Biology
AgriculturalAndFoodSciences,1970-2015-Agricultural And Food Sciences-2003.jsonl.gz.gz
Agricultural And Food Sciences
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Genotypic and Environmental Variation of Lag Period of Pod Growth in Soybean

Abstract Pod growth in soybean (Glycine max(L.) Merr.) begins several days after flower opening, compared with more immediate growth in other beans. We investigated the relationship between genotype, raceme order of pod set, assimilate supply or photoperiod and the length of lag period of pod growth (LP, days from flower opening to the time when pod length reaches 10 mm). Soybean (five cultivars) plants were grown in a greenhouse and in the field in 2001. The lengths of pods developed from 20 flowers which opened on the same day and set on the same raceme order, were measured every other day. The length of LP varied with the cultivar from 5 to 16 days and it was longer in late cultivars. The LP in the primary raceme (early flowers) was 15 days but that in the secondary raceme (late flowers) was 8 days. Both late sowing and short photoperiod (lOh) after the start of flowering shortened the LP by up to 7 days in Enrei and 5 days in Fukuyutaka. However, neither sink (except the target racemes) removal nor BA application to the target racemes at the start of flowering affected the length of LP, even though these treatments were expected to stimulate pod growth.

The period of reproductive growth in soybean is very long (70 to 90 days after the start of flowering) compared with that in cereals. The pod shells in soybean develop after flowering, whereas the husks in rice or wheat, those function is the same as that of pod shell in soybean, are formed before flowering. Therefore, the seed filling begins about four weeks after flowering in soybean (Konno, 1976), but about one week after flowering in rice or wheat (Sofield et al., 1977;Chowdhury and Wardlaw, 1978). In some legumes, such as mung bean, cowpea and common bean, the period from flowering to maturation is also shorter than that in soybean (Egli, 1998).
Several studies documented that the pod formation is very slow after flowering in soybean. Umezaki(1991) reported that the time of pod appearance (the ovary developed more than 10 mm in length) varied from 6 to 18 days after the start of flowering among 71 late maturing cultivars. Heitholt et al. (1986) also reported a difference in time of pod growth between genotypes. The delay of pod growth seems to lengthen the reproductive growth period. On the other hand, Saitoh et al. (1998) indicated that the delay of pod growth is much longer in the low order racemes (early opening flowers) and is reduced in high order racemes (late opening flowers). It is suggested that the timing of pod formation plays a role in the regulation of synchronous pod maturation in soybean (Saitoh et al., 1998). However, there are no further investigations focused on the delay of pod growth in soybean. The understanding of this function may be helpful to understand the mechanism of pod set, seed filling and yield.
It is well known that the vegetative growth continues for a while even after the start of flowering in soybean. Thus, the competition for assimilate supply between reproductive and vegetative growth after the start of flowering could inhibit the pod growth (Brun and Betts, 1984;Baba et al., 2003). Furthermore, the pod set could be stimulated by cytokinin (Peterson et al., 1990), and the pod filling duration is shortened by a short photoperiod (Thomas and Raper, 1976;Raper and Thomas, 1978). In this study, we investigated the process of pod formation and the effects of genotypes, raceme orders of pod set, assimilate supply and photoperiod on the pod growth.

Materials and Methods
Soybean and the other three legumes were grown in a greenhouse (pot experiment) and in the field in 2001 at Kyushu University, Fukuoka of Japan. The varieties used and sowing dates are shown in Table 1. In the pot experiment, paddy soil mixed with 5 g compound fertilizer (N:P 2 0s:K 2 0=3:1O:10) and 3g lime were compacted in a plastic pot (4 L in volume) before sowing. After emergence, one plant per pot was allowed to grow. In the field, the planting density was 60 cm X 15 cm spacing. The same compound fertilizer and lime (100 g m-2 each) were applied before planting. When the plants began flowering, the lengths of the pods from 20 flowers that opened on the same day and on the same raceme order in each experimental group were measured every other day. To determine the environmental and physiological effects on the pod growth, following treatments were applied.

Short photoperiod treatment
After the first flower opened, five pots with plants were moved into the apparatus having the roof that automatically open at 8: 00 and close at 18: 00 everyday. In the apparatus, plants were exposed to a 10hour photoperiod under natural light and temperature conditions. The treatment was continued until the plant matured.
2. Sink removal and BA (Benzyladenine) application After the first flower opened, all of the racemes except two at the target nodes (the lowest two nodes without a branch on main stem), and the tops of main stem and all branches were removed, in order to concentrate the assimilate supply to the target racemes. In the other groups, a water solution containing 200 mg L -) BA (Peterson et aI., 1990) was carefully sprayed on the target racemes 00 mL for each raceme) once. Five plants 00 racemes) were selected from a bordered block   with that in the other three crops. Soybean pods reached the maximum length 15 to 25 days after flower opening against 7 days in mung bean, and 10 days in azuki bean and common bean. The differences in pod growth were also observed among soybean cultivars (Fig. 2). In two early maturing cultivars, Kitamusume and Orihime, the pods began to grow immediately after the flower opened, while in the other three cultivars, the pods began to grow more than 10 days late. Therefore, the lag period of pod growth (LP, days from flower opening to the time when pod length reaches 10 mm) was about 5 days in Kitamusume and Orihime, 10 days in BR37, 15 days in Fukuyutaka and 16 days in Enrei. The elongation rate after the LP seemed to be the same in all cultivars. Thus, the time when the pod reached the maximum length was delayed as the LP was prolonged.
The timing of flower opening also affected the length of LP (Table 2). The pods set on the primary raceme which opened the flowers first had the longest LP in both Fukuyutaka and Enrei independent of sowing date. There were no significant differences in the length of LP among secondary, secondary with a leaf and tertiary racemes in both cultivars except for Enrei when sown on 8 June.
The later the sowing date, the shorter the LP in all racemes in both cultivars (Table 2). However, the secondary and tertiary racemes in the plants sown on 1 August had only a few flowers. Moreover, no pods set were observed on the primary and terminal racemes in Fukuyutaka sown on 8 June, indicating that all of the flowers in these racemes aborted. Ta ble 2. Lag period of' pod growth in di fferent order racemes in two soybean culliva l's. Since the photoperiod du ri ng plant growth va ries with the sowi ng date, we exposed one gro up of the plan ts to a 10-hour photoperiod under natura l ligh t a nd temperature cond itions after the first nower opened . The natural photoperiod was abou t 15 hours in early J uly (the time of nowe ring in Enrei sown on 8 June) and 14 hou rs in midd le August (the time o f Oowering in Fuku yu taka sow n on 9 July) . Expos ure to a short p hotoperiod shortened the LP by 7 days in Enrei and 5 d ays in Fukuyu taka, b ut did not a ffect t he length of LP of the secondary raceme in Enre i, and secondary a nd secondary with a compound leaf racemes in Fukuyutaka (Fig. 3).

Raceme order
Neit her sink removal nor BA a pplication significantly al1cc ted t he lengt h o f LP of Fukuyu taka in t he licld (Fig.  4). However, the elongating ra te after t he LP seemed to be increased by sink removal (data is not shown). Fig. 3. EfTect of' photoperiod on the lag period of pod growth in two soybean cu[t ivat's, Enrci and Fukuymaka, Sown on BJ une and 9 J uly, respectively.

Discussion
In t his stud y, we found that t he pod grow th a fter flower openi ng was ve ry slow in soybean as compared with ot her legumes (Fig. I) . The lengt h of LP varied not o nly with the culti var but also wit h the raceme order of pod set (Fig. 2, T able 2) . Hei tholt et al. ( 1986) reported tha t the accu mula tion of dry matter in the pod was slower in Kent (M aturity group: [V) than in M cCall (M atu rity group: 00). Umezaki ( 1991) recorded a great variation in the timing of pod lo rm ation among cultivars within the same maturity gro up. It seemed t ha t the carlin maturing cu ltivar had a shorter LP was observed. However, in BR37 whic h opened its nowers at the same time as Fuku yutaka, had a shorter LP than Fukuyutaka (Fig. 2), ind icating tha t so me factors which are not associated with the maturing habit also a ffect the length of LP.
Since more t ha n 2/3 of vegeta tive organs d evelop a fter the start of Oowerin g in soy bean (Shimada et aI., 1990;  Asanuma a nd Okumura, 199 1; Baba et a l., 2003), some reports suggested that the abortion of flowers and young pods are caused by the competition for assimilate between reprod uctive and vegetative organs after the start of flowering (Brun and Betts, 1984;Baba et aL, 2003). Therefore, during the LP the assimilate might be assigned lor the vegetative growt h rather than th e pod growth. In this respect, a short LP in early maturing cultivars (Fig. 2) may be attributed to the vegetative organs develop mainly before the start of flowering. However, sink remova l did not shorten th e LP (Fig. 4), suggesting that some vegetative growth is necessary before the pod growth. A short LP in high order raceme (Table 2) may also be a ttributed to the completio n of a large proportion of vegetative growth at that stage. Of g reat interest is how the plant switches from vegetative to reproductive growth. The effect of photoperiod on floral initiatio n of soybean is well known. H oweve r, the effect of photoperiod on pod growt h is obscure. Several reports indicated that a long photoperiod during the reproductive stage delayed the maturation by stimulating the vegetative growth Raper, 1976, 1977;Raper and Thomas, 1978;Guiamet and N akayama, 1984). This effec t seemed to be mainly ca used by the delay of pod initiation (van Schaik a nd Probst, 1958;Yokoyama et aL, 1988Yokoyama et aL, , 1989. However, in the present experiment, the LP was shorte ned either by a short photoperiod or la te sowing (Fig. 3, Table 2), suggesting that the short photoperiod req uirement for pod growth is more critical than that for floral initiation. However, short photoperiod did not reduce the LP in secondary racemes. It is suggested that at least a few days for the morpho logical development, such as the formation of vascular organs in the pedicel or nower, a re necessary before pod growth. The physiological meaning of the LP needs to be inves-tigated further.