Reversible form- und volumenänderungen des Mougeotia-Chloroplasten

Abstract

Summary 1. The chloroplast of Mougeotia contracts in white or blue light of high intensity. This contraction is a reversible physiological response in the range of 10 to 200 kerg/cm2sec blue light and of temperatures 5° to 25°. 2. Since only the short wave part of the visible spectrum is effective, it is suggested that a flavin might be the photoreceptor. Comparing with an analogous response in Mnium, the photoreceptor might be localized in the chloroplast proper. 3. The contraction takes some hours, depending on the temperature, after which a final level will be reached (fig. 1). 4. The amount of contraction increases with the logarithm of the light intensity without reaching a saturation within the physiological range (fig. 2). 5. The temperature influences the contraction in a complex way (fig. 1 and 3): The higher the temperature, the more rapid the contraction proceeds with shortening the lag period and with reaching the final level earlier. At the other hand, the threshold intensity and the intensity needed for a particular contraction value increase with increasing temperature. 6. Detailed measurements revealed that contraction means a decrease in volume, due to a decrease in length (as measured normally in the experiments) and a decrease in width, but accompanied by an increase in thickness (tab. 2). This means an important change of the internal structure of the chloroplast. The decrease in volume must be due mainly to loss of water. 7. No evidence could be obtained for assuming an osmotic mechanism of water efflux from the chloroplast (cf. tab. 3); instead, an active mechanism has to be assumed. 8. In phase contrast observations and scanning electron microscope preparations, cytoplasmic strands have been demonstrated (fig. 4, 5) which, however, are assumed not to represent structures active in chloroplast contraction. Instead, contractile structures are assumed to be localized in the chloroplast proper. 9. Comparing with other responses of the Mougeotia chloroplast and with chloroplast contraction in other plants, it seems reasonable to assume, that light dependent fastening or loosening between the chloroplast and the cortical cytoplasm is an important step in chloroplast contraction.