Effect of different temperatures on seed quality parameters during storage in Soybean (Glycine max (L) Merril)

Introduction

Soybean seed especially the breeder and foundation seed generally stored at cold storage conditions (15⁰C and R.H. 45%) after processing but under informal systems, the seed produced is stored under ambient conditions only without any control over temperature and relative humidity which had a significant effect on seed quality. Production of good quality soybean seeds has always been one of the main challenges in the seed distribution system [1]. It is highly difficult to produce and store the seeds and to maintain the quality of a determined seed lot under ambient conditions [2,3]. The post-harvest storage is important especially in crops like soybean where variations in temperature and relative humidity have negative influences on seed quality [4, 5]. If the storage conditions are not optimum the soybean will lose its activity and in turn result in poor seed quality.

Among the various physiological attributes, vigour has an important role in expressing the physiological potential of the seed, and is associated with essential seed function characteristics such as longevity, germination, rapid and uniform emergence, as well as tolerance to environmental adversity [2, 6]. The evaluation of seedling performance can provide useful results regarding seed quality considering their ability to manifest damage in some points of the production chain such as that generated at harvest and during the processing phase [7, 8].

           For efficient storage not only good initial quality seed is necessary to avoid loss of physiological quality such seed will also be stored under ideal conditions under optimum temperatures below 20⁰C and relative humidity below 60% [9, 10,11]. If proper care is not taken while drying and storage they will harm seed quality [12, 13].

                  Drying soybean seeds at temperatures of 45⁰C and 55⁰C directly affects the viability, germination and vigour of soybean seeds with a further negative impact on the storage potential of the material. Under temperatures greater than 20⁰C and relative humidities that are not controlled will reduce the potential of seed stored [14]. Storage time will also affect the seed quality and increases seed deterioration [15, 16]. In the present study the JS 335 and Basara seed were stored at cold storage conditions (15⁰C and 45% RH), 30⁰C and 40⁰C temperatures (seed stored in two different incubators) and at ambient conditions where there is no control over temperature and relative humidity. The study is conducted to evaluate the effect of different storage temperatures on soybean seeds stored in jute bags and evaluated how the temperatures will affect the physiological quality of soybean seeds the study was taken up for two consecutive years.

B. Material and methods                                                           

The experiment was carried out at Seed Testing Laboratory of Department of Seed Science and Technology, Seed Research and Technology Centre, PJTSAU, Hyderabad for two consecutive years 2020-21 and 2021-22. The seed quality parameters about percent normal seedlings, percent abnormal seedlings, percent dead seeds, Moisture percentage, Seedling vigour index-II and Electrical conductivity were tested using the following methods.

i. Seed Germination (%)

The germination test was conducted as per the ISTA rules [17] by adopting between paper method (BP). Each replication with 100 seeds for a treatment were used for the germination test that was placed in seed germinator and maintained at a constant temperature of 25 ± 2 ℃ and high humidity. On the day of the final count i.e., 7^th day, the number of seeds germinated was counted and the per cent normal seedlings, abnormal seedlings and dead seed were calculated as follows:

Number of normal seedlings

Seed germination (%) =                                                     × 100

Total number of seeds planted

ii. Seedling Dry Weight (mg)

Ten normal seedlings were placed in butter paper bags. These were placed in a hot air oven maintained at 80 ± 1 ℃ for 24 h. After completion of the drying period, these seedlings were kept in a desiccator for cooling. The weight of dried seedlings was recorded mean weight was calculated per seedling and expressed in milligrams per seedling.

iii. Seedling Vigour Index ΙΙ

The seedling vigour indices were calculated as per the method suggested by [18] as given below and expressed in whole numbers.        

 Seedling vigour index ΙΙ = Germination (%) × Seedling dry weight (mg)

iv. Electrical Conductivity (µS cm^-1g^-1)

Electrical conductivity was measured by following the procedure mentioned in Seed Testing Rules [17]. Three replicates of 50 seeds each drawn randomly from pure seed fraction was weighed to two decimal places. Cleaned conical flasks were used to not affect the conductivity of the samples. We have added 250 ml of distilled water with a conductivity of water less than 5 µS cm^-1 to the containers and covered them with aluminum foils to avoid contamination. The containers were placed at 20 ± 2 ℃ for 18-24 h prior to placing the seeds in the water. Two containers filled with only distilled water were used as control. The seed samples were weighed and placed into the prepared containers and swirled gently to completely immerse all the seeds. Each container was covered and placed at 20 ± 2 ℃ for 24 h in an incubator. After 24 h, the containers were swirled gently to mix the leachates and the conductivity was measured using a conductivity meter. The conductivity of the samples was calculated by the following formula:

                                          Conductivity reading (µS cm^-1) – background reading

Electrical conductivity =    ————————————————————

 (µS cm^-1g^-1)                                                    Weight of replicate (g)

v. Seed moisture content (%)

The moisture content of seed was determined by the hot air oven method as per ISTA rules. Five grams of coarsely ground seed material from each treatment in four replications were dried in a hot air oven maintained at a temperature of 103⁰C for seventeen hours. Then samples were cooled in a desiccator for one hour and the moisture content was determined by using the formula given below and expressed in percentage.

This was calculated by the following formula.

                                                    M₂ – M₃

Moisture content (%) =   ————– x 100

                                         M₂ – M₁ 

 Where,

 M₁ = Weight of the metal container along with the lid in grams

 M₂ = Weight of the metal container along with the lid and the sample before drying in grams

M₃ = Weight of the metal container along with lid and the sample after drying in grams

3. Results and Discussion

i. Per cent normal seedlings

The two years data on the interaction effects of temperature with soybean varieties Basara and JS 335 stored for 8 months duration for per cent normal seedlings is depicted in Table.1. There is no significant difference observed for normal seedlings per cent in both JS 335 and Basara varieties stored at 30⁰C and ambient conditions with the Pr (> F) value of 0.0265 after 2 months of storage. While, the interaction effects of year with varieties significant difference is observed in Basara variety with the Pr (> F) value of 0.0377 (Table. 1). After 4 months of storage no significant difference is observed in Basara seed stored at 30⁰C and ambient conditions while JS 335 recorded significant difference at all temperatures with Pr (> F) value of 0.0001. For years x variety interaction Basara and JS 335 showed significant difference with Pr (> F) value of 0.0256. After 6 months of storage JS335 seed recorded no difference in the per cent normal seedlings stored at 30⁰C and 40⁰C but Basara has shown a significant differences at all temperatures with Pr (> F) value of 0.0001and for years x variety interaction both varieties showed significant difference with Pr (> F) value of 0.0299. After 8 months of storage JS335 and Basara showed no difference in the per cent normal seedlings while for years x variety interaction Basara and JS 335 showed significant difference with Pr (> F) value of 0.0198 (Table.1, Fig.1a & 1b). The results were following [19, 11, 20]  who observed that climate-controlled storage at lower temperatures is a suitable method for maintaining seed quality in soybean seed and storage environment will play an important role and beneficial in providing better results in a germination test. Several authors have verified the germination rates of soybean seeds during six months of storage in artificially cooled conditions [19, 21, 11, 22, 23, 24, 25 and 26]. However, [27] found that drying at low air temperatures reduced the germination effects of storage in a non-conditioned environment for six months.

ii. Per cent abnormal seedling

The interaction effect of different temperatures on varieties Basara and JS 335 seeds for per cent abnormal seedlings is presented in Table.2. After 2 months of storage there is no significant difference observed in JS 335 seed stored at 40⁰C and ambient conditions, in Basara, seed stored at 30⁰C and ambient conditions while for the interaction effect of years with varieties there is no significant difference with Pr(> F) value of 0.6094. After 4 months of storage no significant difference is recorded in Basara seed stored at 30⁰C and ambient conditions while in JS335 no significant difference is observed for % abnormal seedlings in at 30⁰C, 40⁰C and ambient conditions. For years x variety interaction significant difference is recorded with Pr (> F) value of 0.0059. After 6 months of storage no significant difference is recorded in Basara kept at 30⁰C and ambient temperatures but in JS335 at 30⁰C and 40⁰C and ambient conditions while for years x variety interaction revealed a significant difference with Pr(> F) value of 0.0136. After 8 months of storage no significant difference is observed in seed stored at 30⁰C, 40⁰C and ambient conditions in both the varieties while for years x variety interaction showed a significant difference with Pr(> F) value of  0.0150 (Table.2, Fig.2a & 2b). [11] found that soybean seeds stored for 180 days maintained their initial germination rate when stored at 20⁰C, but at 27⁰C they suffered reductions in germination and losses in the commercial pattern. According to [20], soybean seeds with an initial 94% germination stored for 225 days in an air-conditioned environment at 20⁰C had a reduced germination rate of 91%, while those stored in a non-conditioned environment were reduced to 84% germination. Soybean seeds stored in environments with ambient temperatures had accelerated deterioration over the storage period.

iii. Per cent dead seeds

There is no significant difference in Basara seeds stored at 40⁰C and 30⁰C for per cent dead seeds at 2 months of storage while a significant difference is recorded in JS335 at all temperatures while for the interaction effect of years with varieties also recorded significant difference with Pr (> F) value 0.0013. After 4 months of storage no significant difference is observed in Basara seed stored at 30⁰C, 40⁰C and ambient conditions but in JS 335 significant difference is recorded at all temperatures with Pr (> F) value of 0.0295. After 6 months of storage no significant difference is observed in per cent dead seeds stored at 30⁰C and ambient temperatures in Basara but in JS335 no significant difference was recorded for the seeds stored at 30⁰C, 40⁰C and ambient conditions whereas for years x variety interaction both varieties showed significant difference with Pr (> F) value 0.0050. After 8 months of storage, no difference observed in the per cent dead seeds of soybean stored at 30⁰C and ambient conditions in JS 335 whereas for years x variety interaction significant difference was observed in both the varieties (Table.3& Fig.3a, 3b). The effect of different drying temperatures on the physiological quality of seeds was also observed by [28] in sweet sorghum seeds, where in a reduction in germination was observed especially at temperatures above 40⁰C each increase in the degree of temperature led to reduction of half germination percentage. The deleterious effect portraying the damage caused by drying in soybean seeds was also observed by [29]. Upon finding that the use of drying temperatures higher than 40⁰C led to cell damage, such as membrane disarray and leaching of solutes, mainly in the region of the embryonic axis, harmful to the development of seedlings and thus reducing the two main physiological properties of germination and vigour.

iv. Moisture percentage

No significant difference is recorded in Basara seeds stored at 40⁰C, ambient conditions and 30⁰C for moisture per cent and for JS 335 no significant difference observed in seed stored at 30⁰C and 40⁰C whereas, the interaction effects of years with varieties has recorded no significant difference after 2 months of storage with Pr (> F) value 0.4026. After 4 months of storage, Basara seed recorded significant difference for moisture percentage at all temperature conditions while JS 335 showed no significant difference in the seeds stored at 30⁰C and ambient conditions. For years x variety interaction has showed no significant difference with Pr (> F) value 0.1755. After 6 months of storage, Basara seed showed no significant difference for moisture percentage but JS335 has shown no significant difference in the seeds stored at 30⁰C & 40⁰C and 40⁰C & ambient conditions whereas for years x variety interaction both the varieties showed no significant difference. After 8 months of storage JS335 recorded no difference in the per cent moisture in the seed stored at 30⁰C and 40⁰C while for years x variety interaction both the varieties showed no significant difference with the Pr (> F) value 0.0565 (Table.4, Fig.4a & 4b).

v. Seedling vigour index-II

After 2 months of storage, there is a significant difference observed for the seedling vigour index-II in Basara seeds but in JS 335 no significant difference is recorded in the seed stored at 30⁰C, 40⁰C and ambient temperatures whereas, for the interaction effect of years with varieties has recorded significant difference with Pr (> F) value of 0.0267. After 4 & 6 months of storage no significant difference is observed in Basara seed stored at 30⁰C and ambient conditions while JS 335 has shown a significant difference at all temperatures whereas, for years x variety interaction significant difference is observed at all temperatures for seedling vigour index-II. After 8 months of storage JS335 and Basara had shown significant differences in the seedling vigour index-II of soybean seed stored at all temperatures whereas for years x variety interaction Basara and JS 335 showed a significant difference with Pr (> F) value of 0.0001 (Table.5, Fig.5a & 5b). [30]Garcia et al. (2004) observed that thermal damages might not manifest immediate effects on germination however, after a period of storage; seed vigour will undergo considerable reductions. [31] noted that analysis of seedling growth and other complementary tests compose a vigour index for evaluation of soybean seed quality and results inferred that the deleterious effects of high temperatures during drying and storage were more expressive on the growth of the shoots and roots.

Vi. Electrical Conductivity

There is no significant difference observed in Basara seed stored at 30⁰C & 40⁰C whereas, JS335 recorded significant difference for electrical conductivity at all temperatures after 2 months of storage while years x variety interactions significant difference is observed at all temperatures with Pr (> F) value of 0.0076. After 4 months of storage no significant difference was observed in Basara seed stored at 30⁰C and ambient conditions while JS 335 has recorded significant difference at all temperatures while for years x variety interaction both the varieties showed significant differences. After 6 months of storage Basara has recorded no significant difference in the seed stored at 30⁰C and ambient conditions while JS 335 has shown a significant difference among all the temperatures whereas, for years x variety interaction both the varieties showed significant difference with Pr (> F) value of 0.024. After 8 months of storage no significant difference is observed in JS 335 seed stored at 30⁰C and ambient conditions but Basara has recorded a significant difference at all temperatures with Pr (> F) value of 0.0314. [18, 10] observed lower electrical conductivity in seeds stored in a refrigerated environment than those stored without climate control. [ 32, 33, 34] found that soybean seeds showed increased electrical conductivity values over time of storage. [23] found that soybean seeds stored in Kraft paper packaging had lower values of electrical conductivity than those in uncooled conditions. According to [22], soybean seed lots with high vigour have high electrical conductivity values below 80 mS cm/1/g. [33] observed that soybean seeds had higher values of electrical conductivity after 180 days of storage.

vii. Correlation analysis

 The correlation analysis showed significant and positive correlation with the majority of seed quality traits (Fig 7). The highest positive correlation is observed between germination per cent and seedling vigour index-II (r = 0.90**) and also between dead seeds and electrical conductivity (r = 0.91**). Significantly positive correlations were observed between abnormal seedlings with dead seeds (r = 0.63**) and moisture per cent (r = 0.85**) also.

viii. Principal component analysis

The principal component analysis has shown that the biplots for PC2 and PC3 had revealed the germination percentage i.e., normal seedlings (28%) and seedling vigour index-II (33%) have contributed more towards the seed quality (Fig.8).

Conclusions

The increase in the storage temperatures above 40⁰C affects seedling performance and in turn the effect is accentuated over the time of storage. Cold storage or ambient storage conditions can be preferred for storage up to 180 days under storage conditions to maintain good seed quality.

Acknowledgements:

The authors are thankful to Professor Jayashankar Telangana State Agricultural University, Rajendranagar, Hyderabad for providing support for doing the research work.

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