Herbertia 2: 109-110. 1935
Artificial Reversal of Growth Dominance in Amaryllids
Hamilton P. Traub, Florida
In connection with the working out of a technic for amaryllid breeding with flowers on excised scapes, an interesting case of the artificial reversal of growth dominance has been observed.
In a large number of trials, self or cross pollinated flowers of excised amaryllid scapes, especially those of Hippeastrum, placed in water or nutrient solution, have in the great majority of cases produced seeds. Within limits, the number of seeds per capsule seems to be largely a function of the relative size ("fleshiness") of the peduncle. Species in five Genera have been used in the experiments,— Hippeastrum, Crinum, Haemanthus, Zephyranthes and Narcissus. In Zephyranthes the number of seeds produced has been below expectancy, especially in the case of Z. Atamasco and Z. treatieae, which may be due in part to the relatively small size of the peduncle. Z. robusta, with a larger peduncle, produces a relatively larger number of seeds per capsule. Although abundant seeds have been secured from excised scapes of Crinum asiaticum, C. longifolium album and C. longifolium roseum, only an abundant number of fleshy fruits without seeds were produced in the case of C. augustum, a doubtful species which does not set seeds under Florida conditions. A Burbank hybrid Crinum produced many small seeds in each pod which were not viable. Approximately 5 percent of flowers on excised scapes of Haemanthus multiflorus have produced seeds.
The behavior of Narcissus tazetta, The Pearl, under Florida conditions, led to the experiments reported below. This variety does not set seeds under Orange County, Florida, conditions, but it was noticed that when excised flower scapes fell to the ground, in the partial shade of the rows of narcissi, in due course, seed capsules were matured containing a few seeds. This suggested the hypothesis that growth dominance for this variety of Narcissus tazetta is normally confined to the lateral growth at the base of the flower scape (2) after flowering in central Florida and that growth dominance in excised scapes can be reversed in favor of the developing ovary under favorable conditions.
To test this hypothesis further an experiment was carried out with excised scapes of Hippeastrum equestre major in water with flowers pollinated. The controls consisted of excised scapes in water with flowers not pollinated and unexcised scapes with flowers pollinated. This variety of H. equestre propagates very rapidly by offsets under Florida conditions but normally does not set seeds.(3) Six flowers were self pollinated; the same number were crossed with pollen from a named hybrid variety. In the first case 66 2/3 percent and in the second, 100 percent of the flowers produced seeds. The ovaries on the scapes used as controls dried up soon after the flowers faded. The ovaries on the excised scapes in water where pollen was applied increased to normal size even when no seeds were produced. The number of seeds was low, usually only one or two seeds in the upper end of each of the three locules. The results indicate that part of the failure to produce seeds in this variety under Florida conditions may be due to self-incompatibility but another important factor is apparently to be found in the growth dominance of the rapidly forming offsets, or the new lateral growth at the base of the declining flower scape.(2)
Although these results clearly indicate that growth dominance may be artificially reversed in favor of the apical stem (peduncle, and ovaries at its periphery) they do not give direct evidence as to the causal mechanism responsible for the reversal. The variety H. equestre major is native to tropical Americas and its failure to set seeds in Florida may be due to the fact that it flowers from February to April inclusive, during the latter part of the dry season in the State. The Narcissus variety, The Pearl, also flowers during the winter. A hormone mechanisn (5,1) on the basis of growth promoting and growth inhibiting substances (4) might be postulated to account for the facts but no experiments with this in mind have been carried out. The plant material may be of value in such studies.