Rooting of flowering Dogwood (Cornus florida) microshoots

Before flowering dogwood (Cornus florida L.) can be commercially propagated by tissue culture methods, a reliable and efficient method of production must be developed. Cornus florida has been micropropagated successfully but with a low rooting efficiency of about 50 %. Rooting has been a major problem in tissue culture of many other plants as well. This study was conducted to increase rooting efficiency of flowering dogwood microshoots over that previously achieved. Five to six week old microshoots originating from acclimatized axillary and nodal bud stock cultures were excised and used in the rooting experiments. Cornus florida microshoots were treated with various root promoting substances and bacteria. Microshoots were dipped for 24 h in aqueous diffiisates that were prepared by leaching stem cuttings of either black locust (Robinia pseudoacacia L.) or contorted willow (Salix x erythroflexuosa Rag.). Explants were then transferred to Woody Plant Medium (WPM) with and without indole-3-butyric acid (IBA). Various concentrations of salicylic acid and acetylsalicylic acid as root stimulating moeities were tested as a continuous exposure and a 24 h pulse treatments. A recently discovered root stimulating bacterium (RSB) promoted rooting in Pinus seedlings was also used to study rooting of flowering dogwood microshoots. Microshoots were treated with RSB cells as well as extracts obtained at pH 3 and pH 7 from the spent medium in which RSB was grown. Rooting efficiencies of microshoots grown on WPM and treated with these root promoting substances, bacterium and bacterial culture extracts were compared to that of microshoots grown on WPM with 4.9 µM IBA. Microshoots formed roots in all the experiments. Locust and willow diffusate slightly inhibited root formation on microshoots. Continuous exposure to 100 µM salicylic acid and 50 and 100 µM acetylsalicylic acid promoted root generation but not significantly different from that achieved with IBA. The highest mean number of roots/explant were produced when pulsed with 10 µM salicylic acid, 25 and 50 µM acetylsalicylic acid. Significant rooting was not observed on direct exposure of microshoots to live RSB. However, high rooting percentages of 70 % and 90 % were observed when microshoots were transferred to WPM amended with 2.5 ml and 0.5 ml of RSB extract obtained at pH 7 respectively. Of all the treatments tested, treatment of 5 to 6 wk old microshoots with 4.9 µM IBA stimulated the best and most consistent rooting efficiency of 70 to 100 %. Histological studies of root formation and leaf growth were also conducted. Root formation from microshoots started within 10 days of transfer to rooting medium. Microshoots grown on WPM amended with 4.9 µM IBA and leaf samples taken from various stages of micropropagation were sectioned and stained using Flemings triple stain. Leaf sections from plants in the greenhouse were found to be fully acclimatized as the anatomy and arrangement of tissues strongly resembled those from mature dogwood trees.