THE ROLE OF AUXIN IN THE REMOVAL OF DORMANCY CALLOSE AND RESUMPTION OF PHLOEM ACTIVITY IN VITIS-VINIFERA

During winter the phloem of Vitis vinifera L. is dormant and is characterized by heavy deposits of callose on the sieve plates and lateral sieve areas of the sieve tubes. After bud break, the maturing leaves stimulate a nonpolar breakdown of this dormancy callose along the branch axis in the internodes located both above and below the leaves. However, the pattern of callose degradation in the radial direction is polar. It proceeds in a centrifugal direction so that the sieve tubes near the cambium become free of callose first and those adjacent to the periderm last. The effect of the leaves on the removal of dormancy callose can be replaced by auxin. Application of naphthaleneacetic acid to either the top or basal ends of excised dormant branches resulted in the removal of callose from sieve tubes, usually in less than a week. The fluorescent dye fluorescein was used to test phloem reactivation. Both acropetal and basipetal fluorescein movement occurred in sieve tubes in branches that were pretreated for 1 week with auxin, while much less movement of fluorescein occurred in the control branches, which remained dormant. Fluorescein translocation was observed in sieve tubes that had a reduced amount of callose and were wider than 20-mu-m, but was not detected in the narrow sieve tubes (diameters less than 15-mu-m) located next to the cambium. The possible roles of auxin, ethylene, and cytokinin in controlling callose levels in the sieve tubes are discussed.