Control of cell elongation in Nitella [axillaris, Algae] by endogenous cell wall pH [hydrogen-ion concentration] gradients. Multiaxial extensibility and growth studies

The multiaxial stress of turgor pressure was stimulated in vitro by inflating isolated Nitella cell walls with mercury. The initial in vitro extension at pH 6.5, 5 atmospheres pressure, returned the wall approximately to the in vivo stressed length, and did not induce any additional extension during a 15-minute period. Upon release of pressure, a plastic deformation was observed which did not correlate with cell growth rates until the final stages of cell maturation. Since wall plasticity does not correlate with growth rate, a metabolic factor(s) is implicated. Walls at all stages of development exhibited a primary yield stress between 0 and 2 atmospheres, while rapidly growing cells (1-3% per hour) exhibited a secondary yield stress of 4 to 5 atmospheres. The creep rate and plastic deformation of young walls were markedly enhanced by acid buffers (10 millimolar, pH ≤ 5.3). Nitella cells produce acid and base "bands" along their length due to localized excretion of protons and hydroxyl ions. Marking experiments showed that growth is largely restricted to the acid regions. Growth in the acid bands was inhibited by alkaline buffers, and growth in the base bands was stimulated by acidic buffers. The two zones have similar mechanical properties. When the proton-binding capacity of the wall was taken into account, the pH of the solution in contact with inner wall surface in the acid band was estimated to be about 4.3, well within the threshold of acid-enhanced creep. Since the inner 25% of the wall controls extensibility, we conclude that growth in the acid band is caused by the action of protons on the wall.