Effect of Seed Tuber Weights on the Development of Tubers

Effect of Seed Tuber Weights on the Development of Tubers and Flowering Spikes in Japanese Yams (Dioscorea japonica) Grown under Different Photoperiods and with Plant Growth Regulators

Yasunori Yoshida1*, Harumi Takahashi1, Hiroomi Kanda1 and Koki Kanahama2

1Akita Prefectural College of Agriculture, Ohgata, Akita 010–0451, Japan 2Faculty of Agriculture, Tohoku University, Aobaku, Sendai 981–8555, Japan

The effect of seed tuber (ST) weight on the development of main shoots, aerial tubers (bulbils, AT), new tubers (below ground, NT), and flowering spikes (inflorescences) was examined in Japanese yam plants (Dioscorea japonica) grown under different photoperiods and with plant growth regulators (PGRs). Within the same PGR and ST-weight group, the main shoot lengths of plants grown under a 24-h photoperiod (constant light, LD) were found to be longer than those grown under an 8-h photoperiod (SD) in all seasons. Furthermore, within the same photoperiod and ST-weight group, except for the plants grown from 25 g of ST (25gST plants) under SD conditions, the main shoot lengths of control and gibberellic acid (GA3)-treated plants were found to be longer than those of uniconazole-P (Uni)-treated plants. These tendencies were stronger in the 50gST plants than in the 25gST plants. In the 25gST plants, the final fresh weight (FW) of NT and the combined FW of AT and NT were greater in plants grown under LD conditions (LD plants) than those of SD plants; however, this was not observed in the case of the 50gST plants. This is because AT and NT development commenced earlier in the 25gST plants than in the 50gST plants. Furthermore, in the case of the 25gST-LD plants, the control plants also produced a few AT. Hence, we could confirm that the yield of NT was slightly increased by the inhibition of AT in the GA3-treated plants. The 25gST or 50gST-SD plants had only one peak of spike development from June through July, whereas the 25gST or 50gST-LD plants had two peaks of spike development, that is, in June and July and then again from September through October. In the season (June to July) of early spike development, the number of spikes per LD plant was greater than that in the SD plants, and the total number of spikes per plant was inhibited by GA3 treatment. These tendencies were stronger in the 50gST plants than in the 25gST plants.

Key Words: aerial and new tuber, Dioscorea japonica, photoperiod, plant growth regulator, seed tuber weight.

Introduction and flowering spikes (inflorescences) of this yam. In our previous study (Yoshida et al., 2004), we Japanese yam (Dioscorea japonica) is the most reported the relationship between the effects of expensive root vegetable in Japan because of its high photoperiods and plant growth regulators (PGRs) on the viscosity and nutritional value, low yield of new tubers development of tubers and spikes in Japanese yams. It (NT), and the extensive labor involved in its cultivation. was clarified that the shoot elongation period under a Few attempts to improve the low yield and disease 24-h photoperiod (constant light, LD) was significantly resistance of Japanese yam have been carried out because extended as compared with that in Chinese yam (D. of a lack of information about growth habits of shoots oppositifolia ‘Ichoimo’ and ‘Nagaimo’) (Yoshida et al., and NT, and its ecological characteristics such as poor 2001, 2002). The continued vegetative growth severely flowering and immature seed development. It is essential delayed the growth of AT and NT under LD conditions. to resolve the abovementioned problems to clarify the Hence, the final fresh weights (FW) of AT and NT growth habits of the shoots, aerial tubers (AT), and NT, remained lower in plants grown under LD conditions (LD plants), which exhibited excellent shoot growth, than that in plants grown under an 8-h photoperiod (SD). Received; July 13, 2006. Accepted; Febrary 15, 2007. The poor yields reflect the fact that the shoot diedback * Corresponding author (E-mail: [email protected]). was due to the low temperature in winter prior to the

230 J. Japan. Soc. Hort. Sci. 76 (3): 230–236. 2007. 231 adequate partitioning of the photosynthates stored in the the base of the pole. In order to eliminate the influence shoots for the growth of NT. of sex on ecological characteristics, only male Yoshida et al. (2001, 2002) showed that the inhibition D. japonica plants were used in the experiment. Initially, of AT by gibberellin A3 (GA3) treatment partitioned the 0.3 g of magnesium lime and 0.6 g of NPK fertilizer photosynthates stored in shoots for the growth of NT, (15 N-15 P2O5-15 K2O) were added to each pot per liter as in the case of D. oppositifolia ‘Ichoimo’ and of potting soil. At the end of July and August, two g of ‘Nagaimo’. Japanese yams generally develop a large fertilizer was added to each pot. The plants were irrigated number of AT. Thus, we postulated that the yield of NT as required. might be improved by GA3 treatment, which inhibits AT development. However, the promotion of NT in Treatment GA3-treated plants was not observed in Japanese yam The plants were grown in an unheated plastic house; LD plants, in contrast to that in SD plants, since the AT photoperiodic treatments were begun on June 24, 2002. of LD plants had scarcely developed. Furthermore, The plants that were exposed to both an 8-h photoperiod Japanese yam SD plants had only one peak of spike (SD) as well as a 24-h photoperiod (constant light, LD) development, whereas LD plants exhibited two peaks of were grown in natural sunlight, between 8:40 and 16:40 spike development. This finding in Japanese yam and were kept covered with a silver polyethylene film significantly differs from the findings in Chinese yam, (100% shading material) between 16:40 and 8:40. The which exhibited only one peak of spike development plants in the LD were irradiated by incandescent lamps (Yoshida et al., 2001, 2002). (approximately 2.0 µmol·m−2·s−1) between 16:40 and As mentioned above, we observed that the develop- 8:40. These treatments were continued until the end of ment of the main shoots, AT, NT, and spikes in Japanese November (SD) and December (LD) when the shoots yam was considerably influenced by the interaction became dormant and diedback. A combination of between the effects of photoperiod and plant growth photoperiod and PGR treatment was started on July 1 regulators. Yoshida et al. (2000) reported that the when the main shoots attained a length of 100 cm. The reaction to photoperiod and plant growth regulators was entire surface of the plant leaves were sprayed at weekly greater in D. oppositifolia plants grown from light- intervals with GA3 (100 ppm) and uniconazole-P (Uni, weight seed tubers (ST) than those grown from heavy- 25 ppm) containing 0.01% Tween20 until run-off. The weight ST. Similarly, shoot growth in D. alata (Toyohara PGR treatments were stopped on September 4 in the SD et al., 1998) and D. rotundata (Njoku et al., 1984) was group and on October 12 in the LD group when the promoted in plants grown from heavy-weight ST. As a main shoots ceased to elongate. In the no-PGR treatment result, the yield of NT was greater in plants grown from group (control), plants were sprayed at weekly intervals heavy-weight ST than those grown from light-weight with distilled water in addition to 0.01% Tween20. Each ST. Thus, the objective of this paper was to investigate treatment group consisted of 12 plants. the effect of ST weights on the development of tubers The length of the main shoots was recorded every and flowering spikes in Japanese yams grown under week after sprouting during the growing period. The FW different photoperiods and plant growth regulators. of the AT and NT were recorded three times: (1) on Materials and Methods September 4 (early development of AT and NT), when the main shoots ceased to elongate in the SD group; (2) on October 12 (middle development of AT and NT), Plant materials and culture practices when the main shoots ceased to elongate in the LD group In early April 2002, Japanese yam (Dioscorea (only 50gST plants); and (3) at the end of December japonica Thunb.) tubers were purchased from a seedling (final development of AT and NT), when the shoots of company. They were selected on the basis of weight both groups had died. Three or five plants were harvested (0.8–1.2 kg); both ends of the tubers were removed. The on each sampling date. tubers were then cut into pieces weighing 25 and 50 g (FW). The ST weight of 25 g (25gST) was used since Results and Discussion we expected that the plants grown from 25gST (25gST Shoot growth plants) developed spikes less easily in contrast to 50gST Within the same ST-weight and PGR-treatment group, plants. These pieces of ST were allowed to sprout for the cessation of main shoot growth (growth cessation) approximately one month in an unheated glasshouse at in both the 25gST and 50gST plants was earlier under the Akita Prefectural College of Agriculture, Japan. SD than under LD conditions (Fig. 1). Consequently, the Subsequently, on June 18, 2002, they were transplanted main shoot lengths of both the 25gST- and 50gST-LD into plastic pots (diameter, 35 cm; height, 50 cm). The plants were longer than those grown under SD conditions most vigorous shoots were retained as the main shoots in all growth periods. The shoot elongation period in and were entwined on a 240-cm pole. The terminal shoot LD plants was significantly extended as compared with (30 to 100 cm) was supported vertically, while the that in Chinese yam (D. oppositifolia ‘Ichoimo’ and remaining main and lateral shoots were coiled around ‘Nagaimo’) (Yoshida et al., 2001, 2002) and was similar 232 Y. Yoshida, H. Takahashi, H. Kanda and K. Kanahama

Similarly, under SD conditions, growth cessation in the 25gST plants (early August, 37 DAPT) occurred approximately 1.5 months earlier than that in the 50gST plants (early September, 81 DAPT). Within the same ST-weight and photoperiod group, on comparing the influence of PGR on the main shoot length at the time of shoot dieback, except for the 25gST-SD plants, the main shoot lengths of the control and GA3-treated plants were found to be longer than those of the Uni-treated plants. Furthermore, in the case of the PGR-treated plants, the difference in the shoot growth was clearly greater in the 50gST plants than in the 25gST plants. On the basis of the above results, we reported that growth cessation in SD and LD plants increased using a small ST. Hence, growth cessation could be regulated by the ST weight. This finding was important for the early production of NT in Japanese yam since growth cessation was caused by the initiation of the development of AT and NT. Fig. 1. Growth curves of the main shoots of Japanese yam (D. japonica) plants grown from seed tubers with weights of 25 and 50 g treated with GA3 and Uni and grown under 8-h (SD) and 24-h Aerial and new tuber development photoperiods (LD). Vertical bars represent SE (n = 10 to 12 (June At the initial harvest on September 4, when the main 24 to September 2), 7 to 9 (September 9 to October 7), and 5 shoots had ceased to elongate in the 50gST-SD plants, to 7 (October 14 to end of December)). Arrows indicate the within the same ST-weight and PGR-treatment groups, harvest dates of aerial and new tubers: I, September 4; II, October 12; III, at the time of shoot dieback at the end of December; the control as well as the PGR-treated SD plants Cont, control plants sprayed with distilled water; GA3, produced 8.4 to 13.6 g of AT, while all LD plants gibberellin A3 (100 ppm); Uni, uniconazole-P (25 ppm). produced a negligible amount of AT (Fig. 2). Similarly, 39.4 to 45.5 g of early NT were produced in all SD to that in our previous study using Japanese yam plants, whereas in all LD plants, 12.6 to 22.9 g of early (Yoshida et al., 2004). However, in the 25gST-LD plants, NT were produced. These differences in the early growth cessation occurred after early September (88 days developments of AT and NT in the 50gST plants between after photoperiod treatments (DAPT)); this was SD and LD conditions corresponded with that of Chinese approximately 1 month earlier than that observed in the yams and D. alata (Garner and Allard, 1923; Hayashi 50gST-LD plants (mid-October, 121 DAPT) (Fig. 1). and Ishihata, 1991; Sato, 1964; Shiwachi et al., 1999,

Fig. 2. Interaction between photoperiods and plant growth regulators on the fresh weight of aerial and new tubers in Japanese yam (D. japonica) plants grown from seed tubers with weights of 25 and 50 g. Initial, second, and final harvest was conducted on September 4 (I), October 12 (II), and at the time of shoot dieback at the end of December (III), respectively. Vertical bars represent SE (n = 3 (I), 3 (II), 3 or 4 (III)). Mean separation of the total fresh weight of aerial and new tubers at each sampling date by Fisher’s protected LSD test (P ≦ 0.05). Photoperiods and plant growth regulators treatments are as described in Figure 1. J. Japan. Soc. Hort. Sci. 76 (3): 230–236. 2007. 233

2000; Yoshida et al., 2001, 2002). However, in the case the 25gST- and 50gST plants in Japanese yam was of the 25gST plants, all LD plants produced few AT; attributed to the fact that even if the plants were grown moreover, all SD plants produced only 0.5 to 3.4 g of under LD conditions, the initiation of AT and NT growth AT. Furthermore, the FW of NT in the 25gST plants was faster in the 25gST plants than that in the 50gST was almost the same in all SD (16.1 to 24.3 g) and LD plants. In other words, the period of development of AT plants (8.7 to 19.3 g). These differences between the and NT until shoot dieback in the 50gST-LD plants was 25gST and 50gST plants account for the time of growth insufficient for the diversion of photosynthates stored in cessation, which was caused by the initiation of the shoots toward AT and NT, as compared to that in the development of AT and NT. As can be seen from 25gST-SD and LD plants and 50gST-SD plants. We Figure 1, the main shoot elongation in the 25gST-SD reported that an 80-day period was sufficient for the and LD plants had ceased until initial harvest on development of AT and NT after the cessation of main September 4, such that even those LD plants had already shoot elongation in Chinese yam (Yoshida et al., 1999). initiated the early development of tubers, particularly In this study using Japanese yam, the AT and NT NT. development periods in the 25gST-SD and LD plants At the second harvest on October 12, when the main and in the 50gST-SD plants were approximately 100 shoots had ceased to elongate in the 50gST-LD plants, and 91 days after growth cessation, respectively, while only the 50gST plants were examined. The middle that in the 50gST-LD plants was only approximately 59 development of AT and NT was significantly greater days after growth cessation (Fig. 1). Within the same than their early development (Fig. 2). Although the AT PGR-treatment groups, under SD conditions, the final and NT development tendency was the same as that FW of NT in the 50gST plants (82.1 to 217 g) was greater observed at the initial harvest on September 4, the middle than that in the 25gST plants (31.2 to 64.3 g) (Fig. 2). development of AT and NT of the SD plants was greater Under LD conditions, the final FW of NT in both the than that of the LD plants. 25gST and 50gST plants was almost equal (70.5 to In the case of Chinese yam (‘Ichoimo’ and ‘Nagaimo’) 150.0 g). In addition, the natural day length for Japanese (Yoshida et al., 2000, 2002), the final FW of NT and yam cultivation is long (12- to 15-h photoperiod) during the combined final FW of AT and NT in LD plants with the early to middle stage of growth (April to September) excellent shoot growth were greater than those of SD and short (13- to 10-h photoperiod) during the end stage plants. However, in our previous findings in Japanese of growth (September to November) in Japan yams, the final FW of LD plants with excellent shoot (Chronological Scientific Table, 2001). Hence, the growth remained lower than that of SD plants (Yoshida growth of NT in Japanese yam was not accelerated by et al., 2004). The difference in the effect of photoperiod the natural day length during the early to middle stage on the final development of AT and NT between of growth in Japan. We would like to emphasize that Japanese and Chinese yam (‘Ichoimo’ and ‘Nagaimo’) with respect to the place of cultivation and the limited is attributed to the continuous elongation of the main period of growth in Japan (temperate zone), the ST shoots in the latter under LD conditions; however, this weight must be considered in order to obtain a high yield is not observed in the case of SD plants. We assumed of NT in Japanese yam. For example, a region with that the continued vegetative growth greatly delayed moderate temperature where a sufficient growing period tuber growth under LD conditions so that the period of for NT development can be maintained is expected to development of tubers was insufficient. Hori (1984) result in a high yield of NT using heavy ST with excellent reported that shoot and AT and NT growth patterns after shoot growth; however, a high yield cannot be expected the germination of seeds or sprouting from ST in from heavy ST grown in a region with a cold climate. Japanese yam were greatly influenced by light intensity; In this experiment, the promotion of NT in GA3- however, whether the light intensity has any effect on treated 50gST plants was not observed in LD plants growth depends on the size of the seeds or ST. Hence, unlike in SD plants, since the AT of the control 50gST- we examined both the 25gST and 50gST plants to LD plants had scarcely developed (Fig. 2); this finding confirm the effect of ST weight on the final development was similar to the results obtained in our previous study of AT and NT under LD conditions (Fig. 2). (Yoshida et al., 2004). However, the control 25gST-LD At the final harvest at the end of December, within plants also produced a few AT so that the yield of NT the same ST-weight and PGR-treatment groups, the FW was slightly increased by the inhibition of AT in GA3- of NT and the combined FW of AT and NT remained treated plants. Hence, we could also confirm that NT lower in LD plants with excellent shoot growth than in development in LD plants was promoted by GA3 those grown under SD conditions, except for the Uni- treatment in Japanese yam using small ST (25gST treated plants (Fig. 2); this finding was similar to the plants). In D. alata, GA3 treatment led to a slight decrease results obtained in our previous studies using Japanese in shoot growth so that the early growth of NT was yam (Yoshida et al., 2004). The FW of AT and NT in promoted by GA3 treatment, while the final development the 25gST-LD plants with excellent shoot growth were of NT in GA3-treated plants was less than that of the greater than those of SD plants. This difference between control plants with excellent shoot growth (Onjo and 234 Y. Yoshida, H. Takahashi, H. Kanda and K. Kanahama v (38.8 f) dieback leaves) (59.7 ef) (64.0 def) (64.0 (98.8 cde) (126.2 bc) (113.2 cd) (140.8 bc) (110.3 cde) (105.0 cde) (Number of Date of shoot plant of spikes/ Total number w ng spike group, and those that exhibit the spike Thunb.) plants grown from seed tubers with of the final of Node position D. japonica spike of the first Late developing spike group Node position in Japanese yam ( 0 ( 0) — — — Nov. 25 Occurrence a spike (%) of plants with ush are referred to as the early developi p 0.05).

plant 37.0 ≦

of spikes/ P Total number x ) s q 44.5 spike 40.5 b 23.7 b 0 ( 0) — — — Nov. 19 of the final of Node position e development of spikes (inflorescences) ) 29 (Jul. t u r spike (Jul. 8) (Jul. 22) (Jul. 8) (Jul. 29) (Jul. 8) (Aug. 6) (Jul. 8) (Aug. 13) 2) (Sep. (Oct. 7) (196.1 a) (Jul. 1) (Aug. 6) (Jul. 8) (Jul. 29) (Jul. 8) (Aug. 6) (Sep. 9) (Sep. 23) ab) (168.9 (Jul. 15)(Jul. 15) (Jul. 29) (Jul. 29) (Jul. 15) (Jul. 22)(Jul. 22) (Jul. 29) (Sep. 9) (Sep. 23) (166.8 ab) (Jul. 15 of the first Node position Early developing spike group different by Tukey’s compromise test ( by Tukey’s different Japanese yam plants. Plants that exhibit the June and July fl Occurrence a spike (%) of plants with at the time of shoot dieback. 9 5 (56) 11.8 a 26.0 b 15.3 b 0 ( 0) — — — Nov. 12 10 7 (70) 13.6 a10 49.3 ab 2 (20) 38.0 ab12 8.5 0 ( 0) 9 (75) — 25.6 a 60.0 ab — 34.6 ab — 0 ( 0) Dec. 17 — — — Dec. 20 te developing spike group. growth regulators and their effect on th % or more plants. Number of plants observed y 3 3 3 3 could not be obtained in the experiment. the same letter are not significantly regulator flower buds on 50 Plant growth are referred to as the la th regulator treatments are as described in Figure 1. r of leaves the main shoot z Photoperiod weights of 25 and 50 g. . Interaction between photoperiods and plant 2525 LD LD Cont GA 8 6 (75) 17.6 a 49.5 ab 33.2 ab 0 ( 0) — — — Dec. 17 25 SD GA 25 SD Cont 9 8 (89) 16.3 a 25 SD25 Uni LD 10 Uni 9 (90) 12.7 a 1050 40.3 b 9 (90) LD 27.5 ab 22.3 a 0 ( 0) 47.1 ab Uni 32.5 ab — 11 3 (30) (91) 10 — 139.0 a 12.2 a 150.3 a — 78.0 ab 12.3 b Nov. 19 66.8 ab Dec. 17 4 (36) 139.5 a 184.8 a a 46.3 Dec. 20 50 SD Cont 10 9 (90) 15.4 a 44.5 ab 32.6 ab 0 ( 0) — — — Nov. 25 50 SD GA 50 SD Uni 11 6 (55) 13.2 a 50.7 ab 33.1 ab 0 ( 0) — — — Nov. 25 50 LD Cont 12 10 (83) 18.5 a a 93.5 76.0 a (42) 5 144.6 a 160.6 a 12.6 b Dec. 20 50 LD GA September and October flush There were two flushes of spike development in the main shoots weight (g) Sufficient data for statistical analysis Sufficient Seed tuber Photoperiods and plant grow Date indicates the emergence of Date indicates the emergence Data indicate the final numbe in the same column followed by Values Table 1 z,y x,w v u t,s r,q,p J. Japan. Soc. Hort. Sci. 76 (3): 230–236. 2007. 235

Hayashi, 2001; Onjo et al., 1999). In the 25gST-SD inhibited by GA3 treatment. Within the same ST-weight plants, the final FW of the NT in the GA3-treated plants and PGR-treatment group, there was a small difference (approximately 31.2 g) was also less than that of the in the rate of spike development between the 25gST-LD control plants (approximately 64.3 g), unlike the 50gST- and the 25gST-SD plants; however, in the 50gST plants, SD plants. The difference between the 25gST-SD plants the rate of spike development was higher in LD plants and 50gST-SD plants could be attributable to an than in SD plants. The position of the node of the first excessive decrease in leaf number. The leaf number was spike (node-first spike) and the date of emergence of decreased by the negative effects of GA3. Furthermore, the first flower buds (date first-flower) did not differ the leaf number was also decreased by 25gST plants or (approximately 16 nodes, July 15) with every treatment. SD (Table 1). Hence, an excessive decrease in leaf With the exception of the GA3-treated 50gST plants, the number was observed in the 25gST-SD-GA3-treated node position of the final spike was higher in the LD plants (approximately 38.8 leaves). The final FW of AT plants than that in the SD plants. As a consequence, the increased in the Uni-treated plants and decreased in the total number of spikes per plant was greater in the LD LD plants. This tendency was greater in the 50gST plants plants than that in the SD plants. These tendencies were than in the 25gST plants. The low FW of AT in the Uni- stronger in the 50gST plants than in the 25gST plants. treated 50gST-LD plants could be attributable to the In the late developing spike group (September to insufficient period of development of AT since December), the Uni-treated 25gST-LD plants, control, vegetative growth in LD plants continued until the late and Uni-treated 50gST-LD plants developed spikes, growth period. whereas the 50gST-SD plants did not develop spikes. This result could be attributed to the extended period of Spike development main shoot growth, that is, by September and October, Both the 25gST- and 50gST-SD plants exhibited only the elongation of the main shoots on the SD plants had one flush of spike development from June through July, already ceased by the start of AT and NT development. whereas both the 25gST- and 50gST-LD plants exhibited On the basis of the above results, we concluded that a flush of spike development in June and July and again the ST weight was a more important factor than the LD from September through October (Fig. 3); this was photoperiod for the two flushes of spike development; similar to the spike development observed in Japanese this is because the start time of AT and NT development, yam LD plants (Yoshida et al., 2004). Hence, there were which led to the cessation of main shoot elongation, was two flushes of spike development in the main shoots. strongly influenced by ST weight. Plants that exhibit the June and July flush are referred The frequency of occurrence of plants that showed to as the early developing spike group, while those that spike development with every treatment was low (3 to show the September and October flush are referred to 5 plants out of 10 to 12, 30% to 42%). The node-first as the late developing spike group (Table 1). spike and date first-flower at every treatment were 139 In the early developing spike group (June to July), to 144.6 nodes and September 2 to 9, respectively. Within the effect of photoperiod and ST weight on the the same PGR group (Uni-treated plants), the total development of spikes was unclear in this experiment, number of spikes was higher in the 50gST plants (46.3 although it appeared that spike development was spikes) than that in the 25gST plants (12.3 spikes). Literature Cited

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