Introduction

South Korea ranks 24th among the 107 countries in terms of watermelon production. Watermelon (Citrullus lanatus) is one of the most cultivated fruit-bearing crops in South Korea, whose total cultivation area was 11,762 ha in 2022 and the total production was 487,167 tons representing a 0.4% decrease in production compared to the previous year (KOSIS, 2023). Recent data on the total cultivation area in 2023 indicates a decreasing trend, with the total area reduced to 9,330 ha. This might be due to various constraints in watermelon production such as extreme weather conditions, especially drought and climate change. As a result, prices of watermelons escalate year by year.

Since watermelon is a warm-temperature crop, it is difficult to produce uniform seedlings due to abiotic stress. The abiotic stress which adversely affects plant growth includes drought, extreme temperature, salinity, and other crucial factors (Hassan et al., 2015; Kramer, 1963; Ledesma et al., 2008). In agriculture, abiotic stresses on standing crops lead to severe economic yield loss and affect plant health (Akrami and Arzani, 2019; Heo et al., 2013). Thus, to overcome these constraints better germination, a uniform crop stand, and favorable growth under adverse conditions need to be achieved. Seed priming is one such treatment that may help to overcome these unfavorable conditions. Priming involves prior exposure to elicitors, which brings a cellular state that reduces the harmful effects of abiotic stress, and plants raised after priming are more tolerant of abiotic stress. Priming induces resistance upon pathogen attack and confers enhanced disease protection to the plant (Jayamohan et al., 2020; Marthandan et al., 2020). Seed priming enhances the germination rate and uptake of nutrients and controls seed-borne pathogens by influencing the pre-germination metabolic activities (Taylor and Harman, 1990). Solid Matrix Priming (SMP), known as Matrix conditioning, is a process in which seeds are mixed with a solid material and water in known proportions and then incubated for a given duration at constant temperature. Solid matrix priming utilizes carriers like vermiculite, diatomaceous earth, or another highly water-absorbent polymer (Celite or Micro cel-E) possessing characteristics such as high-water holding capacities, low osmotic potentials, and low bulk density (Khan, 2010; Mereddy et al., 2000; Taylor et al., 1988). Furthermore, SMP can address the low germination issues observed in osmotic priming.

Thus, this study aims to establish an ideal carrier material for Solid matrix priming while and assess its impact on enhanced germination and healthy growth under unfavorable conditions compared to untreated seeds.

Materials and Methods

1. Watermelon cultivar and germination test

The watermelon cultivars used in this experiment were ‘Haechangual’ (Nonghyup seeds, Korea), ‘Gulnara’ (Nonghyup seeds, Korea), ‘Pomina’ (Nonghyup seeds, Korea), ‘Woorigual’ (Nonghyup seeds, Korea), ‘Hyundaigual’ (Hyundai seeds, Korea), ‘Hwalgichan’ (Hyundai seeds, Korea), ‘Rico fresh’ (Partner seeds, Korea), ‘Rico sweet’ (Partner seeds, Korea), and ‘Heukmi’ (Samsung seeds, Korea).

The seeds used in the experiment were harvested and initially stored at 5℃. As for the germination method, after spreading two pieces of filter paper (Whatman No. 2) on a petri dish (9 cm), thirty seeds were assessed for germination in an incubator at 30℃ in 3 replications. The seeds placed in the petri dish were examined at 12-hour intervals for up to 14 days to assess for any germination, which was determined by the radicle extending more than 1 mm through the seed coat. The number of days (T₅₀) required for 50% of the final germination percentage was calculated by the formula below.

N : Sum of final germinated seeds

Ni : Sum of the number of seeds germinated until just before 50% for N

Nj : Sum of the number of seeds germinated immediately after 50% for N

Ti : Germination period until Ni

Tj: Germination period until Nj

Seedling viability was assessed through the BP (Between paper) test. Heavy paper and regular paper, which are BP test papers, were placed in distilled water to absorb and stored at 25°C for 1 day. Afterwards, thirty seeds were arranged at equal intervals on heavy paper from which water was appropriately removed, covered with regular paper, then placed in an incubator at 30°C, and seed viability was examined.

The seed viability test was conducted twice. The first examination evaluated normal, abnormal, and non-germinating seeds 5 days after seeding, and the second examination was conducted 14 days after seeding. On the second examination day, hypocotyl length, hypocotyl diameter, number of roots, root length, fresh weight, and dry weight were measured.

Seedling emergence and viability tests were performed at the Pusan ​​National University greenhouse (50 Cheonghak-ri, Samnangjin-eup, Miryang-si, Gyeongsangnam-do). To investigate the seedling emergence percentage, thirty seeds were sown three times after filling a 105-cell plug plate with topsoil. Seedling emergence percentage was examined at daily intervals up to 30 days after sowing, and plant height, root length, diameter, fresh weight, and dry matter weight were examined on the 30th day after sowing.

2. Determination of optimum Solid Matrix Priming (SMP) treatment conditions

The watermelon seed used to establish SMP conditions was ‘Hyundaiqual’ (Hyundai seeds, Korea). Micro Cel-E (synthetic calcium silicate), kaolin, vermiculite, diatomaceous earth, zeolite, and talc were used to check for the most ideal solid carrier for the SMP of watermelon seeds.

The treatment ratio was prepared by mixing seed:solid carrier:distilled water in ratios of 10:5:5, 10:5:10, and 10:5:15 (w/w/w). To determine the optimal SMP treatment period, Micro cel-E, which was found to be an ideal solid carrier, was fixed at 10:5:10 (w/w/w) in the solid carrier mixing ratio selection test and the treatment period was 1, 2, 3, 4, and 6 days. To determine the optimum SMP treatment temperature, the treatment with Micro cell-E was fixed at 10:5:10 (w/w/w) for 3 days, and the germination performance was evaluated by varying the treatment temperature at 10, 15, 20, 25, and 30℃.

3. Changes in moisture content of seeds during Solid Matrix Priming (SMP) treatment

For this experiment we have used the cultivar ‘Hyundaiqual’ (Hyundai seeds, Korea). The moisture content of seeds during priming and germination period was investigated. For SMP, seed: solid carrier: water were mixed at a weight ratio of 10:5:10 (w/w/w), and osmotic priming was conducted with 200 mM KNO₃ solution at 25°C for 4 days to measure the moisture content. For this process, untreated seeds stored in cold storage at 5°C were used.

The moisture content of seeds during priming was noted at 15-minute intervals within 1 hour after priming, at 1-hour intervals within 6 hours, at 2-hour intervals within 12 hours, and at 12-hour intervals thereafter.

The moisture content was measured by drying at 130°C for 1 hour according to the high-constant temperature oven method of the International Rules for Seed Testing (ISTA, 2023), and was calculated on a fresh weight basis.

M₁ : the weight in grams of the container and its cover,

M₂ : the weight in grams of the container, its cover and its contents before drying, and

M₃ : the weight in grams of the container cover and contents after drying.

4. Assessing the effect of Solid Matrix Priming (SMP) treated seeds.

After SMP treatment for 3 days with Micro cel-E (a solid carrier) where the mixing ratio of seed: solid carrier: water was at 10:5:10 (w/w/w), the effect on seed germination and seedling viability promotion was observed.

The various cultivars used to assess whether the effect of SMP treatment occurs, are ‘Haechanqual’ (Nonghyup seeds, Korea), ‘Gulnara’ (Nonghyup seeds, Korea), ‘Fomina’ (Nonghyup seeds, Korea), ‘Rico fresh’ (Partner seeds, Korea), and ’Rico sweet’ (Partner seeds, Korea). There were eight cultivars including ‘Wooriqual’ (Nonghyup seeds, Korea), ‘Hwalgichan’ (Hyundai seeds, Korea) and ‘Heukmi’ (Samsung seeds, Korea).

Germination percentage and germination speed were evaluated using a germination test in a petri dish, and seedling viability was investigated through BP tests.

5. Statistical analysis

Statistical analysis was performed using least significant difference (LSD) test, and the SAS program (Statistical Analysis System, Inc., NC. USA) was used for this purpose.

Results and Discussion

1. Selection of optimum Solid Matrix Priming (SMP) carriers for germination promotion.

SMP solid carrier ideal for promotion of germination in watermelon seeds(Hyundaeggual) was investigated, and the mixing ratio of seed: solid carrier: water was set (Fig. 1 and Table 1). There was a slight difference in the germination pattern based on the type of solid carrier used and the mixing ratio, but the germination percentage of SMP-treated seeds was greater up to 27.8 % compared to untreated seeds. In addition, although there was a slight difference depending on the type of solid carriers, the germination speed was higher than untreated seeds by more than 0.3 days. Micro cel-E, which is commonly used as a solid carrier for SMP, showed a higher germination percentage as well as germination speed. Furthermore, in the case of Micro cel-E, despite the increase in the moisture content during the treatment process, no such phenomenon of radicle root protrusion was observed.

Fig. 1

Appearance of SMP carrier type and ratio of seed to carrier to water by weight on the performance of ‘Hyundaeggual’ watermelon seeds for 3 days at 25℃.

On the other hand, SMP treatment using diatomaceous earth in 10:5:10 (seed:solid carrier:water w/w/w) for 3 days was less effective than Micro cel-E, but the germination percentage was high, and the T₅₀ was shortened to promote germination.

Vermiculite was found to be difficult to use industrially because radicle protruded during the 3-day SMP treatment process regardless of the mixing ratio. Overall, Micro cel-E was the most effective SMP solid carrier, followed by diatomaceous earth with reliable results.The ideal SMP treatment carrier to enhance the germination of watermelon seeds was found to be Micro cel-E, which was treated for 3 days by mixing seed: solid carrier: water content at 10:5:10 ratio.

SMP treatment involves mixing seeds, solid carrier, and water and then treat them for a certain period to control the moisture content of seeds to complete physiological germination while not allowing radicle protrusion. Solid matrix priming is similar to osmotic priming, allowing seed to imbibe and attain threshold the moisture content and pre-germination metabolic activities but preventing radicle emergence. However, it has the advantages of allowing aeration, incorporation of biological agents to combat soilborne pathogens, and improved ease of handling (Harman and Taylor, 1988; Parera and Cantliffe, 1994; Taylor et al., 1988). In osmo-priming using a liquid solution, the germination promoting effect is maintained only when the treatment agent is removed by washing the seeds after treatment. Contrary to osmotic priming, in SMP, since the solid carriers attached to the seed surface are easily mixed into the soil after treatment, it is not essential to remove, thus reducing the seed treatment effort

2. Identification of SMP treatment period and treatment temperature

After the initiation of the SMP treatment, once the matric potential balance is established between the seed and the solid carrier, no more moisture content occurs due to the turgor pressure of the seed (Cho et al., 2001). The basic principle of SMP treatment is to promote the metabolic activity of seeds prior to germination as a preparation by prolonging the induction phase (first phase) and the second phase is the moisture content. However, if the treatment period is excessively long, it enters the third stage of water absorption in which radicle protrusion occurs (Suzuki et al., 1990). Therefore, an appropriate treatment period must be set to prevent radicle emergence during the treatment and to improve the SMP treatment effect.

Germination was evaluated at varied treatment periods under the established optimum SMP mixing ratio [Micro cell-E 10:5:10 (w/w/w)] conditions (Table 2). Seeds treated with SMP had more than 95% germination percentage, and regardless of the treatment period, germination ability was improved by more than 30% compared to untreated seeds. The concept of germination speed, T₅₀, was also shortened by 0.8－1.1 days. Overall, the germination percentage was highest in the 3-day treatment period and was effective in reducing T₅₀. However, when the treatment period was extended to more than 4 days, seeds with radicle emergence appeared during the treatment process.

Seed companies sell seeds that have been primed and re-dried to an initial moisture content to improve germination. However, seeds with visible radicle protrusion during processing cannot be sold because they lack resistance to drying.

Under the established optimum SMP condition [Micro cell-E 10:5:10 (w/w/w), 3-day treatment], the germination ability was investigated by varying the treatment temperature (Table 3). The germination percentage of watermelon seeds was over 98%, and the SMP treatment temperature did not have a significant effect on the germination percentage. However, there was a significant difference in T₅₀ due to the treatment temperature. When the SMP treatment temperature was lowered, the number of days required for germination was longer. Specifically, at temperature range of 20－30℃, SMP-treated seeds germinated faster than untreated seeds in 0.5 days. Contrary to the above, during SMP, radicle emergence was observed during the treatment process at 30℃, and thus the appropriate SMP treatment temperature was found to be 25℃.

3. Moisture content of seeds during SMP treatment

The main function of the seed coat is to control water absorption (Haigh and Barlow, 1987), and water absorption of the seed is inhibited by the seed coat. During the SMP treatment process, the seeds increase in size as the cell wall softens and water is absorbed into the cells. Fig. 2 shows the water absorption pattern during the SMP treatment process. The initial moisture content of watermelon seeds was 10.9%, within 1 hour of SMP treatment it has increased to 31% due to rapid water absorption, after which the moisture content decreased (water absorption stage 1). After SMP treatment, during the first hour to the fourth hour gradual water absorption was achieved, and after 4 hours, up to 72 hours, water was gradually absorbed into the seeds, indicating a stabilized water absorption phase (water absorption stage 2).

Fig. 2

Changes in percent of water content of ‘Hyundaeggual’ watermelon seeds during the SMP with ratio of seed: Micro cell-E: H₂O (10:5:10 w/w/w), osmopriming in 200 mM KNO₃ and the imbibition in distilled water in dark at 25℃. Percent of water content was measured every hour for the first 6 hours, and the 2 hours for the 12 hours, and 6 hours for the 24 hours, and then 12 hours thereafter for 72 hours. The initial percent of water content was 10.9% on a fresh weight basis.

Imbibition treatment has shown similar water absorption pattern with that of SMP, while OP treatment has shown a delay in water absorption by 2 hrs in the second stage. In the final stage of treatment, Water absorption percentage of OP treatment was 49%, while SMP treated seeds has shown 41% water absorption. This was lower compared to imbibition treatment which was 52%. The cause is interpreted as the result of salt ions used as OP treatment solution reducing the water potential of the treatment solution, thereby limiting the seed's water absorption. Similarly, SMP treatment also resulted in a lower water absorption rate compared to seeds subjected to imbibition treatments, due to the high matric potential of the solid material.

4. SMP treatment effect by cultivar

Eight cultivars of watermelon seeds were treated with SMP to investigate the effects on percent germination, T₅₀, and seed vigor (Tables 4, 5). In all cultivars, SMP-treated seeds had shown improved germination ability compared to untreated seeds, and the germination speed was also faster than untreated seeds. The cultivars for which a significant germination promotion effect was distinct by SMP treatment were ‘Haechangual’ and ‘Rico Sweet’ (Table 4).

In addition, the results of seed vigor test using BP (between paper) showed that in all cultivars, the germination rate of SMP-treated seeds was higher compared to untreated seeds, while the abnormal ones, which cannot grow into healthy seedlings were reduced. Therefore, SMP-treated seeds indicate an increase in the number of individuals that can grow into healthy seedlings.

SMP-treated seeds had a significant germination promotion effect in the plant growth chamber. Similarly, we sought to examine whether the germination effect obtained through SMP treatment leads to any improvement in early growth even under greenhouse conditions (Table 5).

The seedling emergence percentage of SMP-treated seeds was improved compared to untreated seeds in the ‘Gualnara’ and ‘Rico fresh’ cultivars, and the emergence percentage was improved in the other cultivars, but statistical significance was not recognized.

Watermelon seeds were sown after Solid Matrix Priming and grown for 30 days, and the initial growth was examined. As a result, although there were some differences depending on the cultivar, overall, the fresh and dry weight of the seedlings was higher in the SMP-treated seeds.

In addition, SMP-treated seeds had better growth in terms of plant height, root length, and hypocotyl diameter compared to untreated seeds. The above results were consistent with the results of previous studies showing that SMP treatment was useful for improving seedling viability (Harman and Taylor, 1988; Kim et al., 1998; Taylor et al., 1988; Taylor, 1990).

Conclusion

This research was conducted to establish the most ideal seed treatment conditions to be prevailed for solid matrix priming (SMP) to produce high-quality seedlings and uniform germination under unfavorable conditions. Optimum SMP processing material of watermelon seed was Micro cel-E, and the best processing effect was found in 10:5:10 (w/w/w) in mass ratio of seed: material: water at 25℃ for 3 days. After the SMP treatment process, the moisture content was 41%. During the treatment period, SMP treatment showed a lower water absorption rate compared to seeds subjected to imbibition treatments due to the high matric potential of the solid material. However, it exhibited stable water absorption.

Regardless of cultivars, germinability of SMP- processed seed was enhanced, and the germination speed was also faster than untreated seeds. The cultivars for which a significant germination promotion effect was confirmed by SMP treatment were ‘Haechangual’ and ‘Rico Sweet,’ and germination was promoted. In addition, the results of testing seed vigor by BP (between paper) showed that in all cultivars, the germination rate of SMP-treated seeds was higher compared to untreated seeds, while the abnormal ones, which cannot grow into healthy seedlings was lower.

Solid Matrix Primed watermelon seeds when sown and examined for early seedling growth, have shown better growth than untreated ones when measured in terms of dry weight and fresh weight, though it may vary from cultivar to cultivar.