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

This is the first report of a positive light-growth response(LGR) in tip-growing cells. Tip-growth of a coenocytic fresh-wateralga Vaucheria terrestris was temporarily accelerated by a shortpulse of blue light. The LGR occurred after a lag period shorterthan 1 min, and reached its maximum ca. 3 min after the onsetof blue light. The growth rate then rapidly decreased againand often showed a damped oscillation with a period of about10 min. If the blue light pulse was shorter than 2 min, themagnitude of the LGR seemed to obey the reciprocity law. Anothertype of growth promotion, the apical expansion, is brought aboutwhen 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 thepH of the medium: it was 5 J m ^–2 at pH 7.5, 150 J m^–2at pH 6, and 750 J m^–2 at pH 5. The phototropic responsealso showed a very similar pH-dependency between pH 5.5 andpH 6.5. The relationship between the positive LGR and positivephototropic response was found to be much closer in this tip-growingalga than in diffuse-growing cells. The possibility that theLGR is the primary and essential process preceeding the phototropicresponse is discussed.