Changes in cpDNA content during development as determined by blot-hybridization. Total tissue DNA after digestion with SpeI, gel electrophoresis and blot-hybridization using a cpDNA probe (A; petA) and a nuclear DNA probe (B; DRT100). Lanes 1: First and second leaves from 12-day-old plants. Lanes 2: Entire 16-day-old shoots. Lanes 3: Third and fourth leaves from 36-day-old plants. Lanes 4: First and second leaves from 37-day-old plants. Blot-hybridization using the cpDNA probe was also performed on the DNA samples shown in lanes 2 and 3 after digestion with HindIII and NotI (data not shown). The DNA sample in lanes 2 had a five-fold greater (HindIII) or a two-fold greater (NotI) quantified signal than that in lanes 3. We also performed a dot-blot using a dilution series of the DNA samples in lanes 2 and 3. For both samples, a two-fold reduction in DNA resulted in an approximately two-fold reduction in signal intensity of the petA probe (see Materials and Methods).