Initiation of embryogenic cultures and somatic embryo development in loblolly pine (Pinus taeda)

Immature zygotic embryo explants (isolated or with intact megagametophytes) from 10 loblolly pine (Pinustaeda L.) clones (7-34, 7-56, 11-9, 11-16, 11-25, 10-1003, 10-1007, 10-1011, 10-1018, and 10-1019) were surveyed for their potential to form embryogenic tissue from the suspensor region of zygotic embryos. After over 14 000 explants were cultured, embryogenic cultures were initiated from explants of 8 of the 10 clones; only explants from clones 11-25 and 10-1019 were not responsive. Embryogenic tissue was initiated from zygotic embryos with intact megagametophytes on MSG basal medium with no exogenous plant growth regulators or with 2-5 mg/L 2,4-dichlorophenoxy acetic acid (2,4-D) and 0-1 mg/L N 6 -benzyladenine (BA). The highest initiation frequency (5%) was obtained from isolated zygotic embryos of clone 7-34 less than 0.5 mm in length just prior to cotyledon primordia development on DCR basal medium with 3 mg/L 2,4-D and 0.5 mg/L BA. Two types of embryogenic cultures were maintained on medium with 2,4-D and BA: (i) those that contained pre-embryonal masses of cells interspersed with unaggregated suspensorlike cells, but which rarely contained well-formed somatic embryos, and (ii) those that frequently contained well-formed somatic embryos. Somatic embryo development from both types of cultures progressed to a precotyledonary stage on medium with 2.6 mg/L abscisic acid.