Microautoradiographic study of the incorporation of labelled amino acids into insoluble compounds of the shoot of a higher plant

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Abstract
Radioactive amino acids are fed singly to intact shoots of field pea (Pisum arvense L.) via the transpiration stream. The range of radiosubstrates used is chosen to be representative of those compounds exported in quantity from the root system of field pea. The distribution of radioactivity from each substrate suggests that mature organs actively assimilate materials which enter the shoot through the xylem. During a four hour period 5–20% of the label recovered from mature tissues is present in insoluble form, much of this as protein. A microautoradiographic technique is used to localize insoluble labelled materials in thin (1–2 μ) sections of the plant tissues. Certain cell types (xylem parenchyma, cambial initials, mesophyll of leaves, chlorenchyma of stem and petiole, and all elements of the phloem) appear to be particularly active in elaborating protein and other structural constituents from the labelled substrates. Preferential labelling of chloroplasts is observed in cells of mesophyll, while in all cells the wall components appear to be less readily labelled than their contained protoplasts.