Light detection system in higher plant chloroplasts: Pigment mediated or overall photon flux density related

Plants adapt to short term changes in irradiance and quality of the light environment by modulating the structure of the thylakoid membranes to make the best use of the available light energy. Shade-acclimated chloroplasts develop more thylakoid surface area as compared to those growing in full sunlight. Conversion of sun-type chloroplasts to shade-types and vice versa on the basis of total thylakoid membrane surface area can occur quickly. However, the response mechanism of chloroplasts to changes in light levels is yet to be understood. This short term light detection mechanism may be mediated by a pigment system other than photosynthetic pigments or it may be regulated by change in overall photon flux density. In order to verify the light detection mechanism, short term shade-acclimated sunflower chloroplasts were exposed for 4h to low irradiance white light supplemented with one part of the visible spectrum (blue, red, or yellow-green) enhanced up to sun irradiance. The main purpose of the treatment was to induce sun response in shade-acclimated chloroplasts. At the end the treatment period, leaf samples were taken for stereological analysis. Percent volume of chloroplasts and starch grains, actual volumes of palisade cells, chloroplasts, starch, and vacuoles, surface to volume ratio and actual surface area of stromal and granal thylakoid membranes were compared between control and treatment groups. The blue and yellow-green light treatment showed the most reduction in granal thylakoid surface area among the three treatment groups. The stromal thylakoid surface area, actual volume of chloroplasts, starch grains, palisade cells and vacuoles apparently did not respond to the treatment. The photosynthetic rate, relative quantum efficiency, and chlorophyll content did not change in response to short term change in light quality. The stereological data suggested that the light detection system in higher plant chloroplasts is probably regulated by a pigment system and the overall photon flux density incident on the chloroplasts is also critical. The presence of a different pigment system other than the photosynthetic pigments is yet to be established