Changes in Chloroplast DNA Levels during Growth of Spinach Leaves

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Abstract
In young spinach leaves, 1–4 mm long, 7–10% of the total DNA of the leaf was chloroplast (pt) DNA. Growth in these leaves was mainly by cell division with plastid division keeping pace with cell division and maintaining about 10 plastids per cell. About 1% of the leaf cells were formed in 4.0 mm leaves. Both cell division and cell expansion contribute to the next stage of leaf growth, which was quantitatively the major period of new cell formation, nuclear DNA synthesis and ptDNA synthesis. Relative to the nuclear DNA level ptDNA levels rose to 21% of the total DNA and chloroplast.plastome copy numbers from 1500 to 5000 per cell while chloroplast numbers rose from 10 to 30 per cell. In the final period of leaf growth, cell expansion was the main determinant of growth and chloroplast number per cell rose to 180. In contrast to young leaves, newly emerged cotyledons contained 20% of their DNA as ptDNA and, during cell expansion, cell number per cotyledon doubled. On average, the cells became octoploid, and chloroplast numbers and plastome copy numbers rose to 500 and 22 000 per cell respectively. Similar levels of nuclear ploidy, chloroplast number and plastome copy number were induced in the first leaf pair of spinach following decapitation. When senescence was induced in mature leaves by shading, no loss of nuclear or ptDNA occurred. Following the onset of leaf yellowing and a form of senescence induced by nitrogen deficiency in leaves which had not fully expanded, there was preferential loss of ptDNA which fell from 8200 to 3700 plastome copies per cell over an 11 d period.