REGULATION OF GROWTH ANISOTROPY IN WELL-WATERED AND WATER-STRESSED MAIZE ROOTS .1. SPATIAL-DISTRIBUTION OF LONGITUDINAL, RADIAL, AND TANGENTIAL EXPANSION RATES

As a system to study the regulation of growth anisotropy, we studied t hinning of the primary root of maize (Zea mays L.) occurring developme ntally or induced by water stress. Seedlings were transplanted into ve rmiculite at a water potential of approximately -0.03 MPa (well-watere d) or -1.6 MPa (water-stressed). The diameter of roots in both treatme nts decreased with time after transplanting; the water-stressed roots became substantially thinner than well-watered roots at steady state, showing that root thinning is a genuine response to water stress. To a nalyze the thinning responses we quantified cell numbers and the spati al profiles of longitudinal, radial, and tangential expansion rates se parately for the cortex and stele. The results showed that there was n o zone of isotropic expansion and the degree of anisotropy varied grea tly with position and treatment. Thinning over time in well-watered ro ots was caused by rates of radial and tangential expansion being too l ow to maintain the shape of the root. In response to low water potenti al, cell number in both tissues was unchanged radially but increased t angentially, which shows that thinning was caused wholly by reduced ce ll expansion. Water stress substantially decreased rates of tangential and radial expansion in both the stele and cortex, but only in the ap ical 5 mm of the root; basal to this, rates were similar in well-water ed and water-stressed roots. By contrast, as in previous studies, long itudinal expansion was identical between the treatments in the apical 3 mm but in water-stressed roots was inhibited at more basal locations . The results show that expansion in longitudinal and radial direction s can be regulated independently.