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

Factors involved in the varietal differences in development of erect leaves were analyzed with respect to head formation of Brassica rapa L. The bend in the basal part of the midrib or petiole is the main contribution to the development of erect leaves of headed cultivars. This bend was modeled considering the midrib or petiole as the side face of inversely connected truncated cones. The bend angle of the midrib calculated using the model was highly correlated with the actual bend angle across 14 headed and non-headed cultivars. Thus, the model, named the 'double-truncated-cone model,' approximately accounted for the varietal difference in the bend. Analysis using the model suggested that the following morphological trait of the midrib or petiole causes the conspicuous bend and induces development of erect leaves in head cultivars: the large value 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 or petiole, and the distance from the basal end to the bend position, respectively.