No abstract

Abstract. Localized burning of a leaf causes a rapid change in apoplastic electrical potential throughout the shoot of wheat seedlings (‘variation potential’). It also causes marked increases in turgor pressure in epidermal cells of adjoining leaves. These turgor increases indicate rapid propagation throughout the seedling, of a hydraulic pressure wave from the site of wounding. Evidence is presented that this pressure wave is caused by relief of xylem tension, by water released from damaged cells in the wounded region. It is demonstrated that, in the absence of wounding, pressure waves imposed at the tip of one leaf can travel to neighbouring leaves, and can there induce change in apoplastic electrical potential similar to a ‘variation potential’. This indicates that the hydraulic event produced by wounding is the signal responsible for systemic induction of the ‘variation potential’. This signal has been termed ‘Ricca's factor’. It is suggested that arrival of the hydraulic wave alters leaf water potential and thereby induces stomatal activity. Leaf surface potential may be dominated by electrogenic ion pumping or flux at stomatal cells, and the ‘variation potential’ may therefore be a reflection of stomatal activity induced by the hydraulic signal.

Displacement transducers were used to demonstrate that localised wounding causes a rapid and systemic increase in leaf thickness in seedlings of wheat (Triticum durum Desf. cv. Iva). These increases are interpreted as reflecting wound-induced hydraulic signals. The duration of the wound-induced increase was found to be about 1 h or more, and it was shown that repeated wounds could induce repeated responses. The increase occurred even when plants had no access to an external water supply. Change in leaf thickness was shown closely to reflect change in leaf water potential. The velocity and kinetics of the wound-induced hydraulic signal were measured using multiple transducers ranged along a single leaf. The front of the signal was shown to travel through the plant at rates of at least 10 cm · s(-1). Development of the increase in leaf thickness was found to be relatively faster furthest from the wound. Onset of the change in leaf thickness in leaves remote from the wound was shown to precede onset of changes in surface electrical potential (variation potential) which are also induced by wounding. In contrast to reports from other species, variation potentials in wheat were here shown to spread extremely rapidly, at rates similar to that of the hydraulic signal. These data support the view that wound-induced hydraulic signals are the trigger for variation potentials in wheat.

Localised wounding causes rapid and systemic induction of proteinase inhibitors in tomato (Lycopersicon esculentum L.). The signalling system which coordinates this response is not known. Recent work has shown that systemic hydraulic signals are transmitted from wound sites in tomato, but that these cannot by themselves induce proteinase inhibitors. Here, it is demonstrated that the hydraulic signal is nevertheless an essential requirement for the systemic induction of proteinase inhibitors by localised treatments. It is also shown that mass flows similar to those associated with the wound-induced hydraulic signal, can convey a variety of solutes rapidly throughout the shoot. It is concluded that longdistance wound signalling in the tomato occurs by xylem transmission of soluble elicitors in the mass flows induced by wounding.

summary The mechanism of wound signalling in Mimosa is discussed with a brief historical survey. It is demonstrated that strong wound‐induced hydraulic signals occur in Mimosa pudica L., as in many other plants, and that the basipetal mass flows associated with these events could disperse solutes from the wound site at rates of at least 15 mm s−1 and possibly up to 300 mms−1. When such wound‐induced ‘hydraulic dispersal’ is taken into account, Ricca's theory of chemical signalling can explain long‐distance transmission in Mimosa. The pattern of long‐distance wound signalling in Mimosa is shown to be consistent with such a model. Implications for theories of electrical signalling in Mimosa are discussed and it is concluded, in agreement with some previous workers, that systemic wound‐induced electrical phenomena in Mimosa are not travelling signals, but are local responses to the travelling chemical messengers.