The Light-Growth Response of Vaucheria. A conditio sine qua non of the Phototropic Response?

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This is the first report of a positive light-growth response (LGR) in tip-growing cells. Tip-growth of a coenocytic fresh-water alga Vaucheria terrestris was temporarily accelerated by a short pulse of blue light. The LGR occurred after a lag period shorter than I min, and reached its maximum ca. 3 min after the onset of blue light. The growth rate then rapidly decreased again and often showed a damped oscillation with a period of about 10 min. If the blue light pulse was shorter than 2 min, the magnitude of the LGR seemed to obey the reciprocity law. Another type of growth promotion, the apical expansion, is brought about when the pulse is longer than 5 min. In this report, however, only the LGR which is caused by a short pulse of blue light, is dealt with. The threshold fluence at 456 nm was ca. 10 Jm^<-2> at pH 7.0. The response was very sensitive to the pH of the medium: it was 5 Jm^<-2> at pH 7.5, 150 Jm^<-2> at pH 6, and 750 Jm^<-2> at pH 5. The phototropic response also showed a very similar pH-dependency between pH 5.5 and pH 6.5. The relationship between the positive LGR and positive phototropic response was found to be much closer in this tip-growing alga than in diffuse-growing cells. The possibility that the LGR is the primary and essential process preceeding the phototropic response is discussed.
日本植物生理学会の論文
1987-01-00