No abstract

An apparatus for measuring simultaneously the surface electrical potentials along epicotyl was developed to study the largely bending situation by gravity. Potentials increased on the upper side and decreased on the lower side after the horizontal placement. The time course of electrical changes consisted of two components which correspond to growth movements observed during gravitropism.The Cholodny-Went model of gravitropism in plants suggests that the asymmetric distribution of auxin results in a gravitropic response in shoots (14). The process leading to such an unequal auxin distribution is not known. Some investigators have suggested that auxin accumulation results from an electrical asymmetry between the upper side and the lower side in the horizontal position (1 1).In roots, electrical currents around a primary root after gravistimulation have been studied (1, 3), and also membrane potentials have been measured in lateral statocytes of vertically and nonvertically growing roots (2). The currents flow acropetally on the upper side of the root cap and basipetally on the lower side. Statocytes on the lower side of the root depolarize, and statocytes on the upper side hyperpolarize. This electrical asymmetry is the earliest event in graviperception.In shoots, early workers observed a transverse electrical potential in the horizontally oriented shoots (4,5,16). Grahm and Herz measured the potential between the upper and lower sides for 1 h and observed the potential increase after 15 min (4). Woodcock and Wilkins (15,16) and Wilkins (5) considered that asymmetric IAA distribution produced the electrical field across the gravitropically responding shoot during upward curvature. However, Tanada and Vinten-Johansen ( 11) observed the development of a positive electrical potential on the lower side approximately 1 min after horizontal placement. This time is less than the presumed gravitropic presentation time of the shoots. They suggested that Grahm and Hertz and others might have been unable to detect a fast positive gravielectrical effect. However, this experiment seems to need a reexamination because the electrical potentials on the lower side drift to negative in the vertical position (9).The explanation for the electrical potentials during gravitropism in epicotyls requires long-term, sensitive measurements of the potentials not only on the lower side but also on the upper side throughout the entire time course of gravitropism. We have measured the surface electrical potentials on the lower and upper surfaces of shoots by developing new measuring equipment that was not effected by the amount of gravicurvature. MATERIALS AND METHODS Plant MaterialEtiolated epicotyls of adzuki bean (Phaseolus angularis) seedlings were used. Seeds were soaked in distilled water (40 ± 1°C) for 4 h and were placed in trays on filter papers wetted with 0.1 mm KCI and 0.05 mM CaCl2 solution. The trays were kept in darkness at 30 ± 1°C. After 5 d, the seedlings were moved to plastic cases filled with 0.1 mm KCI, 0.05 mm CaC...

No abstract

No abstract