Crop Science 41:1823-1826 (2001)
© 2001 Crop Science Society of America
CROP PHYSIOLOGY & METABOLISM
Low R:FR Light Quality Delays Flowering of E7E7 Soybean Lines
Elroy R. Cober* and
Harvey D. Voldeng
Eastern Cereal and Oilseed Research Centre, Agric. & Agri-Food Canada, Ottawa, ON, Canada K1A 0C6
* Corresponding author (coberer{at}em.agr.ca)
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ABSTRACT
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A new locus (E7) controlling flowering and maturity time, and photoperiod response in soybean [Glycine max (L.) Merr.] was identified by means of 20-h days of low (0.7) red:far-red (R:FR) light quality. The objective of this study was to characterize light quality and photoperiod responses of E7E7 and e7e7 soybean near-isogenic lines. E7E7 and e7e7 lines, in two genetic backgrounds, were grown in 20-h photoperiods with four R:FR light qualities (0.7, 1.2, 2.3, 2.7) and in 12-h photoperiods with 0.7 and 2.7 R:FR light quality. Under high R:FR, all isolines flowered early and similarly in both 12- and 20-h photoperiods. In 20-h photoperiods, as R:FR light quality approached that of natural day light (1.2), E7E7 lines flowered 6 to 7 d later than e7e7 lines, while under low R:FR (0.7), E7E7 lines flowered about 15 d later than e7e7 lines. Under low R:FR, all isolines flowered later under 20-h compared with 12-h photoperiods. Six ‘Harosoy’ isolines (e7e7 and 5 E7E7 lines) were grown in 1998 and 1999 in the field under natural daylength and daylength extended to 20 h with incandescent lamps (ILD). The e7e7 line matured 6 d earlier than the E7E7 line under natural and ILD photoperiod. An ILD treatment was simulated in growth cabinets with 4+12+4 h of incandescent light and was compared with 12-, 20-, and 24-h photoperiods. All lines flowered early and similarly under 12-h photoperiod. The 4+12+4-h treatment delayed flowering less than the 20-h photoperiod. Flowering was not delayed further when the photoperiod was extended from 20 to 24 h. The E7E7 lines were sensitive to light quality. They did not perceive long days of high R:FR light quality, but under natural daylight and under low R:FR light quality, E7 delayed flowering.
Abbreviations: EOD FR, end of day far-red • ILD, incandescent long daylength • R:FR, red to far-red quantum ratio • SED, standard error of a difference
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INTRODUCTION
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TIME TO FLOWERING AND MATURITY in soybean is controlled genetically by the E series of loci: E1, E2 (Bernard, 1971), E3 (Buzzell, 1971), E4 (Buzzell and Voldeng, 1980), E5 (McBlain and Bernard, 1987), E6 (Bonato and Vello, 1999), and E7 (Cober and Voldeng, 2001). For most of these loci, late flowering and maturity is dominant or partially dominant and early flowering and maturity is recessive, while for the E6 locus, early flowering is dominant. The E1, E3, and E4 alleles detect noninductive long photoperiods and delay flowering while under inductive short photoperiods, E1E1, E3E3, E4E4, and e1e1e3e3e4e4 lines flower earlier and similarly (Cober et al., 1996a; Cober and Voldeng, 2001). These alleles have differential responses to changes in light quality quantified by the red to far-red quantum ratio (R:FR). E3 is insensitive to light quality and flowering is delayed by long photoperiods of all light qualities (Cober et al., 1996b). E1 and E4 alleles have requirements for lower R:FR light quality since long photoperiods of high R:FR light quality did not delay flowering (Cober et al., 1996b). Natural daylight has a R:FR ranging from 1.05 to 1.25 (Holmes and Smith, 1977). E1 required natural-daylight-quality light before long photoperiods delayed flowering (Cober et al., 1996b). E7 was identified by low R:FR light quality produced by only incandescent lamps (Cober and Voldeng, 2001). This low R:FR light quality is characteristic of twilight or canopy shade (Smith, 1982). The alleles at the E7 locus have not been studied under a range of light qualities.
The use of natural daylength extended to 20 h with incandescent lamps (incandescent long daylength, ILD) has been used to study the genetics of photoperiod insensitivity (Saindon et al., 1989; Cober et al., 1996a), and to select for early maturity in wide crosses (Cober and Tanner, 1997). The ILD photoperiod treatment concludes each day with low R:FR irradiance produced by incandescent lamps. This type of treatment, often referred to as end-of-day far-red (EOD FR), is very inductive in long day Arabidopsis (Arabidopsis thaliana L.). In Arabidopsis, a 15 min EOD FR treatment following a short day was inductive compared with the short day treatment alone (Devlin et al., 1998) and approximated a long day. Enrichment in far-red light speeds flowering in Arabidopsis and this response was considered to be a characteristic of the shade avoidance syndrome (Halliday et al., 1994; Simpson et al., 1999). As a result, we hypothesize that ILD should provoke an extreme photoperiod response (large flowering delay compared to natural daylength) in soybean. While this is the case for some genotypes, for others (OT89-5, e1e1e3e3e4e4E7E7; OT93-28, E1E1e3e3e4e4E7E7) there is no or very little flowering delay when comparing natural day length to a 20-h ILD in the field (Cober et al., 1996a). A flowering delay, however, was observed for these isolines when a 20-h photoperiod was compared with a 12-h photoperiod in growth cabinet studies (Cober et al., 1996a, b). An EOD FR growth cabinet treatment has not been studied with these lines.
The objectives of this study were to quantify light quality and photoperiod responses of e7e7 and E7E7 soybean near-isogenic lines and to investigate EOD FR photoperiod effects.
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MATERIALS AND METHODS
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Near-isogenic lines with different combinations of photoperiod-sensitivity alleles were developed in a backcrossing program with the cultivars Harosoy and Maple Presto as recurrent parents (Table 1). OT isolines were developed (OT93-28, OT94-41, Voldeng et al., 1996; OT89-5, Voldeng and Saindon, 1991) at the Eastern Cereal and Oilseed Research Centre, Ottawa, ON. Isolines with an L prefix were developed and described by Bernard et al. (1991).
Experiments 1 and 3 were conducted in growth cabinets. These experiments were replicated in time. For each of the two replicates, two plants of each genotype were grown in the same growth cabinet. Therefore each datum point is the mean of four plants. Experiment 1 compared days to flowering of E7E7 and e7e7 isolines in both Maple Presto and Harosoy backgrounds grown at 25°C constant temperature in growth cabinets with 20-h photoperiods. Red:far-red quantum ratios (R:FR) were 0.7 (8- by 60-W incandescent lamps), 1.2 (VITA-LITE fluorescent lamps; Duro-Test Canada Inc., Rexdale, ON, and 8- by 60-W incandescent lamps), 2.3 (VITA-LITE and 2- by 40-W incandescent lamps), and 2.7 (cool white fluorescent lamps; Sylvania, Mississauga, ON, and 8- by 60-W incandescent lamps). The isolines were also grown in 12-h photoperiods for the two extreme light qualities (0.7 and 2.7). Light quality was measured 50 cm from the source with a radiometer/photometer (EG&G Model 550, Salem, MA) and a monochromometer (ISA Instruments SA Inc., Metuchen, NJ). Incident radiation was measured in 10-nm bandwidths centered at 660 and 730 nm. Quantum spectral ratios were calculated as 89.4% of energy spectral ratios since red light (660 nm) has 10.6% more energy per mole of quanta than far-red (730 nm). Photosynthetic photon flux was measured with a LI-COR quantum sensor (Model LI-200, LI-COR Inc., Lincoln, NE). Experiment 3 was conducted in growth cabinets with a series of Harosoy isolines at a constant temperature of 28°C. Photoperiod treatments of 12, 20, and 24 h were provided by VITA-LITE and eight incandescent lamps (R:FR = 1.2). A second 20-h photoperiod, identified as 4+12+4 h, was imposed consisting of 4 h from incandescent lamps (R:FR = 0.7), followed by 12 h of VITA-LITE plus incandescent lamps, followed by 4 h of incandescent lamps. The VITA-LITE and incandescent combination of lamps produced a photosynthetic photon flux of 200 to 230 µmol m-2 s-1. Under only incandescent lamps, the photosynthetic photon flux was 40 µmol m-2 s-1. For all growth cabinet experiments, seeds were germinated in vermiculite and seedlings were transplanted into 13-cm pots filled with standard phytotron potting mix (3 parts loam: 2 parts vermiculite: 1.5 parts peat moss: 1 part crushed brick). The date of the first open flower was recorded for each plant.
Experiment 2 was carried out in the field at Ottawa, ON, (45°23' N lat) in 1998 and 1999. Two photoperiods were provided: natural day length, where the longest day is about 16.9 h including civil twilight or 15.7 h exclusive of twilight; and natural day length extended to 20 h with incandescent lamps (ILD). Civil twilight starts and ends when the true center of the sun is 6° below the horizon. Inclusion of civil twilight is justified for day length calculations in soybean (Summerfield and Roberts, 1987). ILD was generated with 200-W incandescent bulbs suspended 2.25 m above the soil spaced 3 m apart in ranges 3 m wide. Photosynthetic photon flux at the canopy surface was approximately 3 µmol m-2 s-1. Lights were turned on, from emergence to killing frost, about 0.5 h before sunset until 2200 h, and then turned on again at 0200 h until about 0.5 h past sunrise, leaving a 4-h dark period between 2200 and 0200 h. Under ILD, plots were single rows spaced 50 cm apart and 3 m long with 30 seeds planted m-1 of row. ILD plots were planted on 9 June 1998 and 8 June 1999. Under natural day length, plots were single rows spaced 50 cm apart and 4 m long with 30 seeds plant m-1. Natural day plots were planted 21 May 1998 and 28 May 1999 in a different field. A randomized complete block design with two replications was used for each photoperiod.
For all experiment, the date of 50% of plants with an open flower and the date when 95% of pods reached mature color was recorded. The GLM procedure of SAS (SAS Institute Inc., Cary, NC) was used for analysis of variance. Least significant differences (P = 0.05) were used for mean separation.
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RESULTS AND DISCUSSION
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In Exp. 1, E7E7 (OT96-23 and OT89-5) and e7e7 (Maple Presto and OT94-47) lines flowered at the same time under 12-h photoperiod regardless of light quality (Fig. 1)
. Under 20-h photoperiods with high (2.7) R:FR light quality, both the E7E7 and e7e7 lines flowered in 32 d, which was not significantly different from 29 d under the 12-h photoperiod. Differences between E7E7 and e7e7 lines became apparent as the light quality approached that of natural day light (R:FR of 1.05 to 1.25; Smith, 1982) with E7E7 lines flowering 6 to 7 d later than e7e7 lines. Under incandescent lamps (R:FR = 0.7), where the light quality approximated that found under canopy shade or twilight (Smith, 1982), E7E7 lines flowered about 15 d later than e7e7 lines. This difference in flowering time under incandescent light and a 20-h photoperiod was similar to a previous report (Cober and Voldeng, 2001) and was used to identify the E7 locus. Like the E1 allele (Cober et al., 1996b), the E7 allele is not sensitive to long days of high R:FR light quality.
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Fig. 1. Days to first flower of E7E7 and e7e7 lines in different light quality regimes under 20- (open symbols) and 12-h (closed symbols) photoperiod. The error bars are ± SED.
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Interestingly, when 12- and 20-h photoperiods were compared, all lines exhibited a photoperiod response under low R:FR light quality. This evidence shows that there must be genes which respond to long photoperiods of shade-quality light even in Maturity Group 000 germplasm (Maple Presto).
Experiment 2 compared a series of five Harosoy E7E7 isolines and one e7e7 isoline under natural day length and ILD (Table 2). Under natural daylength, there were no differences between the e7e7 and E7E7 lines for days to first flower, however, there was a significant difference between these lines for days to maturity. The e7e7 line was significantly earlier flowering and maturing compared with lines with late-maturing alleles at two or more loci. For the e7e7, E7E7, and E1E1E7E7 isolines, there were no delays in flowering or maturity when natural daylength was extended to 20 h with ILD. Under ILD, the E3E3E7E7, E4E4E7E7, and E3E3E4E4E7E7 isolines had a flowering delay of 24 d on average and did not mature before frost similar to that reported by Cober et al. (1996a).
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Table 2. Days to first flower and maturity under natural day length and natural day length extended to 20 h with incandescent lamps (ILD) for Harosoy near-isogenic lines grown in 1998 and 1999 at Ottawa, ON.
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For the e7e7, E7E7, and E1E1E7E7 isolines, there were conflicting results when field and ILD flowering results were compared with 12- and 20-h photoperiods in growth cabinets at 28°C (Cober et al., 2001). In this growth cabinet study, a flowering delay was observed when the 20-h photoperiod was compared with 12-h photoperiod; however, in the current study, there was no delay when comparing natural day length to 20-h ILD in the field. The ILD photoperiod treatment concludes each day with low R:FR irradiance produced by incandescent lamps referred to as end-of-day far-red (EOD FR). An EOD FR response had not be studied with this germplasm and was the subject of Exp. 3.
In Exp. 3, we simulated a 20-h ILD-type photoperiod in growth cabinets using 4 h of incandescent lamps only, followed by 12 h of complete-growth-cabinet-canopy lighting, and ending with another 4 h of incandescent lamps. This 4+12+4-h photoperiod treatment was compared to 12-, 20-, and 24-h photoperiods (Table 3). Under 12-h photoperiods all isolines flowered early (Table 3), similar to Exp. 1 (Fig. 1). All isolines flowered later under the 20-h photoperiod. Extending the photoperiod to 24 h did not delay flowering further for any of the isolines. Compared to the 12-h photoperiod, the 4+12+4-h photoperiod delayed flowering significantly for all but the e7e7 isoline. All isolines, however, flowered significantly earlier under the 4+12+4-h photoperiod compared with the 20-h photoperiod. In this study, the 4+12+4-h photoperiod delayed flowering less than a 20-h photoperiod, which is unexpected when compared with reports of as little as 15 min of EOD FR having large photoperiodic effects (Devlin et al., 1998). These results help explain the lack of differences between 20-h ILD and natural daylength in the field for several lines (Table 2) since the effective photoperiod for ILD must be somewhat less than 20 h and natural daylength had a maximum photoperiod of 16.9 h when civil twilight was included. Perhaps the EOD FR treatment in this study was less effective in delaying flowering because it was part of a long photoperiod (20 h) instead of the usual practice of adding a brief EOD FR treatment to the end of a short photoperiod.
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Table 3. Days to first flower for Harosoy near-isogenic lines grown under several photoperiods of VITA-LITE and incandescent light quality in growth cabinets.
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All the soybean isolines in this study displayed elongated internodes under low R:FR (data not shown), which is characteristic of the shade-avoidance syndrome (Smith and Whitelam, 1997). The phenomenon where low R:FR light quality results in early flowering in Arabidopsis was also interpreted as part of the shade- or stress-avoidance response (Halliday et al., 1994; Simpson et al., 1999). Smith and Whitelam (1997), in a commentary, stated that early flowering was an important component of the shade avoidance syndrome and was "readily observable in all shade-avoiding plants." This low R:FR accelerated flowering response cannot be generalized to soybean since low R:FR light quality resulted in delayed flowering for E7 (Fig. 1) and E1 (Cober et al., 1996b) soybean alleles. It appears that low R:FR light quality simply elicits more extreme photoperiod responses.
In soybean, most dominant E alleles studied to date function to detect noninductive photoperiods and delay flowering. E alleles seem to function similarly to phytochrome genes (phyB, phyD, and phyE) in Arabidopsis, which also function to inhibit flowering under noninductive conditions (Devlin et al., 1998). Lin (2000) reviewed the role of phyB in delaying flowering in Arabidopsis and pea (Pisum sativum L.), both long-day plants, and in sorghum [Sorghum bicolor (L.) Moench], a short-day plant. Lin (2000) concluded that phyB inhibition of flowering was more apparent in noninductive photoperiods for both short- and long-day plants. Simpson et al. (1999) concluded that flowering is normally actively repressed in Arabidopsis and this also seems to be the case for soybean. Sorghum phyB mutants and recessive soybean e alleles exhibit similar behavior; they both flower earlier in long days but not in short days.
In summary, E7 is sensitive to light quality and cannot perceive long days of high R:FR light quality. Under long days of natural or low R:FR light quality, E7 delays flowering. Most E alleles studied in soybean, including E7, function to inhibit flowering under noninductive photoperiods, which fits the model of active repression of flowering. In contrast with the Arabidopsis model, low R:FR light does not induce a shade-avoidance, early-flowering response, since some isolines flower later under these conditions. The isolines used in this study did not show an EOD FR response since a 20-h photoperiod with an EOD FR treatment was less inhibitory than a 20-h photoperiod. The E alleles in soybean share characteristics similar to light-stable phytochromes in Arabidopsis, pea, and sorghum; however, confirmation of the function of the E alleles in soybean requires a molecular genetics approach.
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NOTES
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ECORC Contribution no. 11625.
Received for publication January 26, 2001.
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ABSTRACT
INTRODUCTION
