The accumulation of steviol glycosides (SGs) in cells of Stevia rebaudiana Bertoni both in vivo and in vitro was related to the extent of the development of the membrane system of chloroplasts and the content of photosynthetic pigments. Chloroplasts of the in vitro plants, unlike those of the intact plants, had poorly developed membrane system. The callus cells grown in the light contained proplastids of almost round shape and their thylakoid system was represented by short thylakoids sometimes forming a little number of grana consisting of 2 -3 thylakoids. In cells of the etiolated in vitro regenerants and the callus culture grown in the dark, only proplastids practically lacking the membrane system were observed. All the chloroplasts having developed thylakoids and forming at least a little number of grana were equipped with photochemically active reaction centers of photosystems 1 and 2. Leaves of in vivo plants accumulated greater amount of the pigments than leaves of the in vitro plants. In both the callus culture grown in the light and the etiolated in vitro regenerants, the content of the pigments was one order of magnitude lower than that in leaves of the intact plants. The callus tissue grown in the dark contained merely trace amounts of the pigments. Leaves of the intact and the in vitro plants did not exhibit any significant differences in photosynthetic O 2 evolution rate. However, photosynthetic O 2 evolution rate in the callus cells was much lower than that in the differentiated plant cells. The in vitro cell cultures containing merely proplastids did not practically produce SGs. However, after transferring these cultures in the light, both the formation of chloroplasts and the production of SGs in them were detected.Additional key words: photosynthetic rate, photosynthetic pigments, rebaudiosides A and C, stevioside.
Photoreduction of Pheophytin 'a' (Pheo) accompanied by a decrease in the chlorophyll fluorescence yield is observed in photosystem II (PS II) of the whole cells of green algae Chlamydomonas reinhardii (a wild type and a mutant lacking both photosystem I and chlorophyll 'b'), Chlorella pyrenoidosa, Scenedesmus obliquus and cyanobacteria Phormidium laminosum, Anabaena variabilis and Cynechococcus elongatus under anaerobic conditions created by means of the glucose-glucoseoxidase-catalase. The photoreaction is activated by the addition of 1 μM CCCP, inhibited by 10 μM DCMU and reactivated upon subsequent addition of either ascorbate or dithionite. Oxidized NADP, benzyl viologen and methyl viologen accelerate dark oxidation of the reduced Pheo indicating that they are able to accept an electron from [Formula: see text] in PS II.The data on both photoreduction of Pheo in the intact cells in the absence of exogenous reductants, when electron donation to reaction centers of PS II occurs only from water, and the inhibition of this photoreaction by DCMU, show that the Pheo photoreduction is sensitized by the reaction centers of PS II and probably occurs as a result of the electron donation from the water-splitting system being in the state S3, to [Formula: see text] producing the long-lived state [Formula: see text] and O2.
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