By washing out '4C-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 a-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 ofp-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. data; 5), the question arose whether the transfer of amino acids from host to Cuscuta is comparable with the transfer of sucrose. This question has not been adequately resolved to date. In a previous report some relatively short experiments with L-alanine were discussed (23). Although quantitatively different, the effects of Cuscuta on the release of "1C administered to the plant as amino acid (alanine), sucrose, or "'CO2 were similar. However, "1C from L-alanine can easily appear in sucrose (1), so new experiments were conducted to study efflux over a longer time and with other amino acids (AIB' and L-valine). Large differences were found between the results from experiments with amino acids and the results obtained in experiments with sucrose. Especially in the case of AIB, Cuscuta did not clearly enhance the release of labeled solutes during a washout experiment (5 to 6 h). These data have been discussed in a preliminary communication (27) and they have also been mentioned in a recent review on phloem unloading (6). Further work was needed to resolve the question. The present study presents new evidence that phloem unloading of amino acids is strongly enhanced by Cuscuta. MATERIALS AND METHODSThe stem parasite Cuscuta has a deleterious influence, draining solutes from the phloem of the host. Using "'CO2 it has been shown that Cuscuta can withdraw almost all photosynthate which normally moves from a photosynthesizing leaf to developing fruits of Vicia faba (22). In addition to a very efficient absorption by the parasite as suggested by the anatomical details (3), an enhanced unloading rate from the host phloem appears to be essential for the transfer of photosynthate from host to parasite (23,25,27,28,30). Evidence for enha...