ANATOMY OF ADVENTITIOUS ROOT-FORMATION IN MICROCUTTINGS OF MALUS-DOMESTICA BORKH GALA

Anatomical events of adventitious root formation in response to root i nduction medium, observing changes during induction and post-induction stages, were made with microcuttings of 'Gala' apples. Shoot explants on root induction medium containing water, 1.5 muM IBA, 44 mM sucrose , or 1.5 muM IBA + 44 mM sucrose after 4 days of treatment averaged 0, 0.2, 2.2, and 11.9 meristemoids per microcutting, respectively. Meris temoids formed in response to sucrose were confined to leaf gaps and t races. Time-course analysis of root induction with 1.5 mum IBA + 44 mm sucrose over 4 days revealed that some phloem parenchyma cells became densely cytoplasmic, having nuclei with prominent nucleoli within 1 d ay; meristematic activity in the phloem was widespread by 2 days; cont inued division of phloem parenchyma cells advanced into the cortex by 3 days; and that identifiable root primordia were present by 4 days. C ell division of pith, vascular cambium, and cortex did not lead to pri mordia formation. Meristematic activity was confined to the basal 1 mm of microcuttings. Time-course analysis of post-induction treatment re vealed differentiation of distinct cell layers at the distal end of pr imordia by 1 day; primordia with a conical shape and several cell laye rs at the distal end by 2 to 3 days; roots with organized tissue syste ms emerging from the stem by 4 days; and numerous emerged roots by 6 d ays. Root initiation was detectable within 24 hours and completed by d ay 4 of the root induction treatment and involved only phloem parenchy ma cells. Chemical names used: 1H-indole-3-butryic acid (IBA).