Introduction

In many crop species, biotic and abiotic stresses, including diseases and insects, extreme temperatures, soil crusting, excess or limitations of water, and salinity, may individually or in combination adversely affect the germination process, leading to reductions in the rate and final percentage of germination, poor stand establishment, and low crop yield (Ashraf and Foolad, 2005). In horticultural crop production, maximizing crop productivity necessitates the use of seeds that exhibit rapid, uniform germination and strong seedling growth rates. This requirement is particularly vital in the seedling industry, where there is a focus on producing standardized, high-quality seedlings. The use of high-vigor seeds is essential to decreasing labor costs and boosting the efficiency of seedling production (Korkmaz et al., 2004; Nelson, and Sharples, 1980).

Water stress has been shown to adversely affect growth, nutrition and fruit yield of watermelon cultivars (Kaya et al., 2003). Recently, seed priming technology has been used to enhance seedling emergence rates by allowing seeds to absorb a controlled amount of moisture, thus initiating the germination process prior to sowing, and then drying the seeds back to their initial moisture content (Korkmaz et al., 2004; Woodstock, 1988). Solid matrix priming (SMP) is a specific priming technique that enhances seed vitality by controlling water absorption through the matric potential of a solid carrier material, unlike osmo-priming, which uses liquid solutions for water absorption and requires a subsequent cleaning process. The carriers used in SMP are harmless to germination (Kim et al., 1998; Taylor et al., 1988). Seeds treated with SMP, which show enhanced metabolic activity, demonstrate improved seedling rates under adverse environmental conditions such as low temperatures, low humidity, high temperatures, salinity, and pathogen contamination. In osmotic priming, salts or non-penetrating organic solutes are used to establish equilibrium water potential between seed and the osmotic medium needed for conditioning (Khan, 2010). In addition, priming is an important mechanism of various induced resistance phenomena in plants (Beckers and Conrath, 2007; Mereddy, 2000).

Recently, horticultural crop seeds that are typically challenging to germinate have been prepared for sowing by absorbing a controlled amount of moisture to advance the germination process. These seeds are then dried back to their pre-treatment moisture content, which enhances seedling emergence rates and results in uniform seedling growth (Bradford, 1986; Kang et al., 1996; Khan, 1992).

This study aims to examine the effects of SMP treatment on watermelon seeds to enhance germination performance under water stress conditions, and to compare the effect of osmotic priming (OP) and SMP.

Materials and Methods

1. Materials used and germination test

The watermelon seed varieties used in this experiment included Haechanggual, Ggualnara (from Nonghyup Seeds, Gyeonggi-do Anseong-si Gongdo-eup, Republic of Korea), Hyundaeggual and Hwalgichan (from Hyundai Seeds, Gyeonggi-do Yeoju-si Ganam-eup, Republic of Korea). These seeds were collected in February 2014 and stored at 5°C in a cold storage facility until they were used in the experiment.

The seeds were germinated at 30°C in an incubator with three replicates, and the average germination rate was recorded. The germination test involved a 12-hour irradiation period until the 14th day. Germination was considered successful when the root penetrated the seed coat and extended more than 1 mm. All procedures followed the International Seed Testing Association (ISTA) rules (1999). The time required for 50% of the seeds to germinate (T₅₀) was calculated using the method described by (Coolbear et al., 1987).

The seed vitality test employed the Between Paper (BP) method. BP test papers, both heavy and regular, were moistened with distilled water and stored at 25°C for one day. Thirty seeds were then evenly spaced on the heavy paper, which had been properly hydrated. They were covered with regular paper and incubated at 30°C to assess seed vitality. The seed vitality test was conducted twice: the first assessment occurred five days after the initial hydration, examining normal seedlings, abnormal seedlings, and ungerminated seeds. The final assessment was conducted 14 days after initial hydration. Measurements taken on the final day included the length and diameter of the hypocotyl, root number and length, fresh weight, and dry weight.

2. Germination test based on the orientation of the micropyle in watermelon seeds

To explore the germination behavior of watermelon seeds with respect to the orientation of the micropyle, a 0.6% agar solution (Junsei, Japan) was prepared and dispensed in 50 ml increments into plant culture dishes (100 × 40 mm). Once the agar solidified, it served as the germination medium. The study examined both SMP-treated and untreated seeds under two temperature conditions: 25°C and 30°C.

The seeds were placed on the agar medium with their micropyle facing upward to partially restrict oxygen supply, while two other sets were placed with the micropyle facing downward and horizontally for comparison. The seed varieties used for this experiment were Ggualnara (Nonghyup Seeds, Gyeonggi-do Anseong-si Gongdo-eup, Republic of Korea) and Hyundaeggual (Hyundai Seeds, Gyeonggi-do Yeoju-si Ganam-eup, Republic of Korea) (Fig. 1).

Fig. 1.

Germination of watermelon seeds according to the micropyle direction.

3. Comparative test of seed vigor: Osmotic priming (OP) and Solid matrix priming (SMP) in watermelon seeds

In this research, the efficacy of osmotic priming (OP) and solid matrix priming (SMP) on watermelon seeds was investigated using the varieties ‘Haechanggual’ and ‘Ggualnara’ from Nonghyup seeds, and ‘Hyundaeggual’ and ‘Hwalgichan’ from Hyundai seeds. The study focused on evaluating germination, seed vitality, and growth using the BP test. For the OP treatment, 10 grams of seeds were placed in a 100 × 40 mm petri dish and exposed to a 200 mM KNO₃ solution. They were incubated in darkness at 25°C for four days. and then air-dried at room temperature for 12 hours. The SMP treatment involved treating seeds with Micro Cel-E as a solid carrier for three days, with a ratio of 10 parts seeds to 5 parts carrier to 10 parts moisture (w/w). Untreated seeds were stored at 5°C for use as controls. Evaluation of seed vitality and growth was conducted using the BP test.

4. Statistical analysis

Statistical analysis was conducted using Duncan’s Multiple Range Test (DMRT) and Least Significant Difference (LSD) Test. For this, the SAS program (Statistical Analysis System, Inc., Version 9.4, NC. USA) was used.

Results and Discussion

1. Germination of seeds according to the orientation of the micropyle

Oxygen is a prerequisite for germination, and depending on the crop, there are seeds with high oxygen requirements, while others germinate under low oxygen conditions. It has long been known that seed vigor is associated with respiratory capacity (Woodstock and Grabe, 1967). To investigate the oxygen requirements for germination of watermelon seeds, the germination was investigated by measuring the micropyle position of the seeds in 0.6% agar medium.

Tables 1,2,3 examines the germination power of watermelon seeds according to the direction of the micropyle. The ‘Ggualnara’ and ‘Hyundaeggual’ varieties had a big difference in germination ability depending on the location of the micropyle. When the micropyle was placed upward or horizontally where the smooth supply of oxygen was ensured, the germination rate of ‘Hyundaeggual’ was more than 90%, but when the micropyle position was raised downwards to partially cut off the oxygen supply, the germination rate was reduced to less than 20% regardless of the germination temperature. However, SMP-treated seeds showed a germination rate similar to that of untreated seeds, which had a smooth oxygen supply even if the oxygen supply was restricted by pointing the micropyle downwards. In addition, they induced germination faster than untreated seeds.

In the ‘Ggualnara’ variety, the germination rate was the highest at 98.9% when the micropyles of the untreated seeds were placed upward (25°C) and 60.0% when the seeds were placed horizontally. On the other hand, when the micropyle was facing downwards, the germination rate was very low, less than 10%, regardless of the germination temperature. However, the germination rate of SMP-treated seeds was higher than 85% regardless of the direction of the main pores, and 100% germination was observed in the treatment that ensured the smooth supply of oxygen in the direction of the micropyles. In addition, the germination rate was 0.7－1.1 days faster than that of untreated ones.

Prior studies have shown that oxygen stress in quinoa is more damaging to old trees than to seedlings (Shabala et al., 2012). In this experiment, the growth of the seedlings on the 14th day after the watermelon seeds was delayed as the oxygen supply was cut off.

In the both variety, which germinated more than 90% of the time regardless of the direction of the micropyle, when the micropyles were facing down, the root appeared, but the root growth was poor thereafter, and this tendency was similar to that of SMP-treated seeds. The cause of this was judged to be the inhibition of muscle elongation due to the robustness of the AGAR medium used in this experiment, and it could not be ruled out that the oxygen supply of the roots was blocked, resulting in a low metabolism. Overall, SMP-treated seeds also showed increased the height and live weight of the root and inferior axis, regardless of the direction of the main hole (Tables 2－3).

Even in the ‘Ggualnara’ variety, the untreated seeds that were germinated at 30°C had lower seedling growth than the SMP-treated seeds, regardless of the direction of the micropyle. In particular, when the direction of the micropyle was horizontalized, the length of the lower embodiment axis was 1.9 cm without treatment, but the seedling of SMP- treated seeds was increased to 8.4 cm.

In ‘Hyundaeggual’, germination and seedling growth differed depending on the direction of the micropyle, and the germination was fastest when the micropyle was facing upwards, and the seedling growth was also faster. On the other hand, if the oxygen supply was partially cut off by pointing the micropyle downwards, germination was delayed, and seedling growth was reduced. Under oxygen-deficient conditions, SMP-treated seeds promoted germination and improved seedling growth compared to untreated seeds.

Therefore, watermelon crop seeds possess high oxygen requirements for germination, and the oxygen requirements varied depending on the variety. Among the published varieties, compared to the ‘Hyundaeggual’ varieties, the ‘Ggualnara’ variety had a higher oxygen requirement for germination. In addition, SMP-treated seeds had the effect of enhanced germination due to better germination promotion compared to untreated ones even under oxygen-deficient conditions.

2. Osmo priming (OP) and Solid matrix priming (SMP) efficiency comparative test

Four watermelon varieties were treated with OP and SMP to investigate germination and seedling vitality on the 5th day after sowing at 25°C and 30°C (Tables 4－5). Regardless of the variety, primed seeds tended to have improved germination rate compared to untreated seeds at all germination temperatures and were effective in promoting germination. Priming had a greater effect on promoting germination than on increasing germination rate. Additionally, primed seeds tended to have a higher proportion of normal seedlings compared to untreated ones.In spite of many reports of the positive effects of osmopriming on plant growth and development, there is also research evidence that disagrees. For example, osmoconditioning of soybean (Glycine max) seed produced variable results; while priming with mannitol did not alter field emergence, priming with PEG promoted rapid and uniform emergence in early plantings (Helsel et al., 1986).

Looking at the variety wise results, ‘Haechanggual’ has the germination rate was consistently high under all conditions. SMP treatment shortened the germination period, and the normal germination rate was also high. In ‘Ggualnara’, the priming treatment did not significantly improve the germination rate, but it was effective in shortening the germination duration. Overall, the germination and germination promotion effects of SMP treatment were high. The most efficient seed treatment in ‘Ggualnara’ was SMP treatment, followed by OP treatment. In case of ‘Hwalgichan’, there was no tendency of significant improvement in germination rates by SMP and OP treatment though there was enhancement up to a certain extent. There was no clear difference between OP and SMP treatment, but SMP treatment was better in terms of germination promotion. In ‘Hyundaeggual’, the germination rate was not significantly improved by SMP and OP treatment, but it was effective in shortening the germination duration. There was no clear difference in germination rate between SMP and OP treatments, but SMP treatment was better in terms of germination promotion.

As a result of comparing the efficiency between priming treatments in the above results, the treatment effect varied with each variety, and overall, SMP treatment was found to be better in terms of improving promoting germination. In the comparison of seedling vigor on the fifth day after sowing, the seeds that were primed had better overall growth than those of the untreated seeds. These results suggest that the seeds treated with priming were converted into seedling growth following rapid germination, resulting in better seedling growth compared to no treatment (Table 5).