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Rapid Respiratory Changes Due to Red Light or Acetylcholine during the Early Events of Phytochrome-mediated Photomorphogenesis ¹

^a Department of Botany, Ohio University, Athens, Ohio 45701

Two millimeter long secondary root tips of etiolated mung bean (Phaseolus aureus) plants were given 4 minute consecutive treatments of darkness, red light, far red light, and acetylcholine during darkness. We studied the effects of these treatments on exogenous (H⁺) changes, ATP utilization, O₂ uptake, P₁ levels, and ATPase activity. Red light and acetylcholine increased the level of P₁, O₂ uptake, and exogenous H⁺, but decreased ATP concentrations. Darkness and far red light caused the amount of ATP to increase and decreased the O₂ uptake and P₁ level. O₂ uptake of both excised root tips and isolated mitochondria was promoted by acetylcholine levels of the same order of magnitude that promoted the other photomimetic phenomena. ADP-O ratios indicated that acetylcholine did not cause an appreciable decrease in ATP synthesis. The total ATPase activity remained constant throughout all treatments. Ouabain caused no adhesion to negatively charged glass in the dark, while the inhibitors valinomycin, atractyloside, digitoxin, gramicidin, and oligomycin caused immediate adhesion. All of the inhibitors prevented release from the glass. In red light ouabain increased adhesion, whereas the other inhibitors caused caused immediate and complete adhesion.

These data seem to imply that one of the functions of the phytochrome-mediated response to red light in roots, regulated by acetylcholine, is to cause the rapid utilization of ATP pools; far red light appears to inhibit this utilization.