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Nucleic Acid and Protein Metabolism of Senescing and Regenerating Soybean Cotyledons ¹

^a Plant Hormone and Regulator Laboratory, Plant Science Institute, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland 20705

An alternative to the leaf disk system for studies of the metabolism of senescence is described. The progress of senescence of soybean (Glycine max L.) cotyledons is arrested when the epicotyl is removed. Epicotyl removal at 16 or 17 days reversed the decline in nucleic acid, protein, and chlorophyll content in the cotyledon. Epicotyl removal at 18 days did not reverse the decline in the above components, and therefore the progress of cotyledon has passed the point of no return. Cotyledons lost 90% of their nucleic acid and 80% of their protein before senescence became irreversible. The rate of recovery in various macromolecular components after epicotyl removal did not occur in an equal manner. Nucleic acid was regenerated at a faster rate than chlorophyll, which was regenerated at a faster rate than soluble protein. The heavy nucleic acid components (ribosomal and heavy ribosomal messenger fractions) regenerated at greater rates than did the soluble RNA or DNA. No label from ¹⁴CO₂ was incorporated into DNA of the cotyledons when the epicotyl was present but label was incorporated into DNA after epicotyl removal.

The parallels between the mechanisms of cotyledon senescence and apical dominance are discussed.

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