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a Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, 1366 W. Altheimer Drive, Fayetteville, AR 72704
b Univ. of Saskatchewan, Dep. of Plant Sciences, 51 Campus Drive, Saskatoon, SK 5A8 S7N, Canada
c Dep. of Biological and Agricultural Engineering, Univ. of Arkansas, Northeast Research and Extension Center, P.O. Box 48, Keiser, AR 72351
* Corresponding author (lpurcell{at}uark.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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400 MJ m-2. Above 400 MJ m-2, the response was curvilinear with little increases in biomass >700 MJ m-2. Our data clearly indicate that RUE decreased as POP increased and that maximum biomass production in these environments was not limited by intercepted PAR.
Abbreviations: DAE, days after emergence • DOY, day of year • MG, maturity group • LI, light interception • PAR, photosynthetically active radiation • POP, population density • RUE, radiation use efficiency
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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1.3 to 2.5 g MJ-1 of intercepted PAR (Sinclair and Muchow, 1999). Differences in RUE among individual experiments have been attributed to differences in energy content of biomass samples, whether biomass was reported on a shoot basis or a root plus shoot basis, the use of absorbed or intercepted radiation, errors in measurement of biomass and LI due to small sample size and nonuniform stands, and water and nutrient status of the crop (Sinclair and Muchow, 1999). Assuming that RUE is constant and that the length of the crop cycle is not affected by POP, increasing POP would expectantly shorten the time required for maximum LI, increase the total accumulation of PAR for a crop during the course of a season, and result in greater biomass at crop maturity. Shibles and Weber (1965) found for a MG II cultivar in Iowa that RUE was approximately constant in a year with adequate rainfall across a POP range of 6 to 52 plants m-2. In a year with suboptimal rainfall, RUE decreased as POP increased.
There have been no reports of the effects of POP on RUE in soybean at lower latitudes or across a wider range of populations than those used by Shibles and Weber (1965). We found that late-sown soybean required population densities considerably greater than those recommended for full-season production to maximize yield (Ball et al., 2000a). As POP increased, the time required for the crop to intercept light completely was decreased, which shortened the time required for linear biomass accumulation to begin and resulted in greater amounts of biomass at the end of the season. Because harvest index for this production system was relatively constant across population densities (Ball et al., 2000b), biomass increases resulted in yield increases.
We hypothesized that given adequate nutrients and water, RUE for soybean would be constant as POP increased and that increased population densities would result in greater amounts of biomass at the end of the season. Therefore, one objective was to evaluate the response of RUE to POP. A second objective was to characterize the amount of biomass produced during the season as a function of accumulated PAR for a wide range of treatments that affected cumulative LI for the crop, including MG, POP, and sowing date.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Canopy light interception was measured periodically from V3 to R6 using digital imagery (Purcell, 2000). A digital camera (model D-500, Olympus America, Inc., Melville, NY), mounted 1.6 m above the ground, was used to photograph each plot. The digital images were analyzed for the number of green pixels (SigmaScan Pro 4, SPSS, Chicago, IL). The fractional canopy coverage was calculated by dividing the number of green pixels by the total number of pixels for the entire digital image. Canopy-coverage measurements using this technique were similar to LI values using a 1-m line quantum sensor (Purcell, 2000).
Canopy coverage (CC) for each day after emergence (DAE) was predicted as a function of POP and DAE:
 | [1] |
Separate models were constructed for cultivars and sowing dates. Model performance was evaluated by comparing actual canopy coverage measurements at specific dates with predicted values for these dates. Throughout the remainder of the manuscript, canopy coverage is referred to as LI.
Daily totals of solar radiation were recorded by a pyranometer (Licor, Lincoln, NE) with an automated data logger (Campbell Scientific, Inc., Logan, UT). Photosynthetically active radiation was calculated as one-half of the total solar radiation (Monteith, 1972). Canopy LI of a given plot for each day (MJ m-2 d-1) from emergence to R6 was calculated as the product of the predicted fractional LI and the daily PAR total. Intercepted PAR was cumulated for each plot from emergence to R6.
When cultivars reached harvest maturity, 1 m from the center row was harvested by cutting the shoots at the soil surface. Senesced leaves and petioles were not included or collected in the sample. These samples were dried, weighed, and threshed with a combine. Harvest index was determined by dividing seed mass by total shoot mass. The remaining bordered area of the plot (4.64 m2) was harvested, and grain yield was corrected to 130 g kg-1 moisture. Shoot biomass (g m-2) at harvest maturity was estimated as the quotient of grain yield (g m-2) and harvest index.
Fayetteville, Arkansas, Experiment II
A second experiment was sown at Fayetteville on 15 July 1999, and this experiment evaluated responses to population densities across a wide range of MGs. There were two cultivars each from MG 00 (‘Glacier’ and ‘McCall’), 0 (‘Dawson’ and ‘Mn0301’), I (‘Asgrow 1553’ and ‘Parker’), and II (‘Dwight’ and ‘Burlison’), and one cultivar from MG III (‘Williams 82’) and IV (‘A4922’). Seeding rates for this experiment were 37, 62, and 86 seed m-2, which resulted in a population densities ranging from 26 to 90 plants m-2. Plot dimensions, irrigation, and measurements of LI, cumulated PAR to R6, yield, harvest index, and end of season biomass were similar to those described for Exp. I in Fayetteville.
Keiser, Arkansas, 1997 and 1998
Late-planted population experiments at Keiser, AR (35°67' N lat) from 1997 and 1998 have been described in detail previously (Ball et al., 2000a,b, 2001). These experiments were sown 8 July 1997 and 26 June 1998 on a Sharkey silty clay (Very-fine, smectitic, thermic Chromic Epiaquerts). The original experiment included three row spacings and irrigated and nonirrigated treatments, but only results from the irrigated 0.19-m row spacing are discussed in this report. The cultivars were A4922 and Manokin, and the resulting plant population densities ranged from 12 to 135 plants m-2 in 1997 and from 12 to 91 plants m-2 in 1998.
Light interception measurements were made with a line-quantum sensor (model LI-191SA, Licor, Lincoln, NE), 1 m in length, at 14-d intervals (Ball et al., 2000a), beginning at V4 and continuing until R6. Measurements were made between 1100 and 1400 h, in unobstructed light, by taking a measurement above the canopy and three measurements below the canopy. LI was calculated as:
 | [2] |
Total daily radiation (Licor, Lincoln, NE) was logged (Campbell Sci., Logan, UT) at an automated weather station and converted to PAR values as described previously. For any given plot, LI for each day between any two measurement dates was assumed to change linearly with respect to the measured values. PAR intercepted on a given day was calculated as the product of LI and PAR for that day. PAR was accumulated from emergence until the last LI measurement at the R5 developmental stage.
Biomass samples were taken from 1 m2 at 14-d intervals beginning at V4 and continuing until R6. After removing shoots at the soil surface, samples were dried and weighed. Radiation use efficiency (g biomass MJ-1) was determined for each plot by regressing biomass against accumulated PAR (MJ m-2). The response of RUE to POP for each year was determined by covariate regression of RUE against POP, with cultivar and block as covariate factors.
Keiser, Arkansas, 1999
The 1999 experiment at Keiser was similar to the 1997 and 1998 experiments, except that the 1999 experiment had two sowing dates. The MG IV soybean cultivars ‘Hartz 4994’ (H4994), and A4992 were sown in 0.19-m rows for early (26 May) and late (25 June) sowing dates. Five seeding rates were used for each sowing date, resulting in mean population densities of 12, 24, 45, 80, and 101 plants m-2 for the early sowing date, and 7, 14, 27, 45, and 66 plants m-2 for the late-sowing date. The experiment had two levels of irrigation treatment (irrigated and nonirrigated), but only the data from the irrigated treatment are presented in this report. The crop was irrigated with a lateral-move irrigation system when the estimated soil-moisture deficit reached 50 mm (Cahoon et al., 1990). The experiment was a multiple split-plot arrangement of treatments in a randomized complete block design. Individual plot size was 130 m2, and treatment combinations were replicated four times. The hierarchy of treatments from the main plot treatment to that in the final split was sowing date, irrigation, cultivar, and population.
Biomass was sampled seven and five times for the early- and late-sowing dates, respectively, between the V5 and R5 developmental stages. Plants from 1-m length of the plot from five adjacent rows were cut at ground level, dried, and weighed. Light interception was monitored by digital imagery (Purcell, 2000), as described for the Fayetteville experiments. Digital images were made five times during the growing season (usually when biomass was sampled) for the early-sowing date, and six times for the late-sowing date. Weather variables, including minimum and maximum air temperature, wind velocity, and solar radiation were recorded by an automatic weather station.
Daily interception of PAR was calculated as described for the Fayetteville experiments. Grain was harvested from 20 m2, avoiding borders, and yield was expressed at 130 g kg-1 moisture. At maturity, shoots were removed at the soil surface from 6 (lowest POP) to 54 (highest POP) plants plot-1. The wide range of plants for sampling was designed to give similar shoot mass across the POP range. After drying, the samples were weighed and threshed for harvest index determination. Total shoot mass m-2 was calculated as the quotient of seed yield and harvest index.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The length of the period for light accumulation was affected by both planting date and by cultivar (Table 1). Population density had no effect on crop developmental stages or maturity in any of our experiments (data not shown). Early-sown treatments at both the Fayetteville (Exp. 1) and Keiser locations had a longer growing season compared with late-sown treatments (Table 1). In Exp. 1 at Fayetteville, the period from emergence to R7 was 109 d for the early-sown treatment, compared with 96 d for the late-sown treatment. The decreased number of days in the life cycle of the late-sown crop was due to a decreased period of reproductive development from 82 d for the early-sown treatment to 67 d for the late-sown treatment. At Keiser, the shorter season for the late-sown crop was also due to a decreased reproductive period. Despite a shorter reproductive period, the seed-fill period (from R5 to R7) for the late-sown treatment at Fayetteville was actually longer (45 d) than the seed-fill period of the early-sown treatment (40 d). Therefore, the period from R1 to R5 was responsible for the differences in the length of the growing season for the different planting dates.
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The incident PAR at Keiser was highly variable, depending upon cloud cover and day of year (DOY), and ranged from 2 to 15 MJ m-2 d-1 (Fig. 1)
, and incident PAR at Fayetteville during the course of the season was similar to that of Keiser (data not shown). There was little change in the maximum incident PAR 25 d before or 50 d after the summer solstice (DOY = 174). After DOY 225, maximum daily PAR steadily decreased to 9 MJ m-2 d-1 by DOY 300. Therefore, the late-sown treatments had shorter periods for light accumulation and the incident PAR levels during reproductive development were less compared with the early-sown treatments.
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Radiation Use Efficiency and Crop Biomass Accumulation
RUE for an individual plot was determined by regressing biomass sampled periodically throughout the season against the cumulative intercepted PAR. The response of RUE to POP was then assessed by regressing RUE against POP with the factors of block and cultivar treated as covariates. In 1997, we found that for a late-sown crop, RUE was significantly decreased (P = 0.0012) by 0.003 g MJ-1 for each plant m-2 increase in POP (Table 2). The cultivar response to POP was similar with an average RUE of 1.45 g MJ-1 at a POP of 0 (i.e., intercept). The regression equation predicted that RUE would decrease from 1.42 to 1.05 g MJ-1 across the POP range from 12 to 134 plants m-2. For the late-sown 1998 experiment, there was no significant response of RUE to POP, and RUE was constant at 1.33 g MJ-1 (Table 2).
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3.0 g MJ-1. Between 500 and 700 MJ m-2 of intercepted PAR, the biomass response was curvilinear, and at intercepted PAR values greater than 700 MJ m-2, there was little further increase in final crop biomass. The initial slope of the regression equation in Fig. 3 (3.0 g MJ-1), was approximately twice the values of RUE reported in Tables 2 and 3, and of those values generally reported for soybean (Sinclair and Muchow, 1999). The analysis presented in Fig. 3, however, should not be considered as a measure of RUE primarily because the measurements of biomass (at harvest maturity) and of cumulated PAR (from emergence to beginning R6) were not synchronous. Although the total biomass may have stayed approximately the same or decreased during the period from R6 to harvest maturity (Ball et al., 2000a), the seed mass of the crop would have increased considerably during this period (Egli et al., 1984). There also would have been additional PAR intercepted between beginning R6 and harvest maturity, which was not included in Fig. 3.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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We found in previous research (Ball et al., 2000a), based on the 1997 and 1998 Keiser experiments, that the time required to fully intercept light and to begin linear biomass accumulation was shortened as POP increased for a late-sown soybean crop. For a late-sowing date and for relatively early-maturing cultivars (MG IV), yield continued to increase at extremely high population densities (>60 plants m-2) and was closely associated with biomass at harvest (Ball et al., 2000b). The relationship of how biomass responded to intercepted PAR was the focus of our current research.
Our hypothesis was that by increasing POP, greater amounts of light would be accumulated during the season, resulting in increased final biomass, assuming that RUE was not affected by POP. Contrary to our hypothesis, RUE decreased at increasing population densities for an early-sown crop (Table 3), and in three out of four environments, RUE was decreased by POP for a late-sown crop (Table 2). We propose that the decrease in RUE at high population densities is responsible for the yield ceiling commonly observed in population-density experiments (Wiggans, 1939; Shibles and Weber, 1965; Weber et al., 1966; Cooper, 1971b; Egli, 1988; Ball et al., 2000a) and for the asymptotic relationship of shoot biomass at high levels of accumulated PAR (Fig. 3).
The reason for the decrease in RUE with increasing POP is not known. Cooper (1971b) found that lodging increased as POP increased, and he speculated (Cooper, 1971a) that lodging decreased yield because the most photosynthetically active portion of the canopy was shaded when the crop was disrupted. The effect of this canopy disruption would be a decreased RUE and, perhaps, increased pod abortion which would decrease harvest index. Nevertheless, in our experiments, lodging was virtually absent except for the highest POP in 1997 at Keiser. Factors other than lodging appear to account for decreased RUE in these environments.
One possibility for the decrease in RUE at high population densities is that our biomass sampling did not include fallen leaves and petioles. At high population densities there would likely have been an increase in the amount of lower-leaf senescence, which would lead to an underestimation of RUE. Sinclair and Sheehy (1999) proposed that a minimum amount of light at lower strata in a rice (Oryza sativa L.) canopy was required to maintain leaves and prevent senescence. They proposed that more erect leaves would result in a greater leaf area receiving light intensity above this minimum, which would increase the storage pool of vegetative N for remobilization during grain filling. Because soybean has leaves more prostrate than rice, LI at lower strata in the canopy may be particularly important in maintaining leaves, and cultivars with more erect leaves would be expected to maintain RUE at high population densities.
A second possibility for the decrease in RUE at high population densities is that the amount of N that could be fixed or obtained from the soil was limited, and at high population densities this N was distributed across a greater leaf area, which would decrease the specific leaf N (g N m-2) concentration. A decreased leaf N concentration would lessen RUE, and this response has been documented for soybean and other crops (Sinclair and Horie, 1989). A limitation of other nutrients or water at high population densities would have a similar effect to decreased specific leaf N concentration. There were no indications, however, of nutrient-deficiency or water-deficit stresses for high population densities in our experiments.
The asymptotic nature of the biomass response to cumulated PAR indicates that light was no longer a factor limiting crop growth at high levels of intercepted PAR in these environments for these cultivars. It is important to determine if other cultivars have similar responses of shoot biomass to cumulated PAR across a broad range of environments. In the absence of nutrient and soil-moisture limitations, the ability of a soybean crop to increase biomass at high levels of cumulated PAR may be inherently limited by photosynthetic capacity. Typical RUE values for C4 plants are greater than those for C3 plants (Sinclair and Muchow, 1999), and elevated CO2 concentration increases photosynthesis and RUE of C3 plants. Because RUE is highly conserved among species within a photosynthetic pathway and among cultivars within a species, increasing biomass productivity at high levels of cumulated PAR is unlikely.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Received for publication March 13, 2001.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. The a coefficient was 0.42, 0.88, and 0.99, and the b coefficent was 0.015, 0.062, and 0.90 for 11, 22, and 46 DAE, respectively.