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Published online 1 August 2005
Published in Crop Sci 45:1764-1769 (2005)
© 2005 Crop Science Society of America
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CROP PHYSIOLOGY & METABOLISM

Shade and Temporal Distribution of Pod Production and Pod Set in Soybean

Dennis B. Egli* and William P. Bruening

Dep. of Plant and Soil Science, Univ. of Kentucky, Lexington, KY 40546-0312

* Corresponding author (degli{at}uky.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The temporal distribution of pod production and pod survival play an important role in determining pod and seed number in soybean (Glycine max L. Merrill). We investigated the effect of changing photosynthesis at growth stage R1 (beginning flowering) on these temporal distributions in two greenhouse experiments. Plants (‘Elgin 87’) were exposed to two levels of shade (60 and 90%) from growth stage R1 to maturity. Other plants were removed from 90% shade or placed under 90% shade midway through flowering (transfer treatments). Temporal distributions of pod production and pod survival were determined by marking all unmarked pods ≥ 10 mm long on plants every three days with different colored paint. The color of paint on the mature pods identified when they started development. Continuous shade reduced mature pods by 27 (60% shade) and 82% (90% shade), but it shortened the pod-production period in only one of four comparisons. Pod production responded quickly to transfer treatments, and the mature pod load was always greater (nearly three fold) than the continuous 90% shade treatment and less (average of 53%) than the control. The mature pod load failed to recover from early shade because the increase in radiation did not lengthen the pod-production period and not enough pods were produced. Pod production was often more important than pod abortion in determining mature pod number. Adding the temporal distribution of pod production and survival to models predicting pod and seed number will improve their accuracy.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
PODS AND SEEDS per unit area are an important determinant of yield in many crop plants including soybean (Jong et al., 1982; Pandy et al., 1984; Egli, 1998; Frederick et al., 1998). However, the mechanisms by which the plant regulates the number of pods and seeds it produces are not completely understood. Recent evidence (Bruening and Egli, 1999, 2000) suggests that the temporal distribution of flower and pod production should be added to the traditional determinants of pod and seed number—photosynthesis or assimilate availability and sink (seed) characteristics (Charles-Edwards et al., 1986; Egli, 1998).

The asynchronous flowering characteristic of soybean is well documented. Flowering periods (first to last flower on a plant) are frequently 30 d long or longer (Hansen and Shibles, 1978; Yoshida et al., 1983; Gai et al., 1984; Dybing, 1994). The length of the flowering period was sensitive to daylength (Guiamet and Nakayama, 1984) and planting date (Constable and Ross, 1988; Dybing, 1994), but CO2 enrichment (Nakamoto et al., 2001), plant density (Torigoe et al., 1982; Saitoh et al., 1998), and N fertilizer (Torigoe et al., 1982) had no effect. Less is known about the survival of flowers to produce pods, but some results suggest that pod production is equally asynchronous. Pods that survived to maturity and contained seeds were produced for 30 to 50 d in field and greenhouse experiments (Illipronti et al., 2000; unpublished data, 2002).

The temporal distribution of flower or pod production was sensitive to planting date (Constable and Ross, 1988) and varied among years (Saitoh et al., 1998). Variation in plant productivity created by changes in plant density (Torigoe et al., 1982), CO2 enrichment (Nakamoto et al., 2001), or N nutrition (Torigoe et al., 1982) had no effect on the temporal distribution of flower production.

Pod and seed number in soybean respond to changes in photosynthesis that are maintained during the entire flowering and pod set period (Hardman and Brun, 1971; Schou et al., 1978; Egli and Zhen-wen, 1991) or just a portion of the period (Jiang and Egli, 1993). The temporal distribution of flower and pod production may play an important role in these adjustments (Bruening and Egli, 1999, 2000), but little is known of the relationship between these distributions and photosynthesis. Some research suggests that the temporal distribution of flowers is not very sensitive to variation in photosynthesis (Torigoe et al., 1982; Nakamoto et al., 2001), but the effects on the distribution of pod production (appearance of small pods) or pod survival have not been determined. These relationships must be defined before we can completely understand the role these distributions play in determining pod and seed number and yield in soybean. Consequently, our objective was to investigate the effect of large changes in photosynthesis (created by shade treatments) on the temporal distribution of pod production and survival in soybean. Small pods (≥10 mm long) were marked at regular intervals to identify when pods were produced and when the pods that survived until maturity initiated growth.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Soybean plants (Elgin 87, Maturity Group II) were grown in a greenhouse at the University of Kentucky using 3-L pots (one plant per pot after overseeding and thinning) filled with a 2:1 (v:v) mixture of a silt loam surface soil and vermiculite. Experiment 1 was planted on 1 May and Experiment 2 on 14 August 2003. Air temperature in the greenhouse was maintained between 20 and 30°C and the photoperiod was never less than 14 h, but the natural photoperiod exceeded 14 h during Exp. 1. Supplemental radiation (120 µmol m–2 s–1 photosynthetic photon flux density) was provided by high-pressure sodium lamps (430 W). The plants were not inoculated with Bradyrhizobium japonicum and the roots were not nodulated. A complete fertilizer (20–20–20, N–P–K) was applied approximately once every 2 wk.

At the beginning of flowering (approximately growth stage R1, Fehr and Caviness, 1977), plants were placed under black commercial shade cloth (60 and 90%) to reduce photosynthesis. Some plants remained under the shade until maturity, while others were moved from the unshaded control to 90% shade and vice-versa midway in the flowering and pod set period. Air temperature (0.5 h means) was measured with two shielded thermistors per treatment at the top of the plants and the data were recorded with a Li-Cor 1000 data logger. The average daily maximum and minimum temperatures under the shades between growth stage R1 and R6 were within 1.0°C of the controls in both experiments. Average maximum temperature in the control treatment in Exp. 2 was {approx}2°C higher than in Exp. 1. The differences between experiments were smaller under the shade. The control, 60 and 90% shade treatments were each assigned to a single greenhouse bench. Four pots (four replications) of each treatment were randomly assigned to the appropriate bench and a completely randomized design was used for the statistical analysis.

The temporal distribution of pod development and pod survival of each plant was characterized by marking all unmarked pods ≥10 mm long with acrylic paint on the pedicel and the base of the pod at 3-d intervals as described previously (Egli and Bruening, 2002), and the color of the paint was changed at each marking. The number of marked pods was recorded when they were marked to provide a temporal distribution of pod production. The color of the paint on mature pods indicated when pods that survived until maturity (mature full size pods that contained at least one developed seed) began growth, that is, the temporal distribution of surviving pods or pod set.

All surviving pods were harvested at maturity (all pods were brown), separated by paint color, and location (main stem or branches) and counted. Generally, <5% of the surviving pods did not have paint on them at maturity, and these pods were included in the totals but not in the temporal distributions. Seeds were removed from the pods in Exp. 1 and counted. Pod abortion, calculated as the difference between marked pods and surviving pods divided by marked pods, does not include abortion of flowers or pods < 10 mm long.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Pod production (appearance of pods ≥ 10 mm long) by control plants continued for 48 d in Exp. 1 vs. 30 d in Exp. 2 (Fig. 1). In both experiments, pod production on the controls increased to a maximum and then declined to zero. The plants in Exp. 1 started flowering in early June and produced more than twice as many pods as the plants in Exp. 2, which started flowering in October (Table 1). Roughly 30% of the pods were produced after growth stage R5 (beginning of seed filling, Fehr and Caviness, 1977).



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  		Fig. 1. The effect of continuous shade on pod production profiles (each data point represents the number of pods ≥ 10 mm long that were marked on that date). Bars represent, for each treatment, the average standard error of the mean after excluding means approaching zero. Shade treatments were applied at approximately growth stage R1 and maintained until maturity. Times of reproductive growth stages R1, R3, R5, and R6 are shown on the x axis.

 

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  		Table 1. The effect of shade on the distribution of marked and surviving pods on main stem and branches.

 
Continuous shade significantly (P = 0.05) reduced pod production in both experiments (an average of 24% for 60% shade and 71% for 90% shade, Table 1). Shade had a greater effect after peak pod production than before, but only the 90% shade treatment shortened the period and then only in Exp. 1 (Fig. 1).

The continuous-shade treatments also significantly (P = 0.05) reduced the surviving pods (mature pods containing a developed seed) in both experiments (Table 1). Pods on branches made a much larger contribution to the total pod load in Exp. 1 than Exp. 2 (Table 1). Seeds per pod of all treatments in Exp. 1 were within ±10% of the control (data not shown).

The temporal patterns of surviving pods (mature pods) (Fig. 2) closely followed the pod production curves in Fig. 1 in both experiments, and some surviving pods initiated growth after growth stage R5 (29% in Exp. 1 and 22% in Exp. 2). Most pods on control plants survived to maturity (i.e., total pod abortion was low, Table 2). The 60% shade treatment did not greatly increase total pod abortion relative to the levels of control plants (significant, P = 0.05, only in Exp. 2, Table 2). However, continuous 90% shade caused substantial increases in total pod abortion (significant at P = 0.05), and the increases were larger late in the pod-production period where roughly 80% of the marked pods did not survive to maturity (Table 2).



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  		Fig. 2. The effect of continuous-shade treatments on surviving pod profiles (marked pods that were full size and contained at least one developed seed at maturity). For each treatment, bars represent the average standard error of the mean after excluding means approaching zero. Shade treatments were applied at approximately growth stage R1 and maintained until maturity. Times of reproductive growth stages R1, R3, R5, and R6 are shown on the x axis.

 

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  		Table 2. The relationship between the time of pod development, shade, and pod abortion.

 
Variation in environmental conditions in the field can cause large rapid changes in photosynthesis during flowering and pod set that could influence final pod number. We simulated such changes by exchanging control and shade (90%) plants approximately midway through flowering and pod set. Pod production responded rapidly to the drastic increase in solar radiation when plants were removed from the shade (Fig. 3, 90%/control treatment). The increase in radiation did not extend the pod-production period beyond that of the control, but it did extend it beyond the continuous 90% shade treatment in Exp. 1. Pod production after the switch was, at one point, double control levels in Exp. 2, but it never exceeded the control in Exp. 1. The reduction in radiation when the plants were moved under the shade (control/90%) caused an almost immediate decrease in pod production (Fig. 3), but again, it had only minimal effects, if any, on the length of the pod-production period.



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  		Fig. 3. The effect of large changes in the radiation environment midway in the flowering and pod set period on pod production (each data point represents the number of pods ≥ 10 mm long that were marked on that date) profiles. Bars represent, for each treatment, the average standard error of the mean after excluding means approaching zero. The arrows indicate when plants in the transfer treatments were moved into and out of shade. Times of reproductive growth stages R1, R3, R5, and R6 are shown on the x axis.

 
Pod production in the early-shade treatment (90%/control) was significantly (P = 0.05) less than the late-shade treatment (control/90%) in Exp. 1, but there was no significant difference in Exp. 2 (Table 1). Pod production in both partial-shade treatments was significantly (P = 0.05) less than the control (20 to 50%) but substantially larger than the continuous 90% shade treatment.

The temporal distribution of pods that survived to maturity in the transfer treatments (Fig. 4) generally followed the patterns of marked pods (Fig. 3). Abortion of early and late pods was significantly (P = 0.05) increased above the control when plants were moved from the high radiation environment to shade (control/90% shade) midway through the pod-production period (Table 2). The abortion of early or late pods on the early shade (90% shade/control) treatment was not significantly (P = 0.10) different from the control, but it was significantly (P = 0.05) lower than continuous 90% shade treatment.



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  		Fig. 4. The effect of large changes in the radiation environment midway in the flowering and pod set period on surviving pods (marked pods that were full size and contained at least one developed seed at maturity) profiles. Bars represent, for each treatment, the average standard error of the mean after excluding means approaching zero. The arrows indicate when plants in the transfer treatments were moved into and out of the shade. Times of reproductive growth stages R1, R3, R5, and R6 are shown on the x axis.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The number of mature pods and seeds is directly related to photosynthesis during flowering and pod set in soybean (Schou et al., 1978; Egli, 1993; Jiang and Egli, 1995) and this relationship was confirmed here when both continuous-shade treatments significantly reduced the number of surviving pods (up to a 76% reduction). There was little change in seeds per pod in Exp. 1, so the number of seeds was primarily determined by the number of surviving pods. The temporal patterns of pod production and survival may play a role in determining pod number, but little is known about how reductions in photosynthesis and assimilate supplies affect these patterns.

Continuous shade affected the length of the pod-production period in only one of four comparisons. The length of the flowering period was also tolerant of treatments that influence photosynthesis (CO2 enrichment, Nakamoto et al., 2001) or individual plant productivity (plant density or N nutrition, Torigoe et al., 1982). Continuous shade reduced marked pod (small pods) production and increased pod abortion. But, the primary cause of reduced pod load under moderate shade stress seems to have been the production of fewer small pods (lower rate of pod production) with little change in the length of the pod-production period or pod abortion. Abortion made a significant contribution only under severe stress where it was much higher (≥80%) in late than early developing pods, in agreement with previous work (Heitholt et al., 1986; Huff and Dybing, 1980). Shade may have reduced flowers per plant (nodes per plant) or per node, as reported previously (Jiang and Egli, 1993), or it could have stimulated flower and small-pod abortion, which can make a significant contribution to total abortion (Hansen and Shibles, 1978; Huff and Dybing, 1980; Heitholt et al., 1986). It's possible that both flower production and flower and small pod abortion were important.

The pod-production periods were shorter in Exp. 2, which was planted on 14 August (vs. 1 May for Exp. 1), and the first pods were marked 39 d after planting in Exp. 2 vs. 44 d in Exp. 1. Earlier flowering (relative to planting) and shorter flowering and pod-production periods are common with late plantings in the field (Constable and Ross, 1988; Egli and Bruening, 2000). It is reasonable to assume that Exp. 2 probably experienced lower radiation levels during pod production since it occurred in late September and early October compared with June and early July in Exp. 1. Air temperatures were slightly higher in Exp. 2 (average of 26.9°C in the control) than in Exp. 1 (25.8°C), and the natural photoperiod in Exp. 1 was slightly longer (maximum of nearly 2 h) than the 14-h photoperiod maintained in Exp. 2. It is not clear whether the shorter photoperiod (suggested by Kantolic and Slafer, 2001) or lower radiation levels (both would occur normally in late field plantings) were responsible for the shorter pod-production period. If low radiation was responsible, it must have been a cumulative effect from seedling emergence, since lowering radiation levels after growth stage R1 (continuous-shade treatments) had almost no effect on the length of the pod-production period. A shorter pod-production period and a reduced rate of small pod (marked pods) production and survival (a function of lower radiation levels) were probably responsible for the lower mature pod load in Exp. 2 (about half of Exp. 1).

The plants responded almost immediately to large changes in the radiation regime midway in the flowering and pod set period by modifying the production and survival of small (marked) pods. The transfer treatments, however, had essentially no effect on the length of the pod production or pod survival periods. Previous relationships between photosynthesis and pod and seed number were frequently based on static relationships (e.g., average plant or crop growth rates during flowering and pod set; Pandy et al., 1984; Egli and Zhen-wen, 1991; Vega et al., 2001). Our data, however, demonstrate clearly that pod production and survival are dynamic systems that respond quickly (within days) to changes in photosynthesis. In fact, the control/90% shade treatment increased abortion of pods produced before shade was imposed (early pods, Table 2) which is not surprising given evidence that pods are susceptible to abortion until rapid seed development begins (Duthion and Pigeaire, 1991; Westgate and Peterson, 1993). Predictions of pod number from average measures of productivity will be accurate only when environmental conditions are relatively stable during the critical period, not a common occurrence in the field. The magnitude of short-term fluctuations in photosynthesis needed to reduce mature pod number will probably depend on the relationship between photosynthesis, storage carbohydrates, and the assimilate supply to reproductive structures as well as the length of time that a pod is sensitive to low levels of assimilate.

The mature pod load never recovered to control levels in either experiment (mean pod load was 45% less than the control) when the plants were removed from the shade midway through the pod-production period (90% shade/control treatment). Pod production continued for approximately 20 d after the transfer, pod production was well above the continuous 90% shade treatment, and abortion was reduced, but these changes were not enough to recover the pods lost during the early shade. There were not enough small pods produced to replace the lost pods; this failure was partially due to a lack of time as the higher radiation levels did not extend the pod-production period beyond the control. Much higher rates of pod production would be needed without an extension of the pod-production period. The fact that the plants were smaller coming out of the shade, probably with less leaf area (photosynthesis per plant is partially determined by leaf area in spaced plants) and fewer nodes (flowers per plant are related to nodes per plant, Egli, 2005) also could have limited pod production and survival. The exact reasons for the failure of the pod load to recover to control levels when the shade was removed are not known, but it seems that the inability of the plant to extend the pod-production period made some contribution.

It is often assumed that the long period of pod production and pod set in soybean may help stabilize pod number in fluctuating environments (Shibles et al., 1975; Loomis and Conner, 1992). However, if the plant cannot increase late pod production (higher rates or extend the period) enough to overcome early losses, and they could not in our greenhouse experiments, then soybean may be no more tolerant of variable environments than species such as corn (Zea mays L.) that have shorter flowering and seed set periods (Tollenaar and Daynard, 1978; Grant et al., 1989). Although our results with spaced plants cannot be extrapolated directly to the field, there are reports that field communities could not recover from early stress (Kokubun and Watanabe, 1983; Jiang and Egli, 1995). Collectively, these results suggest that maximum pod and seed number, and yield, may require a relatively stress free environment throughout flowering and pod set. Just how stress free (what length and level of stress will still allow a complete recovery?) remains to be determined.

Pod production and survival in these experiments responded dynamically to changes in photosynthesis after growth stage R1, to continuous changes that might differentiate a high- from a low-yield environment, and to shorter fluctuations that could occur in many field environments. The length of the pod-production period was almost completely insensitive to changes in photosynthesis, and most of the variation in mature pods was determined by small pod production. Pod abortion seemed to play a major role only under severe stress. Our results support previous contentions (Egli, 2005) that flower and small pod production are usually more important than abortion in determining the number of mature pods. The dynamic nature of pod production and survival means that models predicting pod and seed number must include the time component of flower and pod production and survival to accurately account for short term variations in photosynthesis. Predictions based on average photosynthesis or crop growth rates during the critical period will probably accurately reflect large changes in environmental conditions (e.g., high- vs. low-yield environments), but they may not accommodate smaller changes resulting from short-term fluctuations in the environment and in photosynthesis.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Published with the approval of the Director of the Kentucky Agric. Exp. Stn. as paper 04-06-147.

Received for publication September 21, 2004.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 


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This Article
Right arrow Abstract Freely available
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Google Scholar
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Right arrow Articles by Egli, D. B.
Right arrow Articles by Bruening, W. P.
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Right arrow Crop Physiology & Metabolism
Right arrow Soybean

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