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Published in Crop Sci 39:1400-1406 (1999)
© 1999 Crop Science Society of America
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Crop Science 39:1400-1406 (1999)
© 1999 Crop Science Society of America

SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY

Survival Characteristics of Corn Seed during Storage

II. Rate of Seed Deterioration

Shande Tanga, Dennis M. TeKronya, Dennis B. Eglia and Paul L. Corneliusa

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

dtekrony{at}ca.uky.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
The Ellis-Roberts deterioration model depends on the assumption that the rate of seed deterioration is the same among seed lots of a crop species when stored in identical environments. The rate of deterioration of 11 corn (Zea mays L.) seed lots from six hybrids was measured during storage in various combinations of constant temperatures (20, 30, 40, and 50°C) and seed moisture contents (100, 120, 140, and 160 g kg-1, fresh weight basis). The seed-survival curves were constructed by conducting successive germination tests at frequent intervals during storage. The rates of seed deterioration were estimated by probit analysis and differences among seed lots were identified by analysis of variance. The deterioration rates were significantly different among seed lots in 16 of 21 storage environments when analyzing full data sets (all data points) and in 14 of 21 storage environments when using truncated data sets (germination percentages between 5 and 95%). Both genotype and initial seed quality affected the rate of deterioration with low-vigor seed lots generally deteriorating at a faster rate than high-vigor seed lots. The rate of deterioration was greatly influenced by storage environment and increased with an increase in storage temperature, seed moisture, or both. The assumption of a constant rate of seed deterioration in identical storage environments was not valid for hybrid corn seed.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
PREDICTING CORN SEED DETERIORATION would be extremely beneficial to seed producers, since seed is frequently stored from one year to the next. Such prediction is dependent on the quantitative relationship between the rate of seed deterioration, seed quality, and storage conditions.

During storage all seeds undergo deterioration, with the rate dependent on storage temperature, seed moisture, and crop species. Attempts to quantify the loss of seed germination during storage have given rise to several prediction equations including the equation (Eq. [1]) proposed by Ellis and Roberts (1980a).

(1)

In this equation v is the probit of germination (%), p is the period of storage (days), m is the moisture content (%, fresh weight basis) and t is temperature (°C). The Ki is the initial seed quality constant of a seed lot (on the probit scale) and KE, CW, CH, and CQ are constants having common values for all seed lots of a species. These constants quantify the deterioration response with respect to seed moisture content and temperature, and determine the slope of the seed-survival curve.

The equation has been applied to many species, including wheat (Triticum aestivum L.), rice (Oryza sativa L.), barley (Hordeum vulgare L.), onion (Allium cepa L.), field bean (Phaseolus vulgaris L.), and soybean [Glycine max (L.) Merrill], to predict seed viability or germination during storage, to analyze seed deterioration data, and to evaluate seed quality (potential longevity) during seed development and maturation (Ellis and Roberts, 1980a, 1980b, 1981; Ellis et al., 1982; Wilson et al., 1989; Ellis et al., 1990a, 1990b; Pieta Filho and Ellis, 1992; TeKrony et al., 1993; Fabrizius et al., 1999).

The viability equation (Eq. [1]) is actually a composite of two separate equations. The first (Eq. [2]) describes the seed-survival curve in terms of the viability ({nu}, probit percentage viability) expected after a given storage period (p, days).

(2)

The assumption is that quality differences between seed lots do not affect {sigma} (standard deviation of the frequency distribution of seed-survival times), but are accounted for by Ki. In contrast, the storage environment has no effect on Ki, and only affects {sigma}. The exponent of 10 in Eq. 1 is the common logarithm of {sigma}. The relationship between {sigma} and constant storage environments is described by Eq. [3].

(3)

The {sigma} can also be perceived as the time required for viability to fall by one probit unit (97.7–84.1% or 84.1–50%). Thus, it is a measure of the longevity of seeds.

Once the seed-survival curve is available, a standard deviation ({sigma}) can be estimated by fitting Eq. [2] as a model. The reciprocal of {sigma} (1/{sigma}) is the rate of seed deterioration in a given storage environment. If plotted, parallel lines should be found among seed lots of a species on the basis of the assumption that all seed lots deteriorate at the same rate irrespective of initial seed quality. Because it is assumed that {sigma} is dependent only on seed moisture content and storage temperature, the four constants in Eq. [3] can be obtained from a series of storage experiments conducted across a wide range of constant temperatures and seed moisture contents (Ellis and Roberts, 1980a). Species constants can be determined from one seed lot; however, several typical seed lots representing a wide range of genotypes and vigor levels would be more desirable (Ellis et al., 1990a).

Although the survival of different seed lots within a species may differ when stored under identical conditions, Eq. [1] is based on the assumption that seed-survival curves are symmetrical sigmoids and follow normal distributions that have the same standard deviation in any given combination of temperature and seed moisture content (Ellis et al., 1990a). The fact that seed-survival curves fall on a normal distribution has been confirmed for many crops by visual examination of seed-survival curves (Ellis and Roberts, 1980b, 1981; Ellis et al., 1982) and by statistical tests for hybrid corn seed (Tang et al., 1999). A more important prerequisite for the use of Eq. [1] is validation of the assumption of the same rate of seed deterioration among seed lots within a species when stored in an identical environment. If this assumption is valid, the constants in Eq. [3] can be determined. Otherwise, the constants will not be universal for all seed lots in a species (Ellis and Roberts, 1980a, 1981).

Considerable evidence is available for several crop species supporting the assumption that all seed lots within a given species deteriorated at the same rate when stored in the same environment (Roberts, 1963; Ellis and Roberts, 1980a, 1981; Ellis et al., 1982; Kraak and Vos, 1987; Parkes et al., 1990). Seed of six rice cultivars deteriorated at the same rate in identical storage environments (Roberts, 1963) as did seed of three corn genotypes, although their longevity differed greatly (Ellis and Roberts, 1981). Results of studies with barley, onion, cabbage (Brassica oleracea L.) and soybean seed (Ellis and Roberts, 1980a, 1981; Ellis et al., 1982) were also consistent with this assumption.

In contrast, two recent reports with six corn (Bruggink, 1989) and 16 soybean seed lots (Fabrizius et al., 1999) suggest that seed lots do not always deteriorate at the same rate in identical storage environments. Likewise, primed lettuce (Latuca sativa L.) and tomato (Lycopersicon esculentum Mill.) seed did not deteriorate at the same rate as unprimed seed (Argerich et al., 1989; Tarquis and Bradford, 1992). Such apparently contradictory results, together with occasional anomalies observed by Ellis and Roberts (1981) and Ellis et al. (1982) have raised concerns regarding the assumption of a constant rate of seed deterioration underlying Eq. [1] (Bruggink, 1989; Fabrizius et al., 1999).

Although hybrid corn seed is routinely carried over in storage, there has been little attempt to predict when reductions in germination will occur. Likewise, there have been few studies to determine the rate of deterioration of seed of corn hybrids during storage. The objectives of this investigation were (i) to determine whether corn seed lots deteriorate at the same rate in identical storage environments and (ii) to evaluate the influence of initial seed vigor and storage environment on rate of corn seed deterioration.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Three storage experiments were conducted in several combinations of constant temperatures ranging from 20 to 50°C and seed moisture contents ranging from 100 to 160 g kg-1 using 11 corn seed lots (six hybrids) as described by Tang et al. (1999). The initial germination of these seed lots was high (>=89%) and there was a range in vigor levels (high, medium, and low), as determined by accelerated aging and cold test germination (Tang et al., 1999). Seed samples conditioned to desired moisture contents were heat-sealed into aluminum foil packets and stored in an incubator (long-term storage) at 20 and 30°C (± 1.0°C) or a water bath (short-term storage) at 40 and 50°C (± 0.5°C). Survival of the seeds was determined by removing samples from storage at regular intervals and determining standard germination as described by ISTA (1993). A completely randomized design was used with two replications of each seed lot in each storage environment. The survival data from three experiments were used to evaluate the rate of seed deterioration among corn seed lots and storage environments.

The three experiments were conducted at different times with slightly different procedures of moisture adjustment and equilibration (Tang et al., 1999). In Exp. 1, four seed lots (Lots 1–4; Tang et al., 1999) representing four hybrids were stored in various combinations of constant temperatures (20, 30, 40, and 50°C) and seed moisture contents (100, 120, 140, and 160 g kg-1, fresh weight basis). Each seed lot was first divided into two replications and conditioned to the four moisture levels at different times. Experiment 2 evaluated five seed lots (Lots 1–5; Tang et al., 1999) at 40°C and 160 g kg-1 moisture content, while Exp. 3 included six seed lots (Lots 6–11 including four from one hybrid and two from a second hybrid; Tang et al., 1999) in a three by three factorial arrangement of temperature (30, 40, and 50°C) and seed moisture content (120, 140, and 160 g kg-1). In Exp. 2 and 3, each seed lot was conditioned to the desired moisture level before dividing it into two replications.

Seed deterioration rates were calculated for each storage environment using probit analysis (Finney, 1971) of full (all germination data) and truncated (included only observations between 95 and 5% germination) data sets. The PROBIT procedure (SAS Institute, 1988) provided slopes and intercepts for each probit survival curve where the slope is an estimate of the reciprocal of the standard deviation of the survival curve, 1/{sigma}, (the rate of seed deterioration).

After the slopes (1/{sigma}) derived from probit analysis were available for each seed lot and storage environment, analysis of variance and regression analysis were used to determine if there were significant differences in rate of seed deterioration among seed lots. The GLM procedure of SAS (1988), the protected LSD procedure, and regression analysis were used. Since the three experiments were conducted with slightly different procedures of moisture adjustment and equilibration, small differences in seed moisture content among experiments were observed; accordingly, each experiment was analyzed separately.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Rate of Corn Seed Deterioration
Previous analysis of the survival curves from these experiments confirmed that they were generally normally or near-normally distributed (Tang et al., 1999). Thus, it was appropriate to use probit analysis to determine the standard deviation of seed deaths in time, {sigma} according to Eq. [2].

Visual examination of the survival curves in Fig. 1 (Exp. 3) indicated that seed lots did not deteriorate at the same rate in these identical constant storage environments (i.e., the lines were not parallel) as hypothesized for Eq. [2] (Ellis and Roberts, 1981). Lot 10 usually had the steepest slope and deteriorated more rapidly than Lots 8, 9, and 11 in all environments. Despite a substantial difference in slopes among all seed lots, some seed lots (Lots 6, 7, and 10 or Lots 8, 9, and 11) had the same or similar slopes (Fig. 1). The differences in rate of deterioration of these six seed lots with the full data sets (Fig. 1) also occurred with the truncated data sets (Fig. 2) .



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  		Fig. 1 Deterioration of six corn seed lots (average of two replications, Exp. 3) stored in six environments (full data set). The scale on the x-axis is not constant, reflecting the large differences in rate of deterioration across storage environments. Lots 6 and 7 (low initial vigor) and 8 and 9 (high initial vigor) are from hybrid CF860, while Lots 10 (high initial vigor) and 11 (medium initial vigor) are from hybrid 63MVP

 


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  		Fig. 2 Deterioration of six corn seed lots (average of two replications, Exp. 3) stored in six environments (truncated data set). The scale on the x-axis is not constant, reflecting the large differences in the rate of deterioration across storage environments. Lots 6 and 7 (low initial vigor) and 8 and 9 (high initial vigor) are from hybrid CF860, while Lots 10 (high initial vigor) and 11 (medium initial vigor) are from hybrid 63MVP

 
Analysis of variance of the slope confirmed that differences in rates of deterioration among seed lots within the same environment were statistically significant (P < 0.05) in 16 of the 21 storage environments for the full data set and 14 of 21 environments for the truncated data set (Table 1) . Significant differences occurred among seed lots of all five hybrids included in Exp. 1 and 2 (data not shown). Modifying the procedure of moisture adjustment and equilibration to reduce variability among replications and adjusting sampling intervals on the basis of initial seed vigor and storage environment to increase the number of samples in Exp. 3 generally resulted in smaller coefficients of variation (CV) than in Exp. 1, which facilitated the detection of differences among seed lots (Table 1). The six seed lots in Exp. 3 (Fig. 1 and 2) had significantly different seed deterioration rates (Table 1) in all eight storage environments for moisture contents ranging from 120 to 160 g kg-1 and temperatures ranging from 30 to 50°C. These significant differences in seed deterioration rate persisted among the four seed lots (Lots 8, 9, 10, 11; Tang et al., 1999) with high or medium initial vigor for both full and truncated data sets (Table 1). Significant differences (P < 0.05) among seed lots (across four hybrids) were also found in eight of the 13 storage environments for full and truncated data sets in Exp. 1 and 2 (Table 1). Most of the nonsignificant differences in the rate of seed deterioration were associated with relatively large CVs.


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  		Table 1 Results of the analysis of variance of the rate of corn seed deterioration (1/{sigma}) of seed lots stored in 21 environments across three experiments for the full and truncated (germination <95 and >5%) data sets

 
Seed Characteristics and Storage Environment
The differences in deterioration rates among seed lots in the same environment and across storage environments were relatively large (Table 2) . For example, at 40°C and 140 g kg-1 moisture content, Lot 6 had a rate of 0.3731 probits d-1 but the rate for Lot 11 was only 0.1403 probits d-1. There were significant differences in the rate of deterioration among hybrids (Table 2). Seed lots with high initial seed vigor (Tang et al., 1999) but from different hybrids had variable deterioration rates, e.g., Lot 10 (63MVP) had a deterioration rate as much as 1.5 times higher than Lot 1 (A6102) and Lot 8 (CF860). A large difference in deterioration rate was also observed between Lots 2 and 3, both of which had medium initial vigor levels, but came from different hybrids (H3315 and P3417, respectively) (Table 2).


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  		Table 2 Mean rate of corn seed deterioration (1/{sigma}) for 10 seed lots from five hybrids stored under 11 storage environments (full data set){dagger}

 
The differences in rate of seed deterioration among Lots 6 to 9 from the same hybrid (CF860) revealed that initial vigor level also played a primary role in determining the rate of seed deterioration. Lots 6 and 7 (low vigor) had deterioration rates approximately two times higher than Lots 8 and 9 (high vigor) (Table 2).

The rates of seed deterioration from truncated data sets were similar to those from the full data analysis for most of the 10 seed lots in the majority of the environments (Table 3) . The relative differences in the rate of seed deterioration among seed lots generally did not change between the two data sets, although a substantial difference occurred in a few cases. For example, the rates of deterioration for the truncated data were much higher for two high vigor seed lots, Lot 10, stored at 40°C and 120 g kg-1 and 50°C and 120 g kg-1, and Lot 4, stored in most environments (Table 3). In general, the conclusions regarding the effect of genotype and seed vigor on corn seed deterioration rates based on analysis of full data sets (Table 2) are in good agreement with those based on analysis of truncated data sets (Table 3).


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  		Table 3 Mean rate of corn seed deterioration (1/{sigma}) for 10 seed lots from five hybrids stored under 11 similar storage environments (truncated data set){dagger}

 
The rate of deterioration was less in the lower moisture content and lower temperature environments (Tables 2 and 3). The rate of seed deterioration at 50°C, 140 g kg-1 was consistently much higher than the rates at 30 or 40°C , 140 g kg-1 for all seed lots. The rate also increased with increasing seed moisture content. Although seed lots had different rates, the effects of temperature and moisture were consistent across seed lots (Tables 2 and 3).

An exception to the effect of increasing moisture content on the rate of deterioration occurred in the high temperature regimes (40 and 50°C ), where an increase in moisture content from 140 to 160 g kg-1 did not always increase deterioration rate and sometimes slowed seed deterioration. The rates at 40°C and 140 g kg-1 moisture content were only marginally different from the rates at 40°C and 160 g kg-1 moisture content (Fig. 1, Tables 2 and 3). The rates at 50°C and 160 g kg-1 moisture content were lower for six of the eight seed lots than rates at 50°C and 140 g kg-1 moisture content (Tables 2 and 3).


   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
An important assumption of the Ellis-Roberts deterioration model (Eq. [1])—that all seed lots within a species deteriorate at the same rate—was not valid for all hybrid corn seed lots evaluated in this study. Significant differences in rates of seed deterioration among seed lots were found in 16 of 21 storage environments with full data sets and in 14 of the 21 storage environments when using truncated data sets (Table 1). These variable rates of deterioration occurred with uniform moisture content among seed lots and seed samples (e.g., the range in average seed moisture contents for Lots 6–11 at three moisture levels was 122–124, 141–144, and 162–163 g kg-1) in Exp. 3, and the differences in the rate of deterioration were not explained by small differences in moisture content in Exp. 1. For example, Lot 3, with a measured seed moisture content of 140 g kg-1, deteriorated at nearly twice the rate of Lot 2 (139 g kg-1 moisture content) stored in 40°C . Likewise, Lot 3 (158 g kg-1) had a rate of 0.0926 probits d-1 compared with a rate of 0.0274 probits d-1 for Lot 1 (159 g kg-1) stored in 30°C . Thus, there were real differences in rate of corn seed deterioration when stored in the same environments, which could not be attributed to minor differences in moisture content as suggested by Pieta Filho (1991) for wheat. The differences in the rate of deterioration were comparatively large in some cases, which would result in dramatically different estimates of seed longevity.

Although the fit of seed-survival curves to normal distribution was significantly improved using truncated data sets (Tang et al., 1999), the rate of deterioration for each seed lot from truncated data was similar to the rate from the full data set in most storage environments. This is not surprising since the fit of the two normal curves was similar and most data points were between 95 and 5% germination.

Our finding that the rates of corn seed deterioration differed significantly among seed lots is in agreement with reports of Bruggink (1989) for corn, Pieta Filho (1991) for wheat, Argerich et al. (1989) for tomato, Tarquis and Bradford (1992) for lettuce, and Fabrizius et al. (1999) for soybean, but they do not agree with other reports of constant deterioration rates for corn (Ellis and Roberts, 1981) and other species, including barley, wheat, rice, and soybean (Ellis and Roberts, 1980b, 1980c).

Since seed longevity is strongly affected by seed quality, we wondered if the deterioration rate may also be influenced by initial seed quality and possibly by genotype. There were differences in seed deterioration rate among hybrids in this study that had similar initial (high or low) vigor levels. Likewise, low-quality seed lots (Lots 6 and 7) almost always tended to deteriorate faster than high-quality seed lots within the same genotype (Tables 2 and 3). This suggests that genotypic background or initial vigor level might control corn seed deterioration, and low-vigor seed lots may have less resistance to stress environments. Thus, the difference in seed longevity among corn seed lots may result from a difference in seed deterioration rate, initial seed quality, or both.

Seed deterioration rate was dramatically changed by altering temperature and moisture content; however, the relative differences in rate of deterioration among seed lots was maintained (Fig. 2 and Table 2). Quantitative relationships between seed longevity and both moisture content and temperature have been extensively investigated (Ellis and Roberts, 1980a, 1980b, 1980c; Ellis et al., 1982). In this study, increasing moisture content of corn seed resulted in higher deterioration rates, except when it increased from 140 to 160 g kg-1 at 40 and 50°C. Similar observations were reported by Ibrahim and Roberts (1983) for hermetically stored lettuce seeds across a range of moisture contents from 150 to 550 g kg-1 and by Tompsett (1984) for New Caledonia pine [Araucaria columnaris (Forst) Hook] seed. Possible explanations include seed cellular ultrastructure repair and limited O2 supply in hermetically stored seeds because of high moisture (Roberts and Ellis, 1982; Tompsett, 1984).

Since the assumption of the same rate of seed deterioration for all seed lots stored in an identical environment used to support Eq. [1] was not valid for hybrid corn seed, it was not possible to develop valid estimates of the viability constants, KE, CW, CH, and CQ of Eq. [1], as has been done for many other crop species (Ellis and Roberts, 1980a, 1980b, 1981; Ellis et al., 1982). Constants derived from data from one seed lot or a few seed lots that happened to have the same deterioration rate would not apply to other seed lots with different rates. Therefore, we conclude that the improved viability equation (Eq. [1]) cannot be used to predict hybrid corn seed deterioration. An alternative model, which does not depend on a constant rate of seed deterioration across seed lots, has been developed to predict corn seed germination (Tang, 1998).

In summary, the assumption of a constant rate of seed deterioration for all seed lots within a species stored in an identical environment was not valid for hybrid corn seed. Visual differences in the rate of deterioration were confirmed by analysis of variance (ANOVA) demonstrating that rates were significantly different among seed lots in 16 (full data set) and 14 (truncated data set) of 21 storage environments. Both genotype and vigor level affected the rate of seed deterioration with low initial quality seed lots deteriorating much faster than high initial quality seed lots. The storage conditions also strongly affected the rate of corn seed deterioration, with the rate increasing as storage temperature, seed moisture, or both increased.International Seed Testing Association 1993; SAS Institute 1988


   ACKNOWLEDGMENTS
 
This research was supported in part by a grant from the American Seed Research Foundation. The authors would like to thank Ms. Marcy Rucker for helping conduct Exp. 1. We want to thank Adler's Seeds, Sharpsville, IN; Beck's Superior Hybrids, Atlanta, IN; Caverndale Farms, Danville, KY; Huber Seeds, West Lebanon, IN; and Pioneer Hi-Bred International Inc., Johnston, IA for supplying the seed lots used in these experiments.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Contribution from Kentucky Agric. Exp. Stn. no. 98-06-121.

Received for publication August 11, 1998.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 




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