Bulbil Formation on Water Yam (Dioscorea alata L.) Is Promoted by Waterlogged Soil

The formation of bulbils, which are storage organs, is an important agronomic trait and a unique morphological feature in the vegetative reproduction of yam. We found a landrace of water yam (Dioscorea alata L.), which rarely forms bulbils, that produces bulbils during periods of high rainfall. We investigated the physiological mechanism of bulbil formation in response to over-moist soil and relevant factors at the single plant level. Waterlogging (WL) treatment markedly increased the number of bulbils initiated, predominantly toward the upper nodes. This formed-bulbil was an accessory bud developed as a storage organ in leaf axils. Photosynthetic capacity decreased under WL, attributed to stress-induced stomatal closure. WL stress also reduced dry matter partitioning to the belowground organs. During tuber enlargement in WL plants, photosynthetic products accumulated in the aboveground organs and were transported to the bulbils as a result of reduced translocation to belowground organs. We investigated the effect of abscisic acid (ABA) on bulbil formation on the basis of changes in the sink-source balance in response to WL stress. ABA treatment of leaf axils enhanced bulbil formation in unstressed plants, suggesting that increased ABA is one of the factors that initiate bulbils. Our study shows that bulbil initiation occurs as a result of changes in physiological conditions in response to WL stress. This finding may provide fundamental information for the control of bulbil production. This response of bulbil formation, as an environmentally adaptive trait of the tropical water yam, may underlie the survival strategy of vegetatively propagated plants.