Morphological changes of Dioscorea trifida cv. Short Neck Yampie and D-cayenensis cv. Round Leaf Yellow Yam linked to the number and size of harvested tubers

Successful use of tissue culture to propagate Dioscorea spp. depends on max imizing subsequent growth in vivo. This paper describes the growth of in vi tro plantlets of the Jamaican yams D. trifida L. cv. Short Neck Yampie (SNY ) and D. cayenensis Lam. cv. Round Leaf Yellow Yam (RLYY) after transfer to in vivo conditions, from day zero (day of transfer) to harvest of the firs t generation tubers at 314 d, and sprouting of these tubers. As the SNY and RLYY plants grew in vivo, abrupt changes occurred in the type of shoot (ch anged from type I to type II) and underground organ (from primary nodal com plex to tuber), and in the rates of stem and root growth. Gradual changes o ccurred in the number of roots. The first observable morphological feature of the SNY and RLYY plants to change was the shoot tip. This changed from a lateral position at the node at the base of a terminal leaf (lateral tip), to the shoot apex (apical tip). It is suggested that the change in shoot t ip position and associated features mark the transition from a juvenile pha se to an adult vegetative phase. The shoot tip position changed earlier on plantlets that had the greater number of roots at day zero (RLYY), and earl ier from heavier first generation tubers (SNY). A decrease in the length of the juvenile phase of micropropagated RLYY plants by 14 d resulted in a he avier tuber yield but did not affect tuber numbers. The significance of the se results on the propagation of Dioscorea is discussed.