The effects of enhanced levels of calcium on the gravireaction of sunflower hypocotyls
A current hypothesis states that there is a redistribution of wall calcium from the lower to the upper sides of horizontal shoots during gravireaction, and because calcium stiffens walls, the unequal calcium distribution results in differential wall extensibility on the upper and lower sides, and thus, causes unequal growth. If this hypothesis is valid, then saturating the cell walls with calcium should minimize the effect of calcium redistribution, and thereby inhibit gravicurvature and stiffen the walls. To test this hypothesis, sunflower seedlings were grown on agar containing 0 to 50molm-3 CaCl2. The wall-bound calcium content of the tissues increased as the external concentration of CaCl2 increased, and the epidermal layers were saturated with calcium by the 10molm-3 CaCl2 treatment. Contrary to the predictions from the hypothesis, the vertical growth and the gravicurvature rate of plants grown in 10molm-3 CaCl2 were actually accelerated, and wall extensibility, as measured by the Instron technique, was unaffected. These results contradict the hypothesis, and provide further evidence that wall-bound calcium is not involved in the reaction phase of gravicurvature.