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Agronomy Journal 92:1156-1161 (2000)
© 2000 American Society of Agronomy

SOYBEAN

Differential Response of Soybean Yield Components to the Timing of Light Enrichment

Jomol P. Mathew, Stephen J. Herbert, Shuhuan Zhang, Andreas A.F. Rautenkranz and Gerald V. Litchfield

Department of Plant and Soil Science, Bowditch Hall, Univ. of Massachusetts, Amherst, MA 01003 USA

sherbert{at}pssci.umass.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Solar radiation is an important environmental factor influencing seed yield in soybean [Glycine max (L). Merr.]. Our objective was to analyze the response of soybean seed yield components to light enrichment initiated at different growth stages. Light enrichment was imposed on the indeterminate soybean cultivars Altona and Evans by installing wire mesh fencing on either side of the center row to push the adjacent rows aside at different growth stages. Fences prevented plants in the neighboring rows from encroaching on the growing space of the center row plants. Pod number per plant and to a lesser extent seed size accounted for variation in seed yield. Light enrichment initiated at late vegetative or early flowering stages increased seed yield 144 to 252%, mainly by increasing pod number, while light enrichment beginning at early pod formation increased seed size 8 to 23%, resulting in a 32 to 115% increase in seed yield. Responses to light enrichment occurred proportionately across all node positions despite the differences in the time (15–20 d) of development of yield components at the different node positions. Although maximum seed size may be under genetic control in soybean plants, our results suggested seed size can still be modified by the environment with some internal control moderating the final size of most seeds in all pods. It indicates that plants are able to redistribute the available resources to components not yet determined, in an attempt to maintain or improve yield.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
ENVIRONMENTAL conditions prevailing during the growth period, especially intensity and quality of solar radiation intercepted by the canopy, are important determinants of yield components and hence the yield of soybean (Taylor et al., 1982; Willcott et al., 1984; Myers et al., 1987; Board and Harville, 1992, 1996). Light enrichment using lamps or reflectors increased the yield of soybean (Johnston et al., 1969; Schou et al., 1978). Shading (49–20% of ambient light) resulted in lengthening of internodes and increased lodging in soybean plants (Ephrath et al., 1993).

Hardman and Brun (1971) proposed that the yield of soybean is controlled by the availability of photosynthates during postflowering stage of development. Schou et al. (1978) observed that light levels during late flowering to midpod formation stages of growth are more critical than during vegetative and late reproductive periods in determining the yield of soybean. Taylor et al. (1982) concluded that pod abortion caused by lack of photosynthate supply late in the growing period is a major factor limiting yield of soybean. Duncan (1986) suggested that light intercepted during and after seed initiation is a major determinant of yield. Jiang and Egli (1993) reported that shade imposed from first flower to early podfill reduced flower production and increased flower and pod abscission, resulting in reduced pod number and yield. They also found canopy photosynthesis during flowering and pod set to be an important determinant of seeds m-2, and that the impact of shading on seeds m-2 depends on duration of shading (Jiang and Egli, 1995). Sharma et al. (1996) observed anthesis to be the most critical stage during which low light intensity can cause severe yield reduction in soybean.

Board et al. (1992) and Board and Harville (1996) suggested that the increased seed yield of soybean from planting in narrow rows (<50 cm) reported by many researchers (Lehman and Lambert, 1960; Costa et al., 1980; Herbert and Litchfield, 1982; Willcott et al., 1984), can be attributed to increased light interception during vegetative and early reproductive periods (first flowering to seed initiation). In most of the above studies, changes in yield was mainly brought about by changes in pod and seed number. However, an increase in seed size compensated for the decreased pod load in some source-sink manipulation studies (McAlister and Krober, 1958; Schonbeck et al., 1986).

Herbert and Litchfield (1982) noticed that pod number per plant was the most important component responsible for differences in soybean yield between different row widths and densities within a particular year, while a change in seed size resulted in the yield difference between 2 consecutive years. Thus, there is a differential response of yield components to changes in environmental conditions. However, the exact nature of response to the timing of light enrichment has not been identified yet. Also most of the studies conducted so far on differential response achieved increases in light interception indirectly by removal of leaves, thereby modifying the source strength.

Our objectives for conducting these studies were to analyze the differential seed yield response of indeterminate soybean to nondestructive light enrichment imposed at different stages during soybean growth, and to examine the effect of light enrichment on nodal development of seed yield components to determine developmental stages most affected by environmental change.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Studies were conducted at the University of Massachusetts Agronomy Farm during 1982–1983, 1987–1988, and 1994–1995 using Altona of maturity group 00, which matures in approximately 100 d at this location, and Evans of maturity group 0, which matures in approximately 115 d. The soil at the experimental site was a fine sandy loam (Typic Udifluent).

In each year a randomized block design was used with three replications in 1982 and four in later years. In 1982, a factorial combination of two cultivars (Altona and Evans) with two light levels, light enrichment initiated at flowering [R1 stage (Fehr and Caviness, 1977)], and a nonenriched control were tested. In all other years there were three light enrichment treatments, no light enrichment, light enrichment initiated 5 to 7 d prior to first flowering (V5 stage), and light enrichment initiated at late flower–early pod formation (R3 stage). In all cases light enrichment, once started, lasted until harvest. In 1983, Altona and Evans were used for the study, and in later years only Evans was grown. Each plot consisted of eight rows, planted 25 cm apart, except in 1982 when each plot had five rows that were 50 cm apart. The planting density was 60 plants m-2 in 1982 and 1983 and 83 plants m-2 in later years. Before planting, the seeds were inoculated with commercial powdered peat based granular Brady Rhizobium. Normal cultural practices were followed in all years. A preemergence mixture of 0.85 kg (a.i.) ha-1 linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] and 1.75 kg (a.i.) ha-1 alachlor (2-chloro-2', 6'-diethyl-N-(methoxymethyl) acetanilide) was used for weed control in all years. Total rainfall varied from 370 mm (1983 and 1995) to 522 mm (1982 and 1994) from May to September. Since soil was near field capacity at planting, water availability was judged as being adequate for growth.

Light enrichment was achieved by installing 90 cm tall wire mesh fencing (mesh hole size 4–5 cm) adjacent to the rows bordering the center sample row, sloping away at a 45° angle from the center row, in each plot. Fences prevented encroachment of plants from the neighboring rows into the growing space, and thus increased the radiation interception area of the sample row. The fences were inspected periodically (1–3 times per week) and all plants in rows bordering the center row were pushed behind the fences to prevent encroachment on the sample row. Light intensity measurements, using a Licor line quantum sensor (LI-188B) placed parallel to, and beside the center row plants, during the period from V5 to R3 showed that the control intercepted 98.5% and light enrichment treatments intercepted 74.2% of the incoming solar radiation. Thus, leaves at the base of the canopy in light-enriched plots were receiving more than 25% available light. Light intensity after R3 was always above 25% available light at the base of the canopy for the light enriched treatments.

Yield was determined by harvesting 3 m of the center treatment row from each plot and recording the seed dry weight after drying the samples to constant weight at 60°C in a forced dry air oven. This sample was used to calculated yield m-1 of the row and the number of plants m-1. Estimates of whole plot yields were not possible, because in every plot where plants were light enriched only the center row received the light enrichment treatment and could only be compared on a row equivalent basis. Hence, yield is expressed as yield m-1 rather than yield m-2. For yield component analysis, 15 plants, selected from a random starting point in the center row, were harvested at maturity from each treatment. For each group of plants, data were recorded according to node position on the main stem and for each branch corresponding to the main stem node from which it arose. Node 1 was the unifoliate node, being the first node above the cotyledons. Among the data recorded were pod number, seed number, and seed dry weight. Statistical analysis of the data was performed using the SAS ANOVA procedure (SAS Inst., 1988). Mean separation was done by using Duncans multiple range test (DMRT).


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
Yield of soybean plants and yield components obtained from the six experimental years have been summarized in Table 1 .


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  		Table 1 Yield component analysis of soybean grown under different levels of solar radiation during different growth stages

 
Yield Per Unit Row Length
Light enrichment initiated at both V5 and R3 increased seed yield m-1 compared with that of the nonlight enriched control in all years, and the extent of the increase was higher when light enrichment commenced at V5. Light enrichment initiated at R3 increased yield 32 to 115% while there was a 144 to 252% increase for light enrichment starting at V5. In 1982, light enrichment initiated at R1 increased seed yield m-1 by 38% over the control.

In 1982, a significant cultivar–treatment interaction was observed, with Evans being more responsive to light enrichment than Altona. This could be attributed to the greater plasticity of Evans (Willcott et al., 1984). Evans, a more profusely branching cultivar, exhibited earlier canopy closure compared with Altona, thus enabling greater exploitation of available light. This difference was not obvious in 1983 because the row spacing was reduced from 50 cm in 1982 to 25 cm in 1983, which enabled Altona to attain canopy closure earlier to more fully utilize the available solar radiation. Thus, light enrichment affected both cultivars similarly in 1983 and data was combined.

Light enrichment during flowering and seed fill of soybean increased yield per plant (Johnston et al., 1969; Schou et al., 1978), while shade during seed fill reduced seed yield (Egli et al., 1980). Data obtained from our experiments showed a greater increase in seed yield when light enrichment was initiated at V5 compared with R1, suggesting that the period starting from late vegetative stage (V5) is important in determining the yield of soybean. Improving efficiency of interception of light at this stage through cultural practices and by selecting cultivars with improved efficiency of light utilization could lead to increased yield.

Yield Components
In all years, pod number per plant was the yield component most responsible for yield increase from light enrichment initiated at either V5 or R1 (Table 1). Light enrichment initiated prior to flowering (V5) increased pod number per plant more than light enrichment beginning at early pod formation (R3). This shows that even though pod formation begins at R3, environmental conditions during the period from V5 to R3 are more critical in deciding the final pod number than the conditions associated with pod filling and retention prevailing after R3. Other studies have also identified pod number per plant as the yield component most influenced by changes in cultural and environmental conditions (Lehman and Lambert, 1960; Dominguez and Hume, 1978; Schou et al., 1978; Herbert and Litchfield, 1982). This suggests that light enrichment imposed during early stages of development of soybean would increase availability of assimilates to the developing reproductive structures, increase flowering, and reduce flower and pod abscission with a resultant increase in final pod number at harvest. Other studies have indicated light interception during vegetative and early reproductive stages to be more critical in determining the yield increase in narrow rows compared with latter stages of growth (Board et al., 1992).

Seed number per pod was least affected by changes in light regime in these experiments (Table 1). In 1982, 1987, and 1995, no significant changes in seed number per pod were observed among the treatments. However, in 1983, light enrichment initiated at V5 increased seeds per pod by 6% and late light enrichment initiated at R3 by 8% over the control. In 1988 and 1994, light enrichment initiated at V5 increased seed number per pod more than light enrichment initiated at R3. As evident from these studies, seed number per pod is a minor component determining the yield of soybean. However, there was a small tendency for seed number per pod to increase with light enrichment. Similar findings of small or no significant changes in seed number per pod have been reported in other studies (Dominguez and Hume, 1978; Schou et al., 1978; Herbert and Litchfield, 1982).

Light enrichment at both V5 and R3 were found to significantly increase seed size in 1994 and 1995 (Table 1). In other years seed weight with light enrichment initiated at V5 was similar to that of the control. In most years, light enrichment initiated at R3 was found to increase seed size more than light enrichment initiated at V5. McAlister and Krober (1958) and Schonbeck et al. (1986) proposed that an increase in seed size is a possible compensation response for the reduction in pod number by pod removal. Egli et al. (1978) and Swank et al. (1987) indicated that in soybean, seed size was a function of the rate of seed growth and the duration of dry weight accumulation in the seed fraction, and that genetic differences in seed growth rate are controlled by the cotyledon cell number (Egli et al., 1981). Later, based on a source-sink alteration study, Egli et al. (1989) concluded that soybean plants respond to changes in their immediate environment not only by changing the number of pods per plant, but also by altering cotyledon cell number, which correlated with seed size. Cotyledon cell number is one of the two main components determining seed size, the other being cotyledon cell volume or weight. Hence, the increased seed size observed in our experiments in response to light enrichment could be attributed to either an increased number of cells per cotyledon, or an increased dry matter accumulation of cells during the seed filling period.

Nodal Analysis
In each year there were similar responses to nodal distribution of yield components. Initiating light enrichment at V5 increased the number of pods somewhat proportionally across all mainstem nodes. Nodes in the midmainstem portion (Nodes 4–7) had the largest number of pods, and showed the greatest increase in the number of pods per node (Fig. 1) . The increase in pod number per node with light enrichment was significantly greater in the case of Evans than Altona, again demonstrating plasticity of Evans. Previous studies (Herbert and Litchfield, 1982; Heindl and Brun, 1984; Jiang and Egli, 1993) also indicated that nodes in the central region of soybean plants have the greatest yield potential among the mainstem nodes. Analysis of pod number per node also revealed that, regardless of the cultivar and treatment, branch pods accounted for >50% of the total pods produced at Nodes 1 to 3. Light enrichment initiated at V5 also increased branching, and thereby increased the number of pods produced from lower nodes, and these branches had 72 to 99% of all pods produced at these mainstem nodes.



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  		Fig. 1 Average number of mainstem pods at each mainstem node of Altona and Evans soybean grown under different levels of solar radiation. LE0 indicates no light enrichment. Light enrichment, LE1 initiated at R1 in 1982 and V5 in the remaining years; LE2 commenced at R3 in all years. Bars indicate ±1 SE of the mean

 
Heindl and Brun (1984) reported that in indeterminate soybean, there is only a slight variation in the number of flowers formed at each node, and they suggested that high rate of flower abscission was the major factor determining the pod number per node. Koller (1971) observed that the central nodes had the most leaf area at the time of rapid seed development. Leaves in the middle portion of the plant are also displayed, due to long petioles, much higher and close to the periphery of canopy where light interception is the greatest (Willcott et al., 1984). This appears important for maintenance of yield since most of the photosynthate (60–70%) produced by a soybean leaf during pod filling is ultimately incorporated into pods and seeds borne on the same node or two nodes above and below this node (Stephenson and Wilson, 1977).

In all the years, seed number per mainstem pod remained relatively constant across all node positions except for the extreme node positions (Fig. 2) . Variation in seeds per pod observed at the extreme node positions could be due to the small number of pods borne at these nodes. Seed number per pod for most nodes was averaged over many pods; however, a smaller number of pods with extreme seed numbers (1 or 4) found at the extreme node positions caused variation in the calculation of mean seed number per pod.



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  		Fig. 2 Average number of seeds per pod at each mainstem node of Altona and Evans soybean grown under different levels of solar radiation. LE0 indicates no light enrichment. Light enrichment, LE1 initiated at R1 in 1982 and V5 in the remaining years; LE2 commenced at R3 in all years. Bars indicate ±1 SE of the mean

 
Seed size also was mostly constant across mainstem nodes, and seed size response to light enrichment was similar across nodes (Fig. 3) . In indeterminate soybean vegetative growth continues during reproductive stages. When the lower most nodes started filling seeds, the upper most nodes were still producing flowers. However, seed size was quite uniform across all node positions in spite of the difference (15–20 d) in the duration of seed filling. Egli et al. (1981) reported genetic differences in seed growth rates were controlled by the cotyledons, not by the supply of assimilate from the plant to the cotyledons. Our data suggests that seed size can still be modified by the environment with some internal control moderating the final size of most seeds in all pods. Soybean seed yield response to changes in light regime was mostly by changes in pod number per plant, and least by seed number per pod. Responses occurred proportionately across all node positions despite differences in the time of development of yield components.SAS Institute 1988



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  		Fig. 3 Average seed weight at each mainstem node of Altona and Evans soybean grown under different levels of solar radiation. LE0 indicates no light enrichment. Light enrichment, LE1 initiated at R1 in 1982 and V5 in the remaining years; LE2 commenced at R3 in all years. Bars indicate ±1 SE of the mean

 

   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
This research is based upon work partially supported by the Cooperative State Research Extension, Education Service, USDA, Massachusetts Agric. Exp. Stn., Manuscript 3259.

Received for publication October 29, 1999.
   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




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