The Relationship between Rooting Cofactors of Easy and Difficult-to-root Cuttings of Three Clones of Rhododendron1

<jats:title>Abstract</jats:title> <jats:p>The level of endogenous root-promoting and inhibiting substances in 3 clones of rhododendron were compared at seasonal intervals in order to study the clonal and seasonal variation in rooting response of cuttings. The highest levels of 4 rooting cofactors in any season were found in both stem and leaf tissue of <jats:italic>Rhododendron</jats:italic> cv. ‘Cunningham’s White’ followed by ‘English Roseum’. The clone ‘Dr. H. C. Dresselhuys’ contained the least amount of the rooting cofactors. An inhibitor was often found in all clones, but it appeared less responsible for clonal differences in rooting response than variation in levels of the rooting cofactors. Rooting cofactor levels contained in the stem tissue of ‘Cunningham’s White’ were not less than those in the leaf tissue. In contrast, cofactor levels present in the stem tissue of ‘English Roseum’ and ‘Dr. H. C. Dresselhuys’ were less than those in the leaf tissue. The promoting activity of rooting cofactors in all tissues of the clones increased in September and decreased again in November to the level of July extract. The inhibitor found in the July extracts disappeared in September and reappeared in November.</jats:p> <jats:p>Rooting of cuttings of ‘Dr. H. C. Dresselhuys’ was significantly improved by grafting a leaf and bud scion of ‘Cunningham’s White’. On the other hand, scions of ‘Dr. H. C. Dresselhuys’ resulted in decreased rooting of cuttings of ‘Cunningham’s White‘. Rooting capacity of ‘English Roseum’ was less affected by a leaf and bud scion of other clones of <jats:italic>Rhododendron</jats:italic>.</jats:p>