ONLY XYLEM-BORNE FACTORS CAN ACCOUNT FOR SYSTEMIC WOUND SIGNALING IN THE TOMATO PLANT

Rapid and systemic defence responses occur in various higher plants, i ncluding the tomato (Lycopersicon esculentum L.). The long-distance si gnalling mechanisms which permit these responses are not clear, but th ree models are currently considered in the literature: phloem transpor t, hydraulic dispersal in the xylem, and electrical transmission. Expe riments presented here are designed to discriminate between these thre e models on the basis of some key predictions. It is demonstrated that wound signalling call be prevented by enclosure of the shoots in poly thene bags, to generate high humidity. This effect can be reversed by addition of mannitol solution to the roots, showing that it depends on saturation of the plant's water status rather than on changes in the gaseous environment of the shoot. In addition, wound signals are shown to pass freely across heat-killed tissue. These results are predicted by the hydraulic-dispersal model of signalling, but they are not comp atible with the other two models. We therefore conclude in favour of h ydraulic dispersal.