Dose-Response Curves for Radish Seedling Phototropism

Abstract

Radish seedlings (Raphanus sativus L.) were grown for 4 days in complete darkness, or in white light, or for 3 days in darkness followed by 1 day of red light. Phototropic dose-response curves for the seedlings grown in these three ways were determined with 460-nm light. Dicot seedlings have occasionally been used for studies of phototropism. Most of these experiments relate to the response mechanism; auxin and gibberellins have been investigated as the causal agents of the unequal growth which leads to phototropic bending (2,5,6,8, 10). However, little information was available on the characteristics of the photoreceptor in these seedlings until Steyer (9) published dose-response curves for phototropism for 13 kinds of dicot seedlings. In these experiments, all the dark-grown seedlings were found to have first positive, then indifferent, and then second positive types of response as the energy in the light stimulus was increased. Steyer's studies indicate that, in sensitivity and speed of response, dicot seedlings are comparable to coleoptiles and that these seedlings would be good objects for more detailed experiments. The Fenway, Boston, Mass. 02115. This paper reports on dose-response curves for phototropism in radish seedlings grown under three light regimes. MATERIALS AND METHODSRadish (Raphanus sativus L.) seeds, Burpee's white radish, were soaked for 2 hr in distilled water, placed in a 5% solution of commercial bleach for 5 min, and then rinsed in distilled water. The seeds were spread out in shallow enamel pans on Kimpak-brand absorbent paper moistened with distilled water. The pans were covered tightly with aluminum foil and placed in a dark cabinet in a room maintained at 25 C. Approximately 24 hr after soaking, seedlings with radicals 3 to 5 mm long and with seed coats removed were selected for transfer to individual metal vials. The vials were 2.8 cm deep and 1.8 cm in diameter and fit snugly into metal racks. Each vial had a handle so that, when placed in the rack, it could be rotated 900 by means of the handle and guides on the rack. The vials were lined with aluminum foil and filled with moist vermiculite.The 24-hr-old seedlings were either grown in the dark, or grown in the dark and then red light treated, or grown in white light. For dark growth, seedlings were transferred to vials under a photomorphogenically inactive dim green safelight described by Withrow and Price (11). The racks holding these seedlings were then placed in deep enamel pans; the pans were covered with aluminum foil and placed in a dark cabinet at 25 C. Seedlings to be treated with red light were transferred to vials under a green light described by Shen-Miller and Gordon (7). The racks of vials were then placed in an enamel pan covered with aluminum foil in a dark cabinet at 25 C. Approximately 72 hr after soaking, the aluminum foil was removed and clear plastic wrap was placed over the pan. The seedlings were then exposed to red light at an intensity of about 0.6 mw cm-2 as described by . After 24 hr of red light,...

Comparison of Fluence-Response Relationships of Phototropism in Light- and Dark-Grown Buckwheat

“…Dark-grown seedlings of Arabidopsis (15), pea (2), radish (7), corn (1 1), and oats (1, 17) have a minimum threshold of from 1 to 10 nmol m-2 below which curvature does not occur. This same threshold appears at about l0-3 and nmol m-2 in etiolated buckwheat.…”

“…Unfortunately, the vast majority of rigorous studies have employed only one kind (usually complete darkness) of growth environment for the experimental seedlings. In those few studies in which the species investigated was grown in a variety of conditions, the value of the comparisons made is restricted by the use, in each case, of only one fluence rate or, at most, an extremely limited range (1,3,7,8,10,12). Further, these rates ranged from 7 (1) to 8000 nmol m-2 s-' (10) about the relative phototropic sensitivity oflight-and dark-grown plants varied equally widely.…”

Phototropism: mechanisms and ecological implications

“…Of the three phases of PIP of oats, fPIPP has been found in various monocots (Briggs, 1960;Steyer, 1967) and dicots (Steyer, 1967;Everett, 1974;Steiniz & Pofl", 1986;Baskin, 1986a;Baskin & lino, 1987;Sakoda et al, 1988). On the other hand, PINP has not been found in any other plant species.…”

Pulse-Induced Phototropisms in Oat and Maize Coleoptiles

“…Substantial evidence has accumulated indicating the widespread occurrence of fPIPP in higher plants (1,2,5,7,21,22). The occurrence of fPIPP has also been demonstrated for fungi, Phycomyces (10) and Pilobolus (15).…”