DO COLEOPTILE TIPS PRODUCE AUXIN?

SUMMARY A re‐examination of the evidence for auxin production by coleoptile tips reveals that it is not conclusive and that several important problems remain unresolved. The possibility that auxin and auxin precursors move acropetally in the xylem was tested by analysing guttation fluid from intact coleoptiles, decapitated coleoptiles and primary leaves of Avena sativa. In all cases two zones of auxin activity were detected on chromatograms of the acidic ether‐soluble fraction, one of which corresponded to the RF of indol‐3‐yl acetic acid (IAA). Similar auxin activity was found in guttation fluid from seedlings of Zea mays, Triticum aestivum and Hordeum vulgare. Evidence that guttation fluid also contains alkali‐labile auxin complexes was obtained. Experiments on the movement of dyes and radioactive IAA introduced into the xylem of transpiring or guttating coleoptiles showed that these substances accumulate at the tip of the coleoptile, or at the apical region of decapitated coleoptiles. The hypothesis that IAA and‘inactive auxins move acropetally in the xylem from the seed to the coleoptile tip where they accumulate and where the ‘inactive auxins’ can be converted to IAA is shown to be consistent with the classical work on coleoptiles; it can also explain the autonomous curvature of coleoptiles and the influence of the roots on the auxin content of coleoptile tips. An analogous accumulation of auxin probably occurs at the tips of primary leaves. The anomalous auxin economy of coleoptile tips is discussed.