bees, they proved that insect pollinators were essential for commercial seed production. In cages excluding all insects, an averae of only 128 pounds of seed per acre was produced. When tiny insects were permitted to visit the flowers 453 pounds of seed per acre developed, open plots exposed to pollinators in the area yielded 711 lb/acre. Hawthorn et al. (1960) concluded from their close studies of the 'Red Core Chantenay' cv. that "limited but significant opportunity existed for self-pollination from one umbellet to another by jarring or wind action, and a greater opportunity ( on a time basis) for cross- pollination by accidential rubbing together of umbels on adjacent plants."
However, their test established that such self or mechanical pollination in the absence of pollinating insects was of little value in the comercial production of seed. Slate (1927) concluded that only about 15 percent of the carrot plants set seed from their own pollen. Even though apparently only two pollen grains are essential in the fertilization of the two ovules of the flower, and the stigma is receptive to pollen either from flowers of the same plant or from others for as much as a week, Paci (1956), Pankratova (1957), and Hawthorn et al. (1960) concluded that there is sufficient transfer of such pollen without pollinating insects. Thompson (1962) reported that more than 95 percent crossing occurred in the field at Ithaca, N.Y., but he gave no indication as to the pollinating agents.
The value of hybrid vigor in carrots has been known for years (Poole 1937) and male sterility, essential in its utilization, was reported shortly thereafter (Welch and Grimball 1947), but only a few hybrids have been produced commercially. Whitaker et al. (1970) stated that the uniform, smooth, highly colored roots produced by superior hybrids cannot be duplicated by the open-pollinated varieties. However, hybrid carrot seed production is so recent that time has not permitted the identification of problems that might be involved in providing adequate cross-pollination for this crop. The relatively long flowering period of carrots is favorable and so is the attractiveness of both the nectar and pollen to a broad spectrum of pollinators, particularly honey bees. For large-scale production of seeds, however, where male-sterile plants are used, there is need for pollinating agents interested only in nectar collection that will freely cross over from the normal to the male-sterile flowers and effect maximum cross-pollination.
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FIGURE 63.- Carrot pollination studies, showing flowers tagged to indicate mode of pollination.Pollinators:
Associated with the studies made by Hawthorn et al. (1960) on the need for insect pollinators, Bohart and Nye (1960) also studied the insect visitors to carrot flowers. They collected on the carrot blossoms 334 species of insects representing 71 families, which in itself shows the attractiveness of these blossoms to a wide variety of insect visitors. Most of the species of visitors were in the superfamily Apoidea, or the Ichneumonidae, Psammocharidae (Pompilidae), Sphecidae, and Vespidae families of the Hymenoptera, and the Bombyliidae, Sarcophagidae, Stratiomyidae, Syrphidae, and Tachinidae families of the Diptera. Bohart and Nye (1960) proposed an efficiency rating for the insect pollinators of carrots, based on the amount of loose pollen on the insects' body, the size of the insect, and its activity on the flower head. By multiplying this rating figure by the numbers of insects observed on the flowers, a pollination index was obtained for each species.
They concluded that several genera in the Apoidea were important pollinators of carrots, but from the practical standpoint the honey bee was the only species that could be manipulated and utilized in commercial seed production (fig. 64).
Pankratova (1958) reported that the chief pollinators of carrots near Moscow were flies (90 percent) and bees (9 percent). No mention was made of the number of honey bee colonies in the area nor the plant competition.
The activity of honey bees on carrot blossoms was studied by Bohart and Nye (1960). They stated that pollen collecting honey bees "literally wade across the heads, swinging their abdomens back and forth and scraping the pollen from stamens with their forelegs. The nectar collectors stand higher on the flowers, move about less, and lap up droplets from the exposed nectaries. In other species of bees, the females usually behave like pollen-collecting honey bees and the males like nectar-collecting honey bees."
Hawthorn et al. (1960) reported that plants caged to exclude pollinating insects apparently reached their peak of bloom a few days earlier and held it more than a week longer than plants in the open or in cages where bees were present. This difference, however, was attributable to the dislodging of petals by bees and was only an "illusion" so far as actual flowering was concerned.
As shown earlier by Hawthorn et al. (1960), bees increased production of carrot seed. As a result of the bee activity, there were fewer undesirable large seed and they matured more rapidly and germinated better than seeds produced where the pollinator level was low. Also, progressive shrinkage in weight of seeds, which following the various cleaning processes, was accelerated with every decrease in pollination level. Both quantity and quality of carrot seeds are improved by high levels of bee pollination. Franklin (1970) reported that at one time in Parma, Idaho, the carrot fields were teeming with bees, and excellent seed crops were obtained. Then pest control methods and materials changed, competitive crops moved in, bee counts dropped, and the seed crops failed.
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FIGURE 64.- Honey bee collecting nectar from carrot flower.Pollination Recommendations and Practices:
As a result of their studies Bohart and Nye (1960) made the following recommendations: "(1) Locate enough colonies of honey bees in the area to provide effective populations on the flower heads; (2) avoid the presence of competing bloom; (3) restrict plantings of carrots for seed to avoid dilution of the pollinator population; (4) choose areas with varied habitats capable of supporting large numbers of a wide variety of pollinators; (5) take steps to increase populations of wild pollinators in the area. For most large seed-producing areas a combination of the first and second methods is likely to prove the most practical."
Hawthorn et al. (1960) gave a little more indication as to the number of pollinators needed. They stated, "Under the cultural conditions of our experiments, a honey bee population of 8 per square yard (the lowest average number for the season in our cages) is apparently as high as the plant can use to advantage. Probably a somewhat smaller number would do just as well, although we have no direct evidence to support such a conclusion."
Pankratova (1957) stated that the most reliable pollinators of carrots are honey bees. He recommended transporting colonies to the field, but the number of colonies was not mentioned. Hawthorn et al. (1956) also recommended movement of colonies of honey bees to carrot fields to provide the large numbers necessary at flowering time but did not designate the number. Naturally, the number needed would be influenced by competition from other flowers, the strength and condition of the colonies, and the attractiveness of the carrot flowers. Under most conditions where carrots are grown for seed and maximum production is desired, the placement of several colonies per acre in and around the field would probably be justified. Eight bees per square yard of flowers should be striven for regardless of the number of colonies required to provide this.
LITERATURE CITED:
BOHART, G. E., and NYE, W. P.
1960. INSECT POLLINATORS OF CARROTS IN UTAH. Utah Agr. Expt. Sta. Bul. 419, 16 pp.BORTHWICK, H. A.
1931. CARROT SEED GERMINATION. Amer. Hort. Sci. Soc. Proc. 28: 310-314.BRAAK, J. P., and KHO, Y. O.
1958. SOME OBSERVATIONS ON THE FLORAL BIOLOGY OF THE CARROT (DAUCUS CAROTA L.). Euphytica 7(2): 131-139.ENZIE J. V.
1943. EXPERIMENTS IN THE PRODUCTION OF CARROT SEED. N. Mex. Agr. Expt. Sta. Bul. 308, 11 pp.FRANKLIN, D. F.
1948 SOME PROBLEMS IN CARROT SEED PRODUCTION. Seed World 63(8): 8-9, 44.FRANKLIN, D. F.
1953. GROWING CARROT SEED IN IDAHO. Idaho Agr. Expt. Sta. Bul. 294, 35 pp.______ 1970. PROBLEMS IN THE PRODUCTION OF VEGETABLE SEED. In The Indispensable Pollinators, Ark. Agr. Ext. Serv. Misc. Pub. 127, pp. 112-141.
GARY, N. E., WITHERELL, P. C., and MARSTON, J.
1972. FORAGING RANGE AND DISTRIBUTION OF HONEY BEES USED FOR CARROT AND ONION POLLINATION. Environmental Ent. 1(1): 71 - 78.HAWTHORN, L. R., BOHART, G. E., and TOOLE, E. H.
1956. CARROT SEED YIELD AND GERMINATION AS AFFECTED BY DIFFERENT LEVELS OF INSECT POLLINATION. Amer. Soc. Hort. Sci. Proc. 67: 384 - 389.______BOHART, G. E, TOOLE, E. H., and others.
1960. CARROT SEED PRODUCTION AS AFFECTED BY INSECT POLLINATION. Utah Agr. Expt. Sta. Bul. 422, 18 pp.PACI, P.
1956. [RESEARCH ON THE FLORAL BIOLOGY OF THE CARROT.] Riv. Ortoflorofruttic. Ital. 40: 414-423. [ln Italian, English summary.]PANKRATOVA, E. P.
1957. [THE EFFECT OF BEE POLLINATION ON THE HARVEST OF CARROT SEED.] Dokl. TSKha 30 (part 2): 332-336. [ In Russian. ] AA-396/61.______ 1958. [DATA ON THE BIOLOGY OF BLOSSOMING AND POLLINATION OF CARROTS.] Dokl. TSKha 36: 118 - 123. [In Russian.] AA-727/62.
POOLE C. F.
1937. IMPROVING THE ROOT VEGETABLES. U.S. Dept. Agr. Yearbook 1937: 300-325.SLATE, W. L.
1927. REPORT OF THE DIRECTOR. Conn. Agr. Expt. Sta. Bul. 291: 91-111.THOMPSON, D. J.
1962 NATURAL CROSS-POLLINATION IN CARROTS. Amer. Soc. Hort. Sci. Proc. 81: 332 - 334.WELCH, J. E., and GRIMBALL, E. L., JR.
1947. MALE STERILITY IN THE CARROT. Science 106: 594.WHITAKER, T. W., SHERF, A. F., LANGE, W. H., and others.
1970. CARROT PRODUCTION IN THE UNITED STATES. U.S. Dept. Agr., Agr. Handb. 375, 37 pp.
