Published online 30 July 2007
Published in Crop Sci 47:1510-1517 (2007)
© 2007 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
CROP PHYSIOLOGY & METABOLISM
Photoperiod and E-genes Influence the Duration of the Reproductive Phase in Soybean
Saratha V. Kumudini*,
Praveen K. Pallikonda and
C. Steele
Dep. of Plant and Soil Sciences, Univ. of Kentucky, 1405 Veterans Drive, Lexington, KY 40546
* Corresponding author (s.kumudini{at}uky.edu).
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ABSTRACT
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Duration of the reproductive phase (DRP) is critical for soybean [Glycine max (L.) Merr.] yield. Manipulation of this phase may benefit breeding for higher yield. The soybean E-gene series control time to flowering and maturity through a photoperiod-mediated response. It is possible that E-genes and photoperiod may control the DRP. Two 2-yr studies were conducted with the objective to assess whether the DRP is influenced by either or both E-gene alleles and post-flowering photoperiod. In a planting-date experiment, the main plots were two planting dates, and sub plots were 15 E-gene near-isogenic lines (NILs) in two genetic backgrounds. In a daylength-extension experiment, seven E-gene NILs were planted to synchronize flowering date. Two photoperiods treatments were imposed after flowering: (i) ambient and (ii) ambient plus 3-h incandescent daylength extension. Early planting and daylength extension exposed plants to a longer post-flowering photoperiod, and resulted in an increase in the DRP. When exposed to the same post-flowering photoperiod, the DRP was positively related to the number of dominant E-gene alleles. Both E-genes and post-flowering photoperiod controlled the DRP and this was in addition to the known impact of E-genes on time to flowering. Hence, E-genes control time to maturity through both their effect on time to flowering and through their effect on the DRP. The results of this study underline the importance of time of flowering for determination of the DRP.
Abbreviations: DRP, duration of the reproductive phase (R1 to R7) • NILs, near-isogenic lines • R1, first flower • R7, physiological maturity
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INTRODUCTION
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LATER-MATURING SOYBEAN [Glycine max (L.) Merr.] cultivars have a longer duration over which assimilates maybe accumulated and partitioned to yield than earlier-maturing soybean cultivars. Although later-maturing soybean cultivars have extended crop growth duration, they do not consistently yield better than earlier cultivars (Kane and Grabau, 1992; Egli, 1993). The lack of yield advantage in cultivars from later-maturity groups may be because much of the variability in days to maturity among maturity groups is attributable to an increase in the duration of the vegetative phase (Kane and Grabau, 1992; Egli, 1994) which is not highly associated with seed yield.
A number of studies have pointed to the reproductive phase, singling out the seed-filling period (R4 to R7 i.e., the beginning of seed filling to physiological maturity) as the critical phase for yield determination in soybean (Dunphy et al., 1979; Gay et al., 1980; Nelson, 1986; Smith and Nelson, 1986). The positive correlation between the duration of the seed-filling period and seed yield (Hanway and Weber, 1971; Gay et al., 1980; Smith and Nelson, 1986) encouraged researchers to attempt to manipulate this phase. Breeders selected for increased seed-filling period but have been unsuccessful in breeding for an increased seed-filling period that was related to yield increase, possibly because of strong environmental effects and low heritability (Salado-Navarro et al., 1985; Pfeiffer and Egli, 1988; Egli, 1998). Considering the potential value of this phase of development, a clearer understanding of the factors that control this period may prove important for breeding programs targeting soybean yield. The duration of the reproductive phase (DRP) ranges from first flower (R1) to physiological maturity (R7) and encompasses the seed-filling period as well as the pod-development phase.
Early studies have shown that time from planting to flowering in soybean was mediated by photoperiod (Borthwick and Parker, 1938). Although there was evidence of photoperiod control of post-flowering reproductive development in soybean under controlled-environment conditions (Thomas and Raper, 1976), this phenomenon has only recently been reported in field studies (Kantolic and Slafer, 2001; 2005).
The series of genes known as the E-genes condition the time to flowering and maturity in a photoperiod-mediated response. The ability of soybean to adapt to a wide range of latitudes is attributable, in part, to these genes. This series of genes, E1 and E2 (Bernard, 1971), E3 (Buzzell, 1971), E4 (Buzzell and Voldeng, 1980), E5 (McBlain and Bernard, 1987), and E7 (Cober and Voldeng, 2001) were identified over the course of the last 30 years. The alleles at the E locus were distinguished originally by their effect on time to flowering and maturity. At each locus, there are two possible alleles. The alleles that conferred late flowering and maturity appeared to be partially dominant and the alleles that conferred early flowering and maturity appeared to be recessive; the heterozygote showed an intermediate phenotype in most cases (Buzzell and Voldeng, 1980; McBlain and Bernard, 1987; McBlain et al., 1987; Cober and Voldeng, 2001). In the interest of simplicity, henceforth, the partially dominant alleles which confers late flowering will be called ‘dominant alleles’ and the alleles conferring early flowering will be called the ‘recessive alleles.’
The E-genes have been studied extensively for their role in time to flowering, a phase critical for plant adaptation. However, E-genes are known to impact both time to flowering and time to maturity. It is unclear whether E-genes control time to maturity by simply their effect on time to flowering or whether E-genes also control the DRP. Not much work has been done on the impact of E-genes on post-flowering reproductive development. An earlier study using potted plants under controlled environment conditions, found evidence that E-genes in a ‘Clark’ background did modify post-flowering development (Summerfield et al., 1998). Field growing conditions and reducing epistatic effects through the use of two genetic backgrounds would do much to not only verify the impact of E-genes alleles on soybean reproductive development but to also extend this to an understanding of the impact of these alleles in an agricultural production environment. Another question of interest is that if E-genes are clearly involved in determining the duration of reproductive development, then later-maturing lines should have longer reproductive development than earlier-maturing lines. However, it has been shown that when cultivars belonging to different maturity groups (and therefore likely to have different E-gene allelic combinations) are planted at the same time, they appear to have similar duration of reproductive development (Egli, 1994).
The objectives of the current study were to determine whether (i) post-flowering photoperiod and (ii) alleles at the E-gene loci either alone or in combination, control the DRP in soybean.
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MATERIALS AND METHODS
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Planting-date experiment. This experiment was conducted at the University of Kentucky's Spindletop research facility in Lexington Kentucky (38°N; 84°W) in 2003 and 2004. The soil at this location was a Maury silt loam (fine, mixed, semiactive, mesic Typic Paleudalfs). Based on soil tests, 84 kg ha–1 of KCl was applied in 2003 and pH was 6.7 so no lime was added. In 2004, based on soil tests, no KCl or lime was added. Weed control was achieved using a combination of pre-plant incorporated Canopy XL (sulfentrazone+chlorimuron; DuPont Crop Protection, Wilmington, DE) at 455 g ha–1 and Dual II Magnum (S-metalachlor+benoxacor; Syngenta Crop Protection, Greensboro, NC) 1.55 L ha–1 in both years. Weeds were also removed using a hoe during the season. The treatments consisted of two planting dates and 15 soybean E-gene near-isogenic lines (NILs). Planting dates were 2 June and 24 June for 2003 and 24 May and 21 June for 2004. In 2003, the plots were machine planted for the first date and hand planted for the second date. Seeding rates were adjusted for germination rates to achieve approximately 344000 plants ha–1. Each plot was 2 m long and four rows wide with a 0.38-m row width. In 2004, plots were machine planted at double density and thinned to approximately 344000 plants ha–1. The plots were 4 m long and four rows wide, with a 0.38-m row width. Fifteen E-gene NILs (i.e., near-isogenic lines, also called isolines) in two different genetic backgrounds (Clark and ‘Harosoy’) were selected for this experiment (Table 1). These NILs were derived by backcrossing to BC5 to the recurrent parent, cv. Clark or cv. Harosoy, with selection for each of the various dominant or recessive genes from selected donors or by recombinations among such NILs. In this test, the NILs were selected to represent pairs of genotypes with the same E-gene composition but in different genetic backgrounds (Harosoy and Clark). OT93-26 and OT93-28 also had the same E-gene composition, both came from a Harosoy background but were different from each other in that they were selected for differences in pubescence color. The objective was to test the impact of the E-gene alleles and separate it from epistatic effects. Phenological stages based on the system developed by Fehr and Caviness (1977), was recorded every 2 to 3 d on a length of bordered rows previously defined and flagged. Weather data was obtained from a weather station located within 1 km of the research plots. The number of growing degree days for each phase of development was calculated using mean daily temperatures and a base temperature of 6°C (on the basis of base temperatures reported in Stewart et al., 2003). The experiment design was a randomized complete block, split-plot design with three replications. The main plots were the two planting dates and the split-plots were the 15 NILs. Asumadu et al. (1998) reported that E-genes were responsive to photoperiod until approximately the appearance of the last flower. Therefore, in the current study, the post-flowering photoperiod was calculated as the average ambient photoperiod between R1 and R5 (approximate time to last flower) for each NIL and each planting date tested. Data were analyzed using Proc Mixed (SAS ver. 8.0, SAS institute, Cary, NC), with years, and blocks being random and blocks nested within years.
Daylength-extension experiment: The field experiment was conducted at the University of Kentucky's Spindletop research facility in Lexington Kentucky in 2004 and 2005. Soil type and agronomic management practices were the same as in the planting-date experiment. Based on soil tests, no fertilizer or lime was recommended nor applied in 2004 or 2005. In addition, Japanese beetles (Popillia japonica Newman) were controlled in 2005 by spraying 1.9 L acre–1 of Sevin (22.5% Carbaryl [1-naphthyl N methyl carbamate]; Bayer Crop Science, Research Triangle Park, NC).The treatments consisted of two photoperiods and a subset of seven of the 15 soybean E-gene NILs used in the previous experiment (Table 1). The seven genotypes were selected to represent three pairs of genotypes with the same E-gene composition but different genetic backgrounds (Harosoy or Clark). The seventh genotype was one genotype that had all recessive alleles in a Harosoy background. The two photoperiod treatments were imposed following the initiation of flowering in all NILs. To obtain synchronous flowering, the model of Stewart et al. (2003) was used together with 20 years of historical weather data for the region to determine the appropriate planting dates for synchronous flowering. In 2004, L66-432 and L67-2324 were planted on 24 May, L80-5914 and L71-802 were planted on 3 June, L71-920 and L62-667 were planted on 9 June, and OT94-47 was planted on 14 June. In 2005, L66-432 and L67–2324 were planted on 10 May, L80-5914 and L71-802 were planted on 25 May, L71-920 and L62-667 were planted on 1 June, and OT94-47 was planted on 8 June. Using the model, historical weather data, and staggered plantings, all genotypes flowered within 8 d of the first genotype to flower. Plots were hand planted at double density and thinned back to achieve approximately 370000 plants ha–1. The plots were 4 m long and 6 rows wide, with a 0.38-m row width. The design was a randomized complete block, split-plot design with three replications. The main plots were the two photoperiod treatments: (i) ambient daylength (Amb) and (ii) ambient plus 3-h incandescent daylength extension (Amb+3). The photoperiod treatments were imposed after all genotypes had reached R1 and continued until at least physiological maturity (R7). A timer was set to begin 1 h before civil twilight (when the center of the sun is geometrically 6° below the horizon) and to continue until 3 h after civil twilight. The timer was re-set once a week to follow dynamic changes in civil twilight. The timer controlled a series of incandescent bulbs (100 W) suspended 2 m above the ground. One bulb was suspended above each plot in 2004 and two bulbs were suspended above each plot in 2005. The red-far red ratio (660nm/730 nm) of the incandescent bulbs was 0.58 in 2004 and 0.51 in 2005, measured 1 m above the ground on a moonless night using a spectroradiometer (Spectrawiz EPP2000 VIS-NIR, Stellarnet, Inc., Tampa, FL). The main plots were separated by large sheets of black plastic that shielded the ambient photoperiod treatment from the impact of daylength extension. One segment of bordered row with 10 plants was marked and all phenology data was collected from these plants. Phenology stages, based on the system developed by Fehr and Caviness (1977), were recorded every 2 to 3 d. Weather data was obtained from a weather station located within 0.8 km of the research plots. The number of growing degree days for each phase of development was calculated using a base temperature of 6°C. Data were analyzed with Proc Mixed (SAS ver. 9.1, SAS institute, Cary, NC).
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RESULTS AND DISCUSSION
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Planting-Date Experiment
The 15 NILs tested were grouped based on number of dominant E alleles ranging from 0 to 5 dominant E-gene alleles. The dominant E1 allele was counted as two dominant E-gene alleles based on results reported by Stewart et al. (2003). Although this classification is an oversimplification since the effect of the E-genes at the various loci on phenology are not the same (Stewart et al., 2003), it is a necessary first step. In this study it was found that the growing degree days required to reach first flower (R1) was positively correlated to the number of dominant E alleles (Fig. 1
). This result is consistent with previous E-gene studies that have shown that the dominant E alleles delay time to first flower (Bernard, 1971; Buzzell, 1971; Buzzell and Voldeng, 1980; McBlain and Bernard, 1987; Cober and Voldeng, 2001; Stewart et al., 2003).
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Figure 1. Time to first flower (R1) in growing degree days of 15 E-gene near-isogenic lines (NILs) grouped by number of dominant E-gene alleles (E1 counted as 2 dominant alleles), planted either early or late in 2003 and 2004.
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Planting date influenced photoperiod; the mean photoperiod between planting and R1 was longer under early planting (15.8 h) than under late planting (15.5 h). Time to R1 was delayed under early planting (longer photoperiod) for genotypes with greater number of dominant E alleles (Fig. 1). In addition, the delay in flowering was positively related to the number of dominant E alleles within a planting date (Fig. 1). The current study supports previous findings that both E-gene alleles and photoperiod condition time to flowering (Cober and Voldeng, 2001; Stewart et al., 2003).
The results of the current study were also consistent with many of the previous studies that reported that E-genes impact both time to flowering and maturity (Buzzell and Voldeng, 1980; McBlain and Bernard, 1987; Cober and Voldeng, 2001). Growing-degree days required to reach R7 were greater under the longer photoperiod of the early planting treatment for NILs with a large number of dominant E-genes relative to the later planting treatment (Table 2). When planted at the same time, time to R1 was the single phase of development that accounted for most of the variation in time to maturity (R2 = 0.77 for early planting, and R2 = 0.75 for late planting treatment). This close association between time to flowering and time to maturity may be considered as evidence that the impact of E-genes and photoperiod on time to maturity is simply a result of the control of E alleles and photoperiod on time to flowering and not as a consequence of a direct control by E-genes and/or photoperiod on DRP. In the planting date experiment there was no apparent relationship between the number of E alleles and the duration of the reproductive phase within a planting date (Fig. 2
), which is consistent with the hypothesis that variation in time to maturity is simply a consequence of E-gene and photoperiod control of time to flowering.
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Figure 2. Duration of reproductive phase (R1–R7), in growing degree days, of 15 near-isogenic lines (NILs) grouped by number of dominant E-gene alleles (E1 counted as 2 dominant alleles), planted either early or late in a) 2003 and b) 2004. The bars represent standard error of the means.
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The photoperiod to which plants are exposed during the post-flowering period (R1–R5) is a function of planting date and the duration of the pre-flowering period. When NILs were planted at the same date, NILs with dominant E alleles flowered later than NILs with recessive E alleles and, consequently, the post-flowering photoperiod experienced by the NILs with dominant E alleles was shorter than that experienced by NILs with recessive E alleles. Therefore, when planted at the same time, the post-flowering photoperiod experienced by the NILs was dependent on the number of dominant E-gene alleles (Fig. 3
). Planting date also affected the post-flowering photoperiod experienced (Fig. 3). Later planting exposed plants to the shorter post-flowering photoperiods that occur late in the season. Consequently, the post-flowering photoperiod experienced would depend on both the number of dominant E alleles and the planting date (Fig. 3).
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Figure 3. Average ambient photoperiod (includes civil twilight), experienced by 15 near-isogenic lines (NILs) grouped based on number of dominant E-gene alleles (E1 counted as 2 dominant alleles), during their respective R1 to R5 stage of development under an early and late planting-date treatment. Data are averaged over 2003 and 2004. The bars represent standard error of the means.
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There was a year x photoperiod x genotype interaction for the duration of the reproductive phase. All NILs except the most photoperiod-insensitive lines had a shorter DRP under late planting (Fig. 2), especially in 2003. These results point to a possible role of post-flowering photoperiod and E-gene control of the DRP. The planting-date study was limited in that the variability in time to first flower, and the post-flowering photoperiod was different for the different groups of NILs tested (Fig. 3). To find a more definitive answer to the question of post-flowering photoperiod and E-gene control of DRP, it was important that the different groups of NILs be tested under the same post-flowering photoperiod.
Daylength-Extension Experiment
The NILs in this experiment were planted at different times to obtain synchronous flowering. Consequently, all NILs were exposed to the same post-flowering photoperiod (Amb or Amb+3) and temperature. This allowed for an assessment of the role of post-flowering photoperiod on DRP, independent of the impact of photoperiod on time to flowering.
Duration of the reproductive phase was dependent on year, post-flowering photoperiod, and E-genes (Table 3). The number of dominant E-gene alleles was positively related to the DRP (Fig. 4
). Photoperiod extension increased the DRP in both years, but the impact was greater in 2005 than in 2004 and for those NILs with multiple dominant E alleles (Fig. 4).
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Table 3. ANOVA of growing degree days required for phenological stages of seven near-isogenic lines (NILs) grown under ambient or ambient + 3-h day length extension treatments in 2004 and 2005.
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Figure 4. Duration of the reproductive phase (R1–R7) in growing degree days of seven soybean near-isogenic lines (NILs) grouped based on number of dominant E-gene alleles (E1 counted as two dominant alleles), and grown in A) 2004 and in B) 2005. The bars represent standard error of the means.
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Increasing the post-flowering photoperiod increased the DRP. Johnson et al. (1960) was the first to show that soybeans were responsive to post-flowering photoperiod in controlled-environment experiments. Researchers interested in modeling soybean development utilized planting date and location studies to infer the existence of post-flowering photoperiod sensitivity (Grimm et al., 1993; Jones et al., 1991). Kantolic and Slafer (2001; 2005) were able to confirm the role of post-flowering photoperiod on the duration of the pod development phase under field conditions using daylength-extension experiments. They reported a significant impact of photoperiod on the duration of the pod development phase of Maturity Group IV and Maturity Group V soybean cultivars grown in Argentina. The current study also provides evidence for the role of post-flowering photoperiod on reproductive development of soybean and further suggests that the E alleles, at least in part, are responsible for the post-flowering photoperiod-mediated conditioning of the DRP.
The E-genes play a key role in the photoperiod-mediated control of the duration of reproductive phase. The dominant E alleles condition an increase in the DRP under both ambient and daylength-extension treatments (Fig. 4). The recessive E alleles have previously been reported to confer increased photoperiod insensitivity for time to first flower, the current study illustrates that the recessive E alleles also confer photoperiod insensitivity to the control of the DRP. Therefore, the results of this experiment provide evidence that the E-genes not only condition time to flowering, but also condition the duration of the reproductive phase.
Summerfield et al. (1998) reported that E alleles (in Clark background) and long-day treatments increased certain phases of post-flowering reproductive development in their controlled-environment, reciprocal transfer studies. Contrary to the current study, Summerfield et al. (1998) noted that even the most recessive NILs were sensitive to the post-flowering photoperiod, that the degree of photoperiod sensitivity was similar among the NILs, and that the sensitivity only increased when two or more dominant E alleles were involved. In their controlled-environment study, Summerfield et al. (1998) used photoperiods of 12 to 15 h that remained fixed throughout the post-flowering period. In the current study, the post-flowering photoperiods ranged from 15 to 19 h (i.e., including the daylength-extension treatment) and followed the natural changes in daylength as the season progressed. The differences in the reporting of photoperiod sensitivity of the recessive alleles between the two studies may be due to a number of reasons including the fact that one study was conducted under controlled-environment conditions while the other was conducted under field conditions. Also, the controlled-environment study used a fixed photoperiod while the current study used a dynamic photoperiod that followed natural daylength changes. Another important difference between the two studies is that they differed dramatically in the range of photoperiods used. The controlled-environment study included the use of photoperiods as short as 12 h, therefore comparing short daylength photoperiods (12 h) to long daylength photoperiods, but this response may not be observed when comparing long daylength photoperiods to even longer daylength photoperiods as was the case in the current study. Based on the results of the current study, it appears that the shorter, post-flowering photoperiods used in the current study did not stimulate an increase of the DRP. This is similar to the effect of short photoperiod on time to flowering previously reported. Under long, post-flowering photoperiods, the dominant E alleles appeared to be receptive to daylength extensions beyond at least 12 h, resulting in a lengthening of the DRP. Summerfield et al. (1998) reported that even recessive E alleles can be shown to increase the DRP when daylength is extended from 12 to 15 h. The current study suggests that recessive E alleles do not increase the DRP when photoperiods are extended from 15 to 19 h in a dynamic manner following ambient daily changes in photoperiod. Under field-growing conditions in most temperate soybean growing regions, post-flowering photoperiods of 12-h photoperiods are generally not encountered.
The current study has confirmed the role of E-genes and post-flowering photoperiod control on the duration of the reproductive phase, and that the number of dominant E alleles and the post-flowering photoperiod extension are positively related to the DRP. The current study confirms that soybean plants are sensitive to post-flowering photoperiod and that the E-genes mediate post-flowering development independent of their impact on photoperiod-mediated conditioning of time to flowering.
Why Do Later-Maturing Soybean Cultivars Mature Later, but Have a Similar DRP to Earlier-Maturing Soybean Cultivars?
Assuming that later-maturing soybean cultivars have more dominant E alleles than earlier-maturing soybean cultivars, one would expect that the DRP is longer for later-maturing cultivars (Fig. 4). However, when soybean cultivars from a range of maturity groups are planted at the same time, their DRP appears to be similar (Kane and Grabau, 1992; Egli, 1994). The results of the current study can explain the observed phenomenon as a function of the following: (i) the post-flowering photoperiod experienced, and (ii) the impact of E-gene alleles on post-flowering photoperiod-sensitivity for DRP.
Later-maturing lines (i.e., NILs with more dominant E alleles) flower later (Fig. 1), consequently these photoperiod-sensitive lines experienced shorter post-flowering photoperiod than earlier-flowering lines (Fig. 3). Earlier-maturing lines (i.e., fewer dominant E alleles) flower earlier, consequently these photoperiod-insensitive lines experienced longer post-flowering photoperiod than later-flowering lines (Fig. 3). Since DRP is a function of the post-flowering photoperiod experienced and the sensitivity of E-gene alleles to post-flowering photoperiod, the reason for the similar DRP among maturity groups could be explained as a result of genetic differences in the DRP being offset by differences in both the post-flowering photoperiod experienced and the impact of E-genes on post-flowering photoperiod sensitivity.
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SUMMARY
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E-genes have been known to control flowering and maturity; dominant E alleles delay both flowering and maturity in a photoperiod-mediated response. However, it was not well established whether the impact of E-genes on time to maturity was simply an indirect effect of a delay in flowering, or whether E-genes control DRP in addition to their control of time to flowering. By synchronizing flowering and imposing the same post-flowering photoperiod, it was possible to establish that the photoperiod experienced during the post-flowering period has a significant impact on the duration of the reproductive phase in NILs with at least one or more known dominant E allele. An increase in photoperiod during the post-flowering period through either daylength extension or earlier planting increased the duration of the reproductive phase in NILs with at least one or more dominant E allele. The results from the current study indicate that the duration of the reproductive phase is controlled by both the E-genes and the post-flowering photoperiod experienced and that E-genes control reproductive duration in addition to their control of time to flowering.
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ACKNOWLEDGMENTS
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The authors would like to thank Drs. Elroy Cober and Randall Nelson for providing the seeds of the NILs. We would like to extend a special note of gratitude to Dr. Richard Bernard for his excellent review and suggestions for the manuscript.
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication October 16, 2006.
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E. Bachlava and A. J. Cardinal
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ABSTRACT
INTRODUCTION
