Published in Agron. J. 96:398-405 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
SEED PRODUCTION
Shading and Thinning Effects on Seed and Shoot Dry Matter Increase in Determinate Soybean during the Seed-Filling Period
Jin Kakiuchi and
Tohru Kobata*
Faculty of Life and Environ. Sci., Shimane Univ., 1060 Nishikawatsu-cho, Matsue 690-8504, Japan
* Corresponding author (kobata{at}life.shimane-u.ac.jp).
Received for publication August 13, 2002.
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ABSTRACT
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The relationship between seed and shoot dry matter increase (RS/W) during the seed-filling period (SFP) can reflect the balance between the assimilate supply and the sink capacity of harvest organs. In a 2-yr experiment, the RS/W in determinate soybean [Glycine max (L.) Merr.] during SFP was investigated under various growing conditions induced by shading and thinning for plants grown under both standard and reduced densities. The RS/W approximated a positive linear regression in both experiments. Slopes of the lines were less than 0.5 and did not vary within the same year even if plant density was reduced. The shoot dry matter increase (W) hence seemed to directly determine seed dry matter increase (S). A positive relationship between total pod number and W or S was observed in both years. Only minimal changes in seed numbers per pod and individual seed weight were observed, regardless of the treatment applied. A positive relationship also existed between total pod number and the number of branch nodes, and hence W increased S via increase in the number of branch node bearing pods. Half the current assimilate product during SFP appeared to be used for increase of vegetative plant parts. This determined total pod number and hence potential S. Determinate soybean is thus very different from cereal crops, in which almost all the current assimilate during the SFP is appropriated to S. The stability of the harvest index in soybean reflects the important effect of the W on pod establishment during SFP.
Abbreviations: HI, harvest index • LD, low density • RS/W, relationship between seed and shoot dry matter increase • S, seed dry matter increase • SD, standard density • SFP, seed-filling period • W, shoot dry matter increase
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INTRODUCTION
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DRY MATTER PRODUCTION during the SFP in many crop plants has a significant effect on seed yield (Evans, 1996). A theoretical expression based on dry matter analysis has been proposed to describe the effects of desiccated soils and shading during the SFP of rice (Oryza sativa L.) (Takami et al., 1990; Kobata and Moriwaki, 1990). This expression predicts S as a function of the total W during the SFP and is based on two parameters: potential S and the quantity of potentially mobilized reserves in the stem. The RS/W during the SFP suggests that S equals the potential S when the total assimilate supply (the amount of potentially mobile reserves in the stem plus the W) is equal to or greater than the potential S (Fig. 1
, Case 1). Below this level of assimilate supply, S decreases in proportion to W during the SFP. When the two rice cultivars studied suffered drought or different radiant conditions due to shading and thinning during SFP, the model was useful in predicting the S response. A similar relationship was observed in wheat (Triticum aestivum L.) during SFP under drought conditions (Kobata et al., 1992). Both cereal crops clearly change their growth phase from vegetative to reproductive stages, and the potential S barely changes once the SFP begins (Evans, 1996). In soybean, however, active stem and leaf growth continue after onset of SFP because flowering starts with vegetative growth earlier in the growing season than in most cereal crops (Ojima and Fukui, 1966). Hence, in soybean, the potential S may change with the W. If the potential S changes with W, the S should not reach a limit such as the plateau observed in Case 1. If so, the RS/W may be typed as three cases (Fig. 1, Cases 2–4). In Case 2, S during the SFP is equal to W (S = W), and there is no net transfer of assimilate reserves to the seed. In Case 3, S is greater than W due to net transfer of assimilate reserves from the stem to the seed. Conversely, in Case 4, S is less than W because excess dry matter is stored in the stem rather than in the seed.
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Fig. 1. Schematic relations between shoot and seed dry matter increase (RS/W) during the seed-filling period (SFP). Case 1 is the situation observed in cereal crops. In Case 1, potential seed dry matter increase (S) does not change during SFP, but in Cases 2, 3, and 4, it is affected by shoot dry matter increase (W). In Case 2, the line S = W indicates that almost all of the current assimilate products during the SFP are used for S. When RS/W is above the S = W line, reserved assimilates in the stem supplement S (Case 3). If RS/W is below the S = W line, part of the W accumulates in the vegetative organs (Case 4).
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The soybean harvest index (HI), the ratio of seed yield to total biomass at maturity, has been considered a fairly stable factor. Harvest index was used in a growth model study by Spaeth et al. (1984) to estimate seed yield from biomass production. Furthermore, the tendency for change in the partitioning coefficient (reproductive mass/total biomass) accompanying growth stage in soybean is similar, regardless of diverse growth conditions (Egli et al., 1985). These results suggest that the W controls the S in soybean. However, HI reflects only the ratio of seed yield to biomass at maturity, and the biomass at harvest includes the W during the SFP and the early vegetative growth period. Moreover, leaf fall and petiole weights are generally not included in biomass production figures (Spaeth and Sinclair, 1985).
Light is one of the most significant factors affecting seed yield in soybean and most other crops when compared with temperature, soil fertility, and soil water conditions (Evans, 1996; Loomis and Connor, 1992). Assimilate supply and the sink capacity of harvest organs are main factors influencing yield. To clarify which of these two mainly determines the S, we investigated the effect on the RS/W of different light conditions introduced by shading and thinning treatments during the SFP in determinate soybean.
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MATERIALS AND METHODS
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Plant Materials
Field experiments were conducted in 1999 and 2000 in Matsue, Shimane, Japan. The objective was to assess the effect of shading and thinning on RS/W under standard (1999 and 2000) and reduced (2000) plant densities. The experimental site was a 20- by 15-m plot of silty clay loam in a well-drained paddy field. Seeds of the soybean cultivar Tamahomare were sown on 16 June 1999 in rows 0.60 m apart, with 0.15-m spacing between plants [11.1 plants m–2, standard density (SD)]. In 2000, the seeds were sown on 7 June in rows 0.60 m apart, with 0.15-m spacing (11.1 plants m–2, SD) and 0.30-m spacing [5.6 plants m–2, low density (LD)]. Two seeds were placed in 0.03-m-deep holes and covered with adjacent soil by hand. After establishment (2 wk after germination), one seedling was removed, and one seedling was left to grow. In both years, 66.7 g magnesium lime m–2, 1.05 g N m–2 [as (NH4)2SO4], 7 g P m–2 (as P2O4), and 6.9 g K m–2 (as KCl) were applied following conventional procedure and mixed with the soil before sowing. Weather conditions differed substantially between the two experimental years. Rainfall from sowing to the R2 growth stage (Fehr and Caviness, 1977) was 470 mm in 1999 and 200 mm in 2000. From R2 to R8, the crops received 360 and 660 mm, respectively. In 2000, 90% of rainfall after R2 was concentrated between R6 and R8. Thus in 1999, it was wet through most of the growing season whereas in 2000, it was relatively dry until wet and humid conditions occurred during the late SFP. Soil water potential monitored by tensiometer fell below –40 kPa only three times in 1999. However, this limit was exceeded at least five times in 2000. When soil water potential decreased below –40 kPa, the field was surface-irrigated to increase the soil water potential to –18 kPa.
Shading and Thinning Treatment
The plants were thinned or shaded from the R2 growth stage (start of SFP) to R8 (maturing) (Fehr and Caviness, 1977) in both years to change W during the SFP.
Treatments of the Standard-Density Plot in 1999
On 7 Aug. 1999, either every other plant was removed, or three plants were removed between each plant. This reduced the plant density to half (5.6 plants m–2, one-half thinning treatment) or one-quarter (2.8 plants m–2, one-fourth thinning treatment). Shading treatment was applied to 1.8- by 4.2-m areas in the SD plots. Wooden frames 1.1 m high were positioned over the plants. These frames were covered with a single thickness of white cheesecloth (light-shading treatment), a single thickness of black cheesecloth (moderate-shading treatment), or a double thickness of the black fabric (heavy-shading treatment). Measurements of short-wave radiation inside the shading frame with a solar meter (SOLAR130, HAENI, Jegenstorf, Switzerland) showed the heavy-shading treatment reduced full-sun radiation by 78%, whereas light reductions in the moderate and light treatments were 47 and 29%, respectively. The control plots were neither thinned nor shaded, and the plants in these plots were grown under full-sun conditions throughout the growing season (control). The experimental plots were laid out in a randomized block design with three replications of the treatment parameters (control, thinning, and shading treatments).
Treatments of the Standard- and Low-Density Plots in 2000
In the 2000 experiments, blocks of two plant densities (SD and LD) were randomly assigned to each replication. Thinning and shading treatments were applied to the SD and LD plots. At 30 July, the SD plants were reduced to half (5.6 plants m–2, one-half thinning treatment) or one-quarter densities (2.8 plants m–2, one-fourth thinning treatment). Densities of the LD plants were only reduced to half (2.8 plants m–2, one-half thinning treatment) because one-fourth thinning treatment in this plot was not expected to effectively increase W, owing to the very low plant density (1.4 plants m–2) that would have resulted. Shade frames with a single layer of black cheesecloth (moderate-shading treatment) were installed in the SD and LD plots. Light-shading treatment was omitted in 2000 because the 1999 light-shading treatment did not significantly decrease the W. Heavy-shading treatment was added to the LD plots to increase the number of treatments available for regression analysis. Shading and thinning treatments were randomly assigned to each density plot (SD and LD).
Statistical Analysis
The experimental design was a randomized block design with three replications. Analysis of the yield components was directed toward the effect of shading and thinning treatments in SD and LD plots. Regression analysis was used to evaluate both the RS/W and the plant characteristics for the same plant density. Linear regression was used for data analysis based on the evaluation of correlation coefficients. The significance of differences between regression lines (P < 0.05) was tested by covariance analysis.
Measurements
In 1999, five plants were harvested from each treatment plot at R2 (5 August), R5 (26 August), R6 (26 September), and R8 (6 November). In 2000, four plants were harvested at R2 (30 July) and R8 (4 November). The sampled plants were dried in an oven at 80°C for 48 h and weighed. Counts were made of the numbers of filled pods, total pods, and seeds from each plant. Abscission organs were carefully gathered from tagged plants two or three times each week, dried at 80°C for 48 h, and then weighed. These weights were added to the appropriate dry sample weights. Flower numbers were counted at R2, R5, R6, and R8 only in 1999. Podding rate was calculated from the ratio of the number of filled pods to the total flower number. Carbon content of each organ sampled in 1999 was measured using an infrared C analyzer equipped with a combustion device (CID-301, Nippon Bunko Co., Tokyo).
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RESULTS AND DISCUSSION
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Dry Matter of Shoots and Seeds
Seed yield of control plants in the SD was 248 g m–2 at 14% water content in 1999 and 267 g m–2 in 2000 (Table 1). The higher seed yield in 2000 resulted from higher total (empty plus filled) and filled pod numbers per plant and individual seed weights than in 1999 even though seed numbers per plant and seeds numbers per pod were slightly lower in 2000 than in 1999. In 2000, seed yields from the LD treatment were 223 g m–2 less than from those from SD (Table 1).
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Table 1. Shoot dry weight at final harvest and yield components of soybean cultivar Tamahomare in thinning and shading treatments after R2 for standard plant density (SD, 11.1 plant m–2) in 1999 and SD and low plant density (LD, 5.6 plant m–2) in 2000.
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Reduced plant density by thinning treatment in the SD (1999 and 2000) and LD (2000) plots increased the total pod, filled pod, and seed numbers per plant but had less effect on seed number per pod and individual seed weight (Table 1). Shading treatment decreased the pod and seed numbers per plant in both years. These decreases were correlated with the level of shade, but seed numbers per pod and individual seed weights were not affected. As a result, seed weight per plant in the SD and LD treatments increased with reduction in plant density by thinning and decreased according to the strength of shading. In the SD and LD treatments, filled pod number increased with increase in total pod number. The ratio of filled to total pod numbers in all treatments fell within a narrow range (0.67 ± 0.08, mean ± standard deviation for all data) in both years, and hence the filled pod number was highly dependent on total pod number.
In 1999, SD shoot dry weight in the R2 stage, when the shading and thinning treatments began, was 17.4 g plant–1 (Table 1). Shoot dry weight per plant at maturity at SD fell between 4 and 48% compared with the control, depending on the level of shading. Although seed dry weight was not affected by light shading, under moderate and heavy shading, it fell by 35 and 64%, respectively. The two thinning treatments also had marked effects. Shoot dry weight increased by 69 and 147% (one-half and one-fourth thinning, respectively), and seed dry weight rose by 100 and 202% compared with the control (Table 1).
In 2000, SD and LD shoot dry weight at the R2 stage were 6.8 and 13.7 g plant–1, respectively (Table 1). Both shoot and seed dry weight at maturity for SD decreased around 35% under moderate shading. The thinning treatments increased shoot dry weights by 42 and 74%, respectively, compared with the control, and seed dry weights rose by 54 and 102%. For the LD plots, shoot and seed dry weights fell by 23 to 50% and 22 to 60%, depending on the level of the shading. For this treatment, one-fourth thinning increased shoot and seed dry weights by 38 and 54% over the control, respectively (Table 1). In both years, thinning and shading in the SD and the LD treatments thus produced significant change in shoot and seed dry weights per plant at harvest.
The Relationship between Seed and Shoot Dry Matter Increase during Seed-Filling Period
A close relationship (S = 0.54W, r2 = 0.998) exists between S and W in SD plants shaded or thinned after R2 in 1999 (Fig. 2a) . Forty-six percent of the W was apparently used to increase dry matter other than seeds, regardless of the shading or thinning treatments. For the soybean grown in SD and LD in 2000 and subjected to shading or thinning treatments, RS/W during the SFP was positive linear (SD: S = 0.41W, r2 = 0.994; LD: S = 0.42W, r2 = 0.996; Fig. 2b and 2c). There was no significant difference between the slopes of the SD and LD regressions for 2000. If all data in the SD and LD sets are combined into a single regression, this yields the equation S = 42W (r2 = 0.994). Consequently, in the 2000 experiments, an average of 58% of the W during the SFP was apparently partitioned into vegetative organs. These results suggest that almost half of the W in determinate soybean is used for seed filling during the SFP, and the remainder is partitioned for stem or leaf dry matter production. However, the slope of the 1999 RS/W regression was significantly greater than that of the combined regression (SD + LD) in 2000.
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Fig. 2. The relationship between seed and shoot dry matter increase (S and W, respectively) during the seed-filling period when shading or thinning was performed on standard-density plants (11.1 plants m–2) in (a) 1999 and (b) 2000 and low-density plants (5.6 plants m–2) in (c) 2000.
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Changes in seed and stem dry matter in cereal crops should reflect quantitative movement of assimilate product as there is little difference in C concentrations between these organs. However, in soybean, partitioning of assimilate products from vegetative organs to seeds based on dry weight changes can be underestimated. The lipid concentration in soybean seeds is much greater (0.18 kg kg–1) than in the stem organs (Taira and Taira, 1971), and therefore C weight per dry weight of the seeds is also greater than that of the stems. To nullify the effect of differing C content in the seeds and stems, RS/W from the 1999 results was also calculated on a C weight basis. This also shows a linear relationship (SC = 0.57WC, r2 = 0.998). Although the slope is slightly greater (3%) than that on the dry matter basis (Fig. 2a), the effect of differing C contents in soybean seeds and vegetative organs on the RS/W is small and therefore can virtually be ignored.
Although the effect of C content on the RS/W is not significant, dry matter accumulation in soybean seeds requires higher respiratory C loss for synthesis of lipids and proteins than is needed for carbohydrates such as starch (Sinclair and de Wit, 1975). Respiratory loss for S in soybean is 1.5 times greater than that in rice or wheat (Sinclair and de Wit, 1975). Consequently, partitioning of assimilate into seeds may be greater than that estimated from the RS/W. The higher respiratory loss for soybean seed production possibly causes the lower slope of the RS/W in soybean compared with other cereal crops.
The Key Factor Determining the Relationship between Seed and Shoot Dry Matter Increase
Total pod number varied greatly with shading and thinning treatment (Table 1). A linear relationship exists between S and total pod number for each plant density (SD and LD) in both years (Fig. 3)
. Thus, total pod number was a critical factor contributing to S because individual seed weights and numbers of seeds per pod are less variable among the treatments (Table 1). Total pod number was highly correlated with the W for each plant density in 1999 and 2000 (Fig. 4)
. These relationships suggest that current assimilate supply during the SFP affects pod numbers, and as a result, S is correlated with W. Nevertheless, the slope of the relationship between pod numbers and W for SD in 1999 was significantly greater than that for SD or LD in 2000.
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Fig. 3. Relationships between seed dry matter increase (S) and the total pod number on the standard-density plants in (a) 1999 and (b) 2000 and low-density plants in (c) 2000. Symbols are as in Fig. 2.
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Fig. 4. Relationships between W during the seed-filling period and total pod number in standard-density plants in (a) 1999 and (b) 2000 and low-density plants in (c) 2000. Symbols are as in Fig. 2.
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The number of nodes, filled pods, and flowers and podding rate on the whole plant increased with increasing W in the 1999 SD set (Fig. 5)
. However, the number of nodes on the main stem did not change with increasing W. Filled pod numbers and node numbers on the main stem and branches in the SD and LD (2000) sets increased similarly to 1999, whereas only minimal change was observed in main-stem nodes (data not shown). Flower numbers and podding rate were not measured in 2000. From the available results, main-stem node numbers were about the same regardless of W, but the number of nodes on the branches increased with increasing W. Thus, a greater number of nodes on the branches led to more flowers and a greater proportion of filled pods. Shoot dry matter increase therefore determines a capacity of the S through increase in branch nodes. Variability in branch node numbers in response to environmental stresses such as drought (Frederick et al., 2001) or low radiation seems to influence the seed yield. Pod decline in soybean usually results from interrupted development of the embryo after fertilization while pod establishment is greater in flowers in the early flowering period (Hashimoto, 1980). Low W by shading started from R2 may inhibit early growth of the embryo, thus reducing podding rate. Alternatively, high W by thinning may increase podding rate.
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Fig. 5. Relationships between shoot dry matter increase (W) during the seed-filling period (SFP) and main-stem node number (solid square), total node number (solid diamond), filled pod number at maturity (solid circle), total flower number (solid triangle), and podding rate (filled pod number/flower number; open circle) when standard-density plants were subjected to several shading and thinning conditions during the SFP in 1999. Data are the means ± the standard error of three replications.
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Reserved assimilate, however, did not contribute to the S in these soybean because the RS/W during SFP was below the S = W line (Fig. 1 and 2). Some soybean experiments have shown that stem dry matter decreases during the SFP (Hanway and Weber, 1971), suggesting contribution of translocated reserved assimilate to the S. In these studies, leaf fall and petioles were not included in stem weights, and therefore stem reduction did not reflect translocation of reserved assimilate from the stem organs to the seeds. It is known that movement of N in soybean from leaves to seeds during the SFP is less important than absorption from soils or N fixation by nodules (Hashimoto, 1971). Contribution of reserved assimilate and stored N in stem organs may thus be less important to seed production in determinate soybean.
The Effect of the Relationship between Seed and Shoot Dry Matter Increase on Harvest Index
Harvest index is defined by the slope of the relationship between S and shoot dry matter at maturity (Fig. 6)
. Harvest index was 0.43 in 1999 under SD and 0.37 under SD and LD in 2000. No significant difference is seen between the slopes (HI) of the regressions of the two differing plant densities in 2000. However, a significant difference in HI exists for the 1999 and 2000 SD data. Seed filling in most soybean cultivars usually starts 30 d after planting, and therefore the SFP occupies most of growth period (about 120 d) (Ojima and Fukui, 1966). As a result, dry matter added to shoots during the SFP constitutes most of the total shoot dry matter at maturity. The stability of HI in soybean should reflect the RS/W (Fig. 2). In some cases, however, HI in soybean is variable when shoot dry weight at maturity is low (Spaeth et al., 1984). Harvest index in 1999 and 2000 decreased with reduction in the ratio of W during the SFP to total dry matter at maturity (WR2/R8) although the range of WR2/R8 in 2000 was less (Fig. 7)
. This suggests that HI may be unstable when W during the SFP is markedly affected by environmental conditions such as solar radiation.
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Fig. 6. Relationships between seed dry matter increase (S) and shoot dry weight at maturity on standard-density plants in (a) 1999 and (b) 2000 and low-density plants in (c) 2000. The slope of the regression reflects harvest index. Symbols are as in Fig. 2.
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Fig. 7. Relationships between ratios of shoot dry matter at the early flowering stage (R2) and maturity (R8) (WR2/R8) and the harvest index of standard-density plants in (a) 1999 and (b) 2000 and low-density plants in (c) 2000. Symbols are as in Fig. 2.
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CONCLUSIONS
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Seed dry matter increase in determinate soybean was less than concurrent W under diverse light and plant density conditions. Half or more of W during the SFP was distributed into vegetative plant parts. Potential S seems to be controlled by W. Soybean differs from cereal crops such as rice or wheat, in which potential S changes little after flowering, and almost all current assimilate during the SFP is used for S. The distribution of assimilates to vegetative matter during the SFP in soybean increased both total and filled pod numbers (particularly on the branch node) and also increased pod setting, thus increasing S. Growth of stem organs during the SFP may contribute significantly to increased pod number and hence to increased seed production. Soybean flowers earlier in the growing season than cereal crops, and hence a stable rate of dry matter partition into seeds during the SFP directly reflects HI.
Although similar response of S to W for different plant densities was observed in both years, slopes of RS/W differed between the experimental years (Fig. 2). These differences might affect other parameters, such as HI (Fig. 6). In 2000, little rain fell from sowing to late growth, but heavy rain fell in the late SFP; in 1999, rainfall was relatively uniform over the same period. Lower R2 shoot dry weight in 2000 than in 1999 suggests that in the second year, the plants suffered water deficit during the early growth stage even though severe drought effects should have been avoided by temporary irrigation. Under low soil moisture conditions, soybean roots penetrate into deep soil layers, and shoot biomass is less than that of well-irrigated plants (Mayaki et al., 1976; Hirasawa et al., 1998). When such plants are watered, dry matter production increases more than in continuously irrigated plants due to retardation of leaf senescence and maintenance of photosynthesis rate (Hirasawa et al., 1998). Intensive rainfall during the late growth stage in 2000 should have increased W, but seed development was possibly almost complete when the rain fell. Dry matter production did not contribute to seed production and hence was retained in plant parts other than the seeds. As a result, the RS/W slope would decrease. The effects of water stress intensity and cultivation conditions are possible causes of variability in the slopes of RS/W and HI. The influence of such factors require further investigation.
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ACKNOWLEDGMENTS
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We thank Dr. Barry Roser for reading our manuscript.
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REFERENCES
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ABSTRACT
INTRODUCTION
