Stem Sensitivity and Ethylene Involvement in Phototropism of Mung Bean

Abstract

A system is described for the examination of phototropism in the epicotyl of a dicot seedling, mung bean (Phaseolus aureus Roxb.), under conditions approximating nature, including the use of intact, nonetiolated plants exposed to elevated, continuous, white, unilateral light. It is found that in this system perception of the phototropic stimulus by the leaves alone cannot account for the curvature, and that exposure of the stem is also necessary. The phototropic response was found to be strongly altered in nonintact plants. Hypobaric Previous research on dicot phototropism (11,17) led to the conclusion that the phototropic stimulus is perceived primarily by the leaves and cotyledons. Our experiments indicate that MATERIALS AND METHODS Plant Material. Seeds of Phaseolus aureus Roxb. were surface-sterilized by soaking them for 5 min in 5% Clorox solution. The seeds were then rinsed and soaked in continuously aerated tap water for 24 hr. The aeration served both to provide 02 and to help dislodge the seed coats. The water was changed after approximately 6 and 18 hr. At the end of the 24-hr period the germinating seeds were removed, rinsed, and planted 0.5 cm deep in medium-grain perlite in 5-cm square plastic pots, four seeds per pot. The plants were grown in a growth chamber under 16 hr light and 8 hr dark, 28 C/20 C, and 65% relative humidity. The light intensity in the chamber was 1500 ft-c provided by incandescent and fluorescent lamps. Plants 7 days old (from the time when inhibition began) and 5 cm + 3 mm in height with straight epicotyls were used for experimental purposes. Only one plant per pot was selected; the other three were discarded. At this stage, the primary leaves were fully developed, but the secondary leaves were still embryonic.Phototropic Illumination. A Precision Scientific Co. oven (inside dimensions 30 x 20 x 20 cm) lined with black paper was used as an illumination chamber. The chamber had a glass door and gas inlet and outlet valves. The light source was a PBL Lab-source QH 150 quartz halogen illuminator equipped with an HM-A90-012-M heat reflecting filter and an HAF-01-M heat absorbing filter. The intensity dial on the instrument was set on "'L,'" giving an intensity of 650 ft-c in the chamber. The lighting configuration is shown in Figure 1. Unilateral illumination was applied continuously for the periods of time indicated. The plants were arranged ir the chamber in two rows of three each, still in the pots, with the pair of primary leaves oriented perpendicular to the direction of the light except where otherwise indicated. During the experiments, fresh air was continuously 286 www.plantphysiol.org on May 10, 2018 -Published by Downloaded from

Role of the Cotyledons in the Phototropic Response of Lavatera cretica Seedlings

“…4) are in agreement with those of other dicot seedlings (2,8,11). These data support the conclusion that the hypocotyl itself contains the photoreceptor for its positive phototropic curvature in unilateral light since this response did not depend on directional information from the cotyledons, on their being in light or on any immediate supplies from them.…”

Phototropism of Selaginella: The Role of the Small Dorsal Leaves and Auxin

“…The removal ofthe leaves ofgreen sunflower seedlings suppressed the phototropic responsiveness by more than half (Shibaoka and Yamaki, 1959). When one of the primary leaves ofuniformly illuminated mung bean plants was masked, no measurable curvature of the epicotyl occurred, and the masking of both of the primary leaves only reduced the curvature of plants exposed to unilateral light by 25% (Brennan et al, 1976). However, the removal of either one or both of the primary leaves did prevent a phototropic response to the mung bean epicotyl.…”

“…Removal of the shoot apex and immature leaves of the apical bud of Se/aginella had no effect upon the positive phototropic response ofthe horizontal stems. With mung bean (Phaseo/us aureus), masking the stem tip with an inert lanolin paste containing charcoal reduced the phototropic curvature of the stem by more than half and removal of the tip completely inhibited phototropic curvature (Brennan, Gunckel and Frenkel, 1976). On the other hand, removal of shoot apex had no effect upon the phototropic response of sunflower (Helianthus) hypocotyls (Bruinsma et al, 1975).…”

“…Furthermore, Franssen and Bruinsma (1981) were unable to reproduce the results of Bruinsma et al (1975). The premise that the cotyledon or leaffurthest from the sun receives less light and that phototropism could then be simulated by covering it with foil has been challenged by Brennan et al (1976) for mung bean.…”

Electrophysiological localization of blue light sensory sites in etiolated dicotyledon seedlings

“…For dicotyledons this issue remains unclear, with some controversy in the literature. Although early research suggested that in dicotyledon species the phototropic stimulus is perceived primarily by the leaves and cotyledons (Shibaoka & Yamaki 1959; Lam & Leopold 1966), subsequent studies on de‐etiolated mung bean, cucumber and sunflower seedlings indicated that the primary zone of phototropic perception is the stem (Brennan, Gunckel & Frenkel 1976; Shuttleworth & Black 1977).…”