The effect of cell age on chloroplast structure and chlorophyll in cultured spinach leaf discs
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Discs from young spinach leaves show a 50-fold increase in fresh weight and a 10-fold increase in chlorophyll over 7 days when cultured in high intensity white light (6.5 mWcm−2). In darkness the increase in fresh weight is 10-fold while chlorophyll decreases. Discs from mature spinach leaves show only a 2-fold increase in fresh weight and a marked decrease in chlorophyll content when cultured for 7 days either in light or darkness. Loss of chlorophyll in cultured discs is closely correlated with leaf and cell age and is affected positively by temperature.
In cultural regimes in which there is a loss of chlorophyll from mature discs chloroplast degeneration occurs. However, degeneration of the cytoplasm does not necessarily occur at the same time as chloroplast breakdown. Chloroplast degeneration involves the loss of thylakoid membranes accompanied with a build up of osmiophilic droplets and membrane bound crystals similar in appearance to fraction I protein. In high intensity light loss of plastid structure occurs within 4 days, but in darkness much of the thylakoidal membrane system persists for as long as 7 days.
Chloroplast formation occurs when discs from mature leaves are cultured in low intensity white light (0.2 mWcm−2). Chloroplast formation in these discs is associated with new cell formation and occurs in callus cells which are mainly formed at vein endings. Three different plastid types were observed in callus cells. It is suggested that chloroplast degeneration in mature leaf discs may be a consequence of these tissues not containing or having access to substances produced by either dividing or expanding cells.
KeywordsChlorophyll Leaf Disc Callus Cell Spinach Leave Spinach Leaf
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