Stimulation of Root Elongation and Curvature by Calcium

Ca²⁺ has been proposed to mediate inhibition of root elongation. However, exogenous Ca²⁺ at 10 or 20 millimolar, applied directly to the root cap, significantly stimulated root elongation in pea (Pisum sativum L.) and corn (Zea mays L.) seedlings. Furthermore, Ca²⁺ at 1 to 20 millimolar, applied unilaterally to the caps of Alaska pea roots, caused root curvature away from the Ca²⁺ source, which was caused by an acceleration of elongation growth on the convex side (Ca²⁺ side) of the roots. Roots of an agravitropic pea mutant, ageotropum, responded to a greater extent. Roots of Merit and Silver Queen corn also responded to Ca²⁺ in similar ways but required a higher Ca²⁺ concentration than that of pea roots. Roots of all other cultivars tested (additional four cultivars of pea and one of corn) curved away from the unilateral Ca²⁺ source as well. The Ca²⁺-stimulated curvature was substantially enhanced by light. A Ca²⁺ ionophore, A23187, at 20 micromolar or abscisic acid at 0.1 to 100 micromolar partially substituted for the light effect and enhanced the Ca²⁺-stimulated curvature in the dark. Unilateral application of Ca²⁺ to the elongation zone of intact roots or to the cut end of detipped roots caused either no curvature or very slight curvature toward the Ca²⁺. Thus, Ca²⁺ action on root elongation differs depending on its site of application. The stimulatory action of Ca²⁺ may involve an elevation of cytoplasmic Ca²⁺ in root cap cells and may participate in root tropisms.