Effect of embryo orientation on the developmental sequence of adventitious organogenesis in jack pine (Pinus banksiana Lamb.)

The developmental sequence of adventitious organogenesis in jack pine (Pinus banksiana Lamb.) embryos was analysed depending on two explant orientations. Embryos were placed vertically (cotyledons downward) or horizontally on half-strength Schenk and Hildebrandt medium containing 10 muM 6-benzylaminopurine. Samples were harvested at 2- to 3-day intervals for a 21-day period and observed using light and electron microscopy. In vertically placed embryos, cotyledons were inserted into the medium and became necrotic, while buds and phylloids developed on the upper part of the hypocotyl and in the apical area, 14 and 9 days after culture initiation respectively. In horizontally cultured embryos, organogenesis started in the intercotyledonary areas (day 11) and spread to the abaxial surface of cotyledonary margins outside the medium (day 14). A few buds also developed on the adaxial surface of cotyledons, starting on day 16. In the apical area, phylloid formation started on day 18. Nodules were induced in tissues in contact with the medium for both orientations, but in distinct sites. Embryo orientation influenced the localization of induction sites and the time sequence of organogenesis. The pattern for the mobilization of proteins, lipids, and polysaccharides prior to organogenesis was similar for both orientations. A depletion in proteins and polysaccharides did not occur in tissues in contact with the culture medium.