A Preliminary Report On the Use of Embyro Rescue Techniques with Intra- and Interspecific Hybrids in Crape Myrtle (Lagerstroemia L)

The flowering shrub or tree, crape myrtle, in the genus Lagerstroemia L. is primarily native to tropical regions of Southeastern Asia and Indo-Malaysia. Intra- and interspecific hybrids within the Lagerstroemia taxa have resulted in the development of cultivars with improved horticultural characteristics.  Combining complementary traits found in the species L. indica and L. speciosa would appear to have great potential for expanding genetic diversity by improving pest resistance as well as introducing novel floral characteristics. However, male and female sterility observed in the F1 progeny have prevented recombination of genomes of the two species and the inability to introgress genes affecting complex traits such as cold hardiness as a part of a cultivar improvement program. No progress of embryo rescue on crape myrtle has been reported so far. The objective of this study was to get survive plants from backcrosses between the sterile F1 resulting from L. indica × L. speciosa and L. indica cultivars. Six backcrosses (including reciprocal) with three L. indica cultivars were attempted in 2009. To prevent the drop of seed capsules and abortion of embryos, a solution containing 5 mg/L 2, 4-D, 100 mg/L AgNO₃, and one drop of Tween 20 per 500 ml was applied to the floral petals 2 hours after pollination and then every day until the capsule was matured.  Ovaries treated this way remained on the plant while those that were untreated fell off the plant in a week. Approximately 30 embryos derived from L. indica × L. indica crosses have been used as a control to improve techniques and develop new media formulations. We cultured young embryos in vitro on a nutrient medium composed of half strength MS basal salts plus 30 g/L sucrose, 30 g/L glucose, MS vitamins, 3% coconut water, and 6 g/L Agargel. The cultures were incubated at 25 °C with a 16 hr photoperiod. Healthy plants were obtained from most embryos of L. indica × L. indica crosses. No viable embryos were found in the crosses of F1 hybrid as female, but some crosses where L. indica was used as the female produced embryos. Two of these embryos have germinated to produce viable plants. Several clones were duplicated from these two viable plants onto a new growth media before transferring to an 18-pot tray on a mist bench. The survived plants were transferred to soil and examined for fertility and horticultural traits in greenhouse condition.