CELLULAR-CHANGES IN EARLY DEVELOPMENT OF REGENERATING THIN CELL LAYER-EXPLANTS OF RAPESEED ANALYZED BY LIGHT AND ELECTRON-MICROSCOPY

Cellular changes in thin cell layer (TCL) explants of stem origin of Brassica napus L. cv. Vega were studied from 0 to 15 days by light and transmission electron microscopy. Apical and basal ends of the old explants were analysed separately. Quantitative and qualitative analyses showed that during the first culture day the parenchyma cells enlarged significantly as did the cytoplasm/vacuole ratio. The cytoplasm contained increased rough endoplasmic reticulum (RER), polysomes and dictyosomes associated with both coated and uncoated vesicles. The cell enlargement continued during the first 5 days of culture. The structural organization of the cell wall became somewhat loose and inhomogeneous. Parenchyma in the basal end divided frequently, resulting in several centres of division, while cell division in apical cells was less frequent and cells there remained enlarged. Starch accumulation started on the first day and increased until the third day, i.e. until cell divisions became more frequent. The starch content of dividing cells gradually decreased and starch was almost totally lacking in 15-day-old explants. Starch grains remained numerous, however, in the large non-dividing apical cells, except in those cells adjacent to the medium. Cell divisions started close to medium in explants containing vascular tissue, but closer to the epidermis in the explants without vascular tissue. The results show how rapid (one day) striking changes in the cells take place and suggest that optimal hormone concentration and intertissue relations between epidermis and parenchyma and between parenchyma and vascular tissues as well as intercellular relations among parenchyma cells determine the first cell division sites and planes in the explants. Although the cells change from elongated to spheroid, their original polarity remains as evidenced by the formation of more numerous basal shoot primordia than in apical shoot primordia.