Phototropism: mechanisms and ecological implications

Abstract. Phototropism in seed plants, either etiolated or de-etiolated, is mediated by unidentified photoreceptor(s) sensitive to blue and near-UV regions of the light spectrum. Green plants may have an additional phototropic system sensitive to red light. Fluence-response studies of the blue light-sensitive phototropism, initially made on oat coleoptiles, have indicated the occurrence of multiple response types. Of those, two are found to be general: the first pulse-induced positive phototropism (fPIPP), or the so-called first positive curvature, and the time-dependent phototropism (TDP) or the second positive curvature. The fPIPP, elicited by a pulse stimulus shorter than a few minutes, is characterized by a bell-shaped fluence-response curve and the validity of reciprocity. The TDP, elicited by prolonged irradiation, is characterized by its dependence on the exposure time and the invalidity of reciprocity. Studies made on these two response types have revealed the following: (1) plants acquire directional light information for phototropism by sensing internal light gradients created by light scattering and absorption; (2) phototropism results from redistribution of growth, i.e. inhibition on the irradiated side and compensating stimulation on the shaded side; (3) lateral movement of growth regulators, the principle of the Cholodny-Went theory, can account for the growth redistribution, and auxin is clearly the mediating regulator in maize coleoptiles. This review further describes some mechanistic implications of fPIPP. Experimental results indicate that (1) fPIPP is mediated by a single step of photoreaction, (2) the responsiveness, reflected in the height of the fluenceresponse curve, is reduced by pre-irradiation with blue light and recovers gradually afterward, and (3) the light sensitivity, reflected in the position of the fluence-response curve along the log fluence axis, is also reduced by the pre-irradiation and recovers gradually. Analyses of these results, based on kinetic models, suggest that the bell-shaped fluence-response curve is caused by the difference in the amounts of a photoproduct between irradiated and shaded sides, and that fPIPP represents a mechanism of TDP. It is also indicated that phytochrome in the red-absorbing form exerts two separate effects on phototropism: reduction of the light sensitivity and enhancement of the responsiveness. Along with the discussion of the mechanisms of phototropism, their ecological implications are considered.