ANALYSIS OF EXTENSION AND CURVATURE DURING THE GRAVIRESPONSE IN LEPIDIUM ROOTS

Intact roots of Lepidium sativum were photographed at 1.5‐ to 6‐min intervals before and after being tilted from a vertical to a horizontal position. Relative extension rates were calculated for segments of the root by measuring the rate of movement of charcoal particles on the surface of the root. Curvature was measured at points separated by 0.8 mm, from the tip to the root hair zone. For the first hour after the root was tilted to a horizontal position, the relative extension rate for the side of the root producing the outer side of the curve is higher than that during straight growth. The relative rate for the inner side falls to near zero during the first hour in the horizontal position. There is thus a positive gradient in extension rate from the inner to the outer side of the curve. For most of the second hour, the two sides have approximately equal relative rates of extension. In the last 12 min of the second hour, as the root tip is moving the final distance to a vertical position, the rate on the inner side shows a high extension rate. The gradient in extension rate from the inner to the outer side of the curve has thus become negative during the second hour. This reversal in the extension gradient is necessary to prevent the root from continuing to grow in a circle past the point where the root tip is vertical. Curvature reaches maximal values of 0.7 mm ‐1 to 1.2 mm ‐1 for the inner side and 0.5 mm ‐1 to 0.7 mm ‐1 for the outer side. (Units express reciprocals of the radius of curvature in mm.) Two locations where curvature is highest are separated by a flatter region. The charcoal particles marking specific locations on the root surface move away from the tip at the same rate as the features of the curvature pattern.