Effect of photoperiod and thermoperiod on microtuberization and carbohydrate levels in Cocoyam (Xanthosoma sagittifolium L. Schott)

Cocoyam (Xanthosoma sagittifolium L. Schott) is an important staple tuber crop for tropic populations and consists of three cultivars with different productivity. In vitro tuber induction of a broad range of genotypes could be a very useful way to propagate and increase valuable cocoyam. Shoot tips were cultured on a multiplication medium containing MS mineral salts, Morel and Wetmore (Ann J Bot 38:141-143, 1951) vitamins, 3% sucrose and 6% agar. Multiple shoots induced with 6-benzylaminopurine (BAP) were allowed to develop on the basal medium without plant growth regulators (PGRs). The subsequent explants obtained were used for the production of microtubers on MS-based PGRs free medium supplemented with 8% sucrose. Microtuberization was evaluated after 60 days of culture under different photo- and thermoperiod regimes. Changes in carbohydrates were also examined enzymatically during this process. Microtuberization response of three cocoyam cultivars (White, Red and Yellow) was influenced by photoperiod and thermoperiod regimes. A short photoperiod (8 h lighting), combined with an application of a day/night temperature regime 25/20°C for 10 days, followed by a continuous darkness at 20°C for 50 days, was the most effective treatment. The best response was observed with White followed by Red and Yellow cultivar. The sprouting frequency of microtubers varied significantly with cvs. after 15 days of culture (56% for White, 50% for Red and 40% for Yellow). Under tuber promoting condition, the onset of the process was characterized by an accumulation of starch and hexoses in leaves. The glucose:fructose ratio changed in favour of glucose earlier in White cultivar leaves where it doubled in 10 days. Moreover, in young developing tubers, the sucrose content increased concomitantly with starch.