At least two root-growth-inhibiting substances have been detected in extracts ofZea maysroot tips. One has chromatographic properties similar to those of abscisic acid (ABA) and is largely confined to the cap. Another, which has not been identified, is present in the root apex but not in the cap. Indole-3yl-acetic acid (IAA) is never detected in the root cap. Xanthoxin may be present in the root tip.
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