Previous studies have examined the effects of red light (R) on phototropism of maize ( Zea mays L. cv. Royaldent Hit 85) coleoptiles. The R effect on time-dependent phototropism (TDP) was further studied by characterizing its fluence-response relationship. The results showed the R effect was a low-fluence-response, unlike those on pulse-induced phototropisms that show a very-low-fluence-response mode. A subsequent pulse of far-red light (FR) could reverse the R effect. TDP responsiveness, however, recovered as the following FR was extended, The FR-dependent increase in TDP responsiveness was obtained even coleoptiles were pretreated only with FR. It suggested that TDP responsiveness could also be established in response to a FR signal. The fluence-response relationship for the effect of FR was then investigated. The effect of FR depended on the time of irradiation and required high photon fluences. Because reciprocity was invalid at the higher fluence range, the effect of FR would be a high-irradiance-response mode. Relation between phytochrome action modes and possible multiple pathways for phototropic signal transduction was analyzed based on the experiment results.
