Gibberellin-induced changes in growth anisotropy precede gibberellin-dependent changes in cortical microtubule orientation in developing epidermal cells of barley leaves. Kinematic and cytological studies on a gibberellin-responsive dwarf mutant, M489

We conducted kinematic and cytological studies on "between vein" epidermal cells of the gibberellin (GA)-deficient M489 dwarf mutant of barley (Hordeu m vulgare L. Himalaya). GAs affect radial and axial components of cell expa nsion and cortical microtubule orientation. Adaxial cells in particular exp and radially after leaving the elongation zone (EZ), probably as part of le af unrolling. Exogenous gibberellic acid corrects the mutant's short, wide blades, short EZ, and slow elongation rate. Cell production rates increase more on the adaxial than on the abaxial surface. Cells spend equal periods of time elongating in dwarf and tall plants, but relative elemental growth rates start to decline sooner in the dwarf. GA increased the rate at which longitudinal wall area increased because the increased axial growth more th an compensated for reduced radial growth. In dwarf leaves, increased radial expansion was detected in basal parts of the EZ before cortical microtubul es lost transverse orientation in the distal elongation zone. We conclude t hat loss of microtubule orientation is not required for low GA levels to re duce growth anisotropy.