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J. Crop Sci. Biotech. 2011 (December) 14 (4) : 289 ~ 295 DOI No. 10.1007/s12892-011-0039-x RESEARCH ARTICLE
Chemical Priming with Urea and KNO3 Enhances Maize Hybrids (Zea mays L.) Seed Viability under Abiotic Stress Hadi Pirasteh Anosheh1, Hossein Sadeghi2*, Yahya Emam1 Department of Crop Production and Plant Breeding, College of Agriculture, Shiraz University, Shiraz, Iran Department of Desert Region Management, College of Agriculture, Shiraz University, Shiraz, Iran
1 2
Received: June 19, 2011 / Revised: July 28, 2011 / Accepted: October 19, 2011 Ⓒ Korean Society of Crop Science and Springer 2011
Abstract Seed priming is a method to improve germination and seedling establishment under stress conditions. The effect of seed priming in chemical solutions such as urea and KNO3, on protein and proline content, germination, and seedling growth responses of four maize (Zea mays L.) hybrids under drought and salt stress conditions was studied in a controlled environment in 2010. Treatments included stress type and intensity at five levels: moderate drought (MD), severe drought (SD), moderate salt (MS), severe salt (SS), and control (C1, without stress), three seed priming types including water (C2, as control), KNO3, and urea (as chemical priming), and four maize hybrids including Maxima, SC704, Zola, and 307. The results showed that the highest germination percentage (Ger %), germination rate (GR), seedling length (SL), radical length (RL), and seedling to radical length ratio (S/R) were achieved in no stress treatments and most proline content in SD treatment. Urea priming led to more Ger%, GR, and SL compared to other primers and treatment under KNO3 priming resulted in higher RL compared to other primers. Chemical priming had no effect on S/R and proline content. Also, in terms of most traits, no difference was found among the four hybrids. Results showed that salt stress could affect GR and RL more than the drought stress. Drought stress affected germination percentage and S/R more than the salt stress. Both stresses decreased all measured parameters, except protein and proline content which were increased remarkably, and more under drought compared to salt stress. Based on proline content, hybrid 304 appeared to be more resistant to stress than other hybrids. Generally, KNO3 and urea alleviated effects of both stresses and led to increased germination and seedling growth as well as the root length. Therefore, priming could be recommended for enhancing maize growth responses under stressful conditions. Key words: germination, potassium nitrate, priming, proline, protein, urea
Introduction Since early migration from aquatic environments to the land, plants have had to cope with periodic and unpredictable environmental stresses. Crop production in arid and semi-arid regions is restricted by soil salinity and soil deficiencies in moisture (ELSiddig et al. 1998; Pessarakli 2001). Water deficit associated with salinity in irrigation water is the major limiting factor in the most regions where plants are subjected to extreme water deficit during the dry seasons (Kerepesi and Galiba 2000). In these climatic conditions, salts may accumulate in the soil because of high evaporative demand and insufficient leaching of ions due to Hossein Sadeghi (
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[email protected] Tel: +98-711 6138171 / Fax: +98-711 2287159
The Korean Society of Crop Science
low precipitation. Therefore, soil salinity is sometimes a key factor in determining the ecological distribution of droughtadapted species (Gucci et al. 1997). Moisture stress at any stage of crop growth can cause an irreversible loss in yield potential (Reginato 1993). Rapid and uniform field emergence is an essential prerequisite to reach the yield potential, quality, and ultimately profit in annual crops (Parera and Cantliffe 1994). Greater and better synchronized germination is crucial for achieving an optimal seedling establishment and better productivity; however, several environmental constraints are great impediments (Wahid et al. 2008). One pragmatic approach to increase crop production is seed invigoration (Basra et al. 2004; Farooq et al. 2006; Lee and Kim 2000). Seed invigoration strategies include hydropriming, osmocondi-
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tioning, osmohardening, hardening, hormonal-priming, matripriming, and others (Chiu et al. 2002; Kao et al. 2005; Windauer et al. 2007). Priming offers a means to raise seed performance in many crop species (Chiu et al. 2002). Heydecker et al. (1973) defined seed priming as a pre-sowing treatment in osmotic solution that allows seeds to imbibe water to proceed to the first stage of germination, however, prevents radical protrusion through the seed coat. Taylor et al. (1998) used a broader term, seed enhancement, which includes presoaking hydration (priming), coating technologies, and seed conditioning. Therefore, seed priming can be accomplished through different methods such as hydropriming (soaking in water), osmo-priming (soaking in osmotic solutions such as polyethylene glycol, potassium salts, e.g. KCl, K2SO4) solid matrix priming, and using plant growth regulators (PGRs) (Capron et al. 2000; Chiu et al. 2002; Dearman et al. 1987; Harris et al. 2002). Priming has been known as a viable technology to enhance rapid and uniform emergence, high vigor, and better yields mostly in vegetable and flower species (Bruggink et al. 1999; Dearman et al. 1987; Parera and Cantliffe 1994) and some field crops (Basra et al. 1988; Chiu et al. 2002; Giri and Schillinger 2003; Harris 1999; Hartz and Caprile 1995; Murungu et al. 2004). Seed priming has been a common seed treatment to reduce the time between seed sowing and seedling emergence and the synchronization of emergence (Parera and Cantliffe 1994). Seed deterioration is associated with loss of membrane integrity, changes in enzymatic activities, decline in protein and nucleic acid synthesis, and lesions in DNA (McDonald 1999). These deteriorative changes have frequently been related to activate oxygen species (AOS)-induced oxidative injury (BernalLugo and Leopold 1998; McDonald 1999). Basra et al. (1989) found that priming of corn seed with using polyethylene glycol (PEG) or potassium salts (K 2HPO 4 or KNO3) resulted in accelerated germination. Researchers have reported that seed priming in rapeseed improved germination percentage and increased seedling establishment and growth (Basra et al. 2003; Mohammadi 2009). Seed priming had positive effect on germination characteristics of other crops, such as corn (Zea mays L.) (Basra et al. 1989; Murungu et al. 2003), pop corn (Zea mays sacharum L.) (Chiu et al. 2002), and cotton (Gossypum hirsutum L.) (Murungu et al. 2003; Toselli and Casenave 2003). There are some studies on the effect of seed priming on germination and early growth rate of maize. Basra et al. (1989) found that priming of corn seed using polyethylene glycol or potassium salts (K2HPO4 or KNO3) resulted in accelerated germination at a chilling germinator. Similarly, it has been reported a marked improvement in germination when the pretreated corn seeds with substituted phthalimide, GA 3, and abscisic acid (ABA) were germinated under sub and super optimal temperature regimes (Basra et al. 1988). There are some reports showing similar benefits of other chemicals and PGRs for seed priming. In field grown maize, Kulkarni and Eshanna (1988) found that pre-sowing seed treated with IAA at 10 ppm (part per million) improved root length, rate of germination, and seedling vigor,
Table 1. Characteristics of the solutions made for pod moisturizing Solution Made state Moderate Drought - 0.075 bar* Severe Drought - 0.15 bar Moderate Salt 100 mM NaCl Severe Salt 300 mM NaCl Control (C1) without stress *Based on Michel and Kaufmann formula (Michel and Kaufmann 1973) by using polyethylene glycol 6000 (PEG).
especially for seeds from poor-quality seed lots. Hastened germination in sweet corn when primed using polyethylene glycol has also been reported (Chiu et al. 2002). The present study was conducted to examinet he effects of different seed priming chemicals such as urea and potassium nitrate and to study their effects on total protein, proline content, germination percentage, and seedling growth responses of four maize hybrids under drought and salt stress conditions.
Materials and Methods This study was carried out as a factorial experiment based on completely randomized design (CRD) in a controlled environment (growth chamber) of the laboratory at the College of Agriculture, Shiraz University, Shiraz, Iran [52º 46' E, 29º 50' N, altitude 1,810 m above sea level (ASL)], 12 km north of the city of Shiraz during 2010. The first factor was stress type and intensity at five levels; moderate drought, severe drought, moderate salt, severe salt, and control (C1, without stress). Seed priming was the second factor; hydro-priming (C2, as control), KNO3 and urea (as chemical priming), and four maize hybrids (Maxima, Zola, SC704, and SC307) was the third factor. In this study, proline content (Pro), germination percentage (Ger%), germination rate (GR), seedling length (SL), radical length (RL), and seedling to radical length ratio (S/R) were measured. The origin of Maxima, Zola, SC704, and SC304 hybrids are Hungary, Greece, Iran, and Iran, respectively. Their height and maturity were standard; late, semi-dwarf; semi-late, standard; late and standard; semi-late, respectively. Habits of all the hybrids are temperate and semi-temperate. Before the experiments, Petri dishes and solution dishes were put in oven for 24 h at 110°C. The seeds were surface sterilized with 5% NaOCl (sodium hypochlorite) for 5 min to avoid fungal invasion, followed by washing with distilled water (Basra et al. 2003). Before planting, seeds were primed with related primer (water, KNO3 or urea), for 24 h. Then seeds were washed twice with distilled water. Twenty-five seeds were placed in each petri dish. Seeds were placed in 9 cm petri dishes on two layers of filter paper (Whatman No.1). During the experiment, the Petri dishes were irrigated with five different solutions consisted of moderate drought (MD), severe drought (SD), moderate salt (MS), severe salt (SS), and without stress as control (C1). Solutions were made based on Table 1 (Michel and Kaufmann 1973). NaCl was used for salt stress and PEG 6000 for the drought stress (Ellis et al. 1987;
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Table 2. Mean squares for measured parameters SOV†
df
Ger%‡
G.R.
SL
RL MS
S/R
Stress (S) 4 12268 . 0555** 202 . 9730** 2454 . 0091** 6921 . 0650** 0. 5554** Priming (P) 2 3448 . 5722** 8 . 6165** 897 . 9102** 5856 . 1115** 0. 2052** Hybrid (H) 3 1821 . 2500** 0 . 5033ns 5. 2562ns 418 . 1360** 0. 0019ns SxP 8 662 . 4680** 1 . 8571** 87 . 5975** 219 . 4321** 0. 0227** SxH 12 279 . 3333** 0 . 6466ns 8. 0778ns 135 . 0110** 0. 0058ns PxH 6 264 . 6166** 0 . 5018ns 4. 3711ns 266 . 0975** 0. 0015ns SxPxH 24 357 . 5125** 0 . 6507ns 4. 8362ns 230 . 3056** 0. 0042ns Error 120 20 . 5722 0 . 5550 6. 23894 37 . 6151 0. 0033 CV (%) 8. 4209 12 . 1967 8. 5477 12 . 0651 18 . 9215 *, ** and ns: Significantly different at 5% and 1% probability level, and no significant difference, respectively. † SOV : Source of variance, df: Degrees of freedom, CV: Coefficient variance, MS: Mean square. ‡ Ger% : Germination percentage, G.R.: Germination rate, SL: Seedling length, RL: Radical length, S/R: Seedling to radical length ratio.
Michel and Kaufmann 1973). Dishes were placed in a germinator at 23 ± 2°C. The filter papers of each Petri dish were replaced every two days to prevent salt accumulation (Rehman et al. 1998). Seed germination was recorded daily up to day 11 after the beginning of the experiment. A seed was considered germinated when radical emerged by about 2 mm in length (Kulkarni and Eshanna 1988). In each recording, four seedlings were randomly selected from each petri dish, and their averages were considered as sample data. According to the daily germination percentage data, germination rate was calculated based on the following formula (Mohammadi 2009):
Where GR is the germination rate, n is the number of seeds germinated on a specific day, and D is the number of days from the start of experiment. Germination percentage was determined at the end of the experiment, based on the following formula (Bajehbaj 2010):
In this formula, Ger% is Germination percentage, n is number of seeds germinated, and N is total number of seeds planted. For the proline and protein measurements, seedlings of three randomly selected plants in each petri dish were individually harvested at the end of experiment. Then samples immediately had frozen in liquid nitrogen, freeze-dried, and stored at -80°C for future extraction. Frozen samples were ground to fine powder in liquid nitrogen using a pestle and mortar and ice-cold extraction buffer containing 0.1 M Tris-HCl buffer (pH = 7.5), 5% (w/v) sucrose, and 0.1% 2-mercaptoethanol (3:1 buffer volume/FW). The homogenate was spun at 12,000 x g for 20 min at 4°C, and the supernatant was used for proline and protein measurements, as described below. All procedures for enzyme extraction and assays were carried out at 4°C (Lowary et al. 1951). The concentration of free proline was determined according to the acid-ninhydrin method (Bates et al. 1973) using by spectrophotometer (Biochrom Ltd, Biowave S2100 - Cambridge, UK), with minor modifications. Total protein was measured according to Bradford (1976) using bovine serum albumin (BSA) as a standard. The optical density was measured at 595 nm using by spectrophotometer.
Proline
Protein
19 . 8565** 5. 2238** 4. 7120** 1. 6932** 1. 8693** 0. 6648** 0. 4773** 0. 1783 22 . 83185
2268 . 1236** 8531 . 3822** 130 . 0563* 936 . 6460** 39 . 0313* 600 . 0013** 17 . 4237ns 56 . 2156 12 . 0876
Statistical analysis was performed for each studied parameter based on a randomized complete block design model with four replications using SAS12 software (SAS Inst. 2004). The results of variance analysis showed in Table 2. Means were compared by LSD Test (Least Significant Difference) at P ≤ 0.05, which is entered in Table 3, separately.
Results Germination The germination percentage was significantly affected by stress, priming, hybrid, and interactions between stress and priming, stress and hybrid, priming and hybrid, and also interaction of stress, hybrid and priming (Table 2). The highest germination percentage was achieved in without stress treatment (99.2%) and the lowest in severe drought (51%) (Table 3). Drought stress affected germination more drastic than salt stress. Among priming treatments, germination percentage was greater in urea priming (Table 3). Among hybrids, germination percentage was more in hybrid 704 (Table 3 and Fig. 1). This finding can be useful for farmers to have better seedling establishment in 704 under undesirable conditions. In control conditions (without stress), no difference in germination between chemical primed and hydro-primed treatments was found. However, in moderate and severe drought stress KNO3 priming had the best germination rate compared to the other primers (Fig. 1). The effects of stress, priming and interactive effects of stress and priming on germination rate was highly significant (Table 2). The highest germination rate was achieved in without stress treatment. Salt stress had greater decreasing effect on germination than drought and germination rate was greater in moderate than severe stresses (Table 3). Urea priming had higher germination rate and there was no significant differen ce between KNO3 and hydropriming (Table 3). Although no significant difference was found between hybrids, hybrid 704 had better germination rate (Table 3). This trait helps maize to emerge quickly and complete canopy at early growth. Regarding the interaction of the stress and priming which is shown in Fig. 2, it could be said that KNO3 has more positive effect on germination rate than any other priming treatments.
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Fig. 1. Relationships between stress levels with water and chemical priming (urea and KNO3) on germination percentage of four maize hybrids (Maxima, Zola, SC704, and SC304) under different stress conditions - Least Significant Difference = 7.3826.
Fig. 3. Relationships between stress levels with water and chemical priming (urea and KNO3) on protein content of four maize hybrids (Maxima, Zola, SC704, and SC304) under different stress conditions - Least Significant Difference = 0.871.
Fig. 2. Relationships between stress levels (with water and chemical priming (urea and KNO3) on germination rate of four maize hybrids (Maxima, Zola, SC704, and SC304) under different stress conditions - Least Significant Difference = 1.1967.
Fig. 4. Relationships between stress levels with water and chemical priming (urea and KNO3) on proline content of four maize hybrids (Maxima, Zola, SC704, and SC304) under different stress conditions - Least Significant Difference = 0.682 µM g-1 DW.
Biochemical changes
slightly more negative water potentials, the amino acid proline begins to increase sharply. Proline synthesis in the cytoplasm is carried out to form and maintain glutamic acids levels (Emam and Seghatoeslami 2005). Both priming agents decreased proline content in this experiment; this means without stress treatments had more proline level than urea and KNO3. Among the hybrids, hybrid 304 had more proline content than the others, indicating that this hybrid might be more resistant to drought stress (Fig. 4). Urea priming resulted in higher proline content compared to the other priming treatments. This might have been due to the role of nitrogen in proline.
Protein level was significantly affected by stress, priming, interaction of stress and priming, interaction of priming and hybrid (1%), hybrid and interaction of stress and hybrid (5%) (Table 2). The highest and the lowest protein levels were observed in severe drought and severe salt stress, respectively (Table 3). Also, the protein level was higher under urea priming treatments (Fig. 3). This may be due to the role of nitrogen in protein structure. Also, some of researchers said that the enhancing in proteins content in these plants could be detrimental due to unfair distribution of carbon resources stored in endosperm. Plants might use their precious carbon resource to produce more proteins and protect against abiotic stress. The hybrid 304 had more protein level than the other hybrids. Priming, stress, hybrid and their interactions had significant effect on proline content. The most proline content was achieved in the severe drought condition compared to the moderate drought. Salinity had negative effect on proline content (Table 3). At
Early growth Seedling length was significantly affected by stress, priming, and the interaction of the stress and priming (Table 2). The longest seedlings were observed in without stress treatment and the shortest ones in the severe drought and salt stress (Table 3).
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Table 3. Mean comparison for the main effects of treatment on proline and protein content, germination percentage, and seedling growth (average of all data) SL RL Proline Protein Ger%† S/R G.R. (cm) (cm) (µM/g DW) (mg/g DW) (%) Stress C1‡ 99 .17a 6.70a 23 .76a 65 .00a 0.37a 1.22d 131.26e MD 66 .33c 3.72b 12 .04b 65 .55a 0.18c 2.35b 271.05b SD 51 .00e 2.22c 3 .88c 43 .45c 0.08d 2.92a 394.23a MS 76 .58b 1.48d 12 .23b 47 .80b 0.26b 1.45c 173.82c SS 61 .80d 0.67e 3 .47c 33 .20d 0.10d 1.37cd 171.42d Priming C2 69 .50b 2.82b 7 .04c 41 .72c 0.26a 2.19a 206.85b KNO3 64 .00c 2.67b 11 .43b 60 .6a 0.15b 1.68b 196.09c Urea 79a 3.39a 14 .76a 51 .56b 0.18b 1.7b 282.14a Hybrid Maxima 67 .93c 2.90a 11 .58a 52 .00a 0.21a 1.99b 244.13c Zola 63 .27d 2.99a 10 .97a 46 .28b 0.20a 1.41b 168.30d SC704 77 .40a 3.09a 10 .81a 52 .68a 0.19a 1.09b 250.49b SC304 74 .73b 2.85a 10 .94a 53 .04a 0.19a 2.04a 250.52a Means with same letter in each column are not significantly different using LSD's multiple range test (P ≤ 0.05). † Ger%: Germination percentage, GR: Germination rate, SL: Seedling length, RL: Radical length, S/R: Seedling to radical length ratio. ‡ C1: Control (without stress), MD: Moderate Drought, SD: Severe Drought, MS: Moderate Salt, SS: Severe Salt, C2: Control (without priming).
Fig. 5. Relationships between stress levels with chemical priming (urea and KNO3) on seedling length of four maize hybrids (Maxima, Zola, SC704, and SC304) under different stress conditions - Least Significant Difference = 3.9339.
Table 4. Correlation (R2 value) between stress values, proline content, germination percentage, and seedling growth Stress G.R. SL RL S/R Proline Protein Ger%† level C1‡ MD SD MS SS Total
0.76 0.11 0.63 0.68 0.23 0.32
0.68 0.48 0.63 0.42 0.56 0.88
0.21 0.69 0.69 0.51 0.21 0.58
0.99 0.98 0.98 0.99 0.96 0.47
0.93 0.94 0.99 0.88 0.73 0.54
0.79 0.59 0.95 0.58 0.65 0.31
0.71 0.68 0.89 0.48 0.94 0.21
† Ger%: Germination percentage, GR: Germination rate, SL: Seedling length, RL: Radical length, S/R: Seedling to radical length ratio. ‡ C1: Control (without stress), MD: Moderate Drought, SD: Severe Drought, MS: Moderate Salt, SS: Severe Salt.
Chemical priming had positive effect on seedling length and urea was more effective than KNO3 (Table 3). There was no different between seedling length reduction due to salt and drought (Table 3 and Fig. 5). All sources of variance had significant effect on radical length. Although stresses decreased length of radical, reduction in radical length was lower than that of seedling length. Priming agents had positive effect on radical length and KNO3 was found better than urea (Fig. 6). Zola hybrid had the lowest radical length (Table 3). Chemical priming decreased seedling to radical length ratio, it was lower than 1 in all cases.
Discussion In the present study, salt stress had more decreasing effect on germination rate, seedling length, and radical length compared to drought stress. Meanwhile, germination percentage and seedling to radical length ratio decreased more by drought stress
Fig. 6. Relationships between stress levels with water and chemical priming (urea and KNO3) on radical length of four maize hybrids (Maxima, Zola, SC704, and SC304) under different stress conditions - Least Significant Difference = 9.9716.
than the salt stress. Moisture stress at any stage of crop growth such as germination can cause an irreversible loss in yield potential (Emam and Seghatoeslami 2005; Reginato 1993). Salt and osmotic stresses are responsible for both inhibition or delayed seed germination and seedling establishment (Almansouri et al. 2001). Under these stresses, there is a decrease in water uptake during imbibitions and furthermore salt stress may cause excessive uptake of ions (Murillo et al. 2002). Germination and seedling establishment are critical stages in the plant life cycle (Emam 2007). In crop production, stand establishment determines plant density, uniformity and management options (Cheng and Bradford 1999; Emam 2007). Germination rate helps maize to fast emergence and canopy closure at early growth. Seed priming has been used to improve germination, reduce seedling emergence time, and improve stand establishment and yield (Murungu et al. 2004). Research has demonstrated that seed priming had been successfully improved germination in many crops (Parera and
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Cantliffe 1994; Singh 1995). Priming has been shown to enhance seedling establishment under stressful conditions (Ashraf and Rauf 2001; Basra et al. 2005). It is very important to determine proper priming rate, because the high level of prime was reported to decrease seedling growth in rapeseed (Omidi et al. 2005) or no beneficial effect on grain yield (Subedi and Ma 2005). Also, it has been reported that longevity of primed sweet corn seed was decreased under improper temperature priming (Chiu et al. 2002). While both stresses increased the proline and protein contents, drought stress had more effect than the salt stress. Urea priming increased germination percentage, seedling length, and protein content remarkably. Potassium nitrate had more effect on radical length. So, urea and potassium nitrate priming might be recommended for normal and stressful conditions, respectively. Among the hybrids, hybrid 704 had the best traits under without stress conditions and hybrid 304 had the best response under stress conditions. Protein synthesis may be an almost equally sensitive to water stress (Pessarakli 2001; Reginato 1993). Hydro-priming had more positive effect than KNO3. Hybrid 304 had more protein level than the other hybrids. The relationship between abiotic stresses, proline content, germination percentage, and seedling growth, according Rsquared value (regression), is shown in Table 4. Salinity had a negative effect on proline content, and decreased it more in severe salt stress treatments. As was stated for protein synthesis, proline content may be an almost equally sensitive to water stress (Reginato 1993). At slightly more negative water potentials, the amino acid proline begins to accumulate sharply; sometimes building up to levels of 1 percent of tissue dry weight (Wahid et al. 2008). Free proline accumulation has been suggested to be an indicator of drought resistance (Emam and Seghatoeslami 2005). In all the stress levels, urea priming, due to the role of nitrogen in proline structure, increased the proline content.
Conclusions Germination percentage was significantly affected by stress, priming, hybrid, interactions between stress and priming, stress and hybrid, priming and hybrid, and also interactions of stress, hybrid, and priming. Drought stress affected germination more drastic than salt stress. Among the priming treatments, germination percentage was greater in urea priming. Among the hybrids, germination percentage was more in hybrid 704. This finding can be useful for farmers to have better seedling establishment in 704 under undesirable conditions. In without stress conditions, no difference in germination between hydro-primed and chemical primed treatments was found. However, in moderate and severe drought stress KNO3 priming had the best germination rate compared to the other primers. Salt stress could affect GR and RL more than the drought stress. Drought stress had more effect on increasing proline content. Generally, chemical priming such as KNO3 and
urea alleviated effects of both stresses and led to increased germination and the seedling as well as the root length. Therefore, priming could be recommended for stressful conditions.
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