Chen, Wang, Sui, Jin, Wang and An (2020). Seed Science and Technology, 48, 3, 355-365. https://doi.org/10.15258/sst.2020.48.3.04
Effects of drought and temperature on the germination of seeds of Seriphidium transiliense, a desert xerophytic subshrub of Xinjiang, China
Aiping Chen1,2,3, Yuxiang Wang2,3, Xiaoqing Sui2,3, Guili Jin2,3, Kun Wang1* and Shazhou An2,3*
1 College of Animal Science and Technology, China Agricultural University, Beijing, 100193, China 2 College of Pratacultural and Environmental Science, Xinjiang Agricultural University, Nongda East Road 311, Shayibake District, Urumqi, Xinjiang 830052, China 3 Key Laboratory of Grassland Resources and Ecology of Xinjiang, Nongda East Road 311, Shayibake District, Urumqi, Xinjiang 830052, China * Authors for correspondence. (E-mail: [email protected]; [email protected])
(Submitted April 2020; Accepted August 2020; Published online September 2020)
Abstract
Global warming has led to changes in rainfall patterns in many regions and it has an increasing impact on the availability of water for plants, especially in the arid and semi-arid regions. Seed germination is the most critical stage in the plant life cycle, it determines whether or not the population can successfully establish. Here, we assessed the seed germination characteristics of Seriphidium transiliense under six water potentials and four temperature regimes. S. transiliense seeds could germinate from 5/15°C to 20/30°C, while the optimum temperature regime was 10/20°C. As water potential decreased, the germination percentage, germination index, germination energy, vigour index, plumule length and radicle length increased and then decreased, while mean time to germinate decreased and then increased. The optimum condition for S. transiliense seed germination was -0.2 MPa at 10/20°C. Some seeds that failed to germinate under drought conditions were transferred to distilled water and recovered germination ability.
Keywords: germination, germination recovery, polyethylene glycol (PEG)-6000, temperature, Seriphidium transiliense
Introduction
According to a report, the global average temperature will increase by 1.0-3.0°C by 2050 (IPCC, 2012). Alterations in rainfall patterns in many regions occur, and drought severity and length are increasing dramatically (Dai, 2011). Climate change has an important impact on all stages of plant development (Battipaglia et al., 2014; Elnaggar et al., 2018).
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355 AIPING CHEN, YUXIANG WANG, XIAOQING SUI, GUILI JIN, KUN WANG AND SHAZHOU AN
Compared with other developmental stages, seed germination is the definitive stage in plant population renewal (Bakhshandeh et al., 2013; Song and Wang, 2015), which directly determines the continuation, dynamics and distribution of a population. At the same time, seed germination is the most sensitive to abiotic stresses (Maraghni et al., 2010; Gurvich et al., 2017). Therefore, studying the effect of abiotic stresses on seed germination is the first step to understand the adaptability of species to adverse environments and the mechanism of plant resistance to stress. Seed germination is a complex process (Hellal et al., 2018). When the external environment is suitable, seeds germinate quickly and increase the seedings survival. In contrast, when the external environment is not suitable for seed germination, seeds remain as part of the soil seed bank and escape the fate of death after germination (Bradford, 2002; El-Keblawy et al., 2017), serving as a strategy of self-protection. The germination characteristics of seeds are closely related to precipitation and pre cipitation cycles in natural distribution areas. Some species cannot germinate at -0.2 MPa (Nascimento et al., 2018), while some species have higher germination capacity when water potential is lower than -1.0 MPa (Lai et al., 2019). Seeds can adapt to water stress by increasing protective enzyme activity and osmotic adjustment substances in vivo to minimise the damage caused by water stress during the germination process. Temperature is a critical ecological factor affecting seed germination (Fakhfakh et al., 2018). Different plant species have different temperature requirements for germination (Gorai et al., 2014). Some plant species only germinate over a narrow temperature range, but others can germinate over a wide range of temperatures (Baskin and Baskin, 1998). Drought and high temperatures are commonly encountered together in arid and semi-arid regions, so it is important to study the interactive effects of water deficiency and temperature on seed germination of desert plants. Understanding the range of suitable water and temperature conditions for seed germination of different species will help to provide information for plant survival strategies in the desert. 7 2 Desert pasture occupies approximately 2.7 × 10 hm in Xinjiang, China, and covers more than 46.9% of Xinjiang grassland (Xu, 1993). There are many excellent annual and perennial grasses in desert pasture. As one of the dominant species of desert pasture, Seriphidium transiliense (Poljakov) Poljakov (Compositae) is a perennial xerophytic subshrub (An, 1999). Because of its good root system, drought resistance and adaptability, it plays an important role in maintaining the stability and sustainability of desert ecosystems (Zheng, 2013), and the production of livestock. Seriphidium transiliense is an 6 2 important component of Xinjiang desert pasture, occupying 1.14 × 10 hm . S. transiliense is located in the transition zone between plains and mountains at an altitude of 500 to 3200 m a.s.l. However, due to the effects of climate change and over-grazing (Manzano and Návar, 2000), S. transiliense desert has undergone severe degradation, which not only restricts the development of Xinjiang animal husbandry, but also affects the ecological balance and ecological security of the desert region in Xinjiang. Therefore, the restoration of degraded grassland is an extremely urgent problem at present. The most cost-effective method is re-seeding of S. transiliense. Bademuqiqige et al. (2018) studied the effect of harvest time on seed germination, and Sun and He (2007) reported that mature seeds of S. transiliense had non-deep physiological dormancy. However, there are few studies on the
356 SEED GERMINATION OF SERIPHIDIUM TRANSILIENSE interactive effects of drought and temperature on the germination of S. transiliense seeds. The objectives of this study were to assess the effects of water potential, temperature and their interaction on seed germination. We hypothesised that drought stress can promote seed germination, and seed germination will be inhibited by high temperatures and low water potentials. The results will reveal the adaptability of germination of S. transiliense seeds to drought and temperature stress, and provide theoretical reference for desert plant protection and population renewal.
Material and methods
Seed collection Mature seeds of S. transiliense were collected on 23 October 2017 in the desert pasture in Sangongtan, Changji city, Xinjiang, China (43°50'9''N, 87°11'21''E, 994 m a.s.l.). Seeds were air-dried and stored in brown paper bags at 4°C until used for the seed germination experiments. The climate of this area is a typical temperate continental dry climate. The annual mean temperature and precipitation are 8.5°C and 180 mm, respectively, and annual evaporation is 1790 mm. The soil type is grey desert soil. The vegetation community is dominated by S. transiliense, with local accompanying species at the study area, such as Tulipa iliensis Regel., Geranium transversal (Kar. & Kir.) Vved. ex Pavlov, Ceratocarpus arenarius L., Salsola affinis C.A.Mey., Eremopyrum triticeum (Gaertn.) Nevski, Atriplex sp., Kochia scoparia L. Schrad. and Lappula sp.
Seed germination experiments The experiments commenced on 18 April 2018. Seeds were incubated at six levels of water potential (ψ): 0.0 (control), -0.2, -0.4, -0.6, -0.8 and -1.0 MPa, and four alternating temperatures, 5/15°C, 10/20°C, 15/25°C and 20/30°C, in plant growth chambers. According to Michel and Kaufmann’s (1973) equations, the different water potentials were acquired with different concentrations of polyethylene glycol (PEG)-6000. The light regimes were 12 hours of white light (approximately 200 μmol m-2 second-1 of photosynthetically active radiation) and 12 hours of darkness. The highest temperatures corresponded to the period of exposure to light. S. transiliense seeds were surface-sterilised with 75% ethanol for one minute and 5% sodium hypochlorite solution for five minutes, and rinsed three times with sterile water. Fifty randomly selected seeds were placed on double-layers of filter paper (Whatman
No. 1) in 90 mm-diameter glass Petri dishes, moistened with 6 mL of distilled water (control) or a solution of PEG-6000. Four replicates were included for each treatment.
Seeds were scored as germinated upon 2 mm-long radical emergence The number of germinated seeds was recorded daily for 16 days; no further seeds germinated over the following three days after which the germination trials were finished. To avoid evaporation, glass Petri dishes were sealed with Parafilm. Ten seedlings were selected at random from each dish and radicle and plumule lengths measured using Vernier calipers.
357 AIPING CHEN, YUXIANG WANG, XIAOQING SUI, GUILI JIN, KUN WANG AND SHAZHOU AN
Germination recovery After 16 days, seeds that failed to germinate in different PEG concentrations were rinsed three times with sterile water, and transferred to distilled water in new glass Petri dishes to assess germination recovery. The seeds were incubated again at the four temperature regimes with a 12-hour/12-hour light/dark regime, for seven days. Germinated seeds were counted and removed daily. Mean time to germinate (MTG) was calculated as: ni × d MGT = ∑ ( N ) × 100%, where “ni” is the number of seeds germinated at ith day, “d” is the germination period and “N” is the total number of seeds germinated. The germination index (GI) was calculated as: nd3 ndi GI = ( nd1 ) + ( nd2 ) + ( ) + ... + ( ), 1 2 3 i where nd1, nd2, …., ndi is the number of seeds germinated on days 1, 2, 3, …i. Germination energy (GE) was calculated as: GE = number of germinated seeds after seven days × 100%. test seed number Vigour index (VI) was calculated as:
VI = GI × (SL + RL), where GI is germination index, SL is plumule length and RL is radicle length. The cumulative germination percentage is the proportion of the accumulated number of germinated seeds in the test seed number to the corresponding number of days. Germination recovery (GR) (%) was calculated as: (a − b) GR = × 100, (c − d) where “a” is the total number of seeds germinated after being transferred to distilled water, “b” is the total number of seeds germinated in PEG solution and “c” is the number of test seeds.
Data analysis Percentage data were arcsine-transformed before analysis, but the results shown are the percentages. GP, MTG, GI, GE, VI, plumule length and radicle length were analysed using general linear models (GLM). A one-way ANOVA was used to determine significant differences of drought and temperature stress for GP, MTG, GI, GE, VI, the cumulative germination percentage, radicle length, plumule length and GR (Tukey test at P < 0.05).
Results
Effect of water potential and temperature stress on germination characteristics
There were significant effects of water potential (P < 0.001) and temperature (P < 0.001) on GP, MTG, GI, GE and VI. There were significant interaction effects of water potential and temperature on MTG, GI, GE and VI (P < 0.001) and on GP (P < 0.01).
358 SEED GERMINATION OF SERIPHIDIUM TRANSILIENSE
As water potential decreased, GP (figure 1) and VI (figure 2C) increased and then decreased at the same temperature regime, GI (figure 2A) and GE (figure 2B) increased and then decreased at both 5/15°C and 10/20°C, and gradually decreased at both 15/25°C and 20/30°C. MTG decreased and then increased at both 5/15°C and 10/20°C, and gradually increased at both 15/25°C and 20/30°C (figure 1). With the temperature rose, GP, MTG, GI, GE and VI increased and then decreased at the same water potential. The optimum water potential and temperature for seed germination of S. transiliense were -0.2 MPa and 10/20°C.
100 10 5/15°C 10/20°C 80 8
60 6
40 4
20 Germination (%) 2 Mean time to germination (days) 0 0 100 10 15/25°C 20/30°C 80 8 Germination (%) Germination
60 6
40 4 (days) germinate to time Mean
20 2
0 0 -0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -0.0 -0.2 -0.4 -0.6 -0.8 -1.0 Water potential (MPa) Figure 1. Germination percentage and the mean time to germinate of Seriphidium transiliense seeds under
different water potentials and temperature regimes. Different letters indicate significant differences (P < 0.05). Lower case letters indicate significant differences in the germination percentage; capital letters indicate significant differences in the mean time to germinate.
The cumulative germination percentage of S. transiliense seeds is shown under different water potentials and temperatures in figure 3. The higher the temperature, the earlier seeds started to germinate. The lower the water potential, the more obvious the seed germination was inhibited or delayed.
Effect of water potential and temperature stress on radicle length and plumule length
There were significant effects of water potential (P < 0.001), temperature (P < 0.001) and the interaction between water potential and temperature (P < 0.001) on radicle length and plumule length. With decrease in water potential, radicle length and plumule length
359 AIPING CHEN, YUXIANG WANG, XIAOQING SUI, GUILI JIN, KUN WANG AND SHAZHOU AN
100 25 (A) (B)
80 20
60 15
40 10
20 5 Germination index Germination Germination energy Germination
0 0 05/10 10/20 15/25 20/30 05/10 10/20 15/25 20/30 Temperature regime (°C) Temperature regime (°C) 400 (C)
300 0.0 MPa -0.2 MPa -0.4 MPa 200 -0.6 MPa -0.8 MPa -1.0 MPa
Vigour index Vigour 100
0 05/10 10/20 15/25 20/30 Temperature regime (°C) Figure 2. Germination energy (A), germination index (B) and vigour index (C) of Seriphidium transiliense under different water potentials and temperature regimes. Error bars represent SE (n = 4). Different letters indicate significant differences (P < 0.05).
100 5/15°C 10/20°C 80 0.0 MPa -0.2 MPa -0.4 MPa 60 -0.6 MPa -0.8 MPa -1.0 MPa 40
20
0 100 15/25°C 20/30°C
80
60
40
Cumulative germination percentage (%) percentage germination Cumulative 20
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Time (days) Figure 3. Cumulative germination percentage of Seriphidium transiliense seeds during a 16-day treatment under different temperature regimes and water potentials.
360 SEED GERMINATION OF SERIPHIDIUM TRANSILIENSE increased and then decreased at the same temperature regime (figure 4). With increase in temperature, radicle length increased and then decreased at the same water potential. Plumule length increased and then decreased as water potential increased except at
5/15°C. Radicle length was 10.83 mm at -0.2 MPa and 10/20°C, but just 1.23 mm at
-1.0 MPa and 20/30°C. Plumule length was 4.28 mm at -0.2 MPa and 10/20°C, but was
2.29 mm at -1.0 MPa and 10/20°C. We found that radicle length was more affected by the interaction between water potential and temperature than that of plumule.
Germination recovery Some seeds that failed to germinate in PEG were transferred to distilled water to recover germination ability. The ability to recover germination at high temperatures is lower than that at low and moderate temperatures, which indicates that high temperature was still an important factor in inhibiting seed germination. The strongest ability to recover germination was observed for those seeds that experienced the lowest water potential (-1.0 MPa) at 10/20°C (figure 5).
15 5 0.0 MPa -0.2 MPa 4 -0.4 MPa -0.6 MPa 10 3 -0.8 MPa -1.0 MPa
2 5 1 Radicle length (mm) length Radicle Plumule length (mm) length Plumule 0 0 05/10 10/20 15/25 20/30 05/10 10/20 15/25 20/30 Temperature regime (°C) Temperature regime (°C) Figure 4. Radicle length (A) and plumule length (B) of Seriphidium transiliense seeds under different temperature regimes and water potentials. Error bars represent SE (n = 4). Different letters indicate significant differences (P < 0.05).
80
60 0.0 MPa -0.2 MPa -0.4 MPa -0.6 MPa 40 -0.8 MPa -1.0 MPa
20 Germination recovery (%) recovery Germination 0 05/10 10/20 15/25 20/30 Temperature regime (°C) Figure 5. Germination recovery of Seriphidium transiliense seeds under different temperature regimes and water potentials. Different letters indicate significant differences (P < 0.05).
361 AIPING CHEN, YUXIANG WANG, XIAOQING SUI, GUILI JIN, KUN WANG AND SHAZHOU AN
Discussion
The germination characteristics of plant seeds are closely related to the climate and habitat conditions in their regions of natural distribution, the result of long-term adaptation of plants (Baskin and Baskin, 2014). Our results showed that GP, GI and GE were higher at higher water potential (-0.2 MPa) than in control and lower water potentials at all temperatures. This indicates that some drought stress can promote seed germination. S. transiliense seeds could also germinate at a low water potential (-1.0 MPa). This shows that they have a high tolerance to water stress at the seed germination stage, which is an evolutionary strategy used by xerophytes in arid desert regions (Zeng et al., 2010). However, GI and GE decreased as water potentials decreased at higher temperatures, and as the germination process slowed down, some seeds became dormant. Dormancy is beneficial to the reproduction of species. Once the environment is suitable, these dormant seeds have the potential to germinate into seedlings, and is conducive to the survival of the species. Temperature is one of the abiotic stresses affecting seed germination. Our results showed that S. transiliense seeds can germinate over a wide temperature range, suggesting the ability of S. transiliense seeds to germinate at any time during the growing season. This germination ability can explain the high adaptation of the species to relatively high temperatures in desert environments. The most suitable temperature regime was 10/20°C, a relatively low temperature, which guarantees germination in early-spring. As long as the appropriate conditions are available, seeds can germinate quickly and can ensure the rapid establishment of seedlings. Germination speed plays a highly adaptive role in arid deserts (Kadereit et al., 2017). Drought and temperature stress not only affects seed germination but also increases the average amount of time required for germination in plants (Vicente et al., 2020). Our results showed that MTG was the shortest at -0.2 MPa and 10/20°C, which will be important for successful establishment and population renewal of desert plants when they are reseeded. Most seeds that failed to germinate under low water potentials and high temperatures recovered their germination immediately in distilled water, which indicates that most seeds were viable. Osmopriming with PEG could enhance the germination speed and germination percentage of S. transiliense seeds upon the arrival of effective rainfall. The assessment of seedling characteristics relating to the radicle and plumule, such as the plumule length and radicle length can be applied to measure the initial plant activity (Bademuqiqige et al., 2018; Lotfi et al., 2019). Low water potential and high temperature significantly reduced the growth indices, including the radicle and plumule length, especially in the tissues of the radicle. Radicle length and plumule length were higher than that other treatments when the temperature regime was 10/20°C or 15/25°C, and the water potential was higher than -0.2 MPa. It showed that the seeds of S. transiliense would grow rapidly upon germination and could ensure seedling survival when the seeds are sown at 10/20°C or 15/25°C and high water potential in the desert. Light conditions provide important information about the best time and place for seedling establishment (Carta et al., 2017). Different plants have different sensitivity to the light environment for seed germination. Some species require light to germinate,
362 SEED GERMINATION OF SERIPHIDIUM TRANSILIENSE some require darkness to germinate and some are photoblastic neutral (Baskin and Baskin, 2014). Many desert species prefer dark conditions to germinate, so it would be necessary to test interactive effects of light, temperature and water potential on seed germinability of S. transiliense.
Conclusions
Our study showed that S. transiliense seeds were affected by changes in water potential and temperature. The seeds of S. transiliense could germinate at low and high temperatures, while the optimum temperature regime was 10/20°C. When the soil water potential was -1.0 MPa, there were still some seeds that could germinate, but the optimum water potential was -0.2 MPa. Seeds of S. transiliense could tolerate high temperatures and drought in the stressful habitats of arid desert ecosystems, and had fast germination recovery after a major rainfall event. These results showed that this species has a germination capacity under adverse environmental conditions, ensuring that the species can be used in the ecological restoration of degraded environments in arid regions.
Acknowledgements
This work was supported by the Special Project of National Science and Technology Basic Resources Survey-Investigation of the Basic Resources in Desert Regions of China - “The Investigation of Desert Main in Plant Communities in Tarim-Junggar Basin ” (2017FY100201).
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