Red light enhancement of the phototropic response of etiolated pea stems

In the subapical third internode of 7-day-old etiolated pea seedlings, the magnitude of phototropic curvature in response to continuous unilateral blue illumination is increased when seedlings are pre-exposed to brief red light. The effect of red light on blue light-induced phototropism becomes manifest maximally 4 or more hours after red illumination, and closely parallels the promotive action of red light on the elongation of the subapical cells. Ethylene inhibits phototropic curvature by an inhibitory action on cell elongation without affecting the lateral transport of auxin. Pretreatment of seedlings with gibberellic acid causes increased phototropic curvature, but experiments using ¹⁴C-gibberellic acid indicate that gibberellic acid itself is not laterally transported under phototropic stimuli. Neither red light nor gibberellic acid treatment has any promotive effect on blue light-induced lateral transport of ³H-indoleacetic acid. Under conditions where phototropic curvature is increased by red light treatment, low concentrations of indoleacetic acid applied in lanolin paste to the apical cut end of the seedling cause an increased elongation response in subapical tissue. This could explain increased phototropic curvature caused by red light treatment.