Interaction of stockplants shading and exogenous auxin on the propagation of Cola anomala K. Schum (Malvaceae) by cuttings

The establishment of an efficient protocol for vegetative propagation is still a challenge for Cola anomala, a priority target for domestication in West and Central Africa. The present study examined the rooting ability of C. anomala leafy stem cuttings in response to different levels of stockplants shading and different concentrations of exogenously applied indole 3-butyric acid (IBA). 900 cuttings were set in three blocks of a split plot experimental design in non-mist propagators. Each main plot contained three shading levels (0%, 50% and 75%), whereas five exogenous IBA concentrations (0, 1000, 2000, 5000 and 10,000 mg/l) were tested at the subplot level. At each level, treatments were assigned at random to experimental units so as to have 3 blocks × 3 levels of shading × 5 IBA concentrations × 20 cuttings. After 16 weeks, cuttings were assessed for mortality and rooting. Results showed that stockplants grown under 50% and 75% shade resulted in a subsequent significantly lower cutting's mortality rate as well as significantly higher percentage of rooted cuttings and number of roots per rooted cutting, compared to stockplants grown under 0% shade. Although IBA concentration had no effect on mortality rate, it significantly affected rooting ability. Considering all levels of shading together, the percentage of rooted cuttings as well as the number of roots per rooted cutting increased with exogenous IBA concentration and reached their maximum values (100% and 6 respectively) at 10000 mg/l IBA. With stockplants grown under high irradiance (0% shade), the maximum rooting percentage (100%) was obtained when cuttings were treated with 10,000 mg/l IBA, whereas with stockplants grown under shade (50% and 75% shade) the IBA concentration necessary to achieve maximum rooting was reduced to 5000 mg/l. These results indicate that stockplants shading increases the subsequent rooting ability as well as sensitivity to IBA of stem cuttings. This is valuable information to optimize the clonal propagation protocol for C. anomala and other species of the humid tropics of West and Central Africa, especially those considered to be difficult to propagate.