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Published in Agron J 91:685-689 (1999)
© 1999 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
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Agronomy Journal 91:685-689 (1999)
© 1999 American Society of Agronomy

OIL & GRAIN CROPS

Physiological Changes from 58 Years of Genetic Improvement of Short-Season Soybean Cultivars in Canada

Malcolm J. Morrisona, Harvey D. Voldenga and Elroy R. Cobera

a Agric. & Agri-Food Canada, Eastern Cereal and Oilseed Res. Ctr. (ECORC), Central Exp. Farm, Ottawa, ON, Canada K1A 0C6

morrisonmj{at}em.agr.ca

Received for publication October 1, 1998.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 Summary
 REFERENCES
 
In Canada, yield of short-season soybean [Glycine max (L.) Merr.] cultivars has increased by approximately 0.5% per year since they were first cultivated in the early 1930s. Future yield gains may be dependent on an understanding of the changes made to soybean cultivars by breeding and selection. Our objective was to examine physiological differences associated with seed yield increase within a group of historical cultivars. At Ottawa, Ontario, we grew 14 cultivars representing seven decades of breeding and selection (1934–1992) in a randomized complete block design with four replications, across 4 years. Growth analysis provided data on leaf area and dry weight. Photosynthetic rate per leaf area was measured at several stages of development each year. Yield and harvest index were determined at maturity. The number of days to maturity and the total plant dry weight were not affected by the year of cultivar release. Seed yield, harvest index, and photosynthetic rate were found to have increased by 0.5% per year, while leaf area index decreased by 0.4% per year. The increase in seed yield with year of release was significantly correlated with an increase in harvest index, photosynthesis, and stomatal conductance and a decrease in leaf area index. Today's cultivars are more efficient at producing and allocating carbon resources to seeds than were their predecessors.

Abbreviations: MG, maturity group • CAP, canopy apparent photosynthesis • NAR, net assimilation rate • LAI, leaf area index • LAR, leaf area ratio • SLA, specific leaf area • HI, harvest index, TDW, total dry weight


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 Summary
 REFERENCES
 
SOYBEAN breeders and physiologists have long been intrigued by the prospect of increasing seed yield by raising photosynthetic rates. Jeffers and Shibles (1969) and Wells et al. (1982) found cultivar differences in canopy apparent photosynthesis (CAP), and Harrison et al. (1981) and Ashley and Boerma (1989) showed a positive association between seed yield and CAP. Studies measuring single-leaf photosynthetic rates have also indicated cultivar differences (Ojima and Kawashima, 1968; Curtis et al., 1969; Dornhoff and Shibles, 1970; and Buttery et al., 1981). Buttery and Buzzell (1972) found a positive association between net assimilation rate (the amount of dry matter produced per unit leaf area) and yield.

Ojima (1972) selected single F2 plants with high photosynthetic rates that maintained this rate in the F3 generation. Wiebold et al. (1981) had poor success when selecting for high photosynthetic rate among early-generation progeny from a cross of adapted soybean cultivars. Secor et al. (1982) found that, although leaf chlorophyll content and total soluble leaf protein were correlated with total photosynthesis, the correlation coefficients seemed too low to be of practical value as selection criteria. They concluded that effective selection for leaf photosynthesis can succeed within a soybean population, but only when it is measured directly.

Few studies have examined physiological changes in soybean cultivars across a prolonged period of breeding and selection. Buttery and Buzzell (1972), using cultivars bred and released over a 30-year period, concluded that selection for yield and other agronomic characteristics had resulted in plants with lower leaf area but higher photosynthetic efficiency. Larson et al. (1981) deliberately selected seven cultivars spanning five decades, but found no distinct relationship between leaf photosynthetic rates and yield. Previously, we examined 41 short-season [maturity groups (MG) 0, 00, and 000] cultivars ranging in release date from 1934 to 1992 and found that seed yield improved by 0.5% per year across that period (Voldeng et al., 1997). There was some indication that yield improvement was correlated with lodging resistance, but we found no other mechanisms to account for the improvement in yield. These historic cultivars provide a unique opportunity to determine the progress of plant breeding and the physiological changes made to the crop over decades of selection within a specific climatic region. Accordingly, our objective was to examine a subsample of 14 cultivars representing seven decades of soybean cultivar development, to determine physiological changes associated with yield improvement.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 Summary
 REFERENCES
 
Fourteen soybean cultivars, representing seven decades of cultivation, were chosen from a list of 41 short-season cultivars assembled by Voldeng et al. (1997) (Table 1) . Two cultivars in either MG 0 or 00 were chosen for each decade. With the exception of McCall, these cultivars were either selected, bred, or had parents from the soybean breeding program at Ottawa. The 14 cultivars were grown in 12-row plots (40 cm between rows) in a randomized complete block design, with four replications, from 1993 to 1996 at the Central Experimental Farm, Ottawa, ON, Canada (45°23' N lat). Plots measured 4.8 m wide and 5 m long. Seed was inoculated with Bradyrhizobium japonicum and sown at 50 seed m-2 to a depth of 2 cm in a Grenville loam (coarse-loamy, mixed, frigid Typic Eutrochrepts) (Canadian classification: Eluviated Melanic Brunisol). Weeds were controlled with recommended herbicides. The experiment was planted on 18 May 1993, 24 May 1994, 19 May 1995, and 23 May 1996. Phenological observations were made using the Fehr and Caviness (1977) growth stage key.


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  		Table 1 Name, year of release, maturity group, and pedigree of 14 short-season soybean cultivars

 
Plant growth was measured eight times during the growing season, starting at the V3 stage (third trifoliate) and progressing to R8 (maturity). Shortly after emergence, eight growth analysis sample areas were identified per plot and the plants were counted and thinned to 10 plants in a 0.5-m length of row. The sample areas were buffered on all sides by at least 1.0 m of an intact row. At each growth analysis, the plants from a sample area were cut at the soil surface, placed in a plastic bag, brought into the lab, and temporarily stored in a cold room (4°C) until dissection and analysis. Plants were separated into stems, leaves, and pods. Leaf area per sample was measured with a leaf area meter (Model 3000, Li-Cor, Lincoln, NE). The components were placed in paper bags and dried at 70°C for at least 48 h before weighing.

Leaf area and component dry weight were calculated on a square meter basis and used to determine leaf area index (LAI, unitless: leaf area/ground area), leaf area ratio (LAR, cm2 g-1: leaf area/total plant dry weight), specific leaf area (SLA, cm2 g-1: leaf area/leaf dry weight), and net assimilation rate [NAR, g m-2 d-1: (total plant dry weight/leaf area)/number of days in sampling period], according to the concepts and formulae of growth analysis described by Hunt (1982)(p. 14–46). Growth analysis parameters were calculated during the period of maximum leaf area (R4–R6), which corresponded to the fourth, fifth, and sixth date of sampling in each year.

A portable photosynthesis system (Li-Cor Model 6200) was used to measure leaf photosynthetic rate (µmol m-2 s-1) and stomatal conductance (cm s-1) per area. The penultimate fully expanded leaf was monitored at the V5, R1, and R4 stages of development in each year of the experiment. Measurements were done close to solar noon and on days with full sunlight.

Combined leaf chlorophyll a and b levels were determined from dried growth analysis samples representing the R1 and R4 stage from 1994 and 1996. Chlorophyll was extracted and measured using the method of Porra et al. (1989). Briefly: dried leaves from the entire plant sample were ground to a powder that could pass through a 40-mesh sieve. A homogeneous subsample of 20 mg was removed and placed in 8 mL of chilled methanol for 30 min. After 10 min of centrifuging at 861 g, a 2-mL aliquot of the supernatant was removed and the absorption at 652 and 665 nm was read using a spectrophotometer. The spectrophotometer was set to zero at 750 nm for each sample. The results were used with the formula of Porra et al. (1989): Chl a + b (nmol mL-1) = 24.23 (652 nm) + 3.26 (665 nm). Chlorophyll measurements were combined for data analysis.

The final sample, collected at maturity (R8), was used to calculate the harvest index (seed yield/total aboveground biological yield). Because leaves senesce and fall to the ground during seed development, leaf dry weight from the sixth growth analysis (R5 to R6) was added to the total biomass dry weight at maturity. This method, although more labor-intensive, provides a more accurate representation of the ratio of vegetative to economic yield than current methods of determining an apparent harvest index, which exclude leaf dry weight. After trimming 0.5 m from either end, four rows were combine-harvested, and the seed was cleaned, weighed, and adjusted to 70 g kg-1 moisture content. To facilitate analysis and presentation, data were combined across growth stages and years and a split-plot analysis of variance done, using year as the main effect and cultivar as the split effect. Means of each characteristic examined were plotted against the cultivar year of release to observe changes in plant characteristics with time. Regression coefficients were examined for significance using n - 2 degrees of freedom. Simple correlation among measured traits were computed to establish relationships.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 Summary
 REFERENCES
 
When data were combined across four years, there were significant (P = 0.05) differences among cultivars for number of days to maturity (Fig. 1a) . Cultivars ranged in maturity from 104 to 124 d. There was no significant relationship between year of cultivar release and the number of days to maturity. There were significant seed yield differences among cultivars (Fig. 1b). The oldest cultivar, Mandarin, did not have the lowest yield; rather, the second oldest cultivar, Pagoda, did. The most recently released cultivar, AC Harmony, had the highest yield. The relationship between yield and year of release was positive and significant. Yield improvement calculated from the regression equation was 0.5% per year. This was the same as the yield improvement found in an associated study with 41 historic cultivars (Voldeng et al., 1997). Total dry weight (TDW) was comprised of leaf dry weight from the sixth growth analysis (R4–R5) and the stem, pod, and seed dry weight at maturity (R8). Although there were significant dry weight differences among cultivars, there was no consistent change in total dry weight with year of release (Fig. 1c). There were significant differences in harvest index among cultivars (Fig. 1d). Harvest index calculated from the regression equation increased linearly by 0.47% per year with year of release.



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  		Fig. 1 Relationship between year of cultivar release and (a) the number of days from seeding to maturity; (b) seed yield; (c) total plant dry weight at maturity plus leaf dry weight from R6; and (d) harvest index. The error bar represents differences between cultivars, LSD (0.05)

 
Leaf photosynthetic rate increased linearly from 1934 to 1992 by 0.52% per year (Fig. 2a) . Stomatal conductance increased by 0.48% per year during the same period (Fig. 2b). Although there were some cultivar differences in chlorophyll a + b concentration, there was no significant relationship between year of cultivar release and chlorophyll content (Fig. 2c).



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  		Fig. 2 Relationship between year of cultivar release and (a) leaf photosynthetic rate; (b) stomatal conductance; and (c) chlorophyll a + b concentration. The error bar represents differences between cultivars, LSD (0.05)

 
Leaf area index decreased significantly with year of release at 0.38% per year (Fig. 3a) . Leaf area ratio represents the leafiness of the canopy. There were significant differences among cultivars for LAR, and LAR decreased with year of release (Fig. 3b). Specific leaf area is a measure of leaf density or thickness. There was no relationship between year of release and SLA, although the newer cultivars, for the most part, had thicker leaves than the older ones (Fig. 3c). There were no significant differences among cultivars for NAR (Fig. 3d), and there was no relationship between NAR and year of cultivar release.



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  		Fig. 3 Relationship between year of cultivar release and (a) leaf area index (LAI); (b) leaf area ratio (LAR); (c) specific leaf area (SLA); and (d) net assimilation rate (NAR). The error bar represents differences between cultivars, LSD (0.05)

 
Yield was positively correlated with photosynthetic rate, stomatal conductance, chlorophyll content, and harvest index (Table 2) . The number of days to maturity was negatively correlated with harvest index and NAR, but positively correlated with LAI, LAR, SLA, and TDW. Photosynthetic rate was positively correlated with stomatal conductance and negatively correlated with LAI, LAR, and SLA.


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  		Table 2 Correlation coefficients of yield, growth, and photosynthetic characteristics of 14 short-season soybean cultivars representing 58 years of genetic improvement

 

   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
Summary
 REFERENCES
 
Our experiment showed that soybean breeding and selection has resulted in a 29% increase in seed yield over 58 years. The yield gain over time was not due to the newer cultivars requiring a greater number of days to reach maturity, but was associated with an increase in harvest index, photosynthetic rate, and stomatal conductivity. There was no significant change in the total amount of dry weight produced by old and new cultivars, and the harvest index increased with year of release, so the yield gain was made through the production and allocation of photosynthate to more seeds or larger seeds per area. An increase in photosynthetic rate, higher stomatal conductance, and thicker leaves accompanied the decrease in LAI. As seed yield increased, so did the chlorophyll concentration. Today's cultivars are more efficient at producing and allocating carbon resources than their predecessors.

Our data revealed a positive correlation between photosynthetic rate and seed yield. It appears that, by selecting for higher yield, Canadian plant breeders have also selected for higher photosynthetic rate. In an earlier Canadian experiment, Buttery and Buzzell (1972) examined 21 soybean lines developed across 30 years of breeding and concluded that, over time, cultivars had been selected for smaller leaf area and higher NAR. We showed no significant increase in NAR with year of release, and our values were two- to threefold less than those calculated by Buttery and Buzzell (1972). The differences in magnitude likely arose from the different densities at which the plants were sown in the two experiments. We planted in rows at 50 plants m-2; they used hill plots at 1.2 plants m-2. Further work by Buttery et al. (1981) showed a significant correlation between leaf photosynthetic rates during seed filling and soybean yield. In Japan, Ojima (1972) reported that breeding and selection for higher soybean yield resulted in cultivars with greater leaf photosynthetic rate. In the United States, research has shown cultivar differences for photosynthetic rate in soybean (Dornhoff and Shibles, 1970) and a positive correlation between CAP and yield (Wells et al., 1982). Wiebold et al. (1981) concluded that early-generation selection for CO2 exchange rate in soybean was ineffective, but Ford et al. (1983) found differences in photosynthetic rates in 20 lines after seven generations of selection for the trait (although there was no significant effect on yield or harvest index). Ashley and Boerma (1989) found a positive correlation between CAP during seed fill and yield from F3–derived lines from two crosses.

Genotypic differences in net photosynthetic rates may be due to direct changes in the photochemical process, changes in resistance to CO2 diffusion, or structural changes such as reduced leaf area or increased sink demand. There is also the possibility that cultivars with greater photosynthetic rate may have better photoassimilate storage and translocation mechanisms (Buttery et al., 1981). In our experiment, increased photosynthesis was correlated with thicker leaves, lower leaf area index and higher stomatal conductance. Previous research has shown a negative correlation between leaf area and photosynthetic rate (Buttery and Buzzell, 1972; Hesketh et al., 1981; Bhagsari and Brown, 1986). Similarly, workers have found that increased leaf thickness correlated with increased photosynthetic rate (Dornhoff and Shibles, 1970; Buttery and Buzzell, 1972).

The higher photosynthetic rates per area found in the newer cultivars are probably the result of reduced leaf area and increased sink demand. Perhaps in response to narrower row widths and higher population density, breeders have selected cultivars with a lower leaf area. The smaller leaf area is more photosynthetically efficient, as evidenced by the lack of change in NAR with year of release. Wells (1991) showed that after canopy closure there was greater shading of the lower leaves than those at the top of the canopy, with subsequent lower CAP rates. The majority of the solar radiation was absorbed by the upper portion of the canopy, with only 10 to 20% of the radiation penetrating beyond 2 LAI units from the top. Therefore, with high-density populations, it is more efficient to have a lower leaf area with a higher photosynthetic rate per area.

In our experiment, there was no significant relationship between leaf chlorophyll concentration and year of cultivar release. Increased chlorophyll concentration was not correlated with higher leaf photosynthesis, as would be expected if higher photosynthetic rates were due only to lower leaf area. Photosynthetic efficiency may also have increased to some extent. When we measured leaf greenness with the SPAD-502 meter in an associated experiment (Ma et al., 1995), we found the same lack of relationship between year of release and leaf greenness, although leaf photosynthetic rate was correlated with the SPAD-502 meter reading. Buttery et al. (1981) also found that chlorophyll concentration was correlated with photosynthetic rate and reduced SLA, but not with yield. The lack of correlation between photosynthesis and leaf chlorophyll in our experiment may have been due to the fact that we determined photosynthesis on the penultimate leaf and chlorophyll on the leaves from the entire plant. It may also have been due to the fewer samples used for chlorophyll determination. The relationship between leaf chlorophyll concentration, photosynthetic rate and year of cultivar release warrants further examination.


   Summary
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 Summary
REFERENCES
 
Soybean yield gain over time in short-season growing regions was associated with reduced leaf area, increased photosynthetic and stomatal conductance rates per area and by greater allocation of photoassimilates to seeds rather than vegetative growth. Selecting for higher photosynthetic rate or stomatal conductance is feasible, although it is less expensive and more reliable to measure yield (Ashley and Boerma, 1989). Further research is required to determine if canopy structure and harvest index have been optimized in today's cultivars. Increasing the photosynthetic rate and the efficiency with which photoassimilate is stored and translocated may become important selection criteria in future cultivars.


   ACKNOWLEDGMENTS
 
The authors thank Brian Couture, Pat Bonnilla, Scott Tomlinson, Ron Guillemette, and Andrew Bird for their technical support.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 Summary
 REFERENCES
 
ECORC contribution no. 971204.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 Summary
 REFERENCES
 




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This Article
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