Effect of Tritiated Thymidine on the Morphogenesis of Lateral Roots in Zea mays

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Abstract
Primary roots of Z. mays were immersed for 8-24 hours in H3T solutions. The roots were grown sufficiently long to permit formation of lateral roots well beyond the region of elongation at time of treatment. No effect on primary root growth during treatment was observed. Lateral roots in the region of differentiation at the time of treatment were inhibited at the early primordium level. Thus a bare region interrupts the normally conical envelope of laterals along the primary root. Lateral primordia established prior to or after the treatment appeared not to be severely affected either in number or length. The distribution of incorporated H3 was studied by autoradiographs of histological sections. The percentage of nuclei with label was determined separately for the cortex and the stele. In the latter, an increased number of labeled nuclei corresponds well with the inhibited region. Although the apical meristem is almost completely labeled, it shows much less inhibition than primordia of lateral roots in the sensitive region. Single applications of X-rays inhibit lateral root formation throughout the treated region. Judiciously chosen fractionations can simulate the H3T-induced bare region.

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