Cell wall extension in Nitella as influenced by acids and ions

The giant internode cells of Nitella axillaris exhibit acid-induced growth similar to that found in higher plants. The threshold pH is 4.5, with a maximum at 3.5. The acid growth effect is transient, lasting no more than 32 min. Extensibility measurements of isolated cell walls showed a similar pattern of acid enhancement. Prolonged boiling in water (12 hr) only partially inhibited the acid-induced wall extensibility and actually increased the extensibility at pH 6. It was concluded that physical, rather than enzymatic, processes were responsible for acid-enhanced continuous extension ("creep") in Nitella walls. A complex cation-sensitive mechanism that affects extensibility was also characterized. Among the stimulatory (wall-softening) cations, divalents were generally more effective than monovalents, with magnesium being the most stimulatory. The inhibitory (wall-hardening) cations included divalents and trivalents, aluminum being the most inhibitory. Ionic effects on extensibility were even less sensitive to prolonged boiling in water than acid effects.