PHOTOTROPIC STIMULATION CAN SHIFT THE GRADIENT OF CROWN ROOT EMERGENCE IN MAIZE

The crown roots in the coleoptilar node of maize emerge asymmetrically : emergence at the dorsal flank of the node (opposite to the caryopsis ) precedes emergence at the ventral flank (facing the caryopsis). This asymmetry can be altered by phototropic stimulation: emergence of cro wn roots is delayed in the lighted flank and promoted in the shaded fl ank causing an inversion of the endogenous asymmetry. The curvature in duced by the phototropic stimulation is transient, the effect on crown root emergence, in contrast, persists. This stable effect is not a co nsequence of curvature per se and becomes irreversibly fixed between o ne and two hours after stimulation. The emergence of crown roots depen ds on directional signalling from the coleoptile to the node. The data are discussed in terms of a stable blue light induced transverse pola rity of the coleoptile that can imprint a stable asymmetry upon the co leoptilar node guiding the emergence of crown roots.