Wall extensibility, wall pH and tissue osmolality: significance for auxin and ethylene‐enhanced petiole growth in semi‐aquatic plants

Abstract

Abstract. Auxin and ethylene both enhance cell elongation in intact petioles of the semi‐aquatic plants Regnellidium diphyllum and Nymphoides peltata. The authors now show that auxin but not ethylene increases the in vitro extensibility of cell walls. No response to ethylene occurs in auxin‐depleted tissue. Neither hormone regulates cell expansion by direct control of internal osmolality (OS). During growth of segments, OS (and hence turgor) declines rapidly over the first 5–6 h with a net loss of osmotic solutes. Thereafter, an apparent threshold OS is maintained with net gains in osmostic solutes (Nymphoides) or further net losses (Regnellidium). Although wall extensibility determines initial rates of hormone‐induced cell expansion, the primary control of wall loosening appears to differ in the two species. Nymphoides shows typical ‘acid growth’, and fusicoccin, auxin and ethylene (with auxin) all enhance proton secretion. In Regnellidium, neither low pH nor fusicoccin (FC) alters the rate of cell expansion, although proton secretion is stimulated by FC. Stress relaxation studies using low pH treatment of living or frozen‐thawed segments show increases in the extensibility of walls in vitro for Nymphoides but not for Regnellidium. The authors propose that extensibility may be controlled by wall pH in Nymphoides but the availability of effective wall‐loosening sites determines extensibility in Regnellidium.