Non-uniformities in the metabolism of excised leaves and leaf discs

In intact tobacco and Chinese-cabbage (Brassica pekinensis) leaves an "ageing" process begins as soon as the leaves are excised. The terminal few millimetres of the petiole increasingly preempt materials such as phosphate and uracil taken up by the leaf. Actinomycin D treatment blocks this process and leads to increased uptake of such materials into the lamina.Immediately after excision there is a radial "geographical" gradient, in the ability of excised leaf discs to accumulate phosphate and uracil from solution. Tissue near the cut edge accumulates much more of these materials than that near the centre, and total nucleic acids isolated from the outer zone have a higher specific activity.Over the first day after excision there is a marked increase in this ability of the outer tissue of discs to accumulate labelled precursors but the changes taking place with time are complex and variable. Specific activity of total nucleic acids increases somewhat less than the increased uptake of labelled precursor. Actinomycin D becomes very unevenly distributed in leaf discs floated on solutions of the drug. These various effects are of sufficient magnitude to affect the interpretation of studies using excised leaf tissues.