Journal of Heredity 16(12): 463-464
PRODUCTION OF TRIPLOID AND TETRAPLOID PLANTS
JOHN BELLING
Carnegie Institution of Washington

TRUE triploid plants are now known in Oenothera, Morus, Canna, Datura, Solarium, and other genera; while tetraploids have been found in Oenothera, Primula, Solarium, Datura, etc. The chief characteristic of true triploids is a partial or total sterility; the triploid cannas, for instance, never (or rarely) producing seeds; and the triploid mulberries being seedless and parthenocarpic.

The characteristic of the tetraploids is the possession of much larger cells; amounting in spherical unvacuolated cells, such as pollen-mother-cells, to twice the volume of those of the diploid, and in vacuolated cells to even more.

The production of a triploid apparently originates with the doubling of the chromosome number in one of the gametes. This doubling is in many cases certainly due to a temporary chill, resulting in non-reduction or non-division during the maturation divisions of pollen-mother-cells, or megaspore-mother-cells (or to non-division in the somatic mitoses which give rise to the sub-epidermal tissue of a branch). The case was well exemplified in a field of Stizolobium (Mucuna) in Florida, where a temporary cold wave, in which however the freezing temperature was not reached, caused abundant production of 2n pollen-grains (and probably of 2n egg-cells also). The same phenomenon has been studied with the microscope in Uvularia (Journal of Genetics, 1925), and also in Datura, after chill in a greenhouse. From triploid plants which have originated in this manner, tetraploids can usually be produced by selfing, as in Oenothera. If tetraploids are of spontaneous occurrence, as in Datura, triploids can readily be obtained by crossing them with pollen of diploids.

Triploid plants are already of commercial value for their flowers in the case of a few cannas. The advantage here consists in the dropping of their young seed-vessels, which set no seeds, and so do not prevent other trusses of flowers from appearing on the same stalk. Even the unremitting removal of all old stalks will not prevent, in many of the ordinary diploid cannas, the production of unripe seeds, to the consequent detriment of the prolonged flowering of the plant. In the triploid mulberries, of which many clones are grown in Japan, no seeds usually appear, although ripe fruits are formed. The production of seedless fruit is sometimes a desideratum.

Pollen-mother-cell of triploid hyacinth, King of the Blues. This cell had divided into 11 and 13 at the first division, and again into 11 + 11 and 13 + 13 at the second division. The three sizes of chromosomes are obvious. The triploid number was 24 chromosomes, made up of 6 small, 6 medium, and 12 large. (Magnified 2,000 times.) Late first prophase of pollen-mother-cell of the triploid hyacinth, King of the Blues. This cell shows eight trivalents, 2 small, 2 medium, and 4 large. Two of the large (on the right) seem larger than the other two. (Magnified about 2,000 times.)

The examples point to the desirability of the artificial production, by the action of temporary cold on the young flower-buds, of many other triploids, both in plants with ornamental flowers and in fruit trees. Triploids of most garden flower plants which are propagated as clones, and which have a prolonged flowering period, such as cannas and dahlias, would be of horticultural value. In cases of trees or shrubs, where seeds are undesirable, such as the Para rubber, for instance (and where the absence of seeds can not be readily procured by making species crosses), triploid plants would he valuable. Even in fruit trees, triploids would he desirable, if parthenocarpy occurred; and the seedless triploid mulberries of Japan might well be paralleled by seedless triploid apples, plums, peaches, mangoes, avocadoes, cherimoyas, etc.

In fact the systematic production of triploidy, by chilling the young flower-buds, presents for the future the pleasing prospect of a method which will probably in time enable the horticulturist to remove the seeds from a number of the temperate, and especially from the tropical fruit trees. There is indeed some reason to think that it is the tropical plants (and several of our fruits and vegetables are essentially tropical plants) which are most readily affected by temporary chills in the desired manner.

With regard to tetraploidy, it offers a method (as yet quite unused) for the horticulturist to employ in increasing the size of the plant cells. This may be of prime importance in fiber plants as cotton, or flax, and perhaps in certain timber trees. But, with the increase in the size of the cells, the size of several plant organs is also increased. The production of larger flowers and of larger fruits is an important aim of horticulture. This will probably be facilitated by the use of tetraploidy; and its employment is not, as is that of triploidy, confined to plants propagated asexually, for tetraploids usually set a fair number of seeds. Hence tetraploidy may in the future he applied to perhaps all the chief fruits of the temperate and tropical regions, with fair prospects of success. Stems, such as those of the sugarcane, may he increased in size by this method, and perhaps the increased size of the cells may facilitate the extraction of the juice. In fact, artificial tetraploidy seems to have before it a wide field in the improvement of the chief economic plants. The method is simple, and the principal material requirement is artificial ice, which is procurable now throughout the civilized world.