Initiation of roots on hypocotyl cuttings of Pinus contorta in vitro

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Abstract
The origin of roots and wound tissue after treatments for induction of roots on hypocotyl cuttings of three‐week‐old Pinus contorta Dougl. ex Loud, is discussed. The cuttings were cultured in vitro and treated with 1.2 μM to 1.5 M IBA (indole‐3‐butyric acid) for 6 h to 10 days. The control, which was not treated with IBA developed a wound tissue from which roots formed. Cuttings treated with IBA developed roots directly from the hypocotyl. Direct rooting was faster than indirect rooting via a wound tissue. Rooting was considered to be optimal if more than 80% of the cuttings rooted within 19 days and half of the cuttings which possessed roots after one month had acquired them within 14 days. This type of rooting was obtained after treatment with either 80 μM IBA for 4 to 6 days or 1.25 to 5.0 mM IBA for 6 h. Suboptimal treatments gave lower rooting percentages and superoptimal treatments resulted in delayed rooting. In IBA‐treated cuttings, large increases in mitotic activity (number of mitoses per mm hypocotyl) were found in the pericycle and parenchyma inside endodermis. However, the control also had similar mitotic activities as the IBA‐treated cuttings but closer to the cut surface. This led us to the conclusion that similar tissues may produce either wound tissue or roots. Almost all roots obtained through direct rooting originated outside resin ducts.