The Effect of pH on the Binding of Calcium to Pea Epicotyl Cell Walls and its Implications for the Control of Cell Extension

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Abstract
Cell walls were prepared from the epicotyls of dark-grown pea (Pisum sativum L.) seedlings. The walls were found to bind externally-added 45Ca2+, with a binding constant of 4 × 10−4 mol dm−3 and a maximum capacity of 1.5 × 10−8 g-ions of Ca2+ per g fresh weight of epicotyl. The binding capacity decreased as the pH of the medium was decreased below 6.0, suggesting that the calcium was bound by an anionic group with an apparent pK of 4.7. More than half the calcium binding was due to polygalacturonic acid in the wall, since up to 60% of the calcium binding capacity was removed by pre-incubation of the cell walls with polygalacturonase (E.C.3.2.1.15). Only small decreases in calcium binding were seen following pre-incubation with protease, nucleases, phospholipase and hemicellulase. These results indicate that calcium will be displaced from the cell wall at hydrogen ion concentrations which are known to occur in the wall during wall extension. They are consistent with a mechanism by which calcium inhibits wall extension by forming ionic bridges between polygalacturonic acid molecules, and also with the hypothesis that calcium and hydrogen ions exert opposing influences on cell wall extension by competing for the same binding sites on the polygalacturonic acid.

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