The effects of gibberellin concentration, treatment intervals, and treatment commencement on the development of the main shoots, aerial and new tubers, and flowering spikes in Chinese yam (Dioscorea oppositifolia L.) cv. nagaimo

The effects of gibberellic acid (GAsub(3)) treatment concentration, GAsub(3) treatment interval, and GAsub(3) treatment commencement on the development of the main shoots, aerial and new tubers, and flowering spikes in Chinese yam (Dioscorea oppositifolia L.) cv. Nagaimo were investigated under natural daylength condition. The foliar application of GAsub(3) (0, 25, 50, 100, and 200 ppm) was carried out every week after emergence. As a result, the final fresh weight of aerial tubers was higher in plants treated with 0 and 25 ppm of GAsub(3) groups than in plants treated with 50 to 200 ppm of GAsub(3) groups. However, the final fresh weight of new tubers was higher in plants treated with 50 to 200 ppm of GAsub(3) groups than in plants treated with 0 and 25 ppm GAsub(3) groups. The foliar application of GAsub(3) (100 ppm) was carried out at one (GAsub(3)E1W), two (GAsub(3)E2W), and four (GAsub(3)E4W) week intervals after emergence. Following the application, aerial tubers having the highest final fresh weight were harvested from the control plants (sprayed with distilled water). This was followed by plants from GAsub(3)E4W and GAsub(3)E2W groups that produced aerial tubers having lower final fresh weights; the plants from GAsub(3)E1W group produced aerial tubers having the lowest final fresh weight. The plants from GAsub(3)E1W group produced new tubers having the highest final fresh weight. In the subsequent experiment, the foliar application of GAsub(3) (100 ppm) was carried out every week at one (GAsub(3)A1W), two (GAsub(3)A2W), four (GAsub(3)A4W), and eight (GAsub(3)A8W) weeks after emergence. Following the application, aerial tubers having the highest final fresh weight were harvested from the control plants (sprayed with distilled water). This was followed by plants from GAsub(3)A8W group; the plants from GAsub(3)A1W, GAsub(3)A2W, and GAsub(3)A4W groups produced aerial tubers having the lowest final fresh weight. The plants from GAsub(3)A1W, GAsub(3)A2W, and GAsub(3)A4W group produced new tubers having the highest final fresh weight. The development of main shoots and spikes did not show significant differences among the above GAsub(3) treatment methods, except for GAsub(3)E4W group. But, the number of leaves was decreased by GAsub(3) treatment methods, which promoted the final fresh weight of new tubers. We concluded that the effective GAsub(3) treatment method in which the new tubers development was significantly promoted by the inhibition of aerial tubers development in cv. Nagaimo grown under natural daylength comprised GAsub(3) concentration of 50 to 200 ppm, one-week treatment interval, and commencement of treatment at no more than four weeks after emergence. Hence, it is essential to investigate the combined effect of the above effective GAsub(3) treatment method on the development of the main shoots, aerial and new tubers, and flowering spikes.