Responses of Elodea densa to Growth-Regulating Substances

Abstract

1. Since Elodea is an aquatic, excised bud-node and apical segments were readily grown in aqueous solutions of some sixteen growth substances and related compounds. The principal gross responses noted were axillary bud development, shoot elongation, the number of roots emerging, time and order of their appearance, their lengths, and root-hair development. 2. The injury of excision provided the initial stimulus for renewed growth in the bud nodes. Although some minor modifications were noted, where damage through treatment had not occurred, the general pattern of vegetative reproduction remained relatively unaltered in both control and test solutions. Excised bud nodes, in tap water alone, and cultured under suitable growing conditions, developed into complete new plants. 3. Nearly all the cultures were grown in 250-ml. glass tumblers under daylight fluorescent light with a daily light period of 13 hours. Experiments with different intensities of light showed that a reasonably good growth was maintained at 400 f.c. 4. In this species four root primordia are formed at each bud node very early in the ontogeny of the primary axis. There are two latent primordia in the primary axis in the region of divergence of the secondary axis, and two in the base of the secondary axis near the point of divergence from the primary axis. Even in rooted shoots as long as 1 m. these primordia remain latent. Upon excision of the bud node the two primary primordia emerge, and later the two secondary primordia may emerge. In control cultures in tap water these secondary primordia rarely appear, at least in the culture period of about 20 days. When these nodes are grown in solutions of certain indole or naphthalene compounds, stimulation of all four primordia may occur, and occasionally fifth roots may emerge. 5. For total root lengths at 16 days (experiment 1, concentration 1 p.p.m.) the decreasing magnitude of response was: indolepropionic, indolebutyric, indoleacetic, l-tryptophane, phenylacetic, control, B1, and naphthaleneacetic. Average individual root length was: l-tryptophane, indolepropionic, control, phenylacetic, indoleacetic, B1, indolebutyric, and naphthaleneacetic. For total number of roots the order was: indolebutyric, indolepropionic, indoleacetic, naphthaleneacetic (l-tryptophane, phenylacetic, and B1 all similar), and control. 6. For total elongation of apical segments in the dark series at 129 hours (experiment 13, concentration 5 p.p.m.) the decreasing magnitude of response was: naphthaleneacetic, indolepropionic, naphthalene acetamide, indolebutyric, indoleacetic, ascorbic, l-tryptophane (B1 and histidine similar), and control. In the light series the order was the same through acetamide, and then the order was indoleacetic, indolebutyric, and control. The remainder of the substances had some inhibitory effect in the light. The highest growth rate measured was 0.30 cm./hr. (control, 0.13 cm./hr.) in the indoleacetic dark series at 21 hours. 7. Root hairs were rarely formed on the free-growing roots of excised bud nodes, larger segments, or shoots, growing under adequate light intensities. Root-hair formation did occur upon treatment with certain types of compounds. Of all the different types used, only those compounds with the indole or naphthalene ring system (and containing acid side chains) were associated with root-hair formation. In cultures in indolepropionic at 5 p.p.m., structural abnormalities developed in the terminal portions of the hair cells-possibly related to the abnormal water relations induced and greatly magnified by this high concentration. In view of the specificity of the response to certain types of compounds, this root-hair test of Elodea would appear to be a useful method of establishing a diagnostic character for the classification and characterization of certain of the growth-regulating substances. 8. The degree of inhibition and stimulation induced by many of the growth substances (aside from concentration) was a function of the light intensity, and to some extent the size of the segment employed. Increased growth of roots and shoots was associated with indolebutyric, indolepropionic, and histidine at low light intensities, where in control cultures little or no growth occurred. The association of growth-regulating substances with the synthesis of total solids, with starch hydrolysis, cellular respiration, salt absorption, and water regulation was considered in relation to cell elongation and to other growth processes.