Crop Science Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published in Crop Sci 39:1361-1368 (1999)
© 1999 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (25)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Egli, D.B.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Egli, D.B.
Agricola
Right arrow Articles by Egli, D.B.
Crop Science 39:1361-1368 (1999)
© 1999 Crop Science Society of America

CROP ECOLOGY, PRODUCTION & MANAGEMENT

Variation in Leaf Starch and Sink Limitations during Seed Filling in Soybean

D.B. Eglia

a Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091 USA

degli{at}pop.uky.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Soybean [Glycine max (L.) Merr.] yield can be limited by the activity of the source (i.e., canopy photosynthesis) or by the ability of the seed (i.e., the sink) to utilize the assimilate produced by the source. An experiment was conducted for 3 yr at Lexington, KY, (38° N latitude) to evaluate potential source-sink limitations of early and late maturing cultivars. Two early maturing cultivars (Kasota and Hardin, Maturity Group I) and two late maturing cultivars (Essex and Hutcheson, Maturity Group V) were planted in mid-May in 0.38-m rows and the plots were irrigated to minimize water stress. Shade cloth that reduced irradiance by 63% was placed over half of the plots from approximately growth stage R6 (early seed filling) to maturity. Shade always significantly reduced the individual seed growth rate (9–32%) indicating that the plants were source limited. Leaf starch levels during seed filling were monitored to evaluate potential sink limitations. Starch levels in the late maturing cultivars usually remained constant or declined during seed filling, providing little evidence of a sink limitation. Starch levels in the early cultivars increased approximately 6-fold in the early stages of seed filling in 1 of 3 yr, suggesting that, in that year, the plants were sink limited, i.e., the seeds could not use all of the available assimilate. Source limitations during seed filling are apparently common in soybean, but sink limitations seem to occur infrequently in environments where water is not limiting.

Abbreviations: SGR, seed growth rate


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
YIELD OF A GRAIN CROP is the net result of the production of assimilate by leaves (the source) and translocation of these assimilates to the developing seed (the sink) where they are used to synthesize starch, oil, and protein. Separation of the processes responsible for yield into those associated with the source or the sink raises the question—is yield source or sink limited? Extensive investigation of this question has failed to produce any clear conclusions. Evans (1993), after a thorough review of the literature, decided that source and sink are not independent and therefore both may limit yield.

The number of seeds produced by a soybean community is related to canopy photosynthesis during flowering and podset. Changes in crop growth rate, an estimate of canopy photosynthesis, (Ramseur et al., 1985; Egli and Zhen-wen, 1991) or environmental factors that affect photosynthesis, such as CO2 levels (Hardman and Brun, 1971) or light (Schou et al., 1978), invariably cause changes in seed number. Seed number is therefore usually limited by the activity of the source. Variation in yield of a single cultivar across locations or years is usually highly correlated with seed number per unit area (Shibles et al., 1975; Egli, 1993) documenting the importance of source activity in determining yield. However, during seed filling, after seed number is established, the ability of the individual seed to utilize assimilate could still limit dry matter accumulation, creating a sink limitation (Jenner et al., 1991).

In vitro soybean seed growth rates (SGR) are related to media sucrose concentration, increasing from essentially zero with no sucrose in the media to a maximum rate at approximately 100 mM, which is maintained at concentrations up to 200 mM (Thompson et al., 1977; Egli et al., 1989). Consequently, the response of SGR in vivo to a change in assimilate supply may depend on the concentration of assimilate or sucrose in the seed (Jenner et al., 1991). If the concentration is high (i.e., above 100 mM and on the plateau portion of the response curve), SGR may not be affected, but at lower concentrations, any change in assimilate supply and sucrose concentration should translate into a change in SGR.

Leaf starch is dynamic and responds quickly to changes in photosynthesis and/or sink availability or activity. Soybean leaf starch levels typically increase during the day and decrease at night (Upmeyer and Koller, 1973; Huber et al., 1984). Increasing source-sink ratios by increasing photosynthesis or decreasing sink size usually results in higher leaf starch levels (Ackerson et al., 1984; Crafts-Brandner et al., 1984; Miceli et al., 1995) while reductions in photosynthesis (e.g., shade treatments or water stress) lower starch levels (Egli et al., 1980; Huber et al., 1984). Thus, changes in leaf starch levels during seed filling should provide some indication of whether or not the soybean plant is sink limited. If the assimilate supply from photosynthesis exceeds the ability of the seeds to utilize assimilate (sink limitation), starch should accumulate in the leaves; consequently, monitoring leaf starch levels during seed filling may provide some indication of relative sink limitations.

The objective of this research was to evaluate potential sink limitations during seed filling of early and late soybean cultivars in a field environment.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Soybean cultivars from Maturity Group I (Hardin and Kasota) and V (Essex and Hutcheson) were grown in the field for 3 yr at Lexington, KY, (38° N latitude). Maturity group I cultivars mature early at Lexington and are not grown commercially, while maturity group V cultivars are late maturing. Seeds were sown on 25 May 1993, 19 May 1994, and 23 May 1995 in 38-cm rows at a rate of 20 seed per m of row. Each plot consisted of 12 rows 6 m long. The soil was a Lanton silt loam (fine-silty, mixed thermic Cumulic Haplaquolls) in 1993, a Donerail silt loam (fine mixed, mesic Typic Argiudolls) in 1994 and a Maury silt loam (fine mixed, mesic Typic Paleudalf) in 1995. The plots were irrigated as needed to minimize water stress. Each cultivar was replicated four times in a randomized complete block design. Additional details on cultural practices were given by Egli (1997).

Reproductive growth stages (Fehr and Caviness, 1977) were determined at weekly intervals (2- to 3-d intervals as the plants approached physiological maturity, growth stage R7) on 10 consecutive plants in an interior row of two replications.

At the beginning of R6 (early seed fill), shade cloth that reduced irradiance by approximately 63% was placed over half of the plots of each cultivar and left in place until maturity.

Individual seeds that were at the same stage of development were identified by marking fully developed pods with acrylic paint (approximately 100 pods in a center row of each plot) when the seeds were just starting to swell. Two 20-pod samples were taken from the marked pods 14 d apart during the linear phase of seed growth to estimate SGR and a final sample was taken at maturity to estimate final weight per seed (seed size). Seeds were removed from the pods and the dry weight determined after drying at 60°C. Effective filling period was calculated by dividing final seed size by SGR (Daynard et al., 1971).

Leaf samples were collected at approximately weekly intervals beginning just after (1993) or before (1994,1995) the shades were installed. The samples, leaflets only from three plants per plot, were collected at mid-day (approximately 1300 h EDST), combined, placed on ice and taken to the laboratory. Two positions on the mainstem were sampled, the node below the node of the uppermost completely unrolled leaf (top leaf) and the node four nodes below the top leaf (middle leaf). The area of the lateral leaflets was measured (LI-COR LI 3100 area meter, LI-COR Inc., Lincoln, NE) before they were freeze-dried and ground for analysis. Soluble sugars, which included reducing sugars, hydrolyzed sucrose and other non-reducing sugars, and total nonstructural carbohydrates (after hydrolyzing the starch with a-amylase and amyloglucosidase) were extracted with benzoic acid and determined colormetrically (Heberer et al., 1985; Egli, 1997). Starch (expressed in glucose units) was taken as the difference between total nonstructural carbohydrates and soluble sugars multiplied by 0.9.

Daily maximum and minimum air temperature and insolation were available from a standard weather station located within 2 km of the experiment.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
On the average, seed filling (R5–R7) in the early cultivars started in late July and was complete by late August (Table 1) . The late cultivars started seed filling in late August and didn't reach physiological maturity until early October. Thus, the seed filling period of the early and late cultivars was separate in time, with seed filling of the late cultivars occurring under lower temperatures and insolation levels (Table 1).


View this table:
[in this window]
[in a new window]
  		Table 1 Air temperatures and insolation during reproductive growth for early and late soybean cultivars, average of 1993 to 1995

 
Starch levels in top leaves of the control plants were highly dynamic, with the temporal variation during seed filling affected by cultivar and years (Fig. 1–3) . The temporal changes in starch levels in middle leaves in 1994 generally paralleled the changes in top leaves, but the levels were usually lower (Fig. 4) . Similar relationships were found in middle leaves in 1993 and 1995 (data not shown). Averaging across samples and years, starch levels in the middle leaves of the early cultivars were 46% less than top leaves, compared with a reduction of 29% in the late cultivars.



View larger version (26K):
[in this window]
[in a new window]
  		Fig. 1 Starch and soluble sugar content (g m-2 leaf area) of top leaves of four soybean cultivars shaded during seed filling, 1993. The arrows indicate when the shade was installed. The bars represent ± standard error of the mean. Only representative bars are shown for starch and the bars for soluble sugars were frequently smaller than the symbol. Significant shade effects within a sample date are shown by * and + . NS = not significant

 


View larger version (27K):
[in this window]
[in a new window]
  		Fig. 2 Starch and soluble sugar content (g m-2 leaf area) of top leaves of four soybean cultivars shaded during seed filling, 1994. The arrows indicate when the shade was installed. The bars represent ± standard error of the mean. Only representative bars are shown for starch and the bars for soluble sugars were frequently smaller than the symbol. Significant shade effects within a sample date are shown by * and + . NS = not significant

 


View larger version (28K):
[in this window]
[in a new window]
  		Fig. 3 Starch and soluble sugar content (g m-2 leaf area) of top leaves of four soybean cultivars shaded during seed filling, 1995. The arrows indicate when the shade was installed. The bars represent ± standard error of the mean. Only representative bars are shown for starch and the bars for soluble sugars were frequently smaller than the symbol. Significant shade effects within a sample date are shown by * and + . NS = not significant

 


View larger version (28K):
[in this window]
[in a new window]
  		Fig. 4 Starch and soluble sugar content (g m-2 leaf area) of middle leaves of four soybean cultivars shaded during seed filling, 1994. The arrows indicate when the shade was installed. The bars represent ± standard error of the mean. Only representative bars are shown for starch and the bars for soluble sugars were frequently smaller than the symbol. Significant shade effects within a sample date are shown by * . NS = not significant

 
Soluble sugars were less variable and did not reach the high levels exhibited by starch (Fig. 1–4), but the temporal variation tended to parallel variation in starch.

Starch levels in top leaves on control plants were highest at the first or second sample early in seed filling in 1993 and 1995 with Essex in 1995 representing the only exception (Fig. 1 and 3). From this maximum level, starch either declined steadily during seed filling (all cultivars in 1993, Fig. 1), followed a bi-phasic pattern with an increase to a second maximum following an initial decline (early cultivars in 1995) or increased or decreased slowly during early seed filling (late cultivars in 1995, Fig. 3). In contrast, leaf starch in the early cultivars in 1994 (Fig. 2) increased dramatically during early seed filling, while little change occurred in the late cultivars. These changes in starch in the early cultivars were generally associated with changes in insolation (Fig. 5) . There was a trend for declining insolation during seed filling in 1993, increasing insolation in 1994 and little overall change in 1995, except for a 4-d period of low insolation in mid-seed filling that was associated with low starch levels. Insolation did not increase during seed filling of the late cultivars in any of the years (data not shown). A variety of temporal patterns of leaf starch during seed filling can also be found in the literature, with reports documenting increases (Mondal et al., 1978; Egli et al., 1980; Crafts-Brandner et al., 1984) or no change (Ciha and Brun, 1978; Ackerson et al., 1984; Havelka et al., 1984).



View larger version (19K):
[in this window]
[in a new window]
  		Fig. 5 Insolation during seed filling of the early cultivars. Day number 200 is July 18

 
The seed is the primary sink for assimilate during seed filling. The seed number component of sink size can no longer change by approximately R6 or slightly thereafter (Board and Tan, 1995; Egli, 1997), thus during much of the seed filling period the seed sink can respond to a change in assimilate supply only by changing SGR. If SGR does not increase when assimilate supply increases, a sink limitation exists and starch should accumulate in the leaves (Ackerson et al., 1984; Crafts-Brandner et al., 1984; Miceli et al., 1995). Starch accumulated in the leaves of early cultivars in 1994 and, to a lesser extent, in Essex in 1995 (Fig. 2 and 3). A large fluctuation in starch occurred in the early cultivars in 1995 (Fig. 3), but the levels following the fluctuation did not exceed the initial levels. Thus, starch levels increased during early seed filling in only 3 of 12 comparisons (four cultivars and 3 yr); however, because leaf starch responds to changes in photosynthesis without an obvious sink limitation (Upmeyer and Koller, 1973; Havelka et al., 1984; Huber et al., 1984), these increases in starch cannot be automatically attributed to a sink limitation. Leaf starch dynamics under the shades may provide a means of evaluating the contribution of changes in photosynthesis to the variation in leaf starch.

Shade Effects
Starch and Sugars
Shade rapidly reduced leaf starch and soluble sugar levels below the controls for all cultivars, leaf positions and years (Fig. 1–4) as reported by Egli et al. (1980). The maximum reduction had always occurred by the first sample after the shade was placed over the plant community, a period of 1 to 7 d. After the initial reductions, starch and soluble sugar levels tended to parallel levels in control leaves. Shade did not increase maximum loss of starch from the leaves as control and shaded leaves usually had similar starch levels at the last sample, taken near physiological maturity (Fig. 1–4), and in yellow leaves that were ready to abscise (Egli, 1997). This reduction in leaf starch in response to shade is consistent with the concept of a dynamic relationship between photosynthesis and leaf starch levels. Changes in leaf starch in response to water stress (Huber et al., 1984) and diurnal variation in insolation (Upmeyer and Koller, 1973; Huber et al., 1984) provide additional examples of this relationship.

Seed Growth Rate
Shade consistently reduced SGR of the four cultivars in all 3 yr by 9 to 32 % (Table 2) and the main effect of shade was always significant . Although the reductions were smaller for the late cultivars in some years, the cultivar x shade treatment interaction was never significant . In contrast, shade treatments applied at the beginning of reproductive growth (R1) did not affect SGR in a 2-yr study (Egli, 1993), probably because seed number was reduced and the supply of assimilate per seed likely remained relatively constant. The shade treatments in this experiment reduced seed number (8–16%, Egli, 1997) in spite of delaying their application until the beginning of R6, but this reduction was apparently not enough to offset the decrease in assimilate supply and SGR was reduced. The effect of shade during seed filling in previous experiments was inconsistent, significantly reducing SGR in one experiment but not in another (Egli et al., 1985).


View this table:
[in this window]
[in a new window]
  		Table 2 Effect of shade during seed filling on seed growth characteristics

 
Air temperature under the shades would probably be slightly lower than in the control plots which could reduce SGR. However, SGR under shade was not affected when seed number was reduced (Egli, 1993), suggesting that shade induced temperature effects were not important. Soybean SGR is relatively insensitive to temperature in the range of air temperatures that occurred in this experiment (Table 1, Egli and Wardlaw, 1980). Thus, the reduction in SGR was probably not a result of lower temperatures, instead, SGR was probably responding to a decrease in assimilate supply as reported by Fader and Koller (1985) following manipulation of CO2 and irradiance levels.

Source-Sink Limitations
The reduction in photosynthesis under shade reduced SGR, indicating that seed growth was limited by the supply of assimilate, i.e., the shaded plant was source limited during seed filling. The lower SGR resulted in significantly smaller seeds because there was no compensation by a longer seed filling period (Table 2), and yield was reduced (Egli, 1997). Consequently, yield of the four cultivars in this experiment was source limited when photosynthesis was reduced during the seed filling period. Board and Harville (1998) reached similar conclusions when they defoliated a maturity group VI soybean cultivar during seed filling.

Although SGR under the shade was apparently limited by the supply of assimilate (i.e., SGRshade < SGRcontrol), leaf starch levels did not decrease, relative to the control, in response to this unrealized potential growth. Leaf starch levels under the shades tended to remain constant or to parallel levels in the control leaves after the initial adjustment to the reduction in irradiance (Fig. 1–4), apparently reflecting the expected dynamic balance between starch and photosynthesis (Wardlaw, 1990). Interpreting the concept of sink demand (Zamski, 1996) as the ability of the sink to cause mobilization of storage carbohydrate from the source may be inappropriate for a seed sink. Under the shade, the seed seemed to be passive, SGR was below its potential while starch was, in some cases, accumulating in the leaf.

Starch levels in shaded leaves of the early cultivars increased in 1994 and 1995 in parallel with the control leaves, when there was no sink limitation, i.e., SGRshade < SGRcontrol. These increases were associated with an increase in insolation, which would increase irradiance and canopy photosynthesis under the shade. The magnitude of the increase in starch in shaded and unshaded leaves was similar in 1995 (Fig. 3), suggesting that the increase was due to an increase in photosynthesis with no sink limitation. However, the increase in the control leaves of the early cultivars was much larger than the shaded leaves in 1994, which may provide evidence for a sink limitation in that the higher leaf starch levels occurred because the seeds could not utilize the extra assimilate. Starch levels provided no indication of sink limitations in 1993 (Fig. 1) or for the late cultivars in 1994 or 1995 (Fig. 2 and 3). The increase in leaf starch levels in Essex in 1995 was exceeded by the increase of leaves under the shade, providing no evidence for a sink limitation.

The results of these experiments provide some evidence for both source and sink limitations in soybean during seed filling, after the seed number component of sink size is fixed. Source limitation seems to be more common as the shade treatment always reduced SGR. Evidence for a sink limitation occurred only with the early maturing cultivars and only in 1 of 3 yr. The source and sink limitations appeared to exist at the same time as reported by Board and Harville (1998). Simultaneous source and sink limitations could occur if the sucrose concentration in the control seeds was close to the level producing maximum SGR so that a decrease in assimilate supply to the seed and in the sucrose concentration would cause a decrease in SGR, demonstrating a source limitation. However, if photosynthesis increased, SGR could not respond (i.e., a sink limitation) to the higher assimilate concentration in the seed because the concentration now falls in the range where SGR is not responsive. The dependence of seed number on canopy photosynthesis and assimilate supply may tend to maintain seed number at a level where the sucrose concentration in the individual seed is near the critical level producing maximum SGR, as suggested by Farrar and Gunn (1996).

This dependence of seed number on canopy photosynthesis would probably minimize sink limitations in a constant environment where canopy photosynthesis would be the same during flowering, podset, and early seed filling. However, in the field, the environment is rarely constant and a change in environmental conditions between flowering and pod set (when seed number is determined) and the early stages of seed filling could increase photosynthesis and create a sink limitation. In 1994 when the early cultivars were apparently sink limited, insolation was increasing when starch was increasing in early seed filling, but there was no consistent increase in 1993 and 1995 (Fig. 5). Although average insolation was less during seed filling than during flowering and pod set for both early and late cultivars (Table 1), the decrease from flowering and pod set to seed filling was larger (27%) for the late cultivars than for the early cultivars (9%), suggesting that sink limitations may be less likely when reproductive growth occurs later in the growing season, as occurred with the late cultivars in these experiments. In contrast, crops that flower in early spring , such as winter wheat (Triticum sp.) or barley (Hordeum vulgare L.) may be more susceptible to sink limitations because average insolation during seed filling would be higher than during seed set.

In summary, the close association between environmental variation in soybean yield and seed number (Shibles et al., 1975), coupled with the dependence of seed number on canopy photosynthesis, provides strong evidence that soybean yields are primarily source limited. Any increase in canopy photosynthesis during the entire reproductive period should be translated into higher yields. Sink limitations can occur during seed filling if canopy photosynthesis increases after seed number is fixed and the seed cannot respond to the increased assimilate supply. The results presented here suggest that such limitations occur relatively infrequently in well-watered soybean. Sink limitations may be more common in rainfed production systems where random periods of moisture stress could easily create higher photosynthesis during seed filling and sink limitations.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Published with the approval of the Director of the Kentucky Agric. Exp. Stn. As Paper 98-06-198.

Received for publication November 23, 1998.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




This article has been cited by other articles:


Home page
 Crop Sci.Home page
V. O. Sadras and D. B. Egli
Seed Size Variation in Grain Crops: Allometric Relationships between Rate and Duration of Seed Growth
Crop Sci., March 19, 2008; 48(2): 408 - 416.
[Abstract] [Full Text] [PDF]

Home page
 ANN BOT (LOND)Home page
A. G. Kantolic and G. A. Slafer
Development and Seed Number in Indeterminate Soybean as Affected by Timing and Duration of Exposure to Long Photoperiods after Flowering
Ann. Bot., May 1, 2007; 99(5): 925 - 933.
[Abstract] [Full Text] [PDF]

Home page
 Crop Sci.Home page
R. E. Brevedan and D. B. Egli
Short Periods of Water Stress during Seed Filling, Leaf Senescence, and Yield of Soybean
Crop Sci., November 1, 2003; 43(6): 2083 - 2088.
[Abstract] [Full Text] [PDF]

Home page
 Crop Sci.Home page
T. Tischner, L. Allphin, K. Chase, J. H. Orf, and K. G. Lark
Genetics of Seed Abortion and Reproductive Traits in Soybean
Crop Sci., March 1, 2003; 43(2): 464 - 473.
[Abstract] [Full Text] [PDF]

Home page
 Crop Sci.Home page
G. Koning, D. M. TeKrony, S. A. Ghabrial, and T. W. Pfeiffer
Soybean mosaic virus (SMV) and the SMV Resistance Gene (Rsv1): Influence on Phomopsis spp. Seed Infection in an Aphid Free Environment
Crop Sci., January 1, 2002; 42(1): 178 - 185.
[Abstract] [Full Text] [PDF]

Home page
 Crop Sci.Home page
G. Koning, D. M. TeKrony, T. W. Pfeiffer, and S. A. Ghabrial
Infection of Soybean with Soybean mosaic virus Increases Susceptibility to Phomopsis spp. Seed Infection
Crop Sci., November 1, 2001; 41(6): 1850 - 1856.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (25)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Egli, D.B.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Egli, D.B.
Agricola
Right arrow Articles by Egli, D.B.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Plant Registrations Soil Science Society of America Journal
Journal of Natural Resources
and Life Sciences Education		 Journal of
Environmental Quality