Air Temperature During Seed Filling and Soybean Seed Germination ...

Published online May 27, 2005

Air Temperature During Seed Filling and Soybean Seed Germination and Vigor D. B. Egli,* D. M. TeKrony, J. J. Heitholt, and J. Rupe

Reproduced from Crop Science. Published by Crop Science Society of America. All copyrights reserved.

ABSTRACT

seed quality (Dornbos et al., 1989; Smicklas et al., 1992; Heatherly, 1993), but Vieira et al. (1991, 1992) found no effect on germination or vigor in field and greenhouse experiments when the stress did not produce shriveled and abnormal seeds. Dry conditions at harvest may increase physical injury and reduce quality if seeds are handled at low moisture levels (TeKrony et al., 1987). Temperature extremes during seed development also affect soybean seed quality. Freeze injury before physiological maturity caused large reductions in germination and vigor (Judd et al., 1982). High temperatures also reduced seed germination and vigor in growth chamber and phytotron experiments (Keigley and Mullen, 1986; Dornbos and Mullen, 1991; Zanakis et al., 1994; Gibson and Mullen, 1996; Spears et al., 1997; TeKrony et al., 2000; Egli et al., 2005). Temperatures of 33/28⬚C (day/ night) (Keigley and Mullen, 1986), 35⬚C (Dornbos and Mullen, 1991), 35/30⬚C (Gibson and Mullen, 1996), 38/ 33⬚C (Spears et al., 1997), and 38/27⬚C (TeKrony et al., 2000; Egli et al., 2005) during seed filling reduced germination of seed from several cultivars. Seed vigor was often more sensitive to high temperature than standard germination. For example, vigor (accelerated-aging germination—assessed by germination after aging the seeds in a warm humid environment) was reduced at 33/28⬚C, while 38/33⬚C was required to reduce standard germination (Spears et al., 1997). Gibson and Mullen (1996), however, reported no difference in the temperature required to reduce standard germination and accelerated-aging germination. Many seeds produced by soybean plants exposed to excessively high temperatures during seed filling are shriveled or abnormal (flattened and wrinkled with depressions in the seed coat), and the quality of these seeds is often much lower than seeds with no visible imperfections. Standard germination of abnormal seeds produced at 33/28⬚C was ⬍50% and approached zero at 38/33⬚C (Spears et al., 1997). Significant levels of shriveled and abnormal seed would reduce the quality of a seed lot; however, high temperature effects on quality were still evident when shriveled and abnormal seeds were removed and only normal seeds were tested (Spears et al., 1997). Temperatures that reduced seed quality in controlled environments (32 to 38⬚C) could occur during seed filling in the field in many soybean production areas. It is difficult, however, to extrapolate the results of growth chamber and phytotron experiments to the field. Temperatures in the field vary diurnally and usually decrease during seed filling in temperate environments. In contrast, in most controlled-environment experiments, plants were exposed to the maximum temperature for the en-

High temperature stress during seed filling in controlled environments reduces soybean [Glycine max (L.) Merrill] seed germination and vigor, but the effect of high temperature in the field has not been determined. Seeds of two soybean cultivars (Hutcheson, maturity group [MG] V, and DP4690, MG IV) were produced in the field in Kentucky, Mississippi, Arkansas, and Texas in 2000 to 2002. Air temperature during seed filling was monitored and brown (mature) pods were harvested, hand threshed, and all shriveled and abnormal seeds were removed before determining standard germination and vigor (accelerated-aging germination). Mean maximum temperatures during seed filling (growth stage R5 to R7) ranged from 24.0 (Kentucky) to 37.6ⴗC (Texas). When seed lots infected with Phomopsis longicolla (Hobbs) were removed from the analysis, standard germination and accelerated-aging germination (AA) decreased as mean maximum temperature during seed filling increased, but the decrease was significant (P ⫽ 0.05) only for Hutcheson. Standard germination of Hutcheson declined linearly (r2 ⫽ 0.49) from near 100% at 24ⴗC to 85% at 36ⴗC, while the decrease in AA was curvilinear (R2 ⫽ 0.86) and germination reached 11% at 36ⴗC. Seeds of Hutcheson were more sensitive to high temperature than seeds of DP4690 and seed vigor (AA) was much more sensitive to high-temperature stress than was standard germination. Our findings support the results of experiments in controlled environments by demonstrating that high temperature during seed filling in the field, without seed infection with P. longicolla or physical injury, reduced soybean seed germination and vigor.

H

igh quality planting seed is a key component of all grain cropping systems. High quality seed is needed to ensure adequate plant populations, with reasonable seeding rates, in a range of field conditions. Seed quality at planting represents the integrated effects of the environment during seed production and the conditions the seeds were exposed to during harvest, conditioning, and storage. Unfavorable environmental conditions (temperature, rainfall, relative humidity) during seed growth and development in the field can reduce germination and vigor of soybean seed. When seeds mature in warm, wet conditions they may be infected with Phomopsis longicolla which reduces germination and vigor (Kmetz et al., 1978; TeKrony et al., 1984, 1987). Some reports suggest that drought stress during seed development also reduces D.B. Egli and D.M. TeKrony, Dep. of Plant and Soil Sciences, Univ. of Kentucky, Lexington, KY 40546-0312; J.J. Heitholt, Texas Agr. Exp. Stn., Dallas, TX 75252; J. Rupe, Dep. of Plant Pathology, Univ. of Arkansas, Fayetteville, AR 72701. Contribution No. 03-06-129 from the Kentucky Agricultural Experiment Station, Univ. of Kentucky, Lexington, KY 40546-0312. Received 13 Jan. 2004. *Corresponding author ([email protected]). Published in Crop Sci. 45:1329–1335 (2005). Seed Physiology, Production & Technology doi:10.2135/cropsci2004.0029 © Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: MG, maturity group.

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CROP SCIENCE, VOL. 45, JULY–AUGUST 2005

tire photoperiod and to temperature regimes that did not vary during seed filling. Thus, temperature treatments in controlled environments may be more severe than similar temperature ranges in the field. Seed quality at harvest (barring mechanical damage) is primarily a function of disease (pod and stem blight caused by P. longicolla), temperature, and moisture conditions. We are not aware of reports describing effects of other aspects of the plant’s environment (e.g., soil conditions, plant population, nutrient availability) on seed quality, so variation in quality in the absence of disease can be related to air temperature during seed development. Consequently, our objective was to evaluate the relationship between air temperature during seed filling and soybean seed germination and vigor of seeds from plants growing in the field. Two soybean cultivars were grown at several locations in four states to create a wide range in air temperature during seed development. MATERIALS AND METHODS Seeds of two soybean cultivars—‘DP4690’ (MG IV) and ‘Hutcheson’ (MG V)—were produced in Kentucky, Texas, Mississippi, and Arkansas in 2000 to 2002 to sample field environments with a potentially large range in air temperature during seed filling (Table 1). Cultural practices typical for soybean at each location were used. Sprinkler (KY, TX) or furrow (AR, MS) irrigation was used during vegetative and reproductive growth at some locations to minimize water stress (Table 1). Reproductive growth stages (Fehr and Caviness, 1977) were determined at approximately weekly intervals. Air temperatures were measured at official weather stations that were 0.8 to 1 km (10 km at Brookston, TX) from the production locations. Maximum pod temperatures in the field with complete crop canopies may be less than air temperatures (up to 2⬚C) (Spears et al., 1997). Cultivars used in these experiments always produced complete canopies by seed filling, so the pods were always shaded, which would help mini-

mize variation in the air–pod temperature differential among cultivars, locations, or years. Three hundred brown (mature) pods of each cultivar– treatment–location combination were hand harvested (to avoid physical injury) at approximately growth stage R7 to R8, air dried, and sent to the University of Kentucky for germination and vigor testing. Pods and seeds were stored at 10⬚C in sealed plastic bags until tested. Seeds were removed from the pods by hand to avoid physical injury. Abnormal seed development is relatively common at high temperatures (Spears et al., 1997), so normal and abnormal seeds were separated by their visual appearance. Shriveled (seeds that were misshapen or had wrinkled seed coats, depressions in the seed, or were not the normal spherical shape) and abnormal (seeds that were severely shrunken, flattened, or diseased, including very small seeds that were nearly flat discs) seeds were removed and only normal seeds (spherical yellow seeds with no visible imperfections) (Spears et al., 1997) were tested. Stink bugs [Acrosternum hilare (Say), Nezara viridula (L.), Euschistus servus (Say)] can injure soybean seed and reduce quality (TeKrony et al., 1987), but no stink bug damage was observed in any of our experiments. Standard germination was determined by planting four 50seed samples on rolled paper towels (50 seeds per towel) which were incubated at 30/20⬚C (8 h at 30⬚C) and seedlings with normal development were counted after 3 and 7 d (AOSA, 2001). The presence or absence of mycelium growth typical of P. longicolla was recorded at the 7-d count. Seed vigor (accelerated-aging germination) was determined by placing 42 g of seed on a wire mesh screen over 40 mL of water in a plastic aging box (11 by 11 by 3.5 cm) and aging them at 41⬚C and nearly 100% relative humidity for 72 h before germination of four 50-seed samples was determined as described earlier (AOSA, 2002). Sigma Plot 2000 (v. 6) Regression Wizard (SPSS, Inc., Chicago, IL) was used for all regression analysis. The PROC CORR in SAS (v. 8.2) (SAS Institute, Inc., Cary, NC), was used for the correlation analysis.

Table 1. Production locations for soybean seed in 2000, 2001, and 2002. Location

Latitude

Cultivar

Planting date

R5–R7

ⴗN Kentucky Texas

Lexington Prosper

38 33

Kentucky Texas

Lexington Prosper Dallas

38 33 33

Arkansas

Brookston Mariana

33 35

Mississippi

Keiser Stoneville

36 33.4

Kentucky Texas

Lexington Prosper

38 33

Mississippi

Stoneville

33.4

Irrigation

d 2000 Hutcheson Hutcheson 2001 DP 4690/Hutcheson DP4690‡ Hutcheson Hutcheson DP4690 DP4690 Hutcheson DP4690/Hutcheson DP4690/Hutcheson 2002 DP4690/Hutcheson DP4690/Hutcheson DP4690/Hutcheson DP4690/Hutcheson DP4690/Hutcheson

16 May 10 May

44 34

yes no

30 16 2 11 20 16/17 17 15/17 6 1

May May May May April May May May April May

41/44† 39 55 42 33 36§ 33§ 28/37# 64/70†,# 56/58†,#

yes no yes no no yes/no¶ yes/no yes/no yes/no yes/no

22 13 15 17 18

May April May April May

36/49 26/58 31/29 62/63†,# 55/53†,#

yes no no yes/no yes/no

† Growth stage R5 to R8. ‡ Two row spacings, 0.35 and 0.71 m, growth stage R5 to R7 averaged over row spacing. § Mean of irrigated and nonirrigated. ¶ Seeds were harvested from irrigated and nonirrigated plots providing two seed lots for each cultivar. # Growth stage R5 to R7 (R8) for DP4690/Hutcheson averaged across irrigation treatments.

EGLI ET AL.: AIR TEMPERATURE AND SEED GERMINATION AND VIGOR


RESULTS The diverse production locations resulted in a large range in air temperatures during seed filling (growth stage R5–R7) for both cultivars, with mean maximum temperatures varying between 37.6 (Texas, 2001) and 25.6⬚C (Kentucky, 2001) for DP4690 (Fig. 1) and 36.0 (Texas, 2000) and 24.0⬚C (Kentucky, 2001) for Hutcheson (Fig. 2). The difference in maturity between cultivars translated into only small differences in mean maximum temperatures during seed filling (mean across locations and years was 32.6⬚C for DP4690 and 31.4⬚C for Hutcheson). Pods were harvested as soon as they matured (growth stage R7–R8) to minimize declines in quality that commonly occur due to field weathering after harvest maturity (TeKrony et al., 1980). Seed infection by P. longicolla is partially dependent on temperature and moisture conditions that occur before as well as after the seeds mature (TeKrony et al., 1984). The environment before maturity at some locations (Mississippi, Arkansas) favored seed infection with P. longicolla, which causes large reductions in seed germination and vigor (TeKrony et al., 1984). The standard germination of some seed lots of both cultivars that were produced at high temperatures (⬎30⬚C)

Fig. 1. Relationship between standard germination or acceleratedaging germination and mean daily maximum temperature during soybean seed filling (growth stage R5–R7) for DP4690. Growth stage R8 was used for the end of seed filling in Kentucky and Mississippi in 2001. All seed lots were included (n ⫽ 19) and regression analysis with several models resulted in no significant relationship between seed quality and temperature.

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exceeded 95%; however, there was a wide range for both cultivars with some lots of DP4690 approaching zero (Fig. 1 and 2). When all seed lots infected with P. longicolla were removed from the analysis, there was no significant change in standard germination of DP4690 as mean maximum air temperatures during seed filling increased (Fig. 3). There was, however, a significant decline in standard germination of Hutcheson (Fig. 4). Accelerated-aging germination of both cultivars varied from near 100% to ⬍10% (Fig. 1 and 2). When all seed lots were included in the analysis, there was a significant (P ⫽ 0.05) relationship between acceleratedaging germination and mean maximum air temperatures during seed filling for Hutchinson, but not for DP4690 (Fig. 1 and 2). Excluding results from seed lots infected with P. longicolla improved the relationship for Hutcheson (Fig. 4, R2 ⫽ 0.86) (P ⫽ 0.01), but there was still only a nonsignificant trend for lower accelerated-aging germination at high temperatures for DP4690 (Fig. 3). Excluding the two observations with accelerated-aging germination ⬍60% (Arkansas in 2001) resulted in a significant relationship for DP4690 between accelerated-aging germination and mean maximum temperature (R2 ⫽ 0.77, P ⫽ 0.01).

Fig. 2. Relationship between standard germination or acceleratedaging germination and mean daily maximum temperature during soybean seed filling (growth stage R5–R7) for Hutcheson. Growth stage R8 was used for the end of seed filling in Kentucky and Mississippi in 2001. All seed lots were included (n ⫽ 20), and the regression analysis for standard germination was not significant (P ⫽ 0.05), but the model for accelerated-aging germination (Y ⫽ ⫺98.966 ⫹ 17.682X ⫺ 0.401X2) was significant (P ⫽ 0.01).


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Fig. 3. Relationship between standard germination or acceleratedaging germination and mean daily maximum temperature during soybean seed filling (growth stage R5–R7) for DP4690. Only those seed lots showing no evidence of seed infection by Phomopsis longicolla in the standard germination test were included (n ⫽ 11). The regression models were not significant (P ⫽ 0.05). Data from irrigated experiments are identified with an asterisk.

Most of the temperature-induced reductions were due to the production of abnormal seedlings (Fig. 5) when accelerated-aging germination was ⬎75%. However, dead seeds also made a significant contribution at lower levels of accelerated-aging germination in both cultivars. Substituting the mean minimum temperature during seed filling for the mean maximum temperature did not improve the relationship between temperature and seed quality. We also evaluated the relationship of seed vigor and mean maximum temperature during the beginning (R5 ⫹ 14 d) and end (R7 ⫺ 14 d) of seed filling, and only the mean maximum temperature during the last 14 d of seed filling of Hutcheson showed a significant relationship with accelerated-aging germination (Fig. 6).

DISCUSSION High air temperatures during seed filling significantly (P ⫽ 0.01) reduced seed vigor in Hutcheson, but the reduction in DP4690 was significant only when the two outliers (AR 2001) were excluded from the analysis (Fig. 3 and 4). Many of the seed lots with low accelerated-aging germination were infected with P. longicolla, and the relationship between mean maximum temperature and accelerated-aging germination was greatly im-

Fig. 4. Relationship between standard germination or acceleratedaging germination and mean daily maximum temperature during soybean seed filling (growth stage R5–R7) for Hutcheson. Only those seed lots that showed no evidence of seed infection by Phomopsis longicolla in the standard germination test were included (n ⫽ 12) and regression models for standard germination (Y ⫽ 135.551 ⫺ 1.409X, P ⫽ 0.05) and accelerated-aging germination (Y ⫽ ⫺724.794 ⫹ 60.952X ⫺ 1.126X2, P ⫽ 0.01) were significant. Data from irrigated experiments are identified with an asterisk.

proved when these infected lots were excluded from the analysis (Fig. 1 and 2 vs. Fig. 3 and 4). Reductions in seed vigor at high temperatures have also occurred in controlled environments (Zanakis et al., 1994; Gibson and Mullen, 1996; Spears et al., 1997; TeKrony et al., 2000; Egli et al., 2005). The two cultivars responded much differently to high temperature, with Hutcheson showing much larger reductions in accelerated-aging germination than DP4690. Spears et al. (1997) and Egli et al. (2005) also found cultivar differences in phytotron and growth chamber experiments where Hutcheson was more susceptible than ‘McCall’, an extremely early maturing cultivar (MG 00). The greater sensitivity of Hutcheson to high temperature was surprising, given that Hutcheson (MG V) is adapted to the mid-south regions of the USA where summer temperatures are probably higher than areas where earlier MGs, such as McCall, are grown. Many more cultivars must be evaluated to fully characterize cultivar variation and to determine if there is any relationship between temperature sensitivity and MG. Seed vigor was much more sensitive to high temperature than standard germination in both cultivars. Spears



Fig. 5. Relationship between accelerated-aging germination and abnormal seedlings in the accelerated-aging germination test for those seed lots with no evidence of seed infection by Phomopsis longicolla. The dashed line represents normal ⫹ abnormal seedlings ⫽ 100% (i.e., no dead seeds).

et al. (1997) and Egli et al. (2005), working in a phytotron and growth chambers with Hutcheson and McCall, also reported that seed vigor was more sensitive to high temperature. These findings are consistent with the concept that vigor declines before germination during seed deterioration (Bryd and Delouche, 1971) and highlight the need to use vigor tests to identify seed lots that have had their performance potential reduced by high temperature, just as vigor tests are needed to detect the early stages of deterioration during storage (TeKrony et al., 1987). There are reports that drought stress during seed filling reduces soybean seed quality (Dornbos et al., 1989; Smicklas et al., 1992; Heatherly, 1993), although Vieira et al. (1991, 1992) found no such effect in greenhouse and field experiments. Drought stress and high temperature are frequently confounded in the field, but some of the seed lots with low accelerated-aging germination at high temperatures in our experiments came from irrigated plots of both cultivars. This finding, coupled with minimal effects of drought stress on seed vigor reported by Vieria et al. (1991, 1992), suggests that the low accelerated-aging germination was a result of high temperature and not drought stress. Seed development begins with a period of cell division when all seed structures are formed, followed by accumulation of storage reserves until physiological matu-

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Fig. 6. Relationship between accelerated-aging germination and mean daily maximum temperature during the last 14 d of seed filling (growth stage R7 ⫺ 14d) for those seed lots with no evidence of seed infection by Phomopsis longicolla in the standard germination test. The regression analysis for DP4690 (Y ⫽ 144.440 ⫺ 2.162X, P ⫽ 0.05) was not significant, but the model for Hutcheson (Y ⫽ ⫺185.875 ⫹ 23.937X ⫺ 0.508X2) was significant at P ⫽ 0.01.

rity, when growth stops and the seed is ready to germinate (Egli, 1998). High temperatures during seed filling frequently disrupt normal seed development, which increases the proportion of seeds that are shriveled and abnormal. The quality of these seeds is usually lower, sometimes much lower, than that of normal seeds (Spears et al., 1997). Abnormal and shriveled seeds were carefully removed from our seed lots before testing, so the reductions in accelerated-aging germination at high temperatures occurred in seeds that were free of physical injury and looked normal (visually free of imperfections). Apparently the processes responsible for dry matter and storage product accumulation in the seed must have proceeded relatively normally in the hightemperature environment. As temperature increased, however, the proportion of the seeds that germinated decreased. The decrease in accelerated-aging germination was larger than in standard germination, and the initial reductions in accelerated-aging germination were associated with the appearance of abnormal seedlings, but eventually dead seeds made a significant contribution in both cultivars (Fig. 5). This pattern suggests a gradual process of deterioration as temperatures increased. Exactly how high temperature stress reduced


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germination and vigor with no visible affects on seed development is not clear. But, stress effects in our experiments seemed to follow the classic deterioration model of seed in storage (Bryd and Delouche, 1971) in that vigor declined faster than standard germination as the seeds gradually lost the capacity to produce normal seedlings, culminating in a complete loss of viability. The deterioration caused by high temperature could represent the cumulative effect of stress throughout seed filling, or only as the seed matured. Keigley and Mullen (1986) found that standard germination decreased as the length of exposure to high temperature increased, but they did not isolate specific growth stages to evaluate their sensitivity. To determine if some stages of seed filling are more sensitive to high temperature stress than others, we evaluated the relationship between accelerated-aging germination and mean maximum temperature from the beginning (14 d after growth stage R5) or the end (14 d before growth stage R7) of seed filling for Hutcheson. Accelerated-aging germination was not related to the mean maximum temperature for the first 14 d of seed filling (data not shown). The relationship with maximum temperatures for the last 14 d was no better than for the mean of the entire filling period (Fig. 6). These results suggest that there may be critical periods during seed development when seeds are more sensitive to temperature, but the high autocorrelations of temperatures during seed development made it difficult to isolate effects during specific stages. Studies targeting specific stages of seed development in controlled environments are needed to resolve this matter. High temperatures during seed filling reduced seed vigor in controlled environments (Zanakis et al., 1994; Gibson and Mullen, 1996; Spears et al., 1997; TeKrony et al., 2000; Egli et al., 2005) and in our experiments in the field; but how likely is it that injurious temperatures will be a factor during seed production? An acceleratedaging germination of 80% can be taken as a minimum acceptable level for planting seed, as it provided acceptable field emergence in a wide range of field environments (Egli and TeKrony, 1995; 1996). Mean maximum temperature during seed filling must be below 31⬚C (88⬚F) in Hutcheson for accelerated-aging germination to be above 80% (Fig. 3 and 4). These critical temperatures agree very closely with estimates from experiments in controlled environments, where Spears et al. (1997) found that accelerated-aging germination of Hutcheson was reduced at 33⬚C, while others found reductions at 35 (Dornbos and Mullen, 1991) or 32⬚C (Zanakis et al., 1994) with other cultivars. Long-term mean maximum temperatures during September in most soybean producing regions of the USA are generally below 31⬚C. August maximum temperatures in the deep south and southwest (e.g., Arkansas, Louisiana, Mississippi, and Texas) are above the critical temperature, sometimes by as much as 4⬚C, while temperature in midsouth areas (e.g., Kentucky, Missouri, and Tennessee) are roughly equal to the critical temperature. Variation in July temperature is similar to the patterns described for August temperature. Mean maximum temperatures in the major soybean production areas in Argentina and Brazil

in January, February, and March are usually not above the critical temperatures. Thus, mean maximum temperatures in some areas, particularly in the U.S. deep south, are high enough to reduce seed quality of the cultivars in this study. In other areas, above-average temperatures are required. Evaluation of the response of many more cultivars is needed to determine the limitations of these conclusions. Delaying planting so that seed filling occurs later in the growing season when temperatures are lower or moving seed production to the northern (southern) limit of adaptation will reduce the chances of high-temperature injury and should result in the production of higherquality soybean seed. Standard germination was always less sensitive to high temperature stress than seed vigor, suggesting that seed lots produced in high-temperature environments could have acceptable or high levels of standard germination, but low vigor levels. Without a vigor test, such seed could lead to stand failure when planted in less-than-ideal conditions. High temperature stress during seed development and maturation should be added to the stresses (P. longicolla seed infection and physical injury) that reduce soybean seed germination and vigor. Evaluation of longterm temperature records suggests that temperature will, in some soybean production areas, routinely reduce seed vigor of some cultivars, although more information on cultivar differences and growth stage-specific effects is needed to precisely predict potential damage. Problems resulting from high temperature can probably be alleviated with proper management practices, but the extent of cultivar susceptibility will determine how often changes in management practices are justified. ACKNOWLEDGMENTS We thank Dr. Larry Heatherly, USDA-ARS, Crop Genetics and Production Unit, Stoneville, MS, for producing seed for this project in 2001 and 2002 and providing advice during preparation of the manuscript.

REFERENCES Association of Official Seed Analysts. 2001. Rules for testing seeds. Assoc. Official Seed Analysts, Las Cruces, NM. Association of Official Seed Analysts. 2002. Seed vigor testing handbook. no. 32. Assoc. Official Seed Analysts, Las Crucas, NM. Byrd, H.W., and J.C. Delouche. 1971. Deterioration of soybean seed in storage. Proc. Official Seed Analysts 61:41–57. Dornbos, D.L.J., and R.E. Mullen. 1991. Influence of stress during soybean seed fill on seed weight, germination and seedling growth rate. Can. J. Plant Sci. 35:373–383. Dornbos, D.L.J., R.E. Mullen, and R.M. Shibles. 1989. Drought stress effects during seed fill on soybean seed germination and vigor. Crop Sci. 29:476–480. Egli, D.B. 1998. Seed biology and the yield of grain crops. CAB Int., Wallingford, UK. Egli, D.B., and D.M. TeKrony. 1995. Soybean seed germination, vigor and field emergence. Seed Sci. Technol. 23:595–607. Egli, D.B., and D.M. TeKrony. 1996. Seedbed conditions and prediction of field emergence of soybean seed. J. Prod. Agric. 9:365–370. Egli, D.B., D.M. TeKrony, and J.F. Spears. 2005. High temperature stress and soybean seed quality: Stage of seed development. Seed Technol. (In press). Fehr, W.R., and C.E. Caviness. 1977. Stages of soybean development. Spec. Rep. 80. Iowa State Univ., Ames.



Gibson, L.R., and R.E. Mullen. 1996. Soybean seed quality reductions by high day and night temperature. Crop Sci. 36:1615–1619. Heatherly, L.G. 1993. Drought stress and irrigation effects on germination of harvested soybean seed. Crop Sci. 33:777–781. Judd, R., D.M. TeKrony, D.B. Egli, and G.M. White. 1982. Effect of freezing temperatures during soybean seed maturation on seed quality. Agron. J. 74:645–650. Keigley, P.J., and R.E. Mullen. 1986. Changes in soybean seed quality from high temperature during seed fill and maturation. Crop Sci. 26:1212–1216. Kmetz, K.T., A.F. Schmitthenner, and C.W. Ellett. 1978. Soybean seed decay: Prevalence of infection and symptom expression caused by Phomopsis sp., Diaporthe phaseolorum var. sojae and D. phaseolorum var. caulivora. Phytopathology 68:836–840. Smicklas, K.D., R.E. Mullen, R.E. Carlson, and A.D. Knapp. 1992. Soybean seed quality response to drought stress and pod position. Agron. J. 84:166–170. Spears, J.F., D.M. TeKrony, and D.B. Egli. 1997. Temperature during seed filling and soybean seed germination and vigour. Seed Sci. Technol. 25:233–244. TeKrony, D.M., D.B. Egli, J. Balles, L. Tomes, and R.E. Stuckey.

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1984. Effect of date of harvest maturity on soybean seed quality and Phomopsis sp. seed infection. Crop Sci. 24:189–193. TeKrony, D.M., D.B. Egli, and A.D. Phillips. 1980. Effect of field weathering on the viability and vigor of soybean seed. Agron. J. 72:749–753. TeKrony, D.M., D.B. Egli, and J.L. Spears. 2000. Seed quality and the early soybean production system. p. 45–57. In Proc. 30th Soybean Seed Res. Conf., Chicago, IL. 6–8 Dec. 2000. American Seed Trade Assoc., Alexandra, VA. TeKrony, D.M., D.B. Egli, and G.M. White. 1987. Seed production and technology. p. 295–353. In J.R. Wilcox (ed.) Soybeans: Improvement, production and uses. 2nd ed. Agron. Monogr. 16. ASA, CSSA, and SSSA, Madison, WI. Vieira, R.D., D.M. TeKrony, and D.B. Egli. 1991. Effect of drought stress on soybean seed germination and vigor. J. Seed Technol. 15:12–21. Vieira, R.D., D.M. TeKrony, and D.B. Egli. 1992. Effect of drought and defoliation stress in the field on soybean seed germination and vigor. Crop Sci. 32:471–475. Zanakis, G.N., R.H. Ellis, and R.J. Summerfield. 1994. A comparison of changes in vigour among three genotypes of soybean (Glycine max) during seed development and maturation in three temperature regimes. Exp. Agric. 30:157–170.