The Development of adventitious buds in Pinus oocarpa

When 10-day/12-16 mm cotyledons of axenically grown seedlings of Pinus oocarpa are excised and placed on a modified MS medium with appropriate concentrations of growth regulators (25 uM BAP and 25 nM NAA) for 14 days then transferred to a basal medium (same medium without growth regulators) adventitious buds are induced. These buds will elongate and root under conducive conditions. The purpose of this research was to document the morphological events that lead to adventitious bud formation. The initial cotyledonary explant has an even and ordered epidermis on all three surfaces. After 3 days in culture, the two epistomatic surfaces become irregular and uneven. The abaxial surface remains ordered. The first evidence of meristemoid formation occurs after 15-19 days in culture. The meristematic area increases in size rapidly and soon forms a dome which gives rise to leaf primordia. Fully developed adventitious shoots are present at day 40 showing the organization typically associated with pine apical meristems. When 8-day old explants 3-7 mm in length are cultured under the same conditions as described above, the most prominent structure observed is an aberrant bud. These buds are asymmetrical and poorly organized. Their leaf primordia have unusually large cells. The ontogenetic events that lead to aberrant buds are indistinguishable from those that lead to the formation of viable adventitious buds up to the point when domes are formed and leaf primordia arise. Other aberrant structures such as phylloids were observed. When 10-day old explants 4-24 mm were cultured under the same regime, viable adventitious buds were produced over the entire range of sizes. Since this range of sizes includes the same as those of the 8-day old explants (3-7 mm), we are able to state that the size of the explant does not effect its ability to produce viable adventitious buds.