Temporal and spatial patterns of twining force and lignification in stems of Ipomoea purpurea

Using the TWIFOR, an electronic device for continuous, in vivo measurement of the forces exerted by twining vines, we examined the forces generated by vines growing on cylindrical poles of slender (6.35 mm) and thicker (19.05 mm) diameter. In stems of Ipomea purpurea (L.) Roth, magnitudes of twining force (axial tensions) were, on average, less at a particular time and location on the more slender poles; while twining loads (normal force per unit length of vine) were much greater on the slender poles because of the greater curvature of the vines. Thus, the geometry of the helix formed by the vine on the pole affects the ability of the vine to maintain a frictional interaction with its support. In addition, the plant-to-plant variation in twining force was twice as great on the thicker support poles. Metaxylem and fibers developed closer to the plant apex in vines on the slender poles. On the thicker poles, a significant fraction of the maximum twining force developed during the establishment of the first gyre, before fibers were lignified, indicating that primary growth can be sufficient to establish high twining forces. On the slender poles, however, twining force increased with developmental stage until the gyre was at least 1.5 m from the apex. Thus, twining force can increase after cessation of primary growth. No simple relationship was found between the site of fiber differentiation and twining force.