научная статья по теме THE EFFECT OF SILICON ON MAIZE GROWTH UNDER CADMIUM STRESS Биология

Текст научной статьи на тему «THE EFFECT OF SILICON ON MAIZE GROWTH UNDER CADMIUM STRESS»

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СТАТЬИ

581.1

THE EFFECT OF SILICON ON MAIZE GROWTH UNDER CADMIUM STRESS1

© 2015 S. Dresler*, M. Wojcik*, W. Bednarek**, A. Hanaka*, A. Tukiendorf*

* Department of Plant Physiology, Institute of Biology and Biochemistry, Maria Curie-Sklodowska University, Lublin, Poland ** Department of Agricultural and Environmental Chemistry, University of Life Sciences in Lublin, Lublin, Poland Received April 16, 2014

The effects of silicon (Si) supply (0, 0.1, 0.5, 1.5, 3.0, and 5.0 mM Si) on maize seedling growth, Si and Cd accumulation, and thiol peptide synthesis under Cd stress conditions were studied. The addition of Si to the growth medium resulted in the significantly higher Si accumulation in plant tissues. The average values of growth parameters (root and shoot fresh weights and root net elongation rates - NER) showed a beneficial role of Si on growth of non-Cd-treated plants, while there was no evidence that silicon mitigated Cd toxicity in maize seedlings. Cadmium exposure depressed plant growth and induced phytochelatin (PC) synthesis. The accumulation of Cd and PCs in roots significantly decreased with increasing Si concentrations in the nutrient solution; however, their accumulation in shoots was not changed in the presence of Si.

Keywords: Zea mays - Cd stress - silicon - growth parameters - phytochelatin

ФИЗИОЛОГИЯ РАСТЕНИЙ, 2015, том 62, № 1, с. 96-102

УДК

DOI: 10.7868/S0015330315010054

INTRODUCTION

Cadmium (Cd) is one of the most toxic trace elements ranked no. 7 among the top 20 toxins [1]. Its increasing level in the soil poses a serious environmental problem. Although Cd has not been considered as an essential element, it is easily absorbed by plants. Excess of Cd in higher plants causes chlorosis and necrosis of leaves, decreases uptake of nutrients, induces growth retardation, and has a disruptive influence on key physiological processes [2]. The negative Cd effects on living organisms necessitate finding a way to decrease its accumulation in and toxicity for plants and, finally, to reduce food-chain contamination.

Over the past few decades, there has been an increasing concern in the possibility of using some soil amendments to alleviate Cd toxicity and control its phytoavailability [3]. Recently, silicon (Si) has been shown to mitigate abiotic stress in plants [4-6]. There is some evidence that Si has a beneficial effect on heavy metal tolerance of certain plant species: for instance, it alleviated the toxicity ofMn in Phaseolus [7], Oryza [8],

1 This text was submitted by the authors in English.

Abbreviations'. GSH — glutathione; NER — net elongation rates; PC — phytochelatin.

Corresponding author. S-lawomir Dresler. Department of Plant Physiology, Institute of Biology and Biochemistry, Maria Curie-Skiodowska University, Akademicka 19, 20-033 Lublin, Poland; e-mail. slawomir.dresler@poczta.umcs.lublin.pl

and Zea mays [9] and Al toxicity in Sorghum [10], soybean [11], and Hordeum [12]. The role of Si in plant resistance to heavy metal stress is attributed to Si deposition into the cell walls of roots, leaves, and stems. Particularly, its deposition in roots reduces apoplasmic bypass flow, resulting in a decreased uptake and translocation of toxic metals [13]. Silicon can also reduce the negative effects of some toxic metals by their com-plexation and compartmentation. The formation of a heavy metal-silicate complexes in the cytoplasm and nuclei and their sequestration in the vacuoles or cell walls can be a part ofAl or Zn tolerance mechanism [6].

Studies have shown that Si increased Cd accumulation in the endodermis of rice seedling roots, restricted the transport of Cd to shoots, and deposited mainly into the cell walls of endodermis, blocking the apoplasmic transport of Cd [14]. Silicon was effective in preventing excessive Cd uptake after Si application to Cd-treated strawberry plants; thus, it was postulated that Si enhanced the tightness and stiffness of cell walls, which formed a natural mechanical barrier for Cd ions [15]. Similar mechanisms of Si-Cd interactions have been suggested for maize plants [16]. Studies with maize plants have shown that Si-enhanced tolerance to Cd can be attributed not only to Cd immobilization caused by a silicate-induced rise in the soil pH but also to Si-mediated detoxification of Cd in plants [9]. Silicon decreases the most bioavailable pools of Cd for maize plants and increases the alloca-

tion of Cd into more stable soil fractions, such as organic matter and crystalline iron oxides [17].

Although various studies have suggested that Cd toxicity is alleviated by the interaction with Si, the mechanisms involved are still poorly understood. There is also limited information in the literature related to optimal Si concentration, Si-Cd interaction on specific physiological parameters, and, finally, practical application in phytotechnologies.

In the present study, we examined the effect of Si on the alleviation of Cd toxicity in young maize plants. The primary objectives of this work were to determine: (1) the influence of various Si concentrations on plants grown in the absence or presence of Cd; (2) the effect of Si concentration on Cd and Si uptake and distribution in maize roots and shoots; and (3) the physiological plant responses to Cd at different Si concentrations by the synthesis of phytochelatins and related peptides.

MATERIALS AND METHODS

Plant culture and treatments. Maize (Zea mays L., cv. Reduta) seedlings were used in the experiment. Seeds were germinated for 4 days on wet filter paper, and then the seedlings were transplanted to plastic pots with aerated Hoagland nutrient solution (9 seedlings per a 1.5 dm3 solution in a pot). After two days, the nutrient solution was changed, and Si and/or Cd were added. The seedlings were cultivated in a growth room at a day/night cycle of 16/8 h at 24/17°C, a relative humidity of 60-70%, and photosynthetic photon flux density of 150 ^mol/(m2 s). The plants were analyzed in 8 days after the addition of Si and/or Cd.

Six levels of Si treatments (0, 0.1, 0.5, 1.5, 3.0, and 5.0 mM) and two levels of Cd (0 and 50 ^M) were arranged in a randomized complete block design. Silicon was added as the potassium silicate (K2SiO3) solution (Vitrosilicon) after neutralization with HCl, while Cd was added as cadmium nitrate solution -Cd(NO3)2. An equivalent amount of KCl was added to the plants treated with less than 5.0 mM Si to compensate for the K content.

Measurement of the growth parameters and root viability. The plants were divided into shoots and roots, and their fresh weights were determined. In addition, the root net elongation rates (NER) were measured. The root and shoot material was oven-dried to a constant dry weight at 105°C. The viability of roots of the control plants and those treated with various Si-Cd combinations was determined using the modified method of Ishikawa and Wagatsuma [18].

Determination of cadmium and silicon contents.

After harvest, the roots were soaked for 30 min in 10 mM CaCl2 solution and then washed thoroughly with distilled water to remove ions absorbed in the free space. Cadmium concentrations in the roots and shoots were measured by atomic absorption spectrophotome-try after wet-ashing of dry plant samples with mixed ac-

ids (HNO3 + HClO4 (1 : 1, v/v)). The Si content was determined in dried plant material after dilution with hydrofluoric acid solution (1.5 M HF-0.6 M HCl) by the colorimetric molybdenum method [19].

Analysis of thiol peptides by the HPLC method.

Sample preparation and chromatographic separation was performed according to the method described by Wojcik and Tukiendorf [20]. Briefly, the fresh plant tissues were homogenized in 0.1 M HCl (1 : 2, w/v) and centrifuged at 14000 rpm at 4°C. Aliquots of 100 ^L of the supernatant were used, and thiol peptides were separated in a linear gradient (0-20%) of acetonitrile (ACN, "Sigma", United States) in 0.05% trifluoro-acetic acid (TFA, "Sigma") on an Ultrasphere C-18 column (4.6 x 250 mm; "Sigma") equipped with a guard-column (4.6 x 10 mm; "Sigma") using a Beckman ("Fullerton", United States) chromatograph (model 126/166). Absorbance was detected at 405 nm after the post column reaction with DTNB (5,5'-dithiobis-2-nitrobenzoic acid, "Sigma"). The chromatogram was recorded and analyzed using 32 Karat 7.0 software (Beckman, "Fullerton").

Statistical analysis. All data presented in this paper are means of at least three independent replications (9 plants each). Differences between Cd accumulation in tissues and PC concentration dependence on the level of Si application were analyzed using one-way Analysis of Variance (ANOVA), whilst the two-way ANOVA was used to explore the Cd x Si interaction and its influence on plant growth, Si and glutathione (GSH) contents. Differences between the factors were determined with Fisher's Least Significant Difference (LSD) at the 0.05 probability level. To evaluate the relationship between the parameters assessed, Pearson's correlation coefficients were computed. The data were analyzed using Statistica ver. 9 ("StatSoft, Inc.", 2009, United States).

RESULTS

Cadmium and silicon content in plants

The relationship between the Si concentration applied and the Si and Cd accumulation in plant tissues is shown in fig. 1 and table 1. The Si concentration in both roots and shoots increased with increasing Si concentration in the growth medium irrespectively of the presence or absence of Cd. At the same time, Si was found to decrease Cd accumulation in the roots (by 30% at 5 mM Si); however, there was no significant Si influence on Cd accumulation in the shoots. Silicon concentrations in the roots and shoots were similar regardless of Cd treatment. Cadmium was deposited mainly in the roots (above 10 times higher concentrations than in the shoots).

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