Abstract
We re-examined the role of light in head formation of Chinese cabbage (Brassica rapa L. var. pekinensis Rupr.) in terms of the hyponastic midrib bending that is induced autonomously during leaf growth. This bending occurs in the basal midrib and can be simulated by a double-truncated-cone geometric model. Before head formation, darkness induced hyponastic bending of the midrib in the uppermost mature leaves. This bending was not simulated by the model, and was not induced autonomously by leaf growth. During early head formation, however, the hyponastic midrib bending of the uppermost mature leaves occurred autonomously, irrespective of irradiation. Light intensity on the abaxial face of erect leaves increased compared with levels before head formation. Alternating light and dark treatments did not affect bending at this stage. Consequently, the main developmental factor for erect leaves during early head formation is likely to be autonomously induced midrib bending during growth rather than a response to reduced light intensity and shading by outer leaves.
