Evaluation of in vitro protocols for effective regeneration of West African Theobroma cocoa (L)

Abstract

Cacao is a perennial tree crop from the Malvaceae family. It is made up of twenty-one species, with Theobroma cacao as the commonest and one of the most economically important crops globally. West Africa is the largest region of producers. Lack of availability of planting materials all year round has been a major factor besetting cocoa production in West Africa and has led to a drastic reduction in production in the last decade. Establishing an efficient in vitro protocol for germinating zygotic embryos of local West African cocoa varieties offers a solution to expedite large-scale planting material production to enhance cocoa sustainable production and material availability. An effective in vitro protocol for germinating zygotic embryos of local West African cocoa varieties will help produce large planting materials within the shortest possible time and promote sustainable supply, which could boost cocoa production within the region and increase the availability of planting materials. Four West African local varieties of Theobroma cacao were cultured in three explant types on both Driver and Kuniyaki Walnut (DKW) basal salts and Murashige and Skoog (MS) culture media without growth regulators for twenty days. The three explant types included seeds with mucilage (SWM), seeds without mucilage (SWtM) and embryo axis (EA). The cultured explants were kept in a growth room of light intensity with a temperature of 25 ℃ ±1, relative humidity of 85% and the three explant types responded differently. Germination responses significantly varied among the explant types. SWtM sprouted earlier with more than 160 plantlets before day 5, followed by EA with 150 plantlets before day 5, while SWM showed the lowest mean germination percentage of 8% compared to SWtM and EA with 9% Early sprouting was observed in the DKW medium with almost 250 (34.72%) plantlets development before day five compared to the MS medium with less than 150 (20.83%) plantlets development, but the MS medium produces the optimal growth performance with the best mean germination time of 0.08 per day, final germination percentage of 71.39%, and development of the growth parameters leading to the development of plantlets, including 2.60 for number of leaves. This study shows that the media for in vitro culture and the source of explants significantly influence seed germination and produce dissimilar effects on the germination of zygotic embryos of Theobroma cacao varieties. The study recommends using SWtM on DKW medium for early sprouting of seeds of Theobroma cacao followed by a transfer into MS medium for seedling development into plants.