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Articles

Transverse Viscoelastic Extension in Nitella

II. Effects of Acid and Ions ¹

Division of Natural Sciences, Thimann Laboratories, University of California, Santa Cruz, California 95064

The transverse viscoelastic extension of isolated Nitella cell walls is stimulated by acid pH and by Mg²⁺ and K⁺ ions. In the presence of 1 millimolar citrate-phosphate buffer the threshold pH in the transverse direction is 3.5, compared to 4.5 in the longitudinal direction. The relative amounts of extension stimulated by acid are comparable in the two directions at their respective thresholds. Longitudinal and transverse Mg²⁺ ion-induced extensibility begins at 10 millimolar and reaches a plateau between 10 and 100 millimolar. The threshold for K⁺ ion enhancement is near 10 millimolar in the longitudinal direction and 50 millimolar in the transverse direction. Maximum stimulation by K⁺ is obtained at 250 millimolar. At their respective maxima, Mg²⁺ and K⁺ induce equal amounts of extension. However, the relative amount of extension induced by ions is significantly less in the transverse than in the longitudinal direction. Ions and acids appear to affect different sites in the wall, inasmuch as neither treatment abolishes the effect of the other. Walls from rapidly growing cells are more sensitive to stimulation than nongrowing cells in the longitudinal direction but not in the transverse direction.