Strong Light Response Research Articles

Strong-light response of Micrasterias thomasiana Archer

Prostrate cells of Micrasterias thomasiana Archer were irradiated from above with intensive blue light. Many of the cells reacted by rising to a profile position. During a period of 15 to 90 min the response is linearly dependent on the duration of irradiation, inferred from the number of rising cells. In a range from 10 to 30 μmol m(-2) s(-1) (equalling 2.7-8.0 W m(-2) at a wavelength of 450 nm) and for an irradiation time of 30 min, the rising reaction was linearly dependent on the quantum flux density. Choosing 30 min irradiation time and a quantum flux density of 30 μmol m(-2) s(-1), the reactive number of rising cells was employed in establishing an action spectrum. As a result of this, a flavin is postulated as the light-percepting pigment in the reaction, whereas chlorophylls do not appear to be involved. The rising reaction can be distinguished from other light-induced movements as a strong-light response, resembling in this respect the movement of chloroplasts within cells. The different sensitivity of individual cells and the importance of this strong-light response for the algal cells is discussed.

The CO2 Effect on Light-Induced Chloroplast Translocations in Higher Plant Leaves

Summary The CO 2 effect on light-induced chloroplast translocation in higher plant leaves was studied photometrically. The main species studied was Tradescantia albiflora . CO 2 was found to act as a factor modifying only the blue-light driven chloroplast response. In the absence of blue light the change of the CO 2 concentration from 0 to 3000 vpm did not influence the dark transmission level. Transient changes of the weak-light response amplitude were within the limits of experimental error. A distinct influence of the CO 2 concentration was visible during the strong-light response. An increase in the CO 2 concentration caused a decrease in the response amplitude, i.e. less complete strong-light arrangement. The main decrease in the amplitude, constituting over 70 % of the total effect, was found when CO 2 , was raised from 0 to 300 vpm. The amplitude decreases observed in the presence of strong red light were about twice as high as those recorded in blue light alone. The greatest effects were found in an anaerobic atmosphere. A hypothesis has been proposed connecting the CO 2 effect with its local gradient in the cell membrane region.

Light-induced chloroplast rearrangements and their action spectra as measured by absorption spectrophotometry

A new combination technique of using both dual-wavelength and opalglas methods for scanning translucent biological samples was applied to leaves of terrestrial plants in order to observe their absorption changes by irradiation and the action spectra for the absorption changes. The measurements of true absorption, free from various effects of scattering, by this technique showed an increase of absorption by weak blue light and a decrease of absorption by strong blue light for almost all of the leaves of 20 plant species examined. These weak- and strong-light responses in absorption were reversible. The fractional increase and decrease of absorbance at 678 nm by weak and strong light were highest, +20% and -31%, for leaves of Begonia semperflorens Link et Otto, and +12% and -13% for leaves of foxtail, Setaria viridis (L.) Beauv., the species examined in further experiments. The response to strong light proceeded to completion earlier than did that to weak light. The strong-light response could be observed separately from the weak-light response by using a leaf pre-irradiated with weak blue light. The responses were measured as a function of light intensity by scanning a single leaf irradiated locally at different intensities, and the action spectra for these responses were measured by scanning a leaf irradiated locally at different wavelengths but at identical intensities. The action spectra for these opposite responses were similar, and showed a band at 450 nm with shoulders but no band in the red region. Microscopic observations of chloroplasts in leaves during irradiation indicated that these changes in absorption are mostly due to rearrangements of chloroplasts in cells caused by irradiation.