Agronomy Journal 93:944-948 (2001)
© 2001 American Society of Agronomy
SOYBEAN
Effects of Photoperiod on Growth and Development of Soybean Floral Bud in Different Maturity
Lingxiao Zhang*,a,
Ruifang Wangb and
John D. Heskethc
a Delta Res. and Ext. Cent., Mississippi State Univ., P.O. Box 197, Stoneville, MS 38776
b Dep. of Agron., Beijing Agric. Univ., Beijing, China
c Dep. of Crop Sci., Univ. of Illinois, Urbana, IL 61801 and USDA-ARS, 1201 W. Goodwin Ave., Urbana, IL 61801
* Corresponding author (lzhang{at}drec.msstate.edu)
Received for publication August 8, 2000.
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ABSTRACT
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Growth and development of most soybean [Glycine max (L.) Merr.] plants is sensitive to photoperiod effects. Therefore, it is important to understand and quantify the processes involved. Studies were conducted to determine growing degree days required for growth of the floral bud from initiation to open flower of different maturity groups (MGs) in both field and controlled environments. Eighteen soybean strains, including ‘Clark’ back-cross near-isolines differing in maturity, were sown in the field at five different dates in 1992 and 1993. In growth chamber studies, plants of two strains differing in maturity were used and moved to 12-, 14-, and 16-h daylengths after floral buds were initiated in 12 h. Results from both field and growth chamber studies indicated that planting dates had a significant effect on the thermal requirement for bud growth in the late-maturing strains used. Shorter photoperiods during the bud growth period accelerated growth rates to open flower. Furthermore, in the growth chamber study, flowering was inhibited under 16-h daylength in a late-maturity (MG V) strain when plants were transferred immediately after floral bud initiation (FBI) under 12 h. Plants remaining 8 d after FBI before they were transferred to 16 h were not significantly delayed in flowering. This study indicated that photoperiod length and treatment duration affects soybean FBI and floral bud development in a quantitative way, which resulted in a profound photoperiod response in late maturity-group soybean under field conditions.
Abbreviations: FBD, floral bud development • FBI, floral bud initiation • GDD, growing degree days • MG, maturity group
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INTRODUCTION
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THE UNDERSTANDING of photoperiod x temperature interaction in controlling flowering processes is one of the key problems in soybean growth and production. Understanding and quantifying this interaction often directly affects soybean breeders and producers when selecting varieties, determining plant dates, and predicting flowering dates, maturity dates, and final yields. Previous studies from laboratory and fields have shown the effects of photoperiod on flowering dates (and subsequent phenological events) and final yields (Board and Hall, 1984; Cregan and Hartwig, 1984; Egli et al., 1989; Jones and Liang, 1978; McBlain et al., 1987; Sinclair et al., 1991; Thomas and Raper, 1976; Wang et al., 1987).
The period from seedling emergence to flowering consists of two phases: (i) emergence to floral bud initiation (FBI) and (ii) FBI to open flower. Most research in the past has focused on mechanisms for photoperiod and temperature effects on FBI, as determined by microscopic dissection (Evans, 1969). However, the effect of photoperiod on soybean floral bud growth, as a sole event, is less discussed. Some efforts have been made to understand and quantify the thermal–photoperiod requirement for the rates of floral bud growth of different soybean MGs (Ephrath and Hesketh, 1991; Zhang et al., 1993, 1995). The objective of this experiment was to further understand the thermal–photoperiod requirements of soybean cultivars with different MGs so that more information can be used for predicting phenology and culture management to properly apply optimal cultural practices.
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MATERIALS AND METHODS
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Field Experiments
In 1992 and 1993, a total of 18 soybean varieties, maturity group (MG) ranging from 00 to VIII, were planted in a Flanagan silt loam (fine montmorillionitic, mesic Aquic Arguidoll) soil at the University of Illinois Crop Science South Farm at Urbana, IL. Seven of them will be discussed in this paper. Their MG ranking, associated maturity genes (if known), and stem termination genes are listed in Table 1. There were five planting dates, ranging from early May to late July, in both years (Table 2). Plots were 8 m long with 0.76 m between rows and 0.05 m between plants in the row. Three replications were used in a randomized complete block design.
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Table 1. Summary of soybean strains planted in 1992 and 1993, with maturity group (MG) ranking and associated termination genes.
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Plants were dissected using a light microscope to determine when a knob-like structure of a floral bud primordium first appeared on the plant (FBI). Flowering dates were recorded when 50% of plants in a plot had open flowers. Fresh and dry weights per plant, plant heights, leaf number (or node number on the main stem), and leaf area of 10 plants were recorded on the flowering date for each plot.
The accumulative growing degree days (GDD) were used to estimate the thermal requirement of each growth stage. Accumulative GDD are calculated by the sum of averages of daily high (
30°C) and low (
10°C) temperatures and then subtracting a base temperature, which is 10°C in this case. Emergence dates were used instead of planting dates for the beginning of the first growth stage because of seedling emergence variations for different planting dates due to soil conditions (especially water availability).
Because the emergence dates of individual correlated plantings of the 2 yr were close (Table 2), the average dates of 2 yr were used for calculations and statistical analysis. Average photoperiods (sums of daily photoperiods divided by days) were also calculated for emergence to FBI and FBI to flowering (Table 3).
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Table 3. Approximate average photoperiod experienced from floral bud initiation (FBI) to 50% of plants flowering for five planting dates and six Clark near-isolines differing in maturity group (MG) genes. The sum of daily photoperiods was divided by days to get the average.
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Growth Chamber Experiment
Seeds of Clark near-isolines L63-3117 (MG II) and L65-3366 (MG V) were sown in 30 and 50 pots (0.3 m high and 0.2 m diam.), respectively. The potting mix was a 1:1:1 ratio of Flanagan silt loam, peat moss, and perlite (vol./vol.). After seedlings emerged, all pots were transferred to one growth chamber with a 3- by 1.5-m2 floor area. Seeds were overly sown, and seedlings were thinned to two plants per pot 7 d after emergence. One plant from each pot was used to determine FBI. The light source was a combination of fluorescent and incandescent lamps providing 450 mol m-2 s-1 photosynthetically active radiation measured with a LI-6200 sensor for 12 h. Pots of the two isolines were transferred to growth chambers under 14- and 16-h photoperiods, with 10 pots per treatment, right after FBI was detected. Another 20 pots were transferred to the longer photoperiods 8 d after FBI. The same number of pots was left in the 12-h treatment each time. Only incandescent lamps of 25 to 30 mol m-2 s-1 photosynthetically active radiation were used to extend daylengths, minimizing photosynthate contributing to biomass production among photoperiod treatments. Temperatures were 25 and 20°C during the light and dark period, respectively, giving means of 22.5, 22.9, and 23.3°C for the 12-, 14-, and 16-h treatments.
Data Analysis
All data were analyzed by one-way analysis of variance (ANOVA), and means were separated by Fisher's Protected LSD (SAS Inst., 1989). Only LSD is listed in the results tables. Regression analysis was also performed between average photoperiod hours experienced and accumulative GDD required for FBI and floral bud development (FBD) in different MGs.
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RESULTS AND DISCUSSION
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Field Experiments
The average daylengths during floral bud initiation were not much different for the first three plantings in early MGs (Table 3). The accumulative GDD required to produce a flower for early MGs (MG IV or less) were less than that required for the MG V near-isoline (Table 4). However, the first planting required fewer GDD for both years for the early MG genotypes. Colder night temperatures and rapidly changing daylengths may have caused this response. We have calculated the GDD requirements for FBI for each MG by using 8°C as the base temperature instead of 10°C. The conclusion was similar.
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Table 4. Average growing degree days (GDD) required for floral bud growth and development (base temperature was 10°C) for soybean Clark near-isolines differing in maturity for planting dates used over 2 yr (1992 and 1993).
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Figure 1 shows the overall relationship between daylength, planting date, and duration of the two growth stages measured for MGs I, IV, and V. The figure shows that the period from emergence to flowering (including floral bud growth) reduced dramatically when photoperiod reduced during the late growing season. This indicated that photoperiod influenced or regulated the total dates of growth of early vegetative growth and floral bud growth. It had showed in all early and later MGs though the degrees of the reduction were varied.
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Fig. 1. Effect of planting date on floral bud development in soybean strains differing in maturity. P1–P5 indicate planting groups. Vertical dash lines indicate emergence dates. Horizontal dash lines indicate the time of floral bud growing period from emergence to open flowers on 50% of plants in a field plot. Roman numerals represent maturity groups (MGs). The curved dot line shows photoperiod changes during the growing season. Numbers in parentheses represent average photoperiod experienced during floral bud growth.
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Effects of photoperiod on floral bud growth rates were shown best when plotted as a function of average daylength during FBI to open flower (Fig. 2). Data in Fig. 2 were the means of eight data points. From the figure, we can clearly see that the slope increases when the MG increases. There was a similar trend in both FBI and FBD. Because early MGs (IV and less) had relatively short periods of FBI, the influence of photoperiod could not been detected clearly, which was indicated by the flatness of the slope of those regression lines (one was even in a negative number). Under field conditions, the duration of bud initiation and development was most affected by photoperiod in the strains from MG IV to VIII, which were late MGs. Both determinate and indeterminate strains showed the same pattern of response, indicating that the stem termination gene did not respond to photoperiod.
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Fig. 2. Growing degree days (GDD) required from emergence (VE) to floral bud initiation (FBI) and floral bud development (FBD) for soybean strains differing in maturity group (MG) and growing under different photoperiods encountered during the growing season at Urbana, IL.
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Effects of planting dates on the nodal position of the first flower were also significant (Table 5). Late MGs produced floral buds at the much lower position when planted after July due to much shorter daylength-initiated floral bud at an earlier stage. It also indicated that the floral bud could be initiated at any plant node if daylength was sufficiently short. Vegetative growth before flowering was influenced by planting dates, FBI, and flowering dates, especially in the late-maturity strains as expected (Table 6). The greater the thermal requirement for flowering, the more the plant produced vegetative biomass. This sink for photosynthate and N may have contributed some to slower bud growth rates.
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Table 5. Vegetative and phenological parameters at flowering for plants that were sown at five different dates. Data represent pooled values from 1992 and 1993 growing seasons.
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Table 6. The effects of planting dates on main-stem nodal position of the first flower for Clark near-isolines differing in maturity. Values are means from 1992 and 1993.
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Growth Chamber Experiment
This growth chamber study demonstrated the relationship of photoperiod and floral bud growth in a more significant way by using artificially increased daylength (Fig. 3). The long-day photoperiod delayed and inhibited floral bud growth, and it largely depended on the starting time of this treatment and how long the treatment was applied and the MGs tested. In early MG soybean (L63-3117, Treatment 1), the prolonged photoperiod (16 h) had less effect on floral bud growth, perhaps due to its relatively short growth period. For late-maturity soybean (L65-3366, Treatment 2), 14 h significantly delayed flowering, whereas 16 h inhibited flowering. However, floral buds one-half to two-thirds full size were observed in the 16-h treatment at the end of the experiment. When exposure to longer photoperiods was delayed 8 d after FBI (Treatment 3), duration of floral bud growth was hardly affected by 14- and 16-h d, but pod set was, as reported earlier (Acock and Acock, 1995; Zhang et al., 1993). Zhang et al. (1995) reported a temperature x photoperiod interaction on the GDD requirement of floral bud growth in these same strains. Duration of floral bud growth in the MG II near-isoline of Clark (L63-3117) was less affected by photoperiod than in the MG V near-isoline (L65-3366). McBlain et al. (1987) showed a similar result in that daylengths of 18 h or longer had a marked effect on time to flower in MG II and earlier MGs, indicating that 16 h is not long enough to induce a photoperiod response in MG II.
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Fig. 3. Summary of the effect of timing of the photoperiod treatment on floral bud growth in Clark near-isolines L63-3117 (MG-II) and Clark L65-3366 (MG-V). All plants were under 12 h before treatments started on the day floral buds were initiated, or for L65-3366, 8 d after floral bud initiation (FBI).
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Photoperiod effect on floral bud growth is a quantitative or accumulative effect. Floral bud growth and development stage is in early growth stage. To an early maturing soybean, this period was relatively short, and the accumulative effect of photoperiod influence was limited and hard to be detected. Therefore, this effect may be invisible or insignificant. Meanwhile, for a later maturing soybean, even the FBD process can be long enough to be affected by photoperiod influence and become significant. For the same reason, pod-setting period was at the late stage of growth. Even an early maturing soybean can accumulate significant photoperiod effect on pod-setting stage. It was indicated that photoperiod effect on soybean from first flowering to maturity is a quantitative process that is controlled by three pairs of genes: E1/e1, E2/e2, and E3/e3 (Summerfield et al., 1998). However, this is not the focus of this paper.
Quantifying the process of floral bud growth and development is a difficult but important step towards accurately predicting flowering and maturity of a soybean variety and MG. Predicting flowering and crop maturity using a computer model is very difficult because of complex effects of daylength and temperature on processes involved. It requires systematic experiments to accurately determine the relationship between daylength hours and MGs. The results reported in this paper can provide valuable information for predicting growth stage and maturity of soybean that can be used by crop managers. More studies are obviously needed before one can predict soybean phenology with confidence, especially for cultivars grown at the lower latitudes.
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REFERENCES
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- Acock, M.C., and B. Acock. 1995. Photoperiod sensitivity during soybean flower development. Biotronics 24:25–34.
- Board, J.E., and W. Hall. 1984. Premature flowering in soybean yield reductions at non-optimal planting dates as influenced by temperature and photoperiod. Agron. J. 76:700–704.[Abstract/Free Full Text]
- Cregan, P.B., and E.E. Hartwig. 1984. Characterization of flowering response to photoperiod in diverse soybean genotypes. Crop Sci. 24:659–662.[Abstract/Free Full Text]
- Egli, D.B., R.A. Wiralaga, T. Bustaman, Z.W. Yu, and D.M. Tekrony. 1989. Time of flower open and seed mass in soybean. Agron. J. 79:697–700.
- Ephrath, J.E., and J.D. Hesketh. 1991. The thermal–photoperiod requirement for floral bud growth. Biotronics 20:1–8.
- Evans, L.T. (ed.) 1969. The induction of flowering: Some case histories. Melbourne Univ. Press, Melbourne, Australia.
- Jones, P.G., and D.R. Liang. 1978. Simulation of the phenology of soybeans. Agric. Syst. 3:295–311.
- McBlain, B.A., J.D. Hesketh, and B.L. Bernard. 1987. Photoperiod and temperature effects on reproductive phenology in soybean isolines differing in maturity genes. Can. J. Plant Sci. 67:105–116.
- SAS Institute. 1989 SAS/STAT software: Usage and reference. Version 6. 1st ed. SAS Inst., Cary, NC.
- Sinclair, T.R., S. Kitani, K. Hinson, J. Bruniard, and T. Horie. 1991. Soybean flowering date: Linear and logistic models based on temperature and photoperiod. Crop Sci. 31:786–790.[Abstract/Free Full Text]
- Summerfield, R.J., H. Asumadu, R.H. Ellis, and A. Qi. 1998. Characterization of the photoperiodic response of post-flowering development in maturity isolines of soybean [Glycine max (L.) Merrill] ‘Clark’. Ann. Bot. (London) 82:765–771.[Abstract/Free Full Text]
- Thomas, F.J., and C.D. Raper, Jr. 1976. Photoperiodic control of seed filling for soybeans. Crop Sci. 16:667–672.[Abstract/Free Full Text]
- Wang, J.B., A. McBlain, J.D. Hesketh, J.T. Wooley, and R.L. Bernard. 1987. A data base for predicting soybean phenology. Biotronics 16:25–38.
- Zhang, L.X., R.F. Wang, and J.D. Hesketh. 1993. Photoperiod effects on soybean floral bud growth. p. 149. In Agronomy abstracts. ASA, Madison, WI.
- Zhang, L.X., R.F. Wang, and J.D. Hesketh. 1995. Separating photoperiod and temperature effects on the degree day requirement for floral bud events in soybean. Biotronics 24:59–64.
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TOP
ABSTRACT
INTRODUCTION
