Initiation, orientation and early development of primary rhizomes in Sorghum halepense (L.) pers.

Axillary buds on the most basal portion of the seedling shoot of Sorghum halepense differentiate directly into rhizome buds. The initial orientation of these buds is upwards, but this orientation starts to be reversed almost immediately. The reversal is caused by the combined effect of differential radial expansion of the basal internodes immediately above and below the bud, and differences in the extent of mitotic activity on the abaxial and adaxial sides of the bud. Reorientation is a geotropic and is progressively less with acropetal nodal position of the bud. Further growth of the rhizomes proceeds in the same orientation as that of the bud from which they had developed, until they change their orientation again by exhibiting diageotropic, or negatively geotropic responses. The second reorientation coincides more or less with the onset of flowering and it exhibits a positional gradient, such that the change is more extensive the higher the nodal position of the rhizome.