Short-term effects of aluminum on osmotic expansion and contraction of red beet (Beta vulgaris L.) protoplasts

Abstract

The hypothesis that Al$\sp{3+}$ interferes with the biophysical properties of the plasma membrane has been tested. Plasma membrane expansion and contraction red beet (Beta vulgaris L.) protoplasts were osmotically induced and quantified with an image analysis system. Under hypotonic conditions, protoplasts exposed to Al$\sp{3+}$ exhibited less osmotic expansion as measured by cross-sectional area, diameter, perimeter, and lysis pattern. Under hypertonic conditions, protoplast roundness was maintained by Al$\sp3$ during osmotic contraction, while Al$\sp{3+}$ enhanced protoplast contraction as measured by cross-sectional area and perimeter. Protoplasts that were pre-exposed to Al$\sp{3+}$ exhibited the same osmotic behaviors: less expansion and more contraction. The observations that Al$\sp{3+}$ consistently minimized the surface area in both hypotonic and hypertonic conditions, suggests that A$\sp{3+}$, due to its strong electrostatic binding property, may bridge neighboring negatively-charged sites on plasma membrane surfaces. To test this hypothesis, several cations bearing different membrane binding affinities were compared to Al$\sp{3+}$. All tested cations reduced osmotic expansion, but with differing effectiveness. The comparative (Ca$\sp{2+}$ = 1) effectiveness of lysis reduction was $\rm HXM\sp{2+} : Ca\sp{2+} : TEC\sp{3+} : Al\sp{3+} : La\sp{3+}$ = 0.67: 1.00: 1.44: 3.04: 3.70, which was consistent with the order of cations' electrostatic binding affinity to negative sites on the membrane surface. Under hypertonic conditions, the $\rm Ca\sp{2+},\ Al\sp{3+},$ and La$\sp{3+}$ equally enhanced the reduction in cross-sectional area. However, trivalent cations (Al$\sp{3+}$ and La$\sp{3+}$) with higher membrane binding affinities maintained the protoplast roundness during hypertonic contraction, while divalent Ca$\sp{2+}$ did not. In conclusion, it is suggested that Al$\sp{3+}$-induced alterations in physical properties and function of membrane may result from the electrostatic binding property of Al$\sp{3+}$ which may either (1) alter the water permeability of the plasma membrane in either increasing or decreasing directions or (2) provide bridging forces between negatively charged sites on the plasma membrane surface which minimize the surface area of the protoplast.