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

Dennis B. Egli* and William P. Bruening

ABSTRACT et al., 1998), and N fertilizer (Torigoe et al., 1982) hadno effect. Less is known about the survival of flowersThe temporal distribution of pod production and pod survival playto produce pods, but some results suggest that pod pro-an important role in determining pod and seed number in soybean

(Glycine max L. Merrill). We investigated the effect of changing duction is equally asynchronous. Pods that survived tophotosynthesis at growth stage R1 (beginning flowering) on these maturity and contained seeds were produced for 30 totemporal distributions in two greenhouse experiments. Plants (‘Elgin 50 d in field and greenhouse experiments (Illipronti et87’) were exposed to two levels of shade (60 and 90%) from growth al., 2000; unpublished data, 2002).stage R1 to maturity. Other plants were removed from 90% shade The temporal distribution of flower or pod productionor placed under 90% shade midway through flowering (transfer treat- was sensitive to planting date (Constable and Ross,ments). Temporal distributions of pod production and pod survival

1988) and varied among years (Saitoh et al., 1998). Vari-were determined by marking all unmarked pods � 10 mm long onation in plant productivity created by changes in plantplants every three days with different colored paint. The color ofdensity (Torigoe et al., 1982), CO2 enrichment (Naka-paint on the mature pods identified when they started development.moto et al., 2001), or N nutrition (Torigoe et al., 1982)Continuous shade reduced mature pods by 27 (60% shade) and 82%

(90% shade), but it shortened the pod-production period in only one had no effect on the temporal distribution of flower pro-of four comparisons. Pod production responded quickly to transfer duction.treatments, and the mature pod load was always greater (nearly three Pod and seed number in soybean respond to changesfold) than the continuous 90% shade treatment and less (average of in photosynthesis that are maintained during the entire53%) than the control. The mature pod load failed to recover from flowering and pod set period (Hardman and Brun, 1971;early shade because the increase in radiation did not lengthen the Schou et al., 1978; Egli and Zhen-wen, 1991) or just apod-production period and not enough pods were produced. Pod

portion of the period (Jiang and Egli, 1993). The tempo-production was often more important than pod abortion in determin-ral distribution of flower and pod production may playing mature pod number. Adding the temporal distribution of podan important role in these adjustments (Bruening andproduction and survival to models predicting pod and seed numberEgli, 1999, 2000), but little is known of the relationshipwill improve their accuracy.between these distributions and photosynthesis. Someresearch suggests that the temporal distribution of flow-ers is not very sensitive to variation in photosynthesisPods and seeds per unit area are an important deter-(Torigoe et al., 1982; Nakamoto et al., 2001), but theminant of yield in many crop plants including soy-effects on the distribution of pod production (appear-bean (Jong et al., 1982; Pandy et al., 1984; Egli, 1998;ance of small pods) or pod survival have not been deter-Frederick et al., 1998). However, the mechanisms bymined. These relationships must be defined before wewhich the plant regulates the number of pods and seedscan completely understand the role these distributionsit produces are not completely understood. Recent evi-play in determining pod and seed number and yield indence (Bruening and Egli, 1999, 2000) suggests thatsoybean. Consequently, our objective was to investigatethe temporal distribution of flower and pod productionthe effect of large changes in photosynthesis (createdshould be added to the traditional determinants of podby shade treatments) on the temporal distribution ofand seed number—photosynthesis or assimilate avail-pod production and survival in soybean. Small podsability and sink (seed) characteristics (Charles-Edwards(�10 mm long) were marked at regular intervals toet al., 1986; Egli, 1998).identify when pods were produced and when the podsThe asynchronous flowering characteristic of soybeanthat survived until maturity initiated growth.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 MATERIALS AND METHODSal., 1984; Dybing, 1994). The length of the flowering Soybean plants (Elgin 87, Maturity Group II) were grownperiod was sensitive to daylength (Guiamet and Naka- in a greenhouse at the University of Kentucky using 3-L potsyama, 1984) and planting date (Constable and Ross, (one plant per pot after overseeding and thinning) filled with1988; Dybing, 1994), but CO2 enrichment (Nakamoto a 2:1 (v:v) mixture of a silt loam surface soil and vermiculite.et al., 2001), plant density (Torigoe et al., 1982; Saitoh Experiment 1 was planted on 1 May and Experiment 2 on 14

August 2003. Air temperature in the greenhouse was main-tained between 20 and 30�C and the photoperiod was neverDep. of Plant and Soil Science, Univ. of Kentucky, Lexington, KY

40546-0312. Published with the approval of the Director of the Ken- less than 14 h, but the natural photoperiod exceeded 14 htucky Agric. Exp. Stn. as paper 04-06-147. Received 21 Sept. 2004. during Exp. 1. Supplemental radiation (120 �mol m�2 s�1

*Corresponding author (degli@uky.edu). photosynthetic photon flux density) was provided by high-pressure sodium lamps (430 W). The plants were not inocu-Published in Crop Sci. 45:1764–1769 (2005).lated with Bradyrhizobium japonicum and the roots were notCrop Physiology & Metabolismnodulated. A complete fertilizer (20–20–20, N–P–K) was ap-doi:10.2135/cropsci2004.0557plied approximately once every 2 wk.© Crop Science Society of America

677 S. Segoe Rd., Madison, WI 53711 USA At the beginning of flowering (approximately growth stage

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EGLI & BRUENING: TEMPORAL DISTRIBUTION OF SOYBEAN POD SET 1765

R1, Fehr and Caviness, 1977), plants were placed under blackcommercial shade cloth (60 and 90%) to reduce photosynthe-sis. Some plants remained under the shade until maturity, whileothers were moved from the unshaded control to 90% shadeand vice-versa midway in the flowering and pod set period. Airtemperature (0.5 h means) was measured with two shieldedthermistors per treatment at the top of the plants and the datawere recorded with a Li-Cor 1000 data logger. The averagedaily maximum and minimum temperatures under the shadesbetween growth stage R1 and R6 were within 1.0�C of thecontrols in both experiments. Average maximum temperaturein the control treatment in Exp. 2 was ≈2�C higher than inExp. 1. The differences between experiments were smallerunder the shade. The control, 60 and 90% shade treatmentswere each assigned to a single greenhouse bench. Four pots(four replications) of each treatment were randomly assignedto the appropriate bench and a completely randomized designwas used for the statistical analysis.

The temporal distribution of pod development and podsurvival of each plant was characterized by marking all un-marked pods �10 mm long with acrylic paint on the pediceland the base of the pod at 3-d intervals as described previously(Egli and Bruening, 2002), and the color of the paint waschanged at each marking. The number of marked pods wasrecorded when they were marked to provide a temporal distri-bution of pod production. The color of the paint on mature

Fig. 1. The effect of continuous shade on pod production profilespods indicated when pods that survived until maturity (mature(each data point represents the number of pods � 10 mm longfull size pods that contained at least one developed seed)that were marked on that date). Bars represent, for each treatment,began growth, that is, the temporal distribution of survivingthe average standard error of the mean after excluding meanspods or pod set.approaching zero. Shade treatments were applied at approximatelyAll surviving pods were harvested at maturity (all pods growth stage R1 and maintained until maturity. Times of reproduc-

were brown), separated by paint color, and location (main tive growth stages R1, R3, R5, and R6 are shown on the x axis.stem or branches) and counted. Generally, �5% of the surviv-ing pods did not have paint on them at maturity, and thesepods were included in the totals but not in the temporal distri- ber (Table 1). Roughly 30% of the pods were producedbutions. Seeds were removed from the pods in Exp. 1 and after growth stage R5 (beginning of seed filling, Fehrcounted. Pod abortion, calculated as the difference between and Caviness, 1977).marked pods and surviving pods divided by marked pods, Continuous shade significantly (P � 0.05) reduceddoes not include abortion of flowers or pods � 10 mm long. pod production in both experiments (an average of 24%

for 60% shade and 71% for 90% shade, Table 1). ShadeRESULTS had a greater effect after peak pod production than

before, but only the 90% shade treatment shortenedPod production (appearance of pods � 10 mm long)the period and then only in Exp. 1 (Fig. 1).by control plants continued for 48 d in Exp. 1 vs. 30 d

The continuous-shade treatments also significantlyin Exp. 2 (Fig. 1). In both experiments, pod production(P � 0.05) reduced the surviving pods (mature podson the controls increased to a maximum and then de-containing a developed seed) in both experimentsclined to zero. The plants in Exp. 1 started flowering in(Table 1). Pods on branches made a much larger contri-early June and produced more than twice as many pods

as the plants in Exp. 2, which started flowering in Octo- bution to the total pod load in Exp. 1 than Exp. 2

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

Surviving pods†

Marked pods Exp. 1 Exp. 2

Treatment Exp. 1 Exp. 2 Main stem Branches Total Main stem Branches Total

Pods plant�1

Control 183 70 52 134 186 38 30 67Shade, %

60 141 53 42 98 141 25 19 4490 44 30 16 14 30 10 5 15

Control/90‡ 119 53 28 47 75 14 8 2290/control§ 90 56 36 48 84 40 16 56CV (%) 13 12 16 24 14 20 32 10LSD (0.05) 23 10 8 25 21 8 8 7

† Full sized pods containing a developed seed at maturity. Includes pods that were not marked (usually �5% of total pods).‡ Shade (90%) from approximately midway in the flowering and pod set period to maturity.§ Shade (90%) from initial bloom (growth stage R1) to midway in the flowering and pod set period.

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1766 CROP SCIENCE, VOL. 45, SEPTEMBER–OCTOBER 2005

Fig. 3. The effect of large changes in the radiation environment mid-Fig. 2. The effect of continuous-shade treatments on surviving podway in the flowering and pod set period on pod production (eachprofiles (marked pods that were full size and contained at leastdata point represents the number of pods � 10 mm long that wereone developed seed at maturity). For each treatment, bars representmarked on that date) profiles. Bars represent, for each treatment,the average standard error of the mean after excluding meansthe average standard error of the mean after excluding meansapproaching zero. Shade treatments were applied at approximatelyapproaching zero. The arrows indicate when plants in the transfergrowth stage R1 and maintained until maturity. Times of reproduc-treatments were moved into and out of shade. Times of reproduc-tive growth stages R1, R3, R5, and R6 are shown on the x axis.tive growth stages R1, R3, R5, and R6 are shown on the x axis.

(Table 1). Seeds per pod of all treatments in Exp. 1creases in total pod abortion (significant at P � 0.05),were within �10% of the control (data not shown).and the increases were larger late in the pod-productionThe temporal patterns of surviving pods (mature pods)period where roughly 80% of the marked pods did not(Fig. 2) closely followed the pod production curves insurvive to maturity (Table 2).Fig. 1 in both experiments, and some surviving pods

Variation in environmental conditions in the fieldinitiated growth after growth stage R5 (29% in Exp. 1can cause large rapid changes in photosynthesis duringand 22% in Exp. 2). Most pods on control plants sur-flowering and pod set that could influence final pod num-vived to maturity (i.e., total pod abortion was low,ber. We simulated such changes by exchanging controlTable 2). The 60% shade treatment did not greatly in-and shade (90%) plants approximately midway throughcrease total pod abortion relative to the levels of controlflowering and pod set. Pod production responded rap-plants (significant, P � 0.05, only in Exp. 2, Table 2).idly to the drastic increase in solar radiation when plantsHowever, continuous 90% shade caused substantial in-were removed from the shade (Fig. 3, 90%/control treat-ment). The increase in radiation did not extend the pod-Table 2. The relationship between the time of pod development,

shade, and pod abortion. production period beyond that of the control, but it didextend it beyond the continuous 90% shade treatmentPod abortion†in Exp. 1. Pod production after the switch was, at one

Early† Late‡ Total point, double control levels in Exp. 2, but it never ex-Treatment Exp. 1 Exp. 2 Exp. 1 Exp. 2 Exp. 1 Exp. 2 ceeded the control in Exp. 1. The reduction in radiation

when the plants were moved under the shade (control/%Control 4 0 3 29 4 7 90%) caused an almost immediate decrease in pod pro-Shade, % duction (Fig. 3), but again, it had only minimal effects,

60 3 7 9 61 4 20if any, on the length of the pod-production period.90 16 32 78 87 30 49

Control/90§ 24 42 54 97 37 58 Pod production in the early-shade treatment (90%/90/control¶ 0 0 13 20 8 9 control) was significantly (P � 0.05) less than the late-LSD (0.05) 13 9 22 19 14 9

shade treatment (control/90%) in Exp. 1, but there wasLSD (0.10) 11 7 18 15 11 8no significant difference in Exp. 2 (Table 1). Pod produc-† Abortion � (marked pods � surviving marked pods)/marked pods � 100.tion in both partial-shade treatments was significantly‡ Abortion of pods marked before (early) or after (late) plants in the

transfer treatments were placed under or removed from the shade mid- (P � 0.05) less than the control (20 to 50%) but substan-way in the flowering and pod set period. tially larger than the continuous 90% shade treatment.§ Shade (90%) from midway in flowering and pod set to maturity.

¶ Shade (90%) from R1 to midway in flowering and pod set. The temporal distribution of pods that survived to

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EGLI & BRUENING: TEMPORAL DISTRIBUTION OF SOYBEAN POD SET 1767

ments that influence photosynthesis (CO2 enrichment,Nakamoto et al., 2001) or individual plant productivity(plant density or N nutrition, Torigoe et al., 1982). Con-tinuous shade reduced marked pod (small pods) produc-tion and increased pod abortion. But, the primary causeof reduced pod load under moderate shade stress seemsto have been the production of fewer small pods (lowerrate of pod production) with little change in the lengthof the pod-production period or pod abortion. Abortionmade a significant contribution only under severe stresswhere it was much higher (�80%) in late than earlydeveloping pods, in agreement with previous work (Heit-holt et al., 1986; Huff and Dybing, 1980). Shade mayhave reduced flowers per plant (nodes per plant) or pernode, as reported previously (Jiang and Egli, 1993), orit could have stimulated flower and small-pod abortion,which can make a significant contribution to total abor-tion (Hansen and Shibles, 1978; Huff and Dybing, 1980;Heitholt et al., 1986). It’s possible that both flower pro-duction and flower and small pod abortion were im-portant.

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

Fig. 4. The effect of large changes in the radiation environment mid- Exp. 2 vs. 44 d in Exp. 1. Earlier flowering (relativeway in the flowering and pod set period on surviving pods (markedto planting) and shorter flowering and pod-productionpods that were full size and contained at least one developedperiods are common with late plantings in the fieldseed at maturity) profiles. Bars represent, for each treatment, the

average standard error of the mean after excluding means ap- (Constable and Ross, 1988; Egli and Bruening, 2000).proaching zero. The arrows indicate when plants in the transfer It is reasonable to assume that Exp. 2 probably experi-treatments were moved into and out of the shade. Times of repro-

enced lower radiation levels during pod production sinceductive growth stages R1, R3, R5, and R6 are shown on the x axis.it occurred in late September and early October com-pared with June and early July in Exp. 1. Air tempera-maturity in the transfer treatments (Fig. 4) generallytures were slightly higher in Exp. 2 (average of 26.9�Cfollowed the patterns of marked pods (Fig. 3). Abortionin the control) than in Exp. 1 (25.8�C), and the naturalof early and late pods was significantly (P � 0.05) in-photoperiod in Exp. 1 was slightly longer (maximum ofcreased above the control when plants were moved fromnearly 2 h) than the 14-h photoperiod maintained inthe high radiation environment to shade (control/90%Exp. 2. It is not clear whether the shorter photoperiodshade) midway through the pod-production period(suggested by Kantolic and Slafer, 2001) or lower radia-(Table 2). The abortion of early or late pods on thetion levels (both would occur normally in late field plant-early shade (90% shade/control) treatment was not sig-ings) were responsible for the shorter pod-productionnificantly (P � 0.10) different from the control, but itperiod. If low radiation was responsible, it must have beenwas significantly (P � 0.05) lower than continuous 90%a cumulative effect from seedling emergence, since low-shade treatment.ering radiation levels after growth stage R1 (continuous-shade treatments) had almost no effect on the length

DISCUSSION of the pod-production period. A shorter pod-productionperiod and a reduced rate of small pod (marked pods)The number of mature pods and seeds is directlyproduction and survival (a function of lower radiationrelated to photosynthesis during flowering and pod setlevels) were probably responsible for the lower maturein soybean (Schou et al., 1978; Egli, 1993; Jiang andpod load in Exp. 2 (about half of Exp. 1).Egli, 1995) and this relationship was confirmed here

The plants responded almost immediately to largewhen both continuous-shade treatments significantly re-changes in the radiation regime midway in the floweringduced the number of surviving pods (up to a 76% reduc-and pod set period by modifying the production andtion). There was little change in seeds per pod in Exp. 1,survival of small (marked) pods. The transfer treatments,so the number of seeds was primarily determined by thehowever, had essentially no effect on the length of thenumber of surviving pods. The temporal patterns of podpod production or pod survival periods. Previous rela-production and survival may play a role in determiningtionships between photosynthesis and pod and seedpod number, but little is known about how reductionsnumber were frequently based on static relationshipsin photosynthesis and assimilate supplies affect these(e.g., average plant or crop growth rates during flow-patterns.ering and pod set; Pandy et al., 1984; Egli and Zhen-wen,Continuous shade affected the length of the pod-pro-1991; Vega et al., 2001). Our data, however, demonstrateduction period in only one of four comparisons. The

length of the flowering period was also tolerant of treat- clearly that pod production and survival are dynamic sys-

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1768 CROP SCIENCE, VOL. 45, SEPTEMBER–OCTOBER 2005

tems that respond quickly (within days) to changes in and level of stress will still allow a complete recovery?)remains to be determined.photosynthesis. In fact, the control/90% shade treatment

Pod production and survival in these experiments re-increased abortion of pods produced before shade wassponded dynamically to changes in photosynthesis afterimposed (early pods, Table 2) which is not surprising givengrowth stage R1, to continuous changes that might dif-evidence that pods are susceptible to abortion until rapidferentiate a high- from a low-yield environment, and toseed development begins (Duthion and Pigeaire, 1991;shorter fluctuations that could occur in many field envi-Westgate and Peterson, 1993). Predictions of pod num-ronments. The length of the pod-production period wasber from average measures of productivity will be accu-almost completely insensitive to changes in photosyn-rate only when environmental conditions are relativelythesis, and most of the variation in mature pods was deter-stable during the critical period, not a common occur-mined by small pod production. Pod abortion seemed torence in the field. The magnitude of short-term fluctua-play a major role only under severe stress. Our resultstions in photosynthesis needed to reduce mature podsupport previous contentions (Egli, 2005) that flowernumber will probably depend on the relationship be-and small pod production are usually more importanttween photosynthesis, storage carbohydrates, and thethan abortion in determining the number of mature pods.assimilate supply to reproductive structures as well asThe dynamic nature of pod production and survival meansthe length of time that a pod is sensitive to low levelsthat models predicting pod and seed number must includeof assimilate.the time component of flower and pod production andThe mature pod load never recovered to control levelssurvival to accurately account for short term variationsin either experiment (mean pod load was 45% less thanin photosynthesis. Predictions based on average photo-the control) when the plants were removed from thesynthesis or crop growth rates during the critical periodshade midway through the pod-production period (90%will probably accurately reflect large changes in environ-shade/control treatment). Pod production continued formental conditions (e.g., high- vs. low-yield environments),approximately 20 d after the transfer, pod productionbut they may not accommodate smaller changes resultingwas well above the continuous 90% shade treatment,from short-term fluctuations in the environment and inand abortion was reduced, but these changes were notphotosynthesis.enough to recover the pods lost during the early shade.

There were not enough small pods produced to replaceREFERENCESthe lost pods; this failure was partially due to a lack of

time as the higher radiation levels did not extend the Bruening, W.P., and D.B. Egli. 1999. Relationship between photosyn-thesis and seed number at phloem isolated nodes in soybean. Croppod-production period beyond the control. Much higherSci. 39:1769–1775.rates of pod production would be needed without an

Bruening, W.P., and D.B. Egli. 2000. Leaf starch accumulation andextension of the pod-production period. The fact that seed set at phloem-isolated nodes in soybean. Field Crops Res.the plants were smaller coming out of the shade, proba- 68:113–120.bly with less leaf area (photosynthesis per plant is par- Charles-Edwards, D.A., D. Doley, and G.M. Rimmington. 1986. Mod-

elling plant growth and development. Academic Press, Sydney,tially determined by leaf area in spaced plants) andAustralia.fewer nodes (flowers per plant are related to nodes per

Constable, G.A., and I.A. Ross. 1988. Variability of soybean phe-plant, Egli, 2005) also could have limited pod production nology response to temperature, daylength and rate of change inand survival. The exact reasons for the failure of the daylength. Field Crops Res. 18:57–69.

Duthion, C., and A. Pigeaire. 1991. Seed lengths corresponding topod load to recover to control levels when the shade wasthe final stage in seed abortion in three grain legumes. Crop Sci.removed are not known, but it seems that the inability of31:1579–1583.the plant to extend the pod-production period made some Dybing, C.D. 1994. Soybean flower production as related to plant

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Field Crops Res. 32:147–158.tion and pod set in soybean may help stabilize podEgli, D.B. 1998. Seed biology and the yield of grain crops. CABnumber in fluctuating environments (Shibles et al., 1975; International, Wallingford, UK.

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Egli, D.B., and W.P. Bruening. 2000. Potential of early maturingperiod) enough to overcome early losses, and they couldsoybean cultivars in late plantings. Agron. J. 92:532–537.not in our greenhouse experiments, then soybean may

Egli, D.B., and W.P. Bruening. 2002. Flowering and fruit set dynamicsbe no more tolerant of variable environments than spe- during synchronous flowering at phloem-isolated nodes in soybean.cies such as corn (Zea mays L.) that have shorter flow- Field Crops Res. 79:9–19.

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