The role of cell clusters and promeristemoids in determination and competence for caulogenesis by Pinus strobus cotyledons in vitro

Exposure of Pinus strobus cotyledons to a modified Schenk and Hildebrandt medium containing N6-benzyladenine revealed that caulogenic determination was present after 4 days and culminated by day 7. Delaying exposure to N6-benzyladenine by preculture on basal medium for 2 days caused a significant reduction in competence for caulogenesis. Competence declined steadily and explants were incompetent by day 14 of culture. Organized three- and four-celled clusters were present by day 4 in the presence or absence of N6-benzyladenine. The number of cell clusters was enhanced on N6-benzyladenine as a result of an increase of three- and four-celled clusters, plus the development of five- and six-celled clusters. The latter were similar to promeristemoids, and their appearance parallelled increasing levels of caulogenic determination. On growth regulator free basal medium, the number and organization of cell clusters did not change from day 4 to 7. Cell clusters were absent by day 14 on growth regulator free medium, when explants were incompetent to respond caulogenically.