COMPARISON OF ELECTRIC AND GROWTH-RESPONSES TO EXCISION IN CUCUMBER AND PEA-SEEDLINGS .2. LONG-DISTANCE EFFECTS ARE CAUSED BY THE RELEASE OF XYLEM PRESSURE

Excision of a growing stem causes local wound responses, such as membr ane depolarization and growth inhibition, as well as effects at larger distances from the cut, In this study, cucumber hypocotyls were excis ed 100 mm below the hook, so that the growing region was beyond the re ach of the wound-induced depolarization (up to 40 mm), Even. at such a distance, the cut still caused a considerable and rapid drop in the h ypocotyl growth rate, This growth response is not a direct wound respo nse because it does not result from the cut-induced depolarization and because it can be simulated by root pressure manipulation (using a pr essure chamber), The results indicate that the growth response resulte d from the rapid release of the xylem pressure upon excision, To test this conclusion we measured the xylem pressure by connecting a pressur e probe to the cut surface of the stem, Xylem pressure (P-x) was found to be +10 to +40 kPa in cucumber hypocotyls and -5 to -10 kPa or lowe r in pea epicotyls, Excision of the cucumber hypocotyl base led to a r apid drop in P-x to negative values, whereas excision in pea led to a rapid rise in P-x to ambient (zero) pressure, These fast and opposite P-x changes parallel the excision-induced changes in growth rate (GR): a decrease in cucumber and a rise in pea. The sign of the endogenous xylem pressure also determined whether excision induced a propagating depolarization in the form of a slow wave potential (SWP), Under norma l circumstances pea seedlings generated an SWP upon excision whereas c ucumber seedlings failed to do so. When the P-x in cucumber hypocotyls was experimentally inverted to negative values by incubating the cumb er roots in solutions of NaCN or n-ethylmaleimide, excision caused a p ropagating depolarization (SWP). The experiment shows that only hydrau lic signals in the form of positive P-x steps are converted into propa gating electric SWP signals, These propagating depolarizations might b e causally linked to systemic 'wound' responses, which occur independe ntly of the short-distance or direct wound responses,