Effects of Salinity and Drought Stress on Seed Germination of Common Purslane (Portulaca oleracea) ^†

Abstract

Common purslane (Portulaca oleracea) is a halophyte, consumed not only as an edible vegetable but also as a traditional medicine. This plant can adapt to extreme salinity and drought conditions and their effects on plant growth, yield and quality were elucidated, but the effects on seed germination was still unclear. Seed germination is an important stage for establishing P. oleracea seedlings that contribute to plant yield and productivity. Thus, the objective of the present study was to examine the individual effects of salinity and drought stress at different levels on the characteristics of seed germination, which the seeds suffered from −0.22, −0.45, −0.89, and −1.78 MPa osmotic stresses induced by polyethylene glycol (PEG), or from 50, 100, 200, and 400 mM NaCl stresses with equivalent osmotic pressures. The seeds sown in petri dishes containing filter paper layers that were flooded with NaCl or PEG-6000 solutions for the treatments and germination parameters were determined daily for 15 days. Data showed that germination percentage (GP), germination rate (GR), germination energy (GE), and mean germination time (MGT) were significantly decreased with increasing levels of salt and osmotic stresses, suggesting that the salinity and drought stress reduced the germination capability of seeds. Moreover, the seeds maintained germination with PEG osmotic pressure above −0.22 MPa, but not with NaCl concentration greater than 50 mM that induced a similar osmotic pressure, suggesting that the ion toxicity effect on the seed germination might be higher than the osmotic effects.

1. Introduction

Salinity and drought are becoming major worldwide threats to agricultural production. Salinity and drought stress interrupt functional cellular processes, such as water absorption, photosynthesis, nutrient uptake, and metabolisms, leading to the inhibition of the growth and development of plants [1]. Plants suffer from osmotic stress induced by a high osmotic pressure of extracellular water deficiency [2]. Meanwhile, under high salinity conditions, mainly caused by NaCl, plants might suffer from both osmotic stress and ion toxicity of Na⁺ and Cl⁻ [3].

Seed germination is the initial stage in a plant’s life cycle and plays an important role to the yield of seed-propagated crops; however, it is sensitive to salinity and drought stress. During germination, water imbibition is the first step required to activate enzymes and promote the development of the embryo [4]. As a result, osmotic stress-inducing factors would reduce cell’s water uptake ability and thus inhibit seed germination [5]. Moreover, the presence of Na⁺ and Cl⁻ ions at certain concentrations might impose toxicity on embryonic metabolisms and seedling formation [3]. The effect level of the stresses on germination is depended on the physiological status of seeds and the plant species. Therefore, understanding the germination capability and characteristics of seeds under unfavorable conditions is significant for the strategic development of agricultural practices.

Common purslane (Portulaca oleracea) is an annual herbaceous creeping plant, commonly cultivated around the world [6]. This is a potential vegetable and herbal crop, owing to its nutritional and medicinal value. The leaves are rich in omega-3 fatty acids, a-linolenic acid, a-tocopherol, ascorbic acid, beta-carotene, and glutathione [6]. Studies with in vitro models showed that the whole plant extract has high antioxidant activity compared to other vegetables due to the high accumulation of antioxidant compounds. In addition, P. oleracea are able to adapt to adverse environmental conditions, such as drought and salinity. Growth responses of P. oleracea plants under salinity and drought stresses have been elucidated in previous studies. It also has demonstrated the ability to maintain growth and enhance synthesis of bioactive substances under severe water-deficient and saline conditions [6,7]. Although the plant is mainly propagated and regenerated by seed, the effects of salinity and drought stresses on the germination of P. oleracea seeds have not been investigated so far.

The objective of the present study was to examine the effects of salinity and drought stress at different levels on the germination of P. oleracea seeds and evaluate the influence level of these two stress factors. For this purpose, seeds were sown under saline conditions induced by NaCl at concentrations of 50, 100, 200, and 400 mM, and physiological drought conditions with osmotic pressure of −0.22, −0.45, −0.89, and −1.78 MPa induced by PEG-6000. The osmotic pressures were respectively equivalent between two salt and osmotic stress levels. PEG-6000 is a water-soluble polymer that was commonly used to simulate the effects of drought stress on seed germination [8].

2. Material and Methods

2.1. Seed Collection

P. oleracea plants were grown in a greenhouse in The University of Danang—University of Science and Education for blossoming. Fruits were collected from the plants after the flower withered and stored in plastic boxes under well-ventilated conditions at room temperature until the fruit opened for seed collection.

2.2. Seed Germination and Treatments

The seeds were sown according to a method described by Dan et al. (2022) [9]. The seed sterilization procedure was carried out to eliminate microorganism contamination prior to sowing. After soaking with Nano Kito 2.6SL fungicide solution prepared according to the manufacturer’s instruction, the seeds were treated with a 0.1% HgCl₂ solution for 10 min and rinsed 3 times with sterile distilled water. About 50 decontaminated seeds were spread in a 9 cm-diameter petri dish containing two filter paper layers flooded with 5 mL of treatment solutions. For the salt treatment, the solutions were prepared with 50, 100, 200, and 400 mM NaCl, and for the drought treatment, the solutions were prepared with amounts of PEG-6000 that produce osmotic pressures at −0.22, −0.45, −0.89, and −1.78 MPa. The seeds sown with only water were considered as a control. The petri dishes were placed in a growth room for 15 days with set conditions such as a temperature of 25 °C and a photoperiod of 14 h/day with light intensity at 2.000 lux. Water was added to the petri dish weekly to maintain the wetness of the filter paper.

2.3. Determination of Germination Parameters

The seeds were considered germinated when shoot and root tips appeared. Germinated seeds were counted daily for 15 days. Germination parameters, such as germination percentage (GP) (%), germination rate (GR) (%/day), germination energy (GE) at the day 7 after sowing (%), and mean germination time (MGT) (days), were calculated as described by Kader et al. (2005) [10] and Shiade et al. (2020) [2].

2.4. Experimental Design and Data Analysis

Seed germination experiments were randomly carried out with 100 seeds and repeated three times (n = 3) at each stress level. The analysis of descriptive statistics was applied to germination parameters, and the significant difference between the stress levels were compared following Duncan’s test with a significance level α = 0.05, using R software (RStudio Cloud Version).

3. Results and Discussions

3.1. Effects of Salinity Stress on Germination of P. oleracea Seed

Data showed that the salt stress levels had significant effects on the germination of P. oleracea seeds. The GP of seeds were decreased with increasing salt stress levels. In the control condition (without salt), the seeds began to germinate two days after sowing with GP up to 66.22%; it was increased in the following days and reached a maximum value of 95.93% on day 9 of treatment time (Figure 1a and Figure 2a). However, the presence of NaCl in the treatment solutions inhibited the germination so that the seeds could germinate only in 50 mM NaCl solution at only the day 5 of the treatment with a low value of GP. The GP of salt-treated seeds was only 4.79% at the first day of germination, decreasing 91.14% compared to the control. The shoots and roots that emerged under this salt treatment showed no morphological difference from those in the control. Remarkably, the seeds did not germinate with 100–400 mM salt levels during the observation time (Figure 1a).

Besides the germination percentage, other germination parameters were also significantly decreased with 50 mM NaCl treatment, in comparison to that in the control (p < 0.05). The GR and MGT of seeds under the control were 41.20%/day and 2.72 days, respectively; however, the GR was reduced to 0.96%/day and the MGT was increased to 5 days when the seeds were treated with 50 mM NaCl concentration (Figure 2b,c). This suggested that salinity stress delayed the seed germination time. In addition, the GE of seeds also significantly decreased with the salt treatment compared to the control, from 91.55% in the control to 4.79% in the salt-stressed seeds, suggesting that salt stress reduced the germination capability of seeds (Figure 2d).

Previous studies have demonstrated that an increase in salinity levels seriously impacted seed germination in various plant species such as Vigna radiate L., Lens culinaris, Elymus junceus, Sorghum species, and pearl millet (Pennisetum glaucum) [11]. Salinity can reduce seed germination capabilities or prolong the germination time because of the reduced water uptake and toxicity of Na⁺ and Cl⁻ ions caused by salt, which lead to limited hydrolysis of nutrients required for energy supplies of seed germination [12]. It is demonstrated that P. oleracea was able to tolerate extreme saline conditions [7], but the results in the present study showed that the germination of seeds was sensitive to salinity. The seed germination was inhibited under salt stress. This result was also similar with the case of Lathyrus sativus L., where saline stress reduced seed the germination capability and delayed the germination process [4].

3.2. Effects of Drought Stress on Germination of P. oleracea Seed

Data showed that the germination of P. oleracea seeds was also negatively affected by PEG-induced osmotic stress. The seed germination capability were decreased with increasing level of osmotic stress (Figure 1b). The seed germination was occurred with the drought stresses of −0.22 MPa and −0.45 MPa osmotic pressures at the day 2 of treatment, but the GP was significantly decreased (60.24%) and (41.32%) compared to the control (66.03%), respectively (Figure 1b). In the following days, the GP of drought-treated seeds was increased and reached maximum values of 93.83% (−0.22 MPa) and 87.96% (−0.45 MPa) on day 9 of the treatment, which decreased 2.10% and 7.07% compared to the control (Figure 3a). The GP significantly decreased at −0.45 MPa osmotic pressures (p < 0.05). The seed germination was inhibited at osmotic pressures of −0.45 MPa or higher (Figure 3a). Remarkably, the seeds still germinated at −0.22 MPa osmotic pressure with a higher GP than that of 50 mM NaCl treatment, but the osmotic pressure was equivalent (Figure 1). This result suggested that the ion toxicity effect on seed germination under salinity was higher than that of osmotic stress, which caused a more serious impact of salinity on the seed germination.

In addition, the result showed that germination parameters, such as GR, also significantly decreased to 30.77%/day, and MGT significantly increased to 3.77 days with the treatment of −0.45 MPa osmotic pressure (p < 0.05) (Figure 3b,c), suggesting that osmotic stress delayed seed germination time. A significant decrease of GE was also observed at −0.45 MPa osmotic pressure (Figure 3d), suggesting that osmotic stress reduced the seed germination capability.

PEG molecules affect germination through limiting the penetration of water molecules into plant tissues, causing physiological drought [12]. Previous studies demonstrated that osmotic stress caused by PEG led to a decrease in the hydrolysis of seeds, thus causing a decline in seed germination [8]. The results in the present study are also consistent with previous studies that showed a reduced seed germination capability of Agropyron elongatum, Agropyron desertourm, and Secale montanum when exposed to high levels of PEG [13].

4. Conclusions

In the present study, the results indicated that salinity and drought stress both negatively affected the germination capability of P. oleracea seeds. The seeds were only germinated under salinity levels not greater than 50 mM with a low germination capacity, and the germination was inhibited with higher salinity levels. Meanwhile, the seeds germinated under osmotic stress of −0.22 and −0.45 MPa with a significant decrease of germination capability at later levels. The seed germination was inhibited with higher osmotic stress levels. The salinity and osmotic stress inhibited germination capability and prolonged seed germination time. Moreover, the ion toxicity effect on seed germination under salinity was higher than that of osmotic stress, which caused a more serious impact of salinity on the germination of P. oleracea seeds.