Geometrical analysis of development of erect leaves as a factor in head formation of Brassica rapa L.

A geometric model simulating the hyponastic bend of the basal part of the midrib was developed to investigate factors involved in head formation of Chinese cabbage (Brassica rapa L. var. pekinensis Rupr.). The model, named the 'double-truncated-cone model', considers the midrib as the side face of inversely connected truncated cones. The convex side face simulates the bend of midrib. The bend angle of the midrib calculated using the model was highly correlated with the actual bend angle over various leaf growth stages. Thus, the model approximately accounted for the bend that induces development of erect leaves. Analysis of midrib growth using the model suggests that development of erect leaves is induced by midrib bend mainly caused by the following dimensional changes in the midrib: (1) an increase of (l1 − l0)d1−1, where l1, l0, and d1 are the length of the cross-sectional arc at the bend position, that at the basal end of midrib, and the distance from the basal end to the bend position, respectively; and (2) a decrease of the central angle of the cross-sectional arc.