The exogenous application of an auxin (α-naphthaleneacetic acid) to the adventitious root of Allium cepa L. promotes lateral root formation and inhibits main root elongation. If the adventitious roots are released from the influence of this auxin, they may resume elongation after 10−5 M but not after 10−4 treatment. Auxin leads to the appearance of late-forming root primordia intercalated between previously formed laterals. The arrangement of laterals along the adventitious root is another feature altered by treatment, i.e., control roots develop the same number of laterals throughout the length of the zone in which laterals appear, whereas in treated roots the number of laterals rises steadily as the distal end of this zone is approached. In 10−5 M auxin-treated roots, no increase in the number of laterals occurs in basal zones of the parent root, whereas treatment with 10−4 M naphthaleneacetic acid leads to a sharp rise in the number of laterals formed at these zones. Our results suggest that the basal part of onion roots is less sensitive than the apical portion of the effects of naphthaleneacetic acid. This chemical seems to have a rejuvenating effect on cells of the pericycle, enabling them to initiate lateral primordia when under normal conditions they would no do so. Key words: Allium cepa, auxin, lateral root, naphthaleneacetic acid, onion.
Endogenous Development of Adventitious Root Primordia in Lettuce Hypocotyls
