In tuberous crops, tuber enlargement is one of the most important target traits for yield formation. It has long been known that tuber growth in yams is enhanced by short-day (SD) conditions, but the mechanism of tuber enlargement remains unknown. Here, we analyzed the photoperiodic regulation of tuber enlargement in water yam (Dioscorea alata L.). The photoperiod experiments in seedlings showed that tuber enlargement is initiated under SD conditions (≤10 h daylength) within 20 days of treatment. DaFT2, a FLOWERING LOCUS T (FT)-like gene, was upregulated in SD and downregulated in long-day (LD) conditions in tubers, suggesting that DaFT2 promotes tuber enlargement. DaFT1, the other FT-like gene, was significantly upregulated only in the leaves under LD, and its expression pattern was opposite to that of DaFT2 in the tubers. A night-break experiment showed that tuber growth was inhibited by red light in the dark period. These results suggest that the tuber enlargement of water yam is completely dependent on the photoperiod and that it involves an FT gene-mediated mechanism in response to the SD condition by red light sensing.

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.

For sustainable food production, labor-saving cultivation systems are required. Yam, a major food crop, is cultivated mostly with supports such as stakes (staking), which increases tuber yield (TY) but involves high material and labor costs. We, therefore, focused on non-staking water yam cultivation, in which no stakes are used. The effects of different cultivation methods (staking vs. non-staking) on TY, yield components, shoot traits, and tuber shape of six cultivars were investigated in a two-year field experiment, and phenotypic traits related to yielding ability in non-staking cultivation were analyzed. Averaged across years and cultivars, TY was significantly lower (by 19%) in non-staking than in staking cultivation because of smaller single-tuber weight. TY was significantly affected by the cultivation × cultivar interaction. We found no difference among cultivars in staking cultivation. In non-staking cultivation, Yamatomakousha and Shirokoushaman 1 cultivars had higher TY than the other cultivars, which was similar to their TY in staking cultivation. Shoot dry weight and vine number were closely associated with TY in both cultivations, whereas lower tuber length-to-width ratio was strongly related to higher TY only in non-staking. Tubers of Yamatomakousha and Shirokoushaman 1 were more rounded than those of other cultivars. In non-staking cultivation, these two cultivars showed a higher vine number and, thus, maintained higher TY owing to higher above-ground growth. Therefore, rounded tubers and high vine number are target traits for non-staking cultivation of water yam.