Alleviation of Adverse Effects of Drought Stress on Growth and Some Potential Physiological Attributes in Maize (Zea mays L.) by Seed Electromagnetic Treatment

ABSTRACT

Effects of varying preseed magnetic treatments on growth, chlorophyll pigments, photosynthesis, water relation attributes, fluorescence and levels of osmoprotectants in maize plants were tested under normal and drought stress conditions. Seeds of two maize cultivars were treated with different (T0 [0 mT], T1 [100 mT for 5 min], T2 [100 mT for 10 min], T3 [150 mT for 5 min] and T4 [150 mT for 10 min]) electromagnetic treatments. Drought stress considerably suppressed growth, chlorophyll a and b pigments, leaf water potential, photosynthetic rate (A), stomatal conductance (g_s) and substomatal CO₂ concentration (C_i), while it increased leaf glycinebetaine and proline accumulation in both maize cultivars. However, pretreated seeds with different magnetic treatments significantly alleviated the drought-induced adverse effects on growth by improving chlorophyll a, A, E, g_s, C_i and photochemical quenching and nonphotochemical quenching, while it had no significant effect on other attributes. However, different magnetic treatments negatively affected the g_s and C_i particularly in cv. Agaiti-2002 under drought stress conditions. Of all magnetic treatments, 100 and 150 mT for 10 min were most effective in alleviating the drought-induced adverse effects. Overall, preseed electromagnetic treatments could be used to minimize the drought-induced adverse effects on different crop plants.

Introduction

Water shortage is a wide-spread problem, which seriously impairs crop growth and yield production (1–3). This water stress-induced growth suppression occurs due to up- or down-regulation of a variety of physiological processes linked with growth and development. It is believed that formation of reactive oxygen species (ROS) as well as alteration in water balance within the plant cells/tissues are the primary plant responses to water stress (3–5). Under mild water stress, plants can maintain photosynthesis as well as turgor but for a short period (6,7). However, prolonged water stress conditions induce damage to cellular macromolecules, membranes and oxidative stress which stop plant growth as well as causes suppression in photosynthesis, stomatal conductance, flowering and seed production (3,6,8–10). So, the development of modern agriculture and the rules of propagation for the rational use of natural resources are necessary to find a safe method for enhancing crop yield production under water limited environment (11).

The presowing seed treatment with magnetic field is one of the most investigated physical treatments in agriculture (12,13). The effects of magnetic treatment depend upon the magnetic field strength as well as time period (14–17). The plant growth, especially seed germination, can be accelerated using an optimal external electromagnetic field (18,19). It is well known that strong magnetic field changes cell membrane characteristics, cell metabolism, cell reproduction and various other cellular functions like mRNA quantity, gene expression, protein biosynthesis and enzyme activities (13,20,21). For example, increased lipid oxidation and ascorbic acid contents were reported in seeds of cucumber (Cucumis sativus) which were exposed to magnetic field as a presowing treatment (17). The presowing seed magnetic field treatment also caused an increase in the sugar contents in sugar beet roots and gluten in wheat (19,22). Magnetic field has been shown to exert a positive effect on cation uptake capacity and immobile nutrient uptake in plants (12,19). The presowing static magnetic field treatment of chickpea (Cicer arietinum L.) seeds caused improvement in root functional parameters, which suggests that magnetically treated chickpea seeds may perform better under unirrigated conditions (23).

Plants accumulate a variety of organic solutes in their cells in response to stressful environments. For example, accumulation of proline, glycinebetaine (GB), trehalose, alanine betaine and proline betaine is a common metabolic response of most plants to saline conditions (24–26). However, very little information on the effect of seed electromagnetic treatment on accumulation of such compounds is available in the literature except of a single report by Dhawi and Al-Khayri (27) who found that in date palm plants at low electromagnetic field intensity (10 mT) for 240 min, proline accumulation increased, while in contrast, beyond this dose, proline accumulation decreased. Thus, one of the objectives of the present study was to examine whether electromagnetic treatment to seed could alter the levels of some key organic osmolytes including proline.

All studies reported earlier clearly show that presowing treatment of seeds with magnetic field could alleviate the adverse effects of drought stress. The mechanisms by which magnetic fields affect growth have not been well explained (16). The paramagnetic properties of some atoms in plant cells and pigments (chlorophyll) can account for the observed positive effects of magnetic field treatment (11). Magnetic properties of molecules enable them to absorb and transform the energy of magnetic field into other form of energy. This transformed energy is later transferred to other plant structures resulting in their activation (11). Keeping in mind the significant effect of magnetic treatment, the objective of the present study was to examine whether the presowing seed treatment with varying electromagnetic field could alleviate the adverse effects of water shortage on growth and various physiological attributes in maize plants.

Materials and methods

The present study was conducted to assess whether seed treated with different magnetic doses could improve plant growth under water stress and nonstress conditions. The experiment was conducted in the Botanical Garden of the University of Agriculture, Faisalabad, Pakistan during August–November, 2010. During the entire period of experimentation the average RH 35% and temperature from 25.5 to 38°C were recorded.

Seeds of two cultivars (Agaiti-2002 [drought tolerant] and EV-1098 [drought sensitive]) (5) were provided by the Ayub Agricultural Research Institute (AARI), Faisalabad. The seeds were subjected to electromagnetic field using an electromagnet at the Department of Physics, University of Agriculture, Faisalabad, Pakistan. The electromagnet consisted of two pairs of energized cylindrical coils, each formed by 4026 turns of 0.41 mm enameled copper wire. Each pair of coils is wound 11 cm apart on an iron bar (dimensions 40 × 3.5 cm). The two bars were placed one above the other and their ends were held by metallic supports. The coils were connected in series and fed through a power source. When electric current was passed through the coils, a full-wave rectified, sinusoidal nonuniform magnetic field generated in the air space between the two bars. For each voltage, the magnetic field generated was measured using a magnetic flux meter (ELWE 8533996; Cerligene, Germany) which was attached manually to the stimulator. The seeds were exposed in two different (100 and 150 mT) magnetic field strengths for varying (5 and 10 min) time periods. The pretreated seeds were grown in plastic pots containing 8 kg sandy-loam soil. The electrical conductivity, 1.39 mS cm⁻¹; pH, 8.42; P, 0.042 mg kg⁻¹; K, 24 mg kg⁻¹ and N, 8.1 mg kg⁻¹ of the soil extract were recorded. The plants were allowed to establish for 25 days before the start of water deficit conditions. Two water deficit treatments were control (normal watering) and 60% of field capacity. The moisture contents of droughted pots were maintained and regularly monitored by keeping the weight of each pot equal to that calculated for 60% field capacity through addition of normal irrigation water if required on daily basis till the maturation of the crop. The plants were clipped so as to maintain uniform plant size before the start of drought stress. Plants were harvested 30 days after the start of drought stress. Plants were uprooted carefully and washed with distiled water. Plant samples were dried in an oven at 65°C to constant dry weight. Before harvesting, the data for the following attributes were recorded:

Water relation attributes.  A fully expanded youngest leaf was excised from each plant at 08:00 hours and leaf water potential (Ψ_w) measured using a Scholander type pressure chamber (Arimad-2-Japan). The same leaf as used for Ψ_w was frozen at −20°C in 2.0 cm polypropylene tubes for 2 weeks, after which time it was thawed, and the sap extracted by pressing it with a glass rod. The sap so extracted was used directly for osmotic potential (Ψ_s) determination using an osmometer (VAPRO, Model 5520). Turgor potential (Ψ_p) was calculated as the difference between Ψ_w and Ψ_s following Nobel (28).

Chlorophyll pigments.  Following the procedure of Arnon (29), the fresh leaves (0.5 g) were extracted with 80% acetone at −4°C and the extract centrifuged at 10 000 g for 5 min and then the absorbance of the supernatant recorded at 645 and 663 nm spectrophotometrically (IRMECO U2020). Both chlorophyll a and b pigments were expressed in mg g⁻¹ fresh weight.

Gas exchange characteristics.  Instantaneous measurements of photosynthetic rate (A), internal CO₂ concentration (C_i), transpiration rate (E) and stomatal conductance (g_s) were performed on a fully expanded third leaf (from top) of each plant from each replicate using a portable infrared gas analyzer (Model LCA-4; ADC, Hoddesdon, England). The other adjustments/specifications of the leaf chamber were as follow: atmospheric pressure (P) 97.9 kPa, leaf surface area 6.25 cm², leaf chamber temperature (T_ch) varied from 29.3 to 35.5°C, gas flow rate of leaf chamber volume (V) 296 mL min⁻¹, atmospheric CO₂ (C_ref) 369 μmol mol⁻¹ and molar gas flow rate of leaf chamber (U) 400 μmol s⁻¹.

Leaf chlorophyll fluorescence.  Following Strasser et al. (30), data for different chlorophyll fluorescence attributes such as photochemical quenching (qP), nonphotochemical quenching (NPQ), coefficient of NPQ (qN) and maximal quantum yield of photosystem II (PSII) (F_v/F_m) were recorded using the OS5p Modulated Fluorometer (ADC BioScientific Ltd, Great Amwell Herts, UK). Before these measurements leaf samples were kept at dark for 30 min.

Leaf proline content.  Leaf samples (0.5 g each) were triturated with 10 mL of 3% (wt/vol) sulfosalicylic acid (MP, Biomedicals, Inc) solution and proline contents were determined following Bates et al. (31) as:

• 

Leaf GB.  To determine the GB concentration in leaf tissues, the procedure described by Grieve and Grattan (32) was employed. Fresh leaf sample (0.5 g) was ground in 10 mL distiled water. After filtration, 1 mL of the extract was treated with 1 mL of 2 N H₂SO₄. Then 0.5 mL of this mixture was taken and mixed with 0.2 mL of potassium tri-iodide (KI₃) solution. The mixture was shaken and cooled in cold water for 90 min. To it, 2 mL ice-cooled distiled water and 20 mL of 1,2 dichloroethane (cooled at −10°C) were added to the mixture. The upper aqueous layer was discarded while the OD of the lower organic layer was noted at 365 nm using a UV–Visible spectrophotometer. The amount of betaine was calculated from a standard curve.

Statistical analysis.  A three factor-factorial completely randomized design with three replicates (n = 3) was used for setting up the experiment. The COSTAT computer package (CoHort Software, Berkeley) was used for working out analyses of variance of all variables.

Results

Drought stress (60% field capacity) considerably suppressed the shoot and root fresh and dry weights of both (Agaiti-2002 and EV-1098) maize cultivars (Fig. 1). However, pretreated seeds with different (100 and 150 mT for 5 and 10 min) electromagnetic treatments had a significant growth promoting effect on both maize cultivars in terms of shoot fresh and dry weights under control (well watered) as well as water deficit conditions (60% field capacity). Of all preseed electromagnetic treatments, 100 mT for 5 min and 150 mT for 5 and 10 min were effective for enhancing the shoot fresh weight but only in cv. EV-1098. With respect to shoot dry weight, 100 mT for 5 min and 150 mT for 5 and 10 min were effective under well watered and water deficit conditions, but with the exception of 100 and 150 mT for 5 min under water stress conditions no effect of electromagnetic treatment was observed on both maize cultivars. Root fresh weight was higher at 100 mT for 5 min and 150 mT for 5 and 10 min under water deficit conditions. In the case of root dry weight, under water stress conditions, 150 mT for 5 min was most effective in both maize cultivars, while under control conditions, root dry weight increased only in cv. EV-1098 at 150 mT for 5 min. Of both maize cultivars, cv. EV-1098 was slightly better in shoot fresh and root dry weights than cv. Agaiti-2002 particularly under water deficit conditions.

Figure 1.  Shoot and root fresh and dry weights and shoot and root lengths of two cultivars of maize when electromagnetically treated seeds were grown under control (normal irrigation) and drought stress (60% field capacity). (T0 [0 mT]; T1 [100 mT for 5 min]; T2 [100 mT for 10 min]; T3 [150 mT for 5 min] and T4 [150 mT for 10 min]), Cvs = cultivars; EMT = electromagnetic treatments; D = drought; ns = nonsignificant; *, ** and *** are significant at 0.05, 0.01 and 0.001 levels, respectively.

Shoot and root lengths of both maize cultivars decreased due to imposition of drought stress. Different preseed electromagnetic treatments enhanced the shoot length significantly only under control conditions and of all the electromagnetic treatments, 150 mT for 10 min was the most effective. However, different seed electromagnetic treatments considerably improved the root length in both maize cultivars under both stress and nonstress conditions. The response of both cultivars was consistent in shoot length, while in root length, cv. EV-1098 was significantly superior to cv. Agaiti-2002 (Fig. 1).

A marked (P ≤ 0.001) reduction in chlorophyll a and b contents in both maize cultivars was observed under water deficit conditions (Fig. 2), while, in contrast, chlorophyll a/b ratio increased under water deficit conditions. Presowing different electromagnetic treatments enhanced the chlorophyll a contents only in cv. Agaiti-2002 under nonstress and drought stress conditions. Of all electromagnetic treatments, 100 mT for 5 and 10 min and 150 mT for 5 min were effective in enhancing chlorophyll b contents in both cultivars under nonstress conditions, while no significant effect was observed under drought stress conditions. Furthermore, chlorophyll a/b ratio increased significantly due to preseed electromagnetic treatments particularly under drought stress conditions and of all preseed treatments, 150 mT for 5 and 10 min were effective for cv. Agaiti-2002. However, the electromagnetic doses 100 mT for 5 and 10 min were effective in enhancing chlorophyll a/b ratio in cv. EV-1098 under water deficit conditions.

Figure 2.  Chlorophyll pigments and water relation attributes of two cultivars of maize when electromagnetically treated seeds were grown under control (normal irrigation) and drought stress (60% field capacity). (T0 [0 mT]; T1 [100 mT for 5 min]; T2 [100 mT for 10 min]; T3 [150 mT for 5 min] and T4 [150 mT for 10 min]), Cvs = cultivars; EMT = electromagnetic treatments; D = drought; ns = nonsignificant; * and *** are significant at 0.05 and 0.001 levels, respectively.

Leaf water potential (Ψ_w) of both maize cultivars decreased considerably (P ≤ 0.001) under water stress conditions. However, leaf osmotic (Ψ_s) and turgor (Ψ_p) potentials remained almost unaffected under drought stress. No significant effect of different preseed electromagnetic treatments was observed on all three water relation attributes (Fig. 2) with the exception of Ψ_w which showed improvement in cv. Agaiti-2002 under control conditions and Ψ_s under drought conditions in the same cultivar. However, leaf Ψ_p increased in both cultivars under drought stress conditions, while, in contrast, it decreased under nonstress conditions due to all the presowing electromagnetic treatments. Of both maize cultivars, cv. Agaiti-2002 was slightly higher in Ψ_p than cv. EV-1098, in contrast, both cultivars were similar in Ψ_w and Ψ_s.

Under water deficit conditions, photosynthetic rate (A), transpiration rate (E), stomatal conductance (g_s), internal CO₂ concentration (C_i), C_i/C_a ratio and water-use efficiency (WUE) measured as A/E declined significantly. Different seed electromagnetic treatments with 100 mT for 5 min and 150 mT for 5 and 10 min enhanced the A in maize cv. EV-1098 under control as well as water stress conditions. Under nonstress conditions, the effect of different electromagnetic treatments was nonsignificant on E, while under stress conditions, preseed treatments with 100 and 150 mT for 5 min in cv. Agaiti-2002 and 150 mT for 10 min in cv. EV-1098 was effective in improving the E. Different electromagnetic treatments negatively affected the g_s in cv. Agaiti-2002, while no such negative effect was observed on cv. EV-1098 under drought conditions. Electromagnetic treatments improved the C_i in both cultivars under nonstress conditions, while they showed negative effect under drought conditions. No change in C_i/C_a ratio was observed in both maize cultivars due to different electromagnetic treatments. Under nonstress conditions, both maize cultivars, while under stress conditions cv. EV-1098 showed improved WUE due to different preseed electromagnetic treatments (Fig. 3). Of both maize cultivars, cv. EV-1098 was considerably higher than cv. Agaiti-2002 in all the above-mentioned photosynthetic attributes under drought stress conditions.

Figure 3.  Different gas exchange characteristics of two cultivars of maize when electromagnetically treated seeds were grown under control (normal irrigation) and drought stress (60% field capacity). (T0 [0 mT]; T1 [100 mT for 5 min]; T2 [100 mT for 10 min]; T3 [150 mT for 5 min] and T4 [150 mT for 10 min]), Cvs = cultivars; EMT = electromagnetic treatments; D = drought; ns = nonsignificant; *, ** and *** are significant at 0.05, 0.01 and 0.001 levels, respectively.

Drought stress significantly increased the NPQ and qP in both maize cultivars. Under drought stress conditions, preseed electromagnetic treatments with 100 and 150 mT for 10 min were most effective in enhancing the NPQ and qP, while a negative or no effect of preseed treatment with 150 mT for 5 min was observed particularly in cv. EV-1098 under nonstress conditions. Furthermore, different preseed electromagnetic treatments had no significant effect on coefficient of NPQ (qN) and efficiency of PSII (F_v/F_m). Although both maize cultivars were similar in qP, qN and NPQ, while in F_v/F_m, cv. Agaiti-2002 was slightly higher than cv. EV-1098 under drought stress conditions (Fig. 4).

Figure 4.  Different leaf fluorescence attributes and leaf glycinebetaine and proline of two cultivars of maize when electromagnetically treated seeds were grown under control (normal irrigation) and drought stress (60% field capacity). (T0 [0 mT]; T1 [100 mT for 5 min]; T2 [100 mT for 10 min]; T3 [150 mT for 5 min] and T4 [150 mT for 10 min]), Cvs = cultivars; EMT = electromagnetic treatments; D = drought; ns = nonsignificant; *, ** and *** are significant at 0.05, 0.01 and 0.001 levels, respectively.

Leaf GB contents increased significantly (P ≤ 0.001) in both maize cultivars due to water deficit conditions (Fig. 4). Of different electromagnetic treatments, preseed treatment with 100 and 150 mT for 10 min significantly enhanced the GB accumulation in both maize cultivars particularly under water deficit conditions. However, no significant effect of different electromagnetic treatments was observed on cv. EV-1098 under nonstress conditions (Fig. 4).

Leaf proline contents increased significantly (Fig. 4) in both maize cultivars due to water deficit conditions. Of different electromagnetic treatments, preseed treatment with 100 and 150 mT for 5 min significantly enhanced the proline contents in both maize cultivars under water deficit conditions. However, no significant effect of different electromagnetic treatments was observed on both maize cultivars under nonstress conditions.

Discussion

It is well established that seed electromagnetic treatment significantly influences the seed germination, seedling growth as well as activities of various enzymes, but in plants the underlying mechanism of action of electromagnetic field under stressful environment is not well understood (17,33–35). In another study with rice, Florez et al. (36) observed that electromagnetic treatment particularly 125 and 250 mT to rice seeds proved to be very effective in improving seed germination and seedling growth particularly under nonstress conditions. In another study, Dayal and Singh (37) exposed tomato seeds to varying electromagnetic treatments (5–155 mT) for different time intervals and observed a significant increase in number of primary branches and plant height. Podleoeny et al. (38) found that an optimal electromagnetic treatment enhances plant growth, total seed germination and rate of emergence in broad bean. In the present study, the effectiveness of preseed electromagnetic treatment was assessed whether it could alleviate the drought-induced adverse effects on maize growth and various physiological attributes. Drought stress (60% field capacity) considerably suppressed the shoot and root fresh and dry weights as well as lengths of both (Agaiti-2002 and EV-1098) maize cultivars. However, seed electromagnetic treatments particularly 100 mT for 5 min and 150 mT for 5 and 10 min were effective for enhancing the maize seedling growth and length under well watered and water deficit conditions. Recently, Vashisth and Nagarajan (39) have shown electromagnetic treatment-induced growth improvement in chickpea (C. arietinum L.) under water deficit conditions. Martinez et al. (40) also reported similar effects of electromagnetic field on barley and wheat growth under control conditions. In view of a number of studies it is now evident that electromagnetic treatments alter relative growth rate, seed germination, chlorophyll contents, root growth, cell membrane characteristics and cell division by inducing changes in plant cell metabolism (35,41–44).

Presowing different electromagnetic treatments enhanced the chlorophyll contents (a and b) under nonstress and drought stress conditions. Of all electromagnetic treatments, 100 and 150 mT for 10 min were effective in improving the chlorophyll contents in both cultivars under nonstress and drought stress conditions. It has already been observed that electromagnetic field causes changes in various functions at the tissue and organ levels (45–47). The electromagnetic treatment-induced improvement in chlorophyll pigments could be due to the presence of paramagnetic properties of chloroplast which can increase the rate of seed metabolism (18,48,49). While using electromagnetically treated water, Hozayn and Qados (50) exhibited a marked increase in carotenoids and chlorophyll a and b contents in wheat and they ascribed this increase to electromagnetic treated-induced better ion mobility as well as uptake during growth. Similarly, Atak et al. (47) observed that chloroplast synthesis was increased in electromagnetically treated soybean plants as compared to nontreated plants.

Leaf water potential (Ψ_w) of both maize cultivars decreased while leaf osmotic (Ψ_s) and turgor (Ψ_p) potentials remained almost unaffected under drought stress. No significant effect of different preseed electromagnetic treatments was observed on all three water relation attributes. In contrast, Reina and Pascual (51) reported that electromagnetic treatments induced changes in osmotic pressure as well as the capacity of the cellular tissues to absorb water in lettuce seeds.

A considerable drought-induced decrease in rate of photosynthesis (A) and other related attributes such as transpiration rate (E), stomatal conductance (g_s) and substomatal CO₂ concentration (C_i) have already been well documented (1,3,52,53). However, very little information exists in the literature on the role of electromagnetic treatment in regulating rate of photosynthesis. In the present study, different seed electromagnetic treatments particularly with 100 mT for 5 min and 150 mT for 5 and 10 min improved drought-induced reduction in A, E and WUE. In a recent investigation, varying (0–300 mT) electromagnetic soybean (Glycine max L.) seed treatments (150 and 200 mT for 60 min) were effective in increasing leaf photosynthesis under field conditions (44).

Preseed electromagnetic treatments with 100 and 150 mT for 10 min enhanced the NPQ and qP under water deficit conditions. But, preseed electromagnetic treatments had no significant effect on coefficient of NPQ (qN) and efficiency of PSII (F_v/F_m). In contrast to our results, Yinan et al. (17) found a significant reduction in maximum quantum efficiency of PSII (F_v/F_m) of cucumber seeds when exposed to 0.2 and 0.45 mT electromagnetic pretreatment. The authors have attributed this reduction to increase in initial fluorescence F_0.

Electromagnetic fields-induced biochemical changes are a well-known phenomenon (19). Generally, to cope with a stressful environment, plants accumulate cellular organic solutes such as GB, proline, trehalose, alanine betaine and proline betaine, etc. (5,54–56). In the present study, leaf GB and proline accumulation increased significantly in both maize cultivars due to water deficit conditions. Furthermore, different electromagnetic treatments significantly enhanced the proline and GB accumulation in both maize cultivars particularly under water deficit conditions. These results corroborate with those of an earlier study on date palm by Dhawi and Al-Khayri (27) who found that at low electromagnetic field intensity (10 mT) for 240 min, proline accumulation increased, while in contrast, beyond this time interval, proline accumulation decreased.

In conclusion, drought stress considerably suppressed growth, chlorophyll a and b pigments, Ψ_w, A, g_s and C_i, while it increased GB and proline accumulation in both maize cultivars. However, pretreated seeds with different electromagnetic treatments particularly 100 and 150 mT for 10 min significantly alleviated the drought-induced adverse effects on growth by improving chlorophyll a contents, A, E, g_s, C_i and qP and NPQ. Overall, preseed electromagnetic treatments could be used to enhance the growth and yield production by minimizing the drought-induced adverse effects on different crop plants.

Acknowledgments

Acknowledgements— The authors acknowledge the financial support from King Saud University Riyadh, Saudi Arabia through research grant KSU-VPP-101.