Negative Phototropism in Vaucheria terrestris Regulated by Calcium I. Dependence on Background Blue Light and External Calcium Concentration :

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In the presence of 1-8 mM Ca^<2+> unilateral 10-15 min irradiation with blue light can elicit negative phototropic bending in the tip-growing coenocyiic fresh water alga, Vaucheria terrestris (Xanthophyceae), when it is simultaneously irradiated with strong blue or green background light. By changing wavelength, fluence rate, and duration of background light and holding those of unilateral light (456 nm, 1.7W・m^<-2>) negative phototropic response was analyzed: the wavelength of the background light had to be shorter than 540 nm; red light (660 nm) was ineffective even at very high fluence rates (≩ 100W・m^<-2>). The negative bending was strongly and specifically dependent on the external Ca^<2+> concentration. Other divalent cations, Mg^<2+> and Ba^<2+>, were either toxic or quite ineffective; Sr^<2+> could partly support the growth, but mediated neither positive nor negative phototropic bending. The rate of tip-growth was not significantly altered between 10^<-6> M and 10 mM Ca^<2+>. Pre-irrradiation with the background light slightly in-creased the negative curvature; whereas that without subsequent simultaneous irradiation does not cause negative bending, but rather increases the positive curvature. Three-min delayed start of background light did not cause negative bending any more. The present results strongly suggest that blue light elicits an influx of Ca^<2+> at the apex of Vaucheria and that the increased cytoplasmic Ca^<2+> regulates the sensitivity and direction of phototropic response.

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