New evidence for the role of mechanical forces in the shoot apical meristem

The mechanism for initiation of lateral organs in the shoot apical meristem is still unknown. In this article we investigate one critical component of a buckling mechanism of organ initiation (that is, the presence and distri bution of compressive stresses in the meristem). Direct evidence for compre ssion in the sunflower capitulum was obtained from the gaping pattern of sh allow cuts and the propagation of fractures. Cuts gaped widely in the centr al region of the capitulum but remained closed, or nearly so, in the genera tive and differentiation regions, suggesting the presence of circumferentia l compression at these locations. Fractures were initiated in the generativ e region and propagated circumferentially over most of their length. They d id not cross the generative region perpendicularly, suggesting again the pr esence of compressive stresses in the circumferential direction. This concl usion was confirmed by the stress distribution computed from the geometry o f the capitulum at three stages of development. One interpretation of these results is that the generative region corresponds to a zone of compression that could control the initiation of new primordia by means of buckling of the tunica layer.