DNA changes during sequential leaf senescence of tobacco (Nicotiana tabacum)

Abstract

Age related DNA changes in tobacco (Nicotiana tabacum) leaf nuclei were investigated by Feulgen cytophotometry, thermal denaturation, renaturation, and DNA‐DNA hybridization studies during sequential leaf senescence. Cytophotometric Feulgen‐DNA comparison measurements between young and senescing nuclei displayed 18% reduction in Feulgen‐DNA values, with a corresponding decrease in nuclear area in senescing nuclei. Hydrolysis kinetics indicated that the loss was not due to compactness of the DNA as the curves for older nuclei were consistently lower than curves generated from younger nuclei. DNA loss in senescing nuclei was associated with a decrease in euchromatin or shift from euchromatin to facultative heterochromatin. Purified DNA from young and senescing leaf nuclei did not display different thermal profiles nor did hydroxylapatite chromatography reassociation curves. DNA‐DNA hybridization in free solution from young and senescing leaf DNA performed by a Gilford thermo‐programmer system indicated that DNA of senescing tobacco nuclei reassociated more slowly than DNA from young nuclei and the mixture of young and senescing leaf DNA displayed intermediate reassociation values. The study indicates that the DNA changes during senescence involve a complex phenomenon which includes the possibility of small single strand nicks undetectable by thermal denaturation, and a loss of small double strand fragments which were detectable only by precise DNA‐DNA free solution reassociation and not by hydroxylapatite chromatography reassociation.