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Phloem Unloading of Amino Acids at the Site of Attachment of Cuscuta europaea

Botanical Laboratory, State University of Utrecht, Lange Nieuwstraat 106, 3512 PN Utrecht, The Netherlands

By washing out ¹⁴C-solutes or ³H-solutes in 0.5 mM CaSO₄ during a period of 5 to 6 hours, the release of amino acids by excised stem segments of broad bean (Vicia faba L. cv Witkiem) was studied. Three hours after pulse labeling with L-valine, L-asparagine, or -aminoisobutyric acid (AIB), hollow stem segments were excised from the plant and incubated in a washout solution.

In experiments with valine and asparagine, stem segments parasitized by Cuscuta europaea released a higher percentage of labeled solutes into the bathing medium than control segments. This can be ascribed to enhanced phloem unloading at the site of attachment of Cuscuta. At low temperature (0°C) and after addition of p-chloromercuribenzene sulfonate to the bathing medium, parasitized segments did not release an enhanced percentage of labeled solutes, in comparison with control segments. These data suggest a metabolic control of the phenomenon of enhanced phloem unloading of amino acids. In experiments with AIB, an enhanced release of labeled solutes could not clearly be observed, but at the site of attachment of Cuscuta an accumulation of labeled solutes was measured. Accumulation of AIB in parenchyma cells, before the start of a washout experiment, will tend to obscure the phenomenon of enhanced phloem unloading.

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