THE EFFECTS OF ENHANCED LEVELS OF CALCIUM ON THE GRAVIREACTION OF SUNFLOWER HYPOCOTYLS

A current hypothesis states that there is a redistribution of wall cal cium from the lower to the upper sides of horizontal shoots during gra vireaction, and because calcium stiffens walls, the unequal calcium di stribution results in differential wall extensibility on the upper and tower sides, and thus, causes unequal growth. If this hypothesis is v alid, then saturating the cell walls with calcium should minimize the effect of calcium redistribution, and thereby inhibit gravicurvature a nd stiffen the walls. To test this hypothesis, sunflower seedlings wer e grown on agar containing 0 to 50 mol m(-3) CaCl2. The wall-bound cal cium content of the tissues increased as the external concentration of CaCl2 increased, and the epidermal layers were saturated with calcium by the 10 mol m(-3) CaCl2 treatment. Contrary to the predictions from the hypothesis, the vertical growth and the gravicurvature rate of pl ants grown in 10 mol m(-3) CaCl2 were actually accelerated, and wall e xtensibility, as measured by the Instron technique, was unaffected. Th ese results contradict the hypothesis, and provide further evidence th at wall-bound calcium is not involved in the reaction phase of gravicu rvature.