Changes in the amount of ribosomal RNA and poly(A)-containing RNA during leaf development.

A rapid increase in RNA content of the primary leaves of Phaseolus aureus Roxb. begins between three and four days after germination. At about the same time there is a corresponding increase in the rate of cell division. The major part of the RNA consists of chloroplast and cytoplasmic rRNA. The accumulation of RNA proceeds more rapidly in the light but substantial amounts of both chloroplast and cytoplasmic rRNA are synthesised in the dark. Developing leaves also synthesise polydisperse RNA containing poly(adenylic) acid. The amount varies in leaves of different ages from 0.6 to 2.2^ of the total RNA. The poly (adenylic) acid regions of these molecules can be detected and measured by a hybridisation assay with [5-(3)H]poly(uridylic) acid. Leaves of dry seeds contain significant amounts of poly(adenylic) acid and additional sequences are synthesised during leaf growth in the dark and in the light. The maximum change, representing a five-fold increase in poly(adenylic) acid content, occurs during cell division in the light. The amount of poly(adenylic) acid declines rapidly soon after cell division ceases, in contrast to the rRNA which is much more stable.