Journal of Heredity 40: 223-227 (1940)
SEED PRODUCTION IN HORSERADISH (PDF)
W. W. WEBER*

*Paper from the Department of Genetics, College of Agriculture, University of Wisconsin, No. 393. The writer wishes to acknowledge his indebtedness to D. C. Cooper, G. H. Rieman and R. W. Hougas for suggesting the subject of this investigation and thanks are here tendered to them for their many helpful suggestions, continued interest and kindly criticisms. Appreciation is expressed to Eugene Hurling, Department of Plant Pathology, for making the photographs used in this paper.

HORSERADISH is an extremely hardy perennial plant. It forms a rosette of erect, long-petioled leaves. The leaf blade is tough, long, dark green, and has crenate, sinuate, or pinnate margins. The plant may or may not form a flower stalk depending on environmental conditions. The inflorescence is a terminal or axillary raceme of short-pedicelled, small white flowers (Figure 9). Horseradish is highly sterile: the anthers are usually abortive, and normally no pods or seeds are formed, although seedless pods have been reported.

Taxonomically, horseradish has been placed in various genera by different botanists. It is known as, Nasturtium armoracia Fries, Roripa armoracia Hitchc., Radicula armoracia Robinson, Cochlearia armoracia L., Armoracia armoracia Britton, and Armoracia rusticana Gaertn. The latter taxonomic designation is preferred by most workers. Cochlearia macrocarpa var. hungarica is considered by Engler as a primitive form of horseradish, but other workers agree that this is the ordinary plant which has escaped from cultivation. Brzezinski1 believes that horseradish is not a natural species but a hybrid.

Horseradish is grown throughout the northern United States. The largest commercial acreage is found in Missouri and Illinois, although considerable horseradish is grown in Michigan, Ohio, Pennsylvania, northern New Jersey, Connecticut, Massachusetts, and Washington. Horseradish is usually cultivated as an annual crop. It produces a fleshy, white taproot which is used as a condiment. The thickened lateral roots are generally used as sets for propagation.

Two types of horseradish are available to the commercial grower. These are known as the “common" and the Bohemian varieties. The "common" horseradish is a broad-leaved, hollow-petioled type and may have originated in eastern Europe. Nearly all of our commercial acreage is planted with this variety. It produces a root crop of good quality in spite of being very susceptible to white rust and horseradish mosaic diseases. It is reported by Pound5 to be approximately 100 percent virus infected. The Bohemian horseradish, also known as the Maliner Kren or Bayersdorf strain, is a smooth, narrow-leaved, solid-petioled variety and is believed to have originated in southern Europe. It lacks root quality and yield but appears to be highly field resistant to both the white rust and horseradish mosaic diseases. The disease resistance of the Bohemian variety makes it a valuable type to use in a breeding program for horseradish improvement.

Seed Production

A number of clones of horseradish were collected from different areas in Wisconsin and planted in the greenhouse during the fall and winter of 1947 and 1948. They came from commercial plantings and from uncultivated areas where the plants had persisted as weeds for many years. The collection of clones contained a fair sampling of both the "common" and the Bohemian varieties. These original selections proved very fortunate, because one clone of "common," though completely pollen-sterile, produced ovaries in which about 5 percent of the ovules contained normal-appearing gametophytes; and two clones of Bohemian matured some functional pollen. Subsequent collections during the spring and summer of 1948 resulted for the most part in completely sterile clones.


INFLORESCENCE OF BOHEMIAN HORSERADISH
Figure 9

SEED DEVELOPMENT
Figure 10
Viable seed is rarely produced on horseradish. A—Diakinesis showing partial pairing of the chromosomes and other irregularities. B—Longitudinal section of developing seed. Seed development appears to be normal if functional pollination and fertilization has occurred.
The inflorescence is a terminal or axillary racene of short-pedicelled, small white flowers. Horseradish seldom blooms under natural conditions. It can be induced to blossom profusely under greenhouse conditions. The photograph is approximately 1/6 natural size.

No clones of "common" thus far examined have produced any functional pollen. The pollen grains show signs of abortion in the early stages of development. Two clones of Bohemian produced some functional pollen. In one about 25 percent, and in the other about 50 percent of the grains were normal in appearance.

It is possible to effect fruit development on several of the pollen sterile "common" clones by applying pollen from one of the pollen producing Bohemian clones to the receptive stigmas. Fruits develop very rarely following selfing of the pollen producing clones. This is surprising since 30-40 percent of the ovules contain normal appearing female gametophytes. A higher percentage of fruit development would be expected unless the phenomenon of self-incompatibility found in the Brassicas by Kakizaki,3 Detjen,2 and Pearson,4 also applies to horseradish. The fruits develop rapidly for a short period following pollination, then development stops in a high percentage of the cases, with usually only a few continuing to grow. The developing fruits (Figure 11, A) are more or less distorted in appearance. Usually only one seed develops within a fruit, with two seeds rarely being present. The fruits mature in three to four weeks, at which time they dehisce, leaving the seed attached to the placenta wall or permitting it to drop. Dehiscence usually occurs while the fruits are still green in color. It is necessary to keep close watch of the maturing pods, if one wishes to collect the few seeds that develop.

The cross between the two types of horseradish proves to be the most effective means of securing seed. The seed harvested varied from fairly well developed to badly shrunken kernels (Figure 11, B). The selfed seeds are smaller and usually not as well developed as those resulting from the cross. Generally the mature kernels are brown to gray in color. Many of the well developed seeds are viable at maturity but lose the capacity to germinate after short periods of storage.


FRUITS, SEEDS AND SEEDLINGS
Figure 11

VARIABILITY OF HYBRIDS
Figure 12
Fruits, seeds and seedlings resulting from a cross between the "common" and Bohemian varieties of horseradish. A—Fruiting branch showing distorted fruits, aborting fruits and naked ovaries which had not been stimulated to develop. B—Seeds collected from ripened fruits. They ranged from fairly plump to badly shrunken specimens. C—Normal horseradish seedling. D—Seedling with fused cotyledons. The fruits and seedlings are approximately natural size. The seed is enlarged about six times.
Hybrid horseradish plants are highly variable and vigorous. A—Selfed offspring from the pollen producing Bohemian clone. It has smooth, narrow leaves; long, solid petioles and it appears to he resistant to white rust and horseradish mosaic. B, C and D are hybrids between the "common" and Bohemian varieties of horseradish. B—A plant with ruffled, broad leaves and long, partly hollow petioles. C—A plant with smooth, broad leaves and short, hollow petioles. D—A plant with ruffled, very broad leaves and short hollow petioles. Plants B, C and D are susceptible to white rust and appear to be resistant to horseradish mosaic.

Seedlings

A few seeds were obtained from the small percentage of pollinated flowers that produced fruits. Some of these seeds were planted. Irregularities noticed in the seedling stage, such as fused cotyledons (Figure 11, D) are similar to those common among Brassica seedlings. Twelve offspring are being grown. Eleven of these plants represent crosses between the "common" and the Bohemian types of horseradish. The twelfth plant is an offspring of the pollen producing clone (Figure 12, A). These plants vary widely in morphological characters as shown in Figure 12 B, C, and D. The selfed offspring has petioles with solid piths while the crosses have hollow petioles. The leaf blades of the various plants differ in size, shape, smoothness, and color. The offspring are vigorous and show no evidence of the presence of virus. Preliminary tests indicate that the F1 plants, of the cross between the white rust and virus-susceptible "common" clone and the resistant Bohemian clones, are segregating for these characters.

Cytological Studies

The somatic number of chromosomes as determined by aceto-carmine root tip smears is 32 in both the "common" and Bohemian varieties of horseradish. Irregularities occur in the premeiotic and meiotic divisions in microsporogenesis. Partial pairing of the chromosome is to be seen at diakinesis (Figure 10, A).

There are numerous lagging chromosomes on the meiotic spindles. These spores with the requisite number of chromosomes develop into pollen grains. The gametes are formed in the pollen grains prior to dehiscence of the anthers.

The ovary contains 16 to 20 ovules. Meiosis is likewise irregular in megasporogenesis. Oftentimes the spore fails to continue growth or the gametophyte starts to develop and breaks clown prior to maturity so that only a small percentage of the ovules contain female gametophytes that appear normal. There is the occasional accumulation of food materials in the nucellar regions of nonfunctional ovules following pollination.

Elimination of the virus from suitable types through the production of seed should increase commercial yields. The production of fertile lines of horseradish through cross pollination and selection would greatly stimulate improvement work with this plant. Fertile lines would furnish material for a study of the inheritance of disease resistance. It would also furnish material to investigate the status of horseradish as a natural species and its taxonomic position in the scheme of classification.

Literature Cited

  1. BRZEZINSKI, M. J. Akad. Umiejetnosci, Cl. des Sci. Math. et Nat. Bul. Internatl. Ser. It. Sci. Nat. 2, 392-408. Cracaw. 1909.
  2. DETJEN, L. R., and C. A. McCUE. Del. Agr. Expt. Sta. Bull. 180, 147 pp. 1933.
  3. KAKIZAKI, Y. Japan. Jour. Bot. 5: (133)-208. 1930.
  4. PEARSON, O. H. Amer. Soc. Hort. Sci. Proc. (1930) 27: 337-342. 1931.
  5. POUND, G. S. Jour. Agr. Research 77: 97-114.1948.


Consider also the following:

The American Naturalist 49: 5-21 (January 1915)
Some Fundamental Morphological Objections to the Mutation Theory of De Vries
Prof. Edward C. Jeffrey

Physiological sterility is frequently due to entirely different causes than genetical lack of harmony, as for example in the horseradish or the potato (Solanum). In the former it has been found possible to bring about the formation of fertile seed by simply girdling the top of the subterranean storage region of the plant, so as to prevent the undue descent of assimilates. The common white lily, Lilium candidum, presents a similar condition, for here the setting of seed takes place only when the leafy flowering axis is severed from its bulb and kept in water. So far as I am aware, there have been no experiments as to the result of severing the continuity of the phloem (girdling), in relation to the restoration of seed production in the potato. The common yellow day lily (Hemerocallis) possibly presents a case similar to that of Lilium candidum, for it does not ordinarily set seed, although in all the examples I have examined the pollen was morphologically perfect.