Introduction

Improving productivity of crop plants under salt stress has been addressed through various means but still remains a challenge for modern agricultural development. Expensive measures like soil amendments, selection and breeding of cultivars that can give a good economic yield under salinity, offer a solution to minimize the losses incurred due to salinity but have met with limited success due to lack of efficient selection criteria, time and labour consuming procedures (Shannon and Greive 1999). Seed priming is an alternative method of seed treatment that is applied to seeds, which are sown under different stress conditions to improve the rate and synchronization of seed germination.

Wheat (Triticum aestivum L.) is a moderately salt-tolerant crop (Maas and Hoffman 1977) and can tolerate salinity levels up to 100 mM NaCl (about 10 dS m−1). Most of the salt tolerant germplasm is derived from selections made from Kharchia 65, a line developed from saline–sodic soils on farmer’s field (Rana 1986). Physiological traits like low transport of Na+ to the shoots, with high selectivity for K+ over sodium has been associated with salt tolerance in wheat (Gorham et al. 1981; Ashraf and Khanum 1997) although it does not hold across all genotypes (El-Hendawy et al. 2005). Osmo-priming of wheat seed with CaCl2 improved the leaf K+ contents with simultaneous decrease in Na+ concentration that led to increased grains per spike, 1000-grain weight, grain yield and harvest index in plants from primed seeds (Jafar et al. 2012). Seed bio-priming with different salinity tolerant isolates of Trichoderma also improved germination and emergence rate during salinity stress in wheat (Rawat et al. 2011). All these techniques involve partial hydration of the seed until germination process begins (Bradford 1986) but can pose a problem for storage of the primed seeds. Magneto-priming (exposure to static magnetic field) is a non invasive dry seed priming treatment used for improving vigour and emergence in seeds under non stressed and stressed condition (Harichand et al. 2002; Ruzic and Jerman 2002; Shine et al. 2011; Bhardwaj et al. 2012; Bilalis et al. 2013; Radhakrishnan and Ranjitha-Kumari 2013). Magnetic field (MF) treatment of maize seeds reflected the potential to enhance seedling growth, leaf water status, and photosynthesis rate and lower the antioxidant defense system of seedlings when grown under soil moisture stress (Anand et al. 2012). The present study was aimed to evaluate the role of MF on wheat seed priming for ameliorating the detrimental effect of salinity.

Materials and methods

Wheat seeds of genotypes Kharchia 65 (salt tolerant) and HD 2967 (salt sensitive) were procured from Division of Genetics, Indian Agricultural Research Institute, New Delhi. Healthy seeds of both the genotypes were exposed to static MF of 50 mT for 2 h by placing in a cylindrical-shaped sample holder of 42 cm3 capacity made of a non-magnetic thin transparent plastic sheet between the poles of the electromagnet having a uniform MF (Rathod 2013). For parallel control, seeds from the same lot were kept under non-magnetic field condition. Salinity was artificially created in the soil filled in the 12″ plastic pots lined with polythene bag. The soil was irrigated with 150 mM NaCl solution at seedling stage (10 DAS) and pH and EC of 8.1 and 13.6 dS m−1, respectively, was maintained till maturity following the procedure of Munns (1993). The pH and EC of non saline soil was 7.9 and 2 dS m−1, respectively. Five pots were kept per treatment, with three plants per pot. Two experiments were conducted side by side: one for the evaluation of biomass in 1 month old plants and the other for harvest data at maturity. Growth parameters like plant height, total leaf area and biomass were measured in 1 month old plants. Total leaf area was measured using leaf area meter (LICOR-100 automatic leaf area meter, Lincoln, USA). These leaves were then dried along with stem part in a hot air oven at 60 °C till constant weight of shoot was obtained. Scanning and image analysis for root characteristics (surface area, volume, total average diameter) was carried out using Root Scanner (LA 1600). Net photosynthesis (Pn) was measured in vivo on the second mature leaf from the top with LI-6400 system (LICOR, USA) by giving constant light of 1000 µmol m−2 s−1. Chlorophyll was determined by the non maceration technique given by Hiscox and Israelstam (1979). Five replicates per treatment were harvested at physiological maturity and number of productive tillers, grain number, seed weight per plant and thousand grain weight were recorded. Sodium and potassium content were measured in dried samples of root, stem and leaves of the plant after harvest following the procedure of Miller (1998). As the experimental design was completely randomized, the data were analyzed by two way analysis of variance (ANOVA) and least significant difference between means performed at probability level of p < 0.05, using the statistical software SPSS version 16.0.

Results and discussion

Plant height, leaf area and shoot dry weight of plants

Magneto-priming resulted in a significant increase in height of the plants of both the genotypes under non saline and saline condition (Table 1). HD 2967 showed an increase of 16–17 % whereas in Kharchia 5–7 % increase was observed (Table 1). Leaf area per plant increased by 40.7 and 34.5 % in HD 2967 and Kharchia 65 respectively, in plants from magneto-primed seeds grown under normal conditions. The magnitude of increase was less under saline conditions in both the genotypes. Shoot dry weight also showed a significant increase in plants from magneto-primed seeds of Kharchia 65 (Table 1). The mechanism for the improved performance of magneto-primed seeds under non saline and saline condition can be explained on the basis of a hypothesis that the ion-cyclotron resonance may interfere with the Ca2+ ion sequestering, thereby, enabling the rise in free Ca2+ concentration in the system. The increased Ca2+ concentration may signal the cell to enter into early mitotic cycle thus accounting for more growth (Rajendra et al. 2005). Also, increased uptake of Ca2+ ions in rice seedlings grown from seeds exposed to pulsed MF were found to be associated with better growth of leaf, meristematic tissues in stems and roots (Saktheeswari and Subrahmanyam 1989).

Table 1 Effect of magneto-priming on plant height, leaf area per plant and shoot dry weight per plant at 30 DAS of in wheat genotypes HD 2967 and Kharchia 65 under control and saline conditions
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Root growth characteristics

A marked increase in root growth parameters promoted by magneto-priming indicates that magneto-priming helped in adaptation to salinity stress. The magnitude of increase in root surface area between the magneto-primed and unprimed seed was more under normal and saline soil in Kharchia 65 compared to HD 2967 (Fig. 1A). Total root length and root volume increased by 114 and 93 % respectively, in magneto-primed Kharchia 65 under saline conditions (Figs. 1B, 2A). Average root diameter decreased in magneto-primed seeds of both the genotypes compared to unprimed seeds, indicating towards development of finer roots in plants post-priming treatment (Fig. 2B). The increase in these root characteristics thus helps the plants to absorb moisture under salinity induced osmotic stress. Maize seedlings were found to adapt to low water potential by making the walls in the apical part of the root more extensible (Wu and Cosgrove 2000). This was accomplished in part by increase in expansin activity and partly by other more complex changes in the cell wall. It is possible that plants from magneto-primed seeds caused greater modulation of activities related to cell wall enzymes and physical properties of the wall matrix and therefore, promoted better root length and root surface area. Rajendra et al. (2005) correlated the increase of root growth to increase in mitotic index as well as 3H-thymidine incorporation into DNA in seeds of Vicia faba exposed to 100 µT MFs.

Fig. 1
figure 1

Effect of magneto-priming on A root surface area and B total root length at 30 DAS in wheat genotypes HD 2967 and Kharchia 65 under non-saline and saline conditions. Values are means of 3 measurements. Bars represent ± SE (n = 3). UPC unprimed non saline, MPC magneto-primed non saline, UPS unprimed saline, MPS magneto-primed saline

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Fig. 2
figure 2

Effect of magneto-priming on A root volume and B average root diameter at 30 DAS in wheat genotypes HD 2967 and Kharchia 65 under non-saline and saline conditions. Values are means of 3 measurements. Bars represent ± SE (n = 3). UPC unprimed non saline, MPC magneto-primed non saline, UPS unprimed saline, MPS magneto-primed saline

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Leaf chlorophyll content and rate of photosynthesis

Total chlorophyll content of the leaf increased in leaves of magneto-primed seeds of HD 2967 under non-saline and saline conditions (Fig. 3A). However, in Kharchia 65 the differences were less between the magneto-primed treatments under both the growing environments. This trend was also evident in case of chlorophyll a content of the leaves (Fig. 3B). The increase in chlorophyll b was more (12.5 %) than chlorophyll a (4 %) in magneto-primed plants compared with unprimed plants in Kharchia 65 under normal condition (Fig. 3C). The overall effect of priming was more marked in HD 2967 as compared to Kharchia 65 under all treatments. The positive effect could be an integration of all growth components starting from improved vigour and better root and shoot growth under magneto-priming. Our results corroborate a previous study where exposure to MF increased photochemical activities in a unit of chlorophyll molecule resulting in increase in green pigment of wheat and bean (Lebedev et al. 1977).

Fig. 3
figure 3

Effect of magneto-priming on A total chlorophyll B chlorophyll a and C chlorophyll b content in leaves of 30 days old magneto-primed and unprimed wheat genotypes HD 2967 and Kharchia 65 under non-saline and saline conditions. Values are means of 3 measurements. Bars represent ± SE (n = 3). UPC unprimed non saline, MPC magneto-primed non saline, UPS unprimed saline, MPS magneto-primed saline

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Salinity resulted in approximately 30 % reduction in net rate of photosynthesis in both the varieties (Fig. 4). This was compensated by magneto-priming as net photosynthesis rate increased in leaves from magneto-primed plants of all treatments of both the genotypes. HD 2967 showed 20 and 25 %, whereas Kharchia, 39 and 16.5 % increase in rate of photosynthesis in plants from magneto-primed seeds under normal and saline condition, respectively. Earlier reports have shown that magneto-primed seeds have a long lasting stimulatory effect on plant’s performance index for photosynthesis and contributed to higher efficiency of light harvesting that consequently increased biomass in plants (Shine et al. 2011). Under drought stress, the photosynthetic parameters improved in maize pretreated with MF (Javed et al. 2011). In our studies also, an increase in net photosynthetic rate could explain increased biomass, reflected as increase in height, leaf area and number of branches in plants from magneto-primed seeds of both the genotypes under non saline and saline soils.

Fig. 4
figure 4

Effect of magneto-priming on rate photosynthesis in leaves of 30 days old magneto-primed and unprimed wheat genotypes HD 2967 and Kharchia 65 under non-saline and saline conditions. Values are means of 3 measurements. Bars represent ± SE (n = 3). UPC unprimed non saline, MPC magneto-primed non saline, UPS unprimed saline, MPS magneto-primed saline

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Na+/K+ ratio in leaf, stem and roots of plants

Na+/K+ ratio is used as a selection criterion for screening of genotypes under salinity. It was interesting to note that all plant parts from magneto-primed seeds showed low Na+/K+ ratio than unprimed counterparts, the magnitude of difference being maximum in root and stem of Kharchia 65 under saline environment. This indicates that Na+ exclusion mechanism may be more active in plant parts of magneto-primed seeds of Kharchia 65 than unprimed that led to lower Na+/K+ levels (Fig. 5).

Fig. 5
figure 5

Effect of magneto-priming on Na+/K+ ratio in root, stem and leaves of 30 days old magneto-primed and unprimed wheat genotypes HD 2967 and Kharchia 65 under saline conditions. Values are means of 3 measurements. Bars represent ± SE (n = 3). UPS unprimed saline, MPS magneto-primed saline

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In general, the stem of both the cultivars had higher Na+/K+ ratio in plants from primed and unprimed seeds, which may be due to lower rate of Na+ transfer from stem to leaves. There was higher retention (~70 %) of Na+ in stem of Kharchia 65, allowing lesser amount to be observed in leaves in comparison to HD 2967 under both treatments. It has been observed that salt tolerant genotypes have lower rate of Na+ loading and better capacity to sequester it as it enters the leaf (Davenport et al. 2005). Also in magneto-primed plants, lower Na+/K+ content accompanied with better sequestration in stem may have helped in imparting tolerance.

Yield components

The number of productive tillers per plant increased in plants from magneto-primed seeds under saline and non- saline conditions in both the genotypes. Under non-saline condition magneto-priming resulted in 26 % increase in productive tillers in both the genotypes, whereas under salinity an improvement of 15.8 % was observed in HD 2967 and 33.3 % in Kharchia 65 compared to their respective unprimed seeds. Magneto-priming also resulted in increase in the number of seeds per plant under both the growing conditions with marked (31.5 %) improvement in grain number from magneto-primed plants of HD 2967 under normal conditions (Table 2).

Table 2 Effect of magneto-priming on no. of productive tillers per plant, no. of grains per plant, grain yield per plant and thousand grain wt. in wheat genotypes HD 2967 and Kharchia 65 under control and saline conditions
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Seed yield per plant increased significantly in plants from magneto-primed seeds with 33.3 and 37.7 % in HD 2967 and Kharchia 65 respectively under normal conditions. Under saline conditions, HD 2967 showed a pronounced increase of 42.1, followed by 16.4 % in Kharchia 65 (Table 2). Kharchia 65 was not benefitted by priming to the same extent as HD 2967 as it already had an inherent potential for salinity tolerance but in HD 2967 priming could enhance salinity tolerance to a greater extent due to genotype x environment effect. Increase in thousand grain weight was also observed under all treatments, with both the varieties showing approximately 7 % increase under saline conditions (Table 2). Yield increase was a consequence of increase in number of productive tillers, number of grains and thousand grain weights that finally led to increased yield in plants from magneto-primed seeds under saline and non saline conditions. The improvement induced by the magnetic treatment was consistent with the results from other studies (Phirke et al. 1996; Amaya et al. 1996) where root, stem and plant fresh weight increased in plants from primed seeds. HD 2967 showed better enhancement in yield under saline condition, comparable to the unprimed plants under normal conditions. Priming of seeds with water has been reported to promote seedling vigour, yield and crop establishment of chickpea, maize and rice in India (Harris et al. 1999). Magneto-priming of dry seeds of chickpea was effective in mitigating adverse effects of salinity and improved yield by 33 % under saline conditions (Thomas et al. 2013). Our study elucidates that magneto-priming of wheat seeds with static MF of 50 mT for 2 h has the capacity to increase the root growth parameters, rate of photosynthesis and chlorophyll content that help the plants to adapt to salinity stress. Lower Na+/K+ ratio in different plant parts helped in imparting tolerance to magneto-primed plants of wheat under salinity stress. All these responses lead to increase in yield in crop raised from magneto-primed seeds under non saline and saline conditions. Future validation of the results under field condition can help in promoting magneto-priming as a feasible option for alleviating adverse effect of salinity stress.