COMPARISON OF ELECTRIC AND GROWTH-RESPONSES TO EXCISION IN CUCUMBER AND PEA-SEEDLINGS .1. SHORT-DISTANCE EFFECTS ARE A RESULT OF WOUNDING

The local electric response to stem excision in both pea epicotyls and cucumber hypocotyls is a depolarization of the cells in the wound are a. If we define wound area as the region of local depolarization, we f ind that it extends for approximately 10 mm from the cut or wound site in pea epicotyls, whereas it can reach up to 40 mm in cucumber hypoco tyls. The wound-induced depolarization in pea cells is transient, reac hing its maximal amplitude within 1-2 min, whereas in cucumber cells t his depolarization is more sustained. A third difference between wound responses in pea and cucumber is the intermittent appearance of spike s, i.e. very short, rapidly reverted depolarizations which frequently accompany the basic depolarization in cucumber but not in pea cells. T hese spikes can propagate in both directions along the hypocotyl axis. The cause of the different responses of pea and cucumber cells is unk nown. A possible explanation might be found in different degrees of el ectrical cell coupling in the two species. This possibility was invest igated in cucumber hypocotyls by measuring the cell input resistance ( R(in)) of epidermal cells at various axial distances from the cut. Sho rter distances increase the likelihood of shunting the cell membrane r esistance through the shortened symplastic path to the cut surface. Wi th a series of cuts made at decreasing distances from the measured sit e, cell depolarization increased without comparable changes in R(in). Two conclusions were drawn. Firstly, wound-induced depolarizations are not brought about by shunting of the cell resistance in the wound are a. Secondly, the depolarization is probably not carried by ion channel s but may be caused by an inhibition of proton pump activity. Parallel to its depolarizing effect on the membrane potential, excision led to a severe and sustained decline in the cucumber hypocotyl growth rate only when carried out sufficiently close to the growing region (45 mm from the hook). Similar excision in pea epicotyls failed to change the growth rate. Both electrical and growth data support the concept that the high and sustained responsiveness of cucumber seedlings to woundi ng is caused by a particular sensitivity of their proton pump mechanis m.