In vitro control of caulogenesis by growth regulators and media components in embryonic explants of eastern white pine (Pinus strobus)

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Abstract
Horizontally oriented embryos of Pinus strobus (L.) produced shoots on Schenk and Hildebrandt medium containing cytokinin. Shoots developed primarily from cotyledons in contact with the medium. Seed pretreatments at 5 or 27 °C did not affect caulogenesis. N6-Benzyladenine (BA) and N6-(Δ2-isopentenyl)adenine (2iP) both induced caulogenesis, with BA being 10–20 times more potent than 2iP. High BA levels caused callus formation. BA exposures from 1 to 8 weeks were equally caulogenic with horizontal explants, but exposures longer than 2 weeks led to increased variability and callus formation. A 1-week, upside-down, vertical orientation during BA treatment increased the uniformity of cotyledon response and was as caulogenic as a 4-week horizontal BA exposure. Neither auxins nor triiodobenzoic acid induced or significantly enhanced shoot formation. Full-strength Schenk and Hildebrandt medium was superior to Murashige and Skoog medium for shoot induction. Dilution of Schenk and Hildebrandt medium had no significant effect on shoot production, but shoot elongation was suppressed on one quarter strength Schenk and Hildebrandt medium. Half-strength Murashige and Skoog medium was as caulogenic as Schenk and Hildebrandt medium. The NH4 level of the macronutrients was responsible for the difference between Schenk and Hildebrant medium and Murashige and Skoog medium. The higher NH4 concentration of Murashige and Skoog medium inhibited shoot formation.