Effects of Seed Priming on Seedling Growth of Isabgol under Salinity Stress

Isabgol (Plantago ovata Forsk.) is an annual species grown in the arid and semi-arid regions and is used widely in traditional and Industrial Pharmacology (Patel et al., 1996; Mahdavi, 2013). Salinity is one of the significant abiotic stresses, limits crop growth and production. Additionally, secondary salinization caused by poor irrigation and/or drainage practices is a continuing natural process in agriculture (Shabala, 2013; Qadir et al., 2014; Shaikh-Abol-hasani and Roshandel, 2019). Bray (2000) reported that salinity has been a restricting factor that affects all plant growth stages. Germination failure in saline soils results from high salt concentrations in the seed planting zone because of the upward movement of soil solution and subsequently evaporation of the soil surfaces. This has been attributed to both osmotic and toxic effects (Song et al., 2005; Farahbaksh and Pour, 2012; Nedjimi, 2013).
       
Seed priming is one of the biological strategies by which salinity tolerance of potential crops can be increased. In priming seeds are exposed to restricted water availability under controlled conditions which allow some of the physiological processes to occurs before complete germination (Sharma et al., 2014; Gholami, Mokhtarian and Baninasab, 2015). The main advantage of this technique is the plant protection against diseases and pests and also reduction in the use of fertilizers and pesticides. Therefore, the farmer can reach a crop with more quality and quantity with expensing less time, cost and effort (Fehhenabi, 2007; Hoseini, Kouchebagh and Jahandidet, 2013). Demir and Mavi (2004) found watermelon seed priming with KNO₃ solution effectively improved germination and seedling growth of the seeds under salinity compared to non-primed seeds. Cayuela et al., (1996) found that tomato seed priming improves germination, seedling emergence and seedling growth under saline conditions.
       
Seed priming provides faster and synchronized seed germination and growth. Therefore, the present study was conducted to study the effects of seed priming on seedling growth of Plantago ovata Forsk. var. GI-3 under salinity stress.

Plant material
 
Seeds of Plantago ovata Forsk. var. GI-3 obtained from seed Spices Research Station, Jagudan, Gujarat, served as the primary material for the present experimentation.
 
Salt screening test
 
To determine the threshold level of salt concentration at which GI-3 seed germination decreased, solution of NaCl were prepared for the different EC of 0, 4.2, 4.6, 5.1, 5.3 dS/m. Seeds were surfaced sterilized with 5% of sodium hypochlorite for 2 minutes. 20 seeds of GI-3 were placed in Petri dishes (100 ×15 mm) with Whatman’s Filter Paper No. 2. Each dish was moistened with uniforms amounts of desired salinity level. Petri dishes were placed at room temperature in the laboratory. 5 ml of the appropriate solutions were added every alternate day to each petri dish. Seeds were considered germinated once the radicle was at least 2 mm long. The final germination percentage was determined after 7 days. There was 60% reduction in germination percentage at the salinity level of 5.3 dS/m.
 
Priming treatment
 
The seeds were sterilized with sodium hypochlorite (4%) solution for 3 minutes and dried on filter paper. Seeds were treated with the following (i) unsoaked seeds (Control); (ii) 0.5 mM of salicylic acid (T1) for 22 h; (iii) 10% KNO₃ (T2) for 22 h and (iv) 10 ppm Gibberellic acid (T3) for 22 h. Priming treatments have been shown in Fig 1. During priming, containers were kept in the dark condition and well aerated with an aquarium water pump because sufficient oxygen is required for seed respiration in osmotic seed priming. After priming, the seeds were rinsed twice with distilled water and dried to the original moisture content as the unprimed seeds and immediately used for germination test.
 

 
Seed germination experiment
 
The experiment was carried out in the Physiology Laboratory of the Department of Botany, University School of Sciences, Gujarat University during Rabi season of 2020-21. The experiment was conducted in completely randomized design with four replications. Two salinity levels of 0 dS/m and 5.3 dS/m have been decided from the above-mentioned salt screening. Unprimed seeds were used as control. Total 20 seeds were placed on Whatman’s No. 1 filter paper per petri dish. The petri dishes were kept under lab conditions i.e. at 25±2°C and humidity 40%. For the salt stress 10 ml of NaCl solution prepared for 5.3 dS/m has been added to the petri dish in the interval of every 2 days. Data were recorded daily on germination for 7 days. After that, data collected on various parameters such as germination percentage, mean germination time, plumule length, radicle length, vigour Index and fresh weight were recorded (Fig 2). Formulas used for the calculation have been mentioned below: 
 
 
  

 
(b) Mean germination time (days)= d1/n1+d2/n2+d3/n3+……
 
Where,
d = Number of days.
n= Number of germinated seed.
 
(c) Seed vigour= Germination % * (RL+PL)
 
Where,
RL= Radicle length.
PL=Plumule length.
 
Statistical analysis
 
Two-way ANOVA was performed to analyse the data by using SPSS software version 26 and means were compared with Duncan’s multiple range test at 5% statistical probability levels.

Germination percentage     
 
The result showed the effect of salinity and interaction of salinity with priming was significant on Isabgol seed germination percentage (Table 1). The highest germination percentage has been observed in control without salinity with an average of 91.25% (Table 2). The germination percentage of GI-3 significantly is influenced by different priming treatments under salinity stress. All the priming treatments increased the germination percentage in primed seeds over control (26.3%) under salinity stress (Fig 3); being maximum in salicylic acid (62.5%) followed by KNO₃ (60%) then gibberellic acid (45%). Munns et al., (1988) stated that the reduction in germination under salinity is due to less water availability to plant roots via low osmotic potential and ionic toxicity of Na⁺ and Cl^-. Salicylic acid is one of the promising priming agent which promotes germination percentage in saline condition. Similar kind of results were obtained by Afzal et al., 2005, observed that Salicylic acid treatment increases germination percentage under salinity conditions. The germination percentage decreases with increased Gibberellic acid concentration, these findings are in accordance with Chauhan et al., (2009) stated that higher concentration inhibits germination in Black gram and Horse Gram.
 

 

 

 
Mean germination time
 
Mean Germination Time of Isabgol was affected by priming (Table 1). Priming with KNO₃ has significantly reduced Mean Germination Time under saline condition i.e. 2.8 days as shown in Table 2. However, priming with Gibberellic acid has shown increased MGT in salt stress compared to primed seeds without any salinity stress (Fig 4). Decrease in MGT was also observed by Karimi and Varyani (2013) in Marigold and in rapeseed by Ghassemi-Golezani et al., (2010).
 

 
Plumule length
 
Effect of priming was significant on Plumule length (Table 1). Seeds treated with Salicylic Acid and Gibberellic acid produced the higher plumule length than untreated. The highest Plumule length was observed in Salicylic Acid without salinity stress with an average of 2.325 cm (Table 2). The mean of Plumule length has not been affected by the salinity stress. Although priming with KNO₃ in salinity stress (Fig 5) significantly increased in the mean Plumule length (2.13 cm) as compared to primed seeds under no salinity stress (1.96 cm). The results are in accordance with Mavi et al., (2006) and Shehzad et al., (2012), who reported that priming with KNO₃ increased seedling size in tomato and sorghum.
 

 
Radicle length
 
Effect of salinity, priming and interaction of salinity with priming were significant on Radicle length (Table 1). The lowest radicle length was in control under salinity stress while the highest was achieved in KNO₃ without salinity stress (Table 2). The mean Radicle Length during the seed germination of unprimed seeds without any salinity stress is 0.98 cm, while under salinity stress, the mean radicle length of unprimed seeds declines to 0.53 cm. Priming with KNO₃ has shown an increased mean radicle length of 1.66 cm at 0 dS/m (Fig 6). These results confirm the finding of Stofella et al., (1992), who reported that priming has significantly improved root length in pepper seeds.
 

 
Seed vigour
 
Priming has shown significant effect on seed vigour (Table 1). The highest seed vigour was 298.725 on an average in KNO3 under salinity stress (Table 2). Priming with Salicylic acid-induced maximum seed vigour i.e. 352 under salinity stress (Fig 7). This finding is in accordance to Afzal et al., (2005), who reported positive effects of priming with salicylic acid on seed vigour in Wheat under salinity stress. Priming with KNO₃ also improved vigour over the unprimed seeds under salinity stress i.e. 299.
 

 
Fresh weight
 
Priming has shown significant effect on Seedling fresh weight (Table 1). Seeds primed with KNO₃ produced seedling having maximum fresh weight, i.e. 0.139 gm followed by Gibberellic acid (0.12 gm) as shown in Fig 8. However, unprimed seeds do not show any decrease in fresh seedling weight under salinity stress. The increased fresh weight with KNO₃ was also observed by Jabeen and Ahmad (2011) in Helianthus annus L. and Carthamus tinctorius L. Similar kinds of result was also obtained by Hamayun et al., (2014) who reported priming with KNO₃ has increased the fresh weight of Glycine max L. seedlings under saline condition.
 

Considering the above results obtained from the present investigation, it could be concluded that among the various priming treatments viz. salicylic acid, KNO₃ and gibberellic acid, KNO₃ was the most effective in alleviating the adverse effects of salt stress during seed germination of Gujarat Isabgol-3. However, after KNO₃, Salicylic acid was also influential in promoting various parameters during the germination under saline conditions. Therefore, it could be concluded that seed priming with KNO₃ and salicylic acid is an effective approach to overcome salinity in Gujarat Isabgol-3 and recommended as a pre-sowing treatment under salinity stress.

The authors are very grateful to UGC, New Delhi, India for the financial assistance to carry out the present investigation.

None.