Light dependence of protoplasmic streaming in Nitella flexilis L. as measured by means of laser-velocimetry.

Laser-velocimetry was applied in order to study the effect of light on the velocity of protoplasmic streaming (pps) in Characean cells. A change from dark to light (= 6 W · m(-2)) leads to an acceleration of streaming by about 15-30% with a time-constant of approx. 300 s. The transition from light to dark causes a transient decrease of velocity below the original dark level. This response occurs with a time constant of about 500 s. It returns to its initial value with a time-constant of about 2000 s. This may indicate that a control loop of cytosolic homeostasis takes a decrease in pCa more seriously than an increase. A possible involvement of temperature effects caused by illumination was excluded by measuring the influence of temperature. Steady-state velocity of streaming changed by 5% per 1° C. Irradiation with infra-red light (λ > 780 nm) did not cause a change in velocity. The absence of a light effect on streaming velocity in the presence of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) shows that photosynthesis and not phytochrome is involved. The role of light-induced changes of pCa is discussed, especially with respect to the hypothesis of Vanselow and Hansen (1989, J. Membr. Biol. 110, 175-187) that photosynthesis acts on the plasmalemma K(+)-channel via light-induced uptake of Ca(2+) into the chloroplasts.