Increase in DNA Content of Primary Leaves ofPhaseolus vulgarisupon Decapitation

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Abstract
Changes in DNA content of bean (Phaseolus vulgaris) primary leaves after decapitation were investigated. When apical buds were removed at 11 d, DNA content per leaf increased by about 20% at 15 d and then decreased in parallel with the controls. The RNA and chlorophyll contents, fresh weight, and leaf area expressed on a single leaf basis changed in the same manner as the DNA content in response to decapitation. But when both apical and lateral buds were removed, all these values continued increasing during the test period. Thus, growing lateral buds and apical buds have the same effect on the DNA change in primary leaves as that due to ageing of the leaves. Cell number per leaf was not increased by any treatment, indicating that the observed increase in the DNA content of primary leaves is ascribable to an increase in DNA per cell. Next, the whole shoots above the nodes of primary leaves were removed at various ages. The response of primary leaves to decapitation varied according to their age. With age, they lost the ability to increase their fresh weight, area, and chlorophyll content but not their DNA and RNA contents in response to decapitation. Decapitation stimulated chloroplast replication only within the period in which chloroplasts were replicating in control leaves, but it induced chloroplast enlargement at any age. Therefore, the increase in DNA content after decapitation may be partially due to an increase in the amount of chloroplast DNA. When stems were heat-girdled above the nodes of the primary leaves, these leaves showed responses similar to but smaller than those to decapitation. The senescence of primary leaves seems to be controlled by the distribution of substances which are transported from the roots.