OVULE ABORTION IN TRADESCANTIA AS AFFECTED BY AQUEOUS SOLUTIONS OF 2,4-DICHLOROPHENOXYACETIC ACID

THE INCREASING importance of the synthetic plant growth regulators, of which 2,4-dichlorophenoxyacetic acid is a notable example, makes it imperative that information relative to the effects of these compounds on all stages of growth and development be assembled if proper and effective use is to be made of them in the many and diverse fields of agriculture. Since, in addition to use of these compounds as selective herbicides, the synthetic growth regulators have been successfully employed in preventing preharvest drop of fruit (Batjer and Thompson, 1946), in inducing flower formation in pineapples (van Overbeek, 1945), and in a variety of other ways, the necessity of gathering detailed data on the responses of floral and fruit structures to spray applications is obvious. The studies, for example, of Weaver et al. (1946) have shown that spray applications of aqueous solutions of 2,4-dichlorophenoxyacetic acid to plants in the early flowering and fruiting stages produced a very marked reduction in crop yield of soybeans and tomatoes, data which demonstrate that caution must be exercised when applying sprays of growth regulators to economic plants. No stimulation of ovarial growth, tumorous or otherwise, was noted by these investigators. Britten (1947) on the other hand, has presented evidence to the effect that the application of naphthaleneacetic acid to the developing maize caryopsis produces a marked stimulation of maternal tissue with a correlated abnormal development and differentiation of the embryonic tissues. Experimental support is thus given to the Brink-Cooper theory of somatoplastic sterility which states, in essence, that the abortion of seeds results from a failure of the endosperm to develop, this failure resulting from, starvation brought on by an overdevelopment of adjacent maternal tissues (Brink and Cooper, 1941; Cooper and Brink, 1940). Britten is careful to point out, however, that his evidence is not critical. Since naphthaleneacetic acid and the substituted phenoxyacetic acids, as synthetic growth regulators have been shown to possess somewhat similar effects on the bean plant although their concentration-activity response is vastly different (Hamner and Kraus, 1937; Kraus and Mitchell, 1939; Swanson 1946), a study of the reaction of developing ovules and ovaries to the substituted phenoxyacetic acids should yield information of both practical and theoretical interest. In an effort to determine the cause, or causes, of the high sensitivity of some flowering plants to applications of 2,4-dichlorophenoxyacetic acid, a 1 Received for publication July 19, 1948. This paper is based on work done for Camp Detrick, Frederick, Maryland, under contract No. W-18-035-CM112 with the Johns Hopkins University. study of this problem was initiated since it was apparent that it must be related to the failure of development of ovaries and/or ovules which at the time of treatment were in preto slightly post-fertilization stages of growth. At these stages, the ovary and the ovules undergo marked increases in size, and previous work (Swanson, 1946) had demonstrated that tissues showing rapid cell division and considerable elongation were greatly affected by this agent. On a priori grounds, therefore, it would be expected that such structures would show a marked response to the growth regulators even in rather low concentrations. This has been found to be the case, although the response pattern was different from that expected, and this report will concern itself with the material obtained from an investigation of the effects of 2,4-dichlorophenoxyacetic acid on early ovule development. MATERIALS AND METHODS.-Because of their ready availability, and because of the cytological advantages of Tradescantia in general, the flowering heads of Tradescantia paludosa Anders. and Woodson were selected for study. Single drop applications of an aqueous solution of 2,4-dichlorophenoxyacetic acid of 10 p.p.m. concentration were made by placing the drop at the base of the sepals, with the age of the ovule being computed from the day of flowering. Since the flowers of Tradescantia are available for pollination but a single day, following which the petals deliquesce, the time intervals after fertilization could be readily determined. Samples were fixed in acetic-alcohol, sectioned transversely at 15 %,u and stained according to Johansen's safranin-fast green schedule. To insure uniform data, all observations and comparisons were made at the level of the micropyle. NORMAL DEVELOPMENT.-Schnarf (1929) has summarized the information in existing literature relative to the embryology of Tradescantia. Embryo sac development follows the normal type as outlined by Sharp (1943), and the endosperm is of the nuclear type, according to Schnarf's definition. In our material no cell walls were formed in the endosperm except in occasional treated cells. Fig. 1 shows a transverse section of an untreated ovule taken through the micropylar region shortly after fertilization. The two integuments, each consisting of several layers of cells, enclose an embryo sac surrounded by a nucellus which, in the micropylar region at least, is two cell layers in thickness. A rather heavy chalazal region lies between the embryo sac and the ovular attachment to the placenta. At this stage when both the zygotic and endospermic nuclei are undi-vided, although the synergid and antipodal nuclei are disintegrated, the integuments are beginning to move inward, a process