The action spectrum for the light depended movements of chloroplasts in Lemna trisulca L.

Abstract

1. The problem studied in this investigation was the influence of irradiation in various spectral regions and of different intensities on chloroplast movements in cells of Lemna trisulca leaves. The distribution of chloroplasts in a cell was established from counts of the number of chloroplasts on the proximal and distal (with regard to the source of light) cell walls. 2. It was found that the course of the displacements from the position of weak light to the position of darkness varied greatly and depended on the material used in the experiments, nevertheless the final result was almost always the uniform distribution of the chloroplasts on all the cell walls. 3. Experiments with five widely spaced wave regions showed that the phototactic activity was exerted by the visual and the near-UV parts of the spectrum. The near-as well as the far infra-red radiation was inactive the displacements being the same as those taking place in darkness. The short TXV radiation (240-350mμ) destroyed the ability to assume phototactic arrangements. 4. Radiation in the spectral region 360-530 mμ induced typical phototactic arrangements. The action spectrum in the reaction from the darkness to the weak light arrangements was exactly the same as in the reaction from the weak light arrangement to that of strong light with one peak at 450 mu and two lower ones at 485 and 380 mv-. The region 550-715 mμ did not induce the formation of phototactic arrangements but had a regulating influence on the speed of the displacements. The action spectrum of photokinetic activity had a peak at 680 mμ 5. Motion film analysis showed that in the short-wave region the pattern of the displacements was differentiated into various types of movements according to the intensity of incident light. In the longwave region the pattern of the movements was essentially the same as in darkness and only the speed of the movements was higher. 6. Experiments with the centrifugation of chloroplasts showed that the weak light arrangement was accompanied by a drop in the ability of chloroplasts to be displaced by centrifugation. The parallelism of the two processes was reflected by the action spectrum as well as by the range of light intensities. 7. The phototactic arrangements were only indirectly related to photosynthesis. The arrangement of chloroplasts was controlled by the short-wave part of the spectrum, but its sensitivity in white light of colour temperature about 6000°K was such that the full strong light arrangement was reached near the saturation point of photosynthesis. 8. On the ground of the assembled results an attempt has been made to develop a working hypothesis explaining the mechanism underlying the movements of chloroplasts.