The Interaction of Calcium and Auxin in the Gravitropic Response of Roots

The role of calcium redistribution in the responding region of the root is examined, however, the potential connection between calcium and auxin redistribution in the elongation zone is not found. The following items are examined: (1) the effect of gravity on calcium movement across the elongation zone; (2) the effect of gravity on auxin movement across the elongation zone; and (3) the effect of calcium on auxin movement across the elongation zone. It is indicated that gravistimulation induces a physiological asymmetry in the auxin transport system of maize roots and that calcium increases the total transport of auxin across the root. Gravistimulation is apparently necessary for the enhancing effect of calcium on lateral auxin movement, and it is possible that the preferential downward movement of calcium across the elongation zone of gravistimulated roots plays a role in establishing the auxin asymmetry proposed to cause positive gravitropic curvature.