CHANGED ORIENTATION OF THE MITOTIC SPINDLE OF STOMATAL GUARD CELL DIVISIONS IN HORDEUM VULGARE

Three-day-old seedlings of ‘Atlas’ barley were treated for 1 hr with an aqueous solution of 2-mercaptoethanol at various concentrations. This caused a general delay of mitosis, suppression of subsidiary cell formation in manystomatal complexes, and complete inhibition of the formation of some stomatal complexes. In a small proportion of stomatal complexes the orientation of the division of the guard cell mother cell was changed so that its mitotic spindle was no longer transverse but parallel to the long axis of the leaf and produced daughter cells proximally and distally to each other. The highest frequency of such proximal-distal orientations, a mean of 2.23 per stomatal row, was produced by concentrations of 0.175–0.2 m. Proximal-distal reorientations were concentrated in a region of the leaf distal to that in which the greatest inhibition of stomatal complexes occurred. Susceptible complexes were those at early stages of subsidiary formation when exposed to mercaptoethanol; this is long before prophase of GCMC division. The mean length/width ratio of cells at this stage was significantly greater in treated than in control material. Gibberellic acid at 550 ppm significantly increased both the 1/w ratio and the frequency of proximal-distal divisions. A much higher proportion of proximal-distal divisions was obtained in upper leaf sheaths excised and placed for 24 hr in a moist atmosphere. This effect was associated with a period of quiescence following excision, after which mitoses were resumed, many of them with proximal-distal reorientation.