Foliar application of potassium silicate induces

Journal of Plant Nutrition

ISSN: 0190-4167 (Print) 1532-4087 (Online) Journal homepage: http://www.tandfonline.com/loi/lpla20

Foliar application of potassium silicate induces metabolic changes in strawberry plants S. Y. Wang & G. J. Galletta To cite this article: S. Y. Wang & G. J. Galletta (1998) Foliar application of potassium silicate induces metabolic changes in strawberry plants, Journal of Plant Nutrition, 21:1, 157-167, DOI: 10.1080/01904169809365390 To link to this article: http://dx.doi.org/10.1080/01904169809365390

Published online: 21 Nov 2008.

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JOURNAL OF PLANT NUTRITION, 21(1), 157-167 (1998)

Foliar Application of Potassium Silicate Induces Metabolic Changes in Strawberry Plants S. Y. Wang and G. J. Galletta Fruit Laboratory, Beltsville Agricultural Research Center, United States Department of Agriculture, 211 Building 010A, BARC-West, 10300 Baltimore Avenue, Beltsville, MD 20705-2350

ABSTRACT The effect of foliar silicon (Si) applications on metabolic changes in strawberry plants (Fragaria x ananassa Duch.) was studied. Silicon was used in the form of the potassium (K) salt. Foliar spray with K silicate (containing 0, 4.25, 8.50, 12.75, or 17.00 mm of Si) showed increased chlorophyll content and plant growth. Potassium silicate treatments also induced metabolic changes such as increases in citric acid and malic acid levels, and decreases in fructose, glucose, sucrose, and myo-inositol contents. The treated tissues also had higher ratios of fatty acid unsaturation [(18:2+18:3)/18:1] in glycolipids and phospholipid and elevated amounts of membrane lipids. These results suggest that Si has beneficial effects on strawberry plant metabolism.

157 Copyright © 1998 by Marcel Dekker, Inc.

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INTRODUCTION Silicon has long been known to be present in strawberry plants (Lanning, 1960) and has been reported to increase plant defense systems against environmental stress and pathogenic invasion (Belanger et al., 1995; Epstein, 1994). Silicon has also been shown to induce higher mechanical stability of stems and leaf blades, enhance light interception (Yoshida et al., 1969), and improve growth of a wide variety of plant species (Adatia and Besford, 1986; Epstein et al., 1988; Emadian and Newton, 1989; Lewin and Reiman, 1969; Miyake and Takahashi, 1986; Okuda and Takahashi, 1965; Vlamis and Williams, 1967). Our study showed foliar application of K or sodium (Na) silicate reduced the severity of powdery mildew in strawberry plants (unpublished data). The objective of this study was to determine the effect of silicon on growth and metabolic changes in strawberry plants. MATERIALS AND METHODS Plant Material and Silicon Treatments 'Earliglow' strawberry (Fragaria x ananassa Duch.) [(Fairland x Midland) x (Redglow x Surecrop)] plants were propagated by runner cuttings and were three months old with three expanded leaves when the experiments were started. The plants were grown in pots (6.5x8.25 cm, E.C. Geiger, Inc., Harleysville, PA) containing Pro-Mix BX (Premier Brands, Inc., Stamford, CT) and were grown in a greenhouse. Radiation sources in the greenhouse consisted of natural daylight and incandescent lamps which provided a PAR level of about 400 to 500 umol nv2 s"1 for 14 hours/day (0600-2000 h). Temperatures were approximately 25°C during the day and 20°C at night. All plants were watered daily and fertilized biweekly with a Peter's nutrition solution (20-20-20, N-P-K). Silicon was used in the form of K salt (Kasil #6) (The PQ Corporation, Chester, PA). Soluble Si solutions were prepared by diluting the predissolved K silicate in distilled water and adjusting to pH 5.5 with phosphoric acid. Tween-20 (0.1%) was added as surfactant. Foliar spray containing 0,4.25,8.50,12.75, and 17.00 mM of Si were applied (also containing 0, 1.25, 2.5, 3.75, 5.0 mM K, respectively). In this experiment, sprays also included a distilled water spray (pH 5.5), a KOH spray containing 5 mM K adjusted to pH 5.5 with phosphoric acid, a phosphoric acid spray containing 5.5 mM P adjusted to pH 5.5 with NaOH, and a phosphoric acid spray containing 5.5 mM P adjusted to pH 5.5 with KOH. The 5 mM K and 5.5 mM P in the sprays were equivalent to the amounts of K and P available in the 17 mM Si spray used in the experiments for determining whether K and P induces metabolic changes in strawberry plants. Twenty plants were used for each treatment. The sprays were applied, until run off, to the upper surface of leaves. The plants were then grown in the greenhouse. The Si and K spray treatments

FOLIAR APPLICATION OF POTASSIUM SILICATE

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were applied every week. The experiment was carried out for 7 weeks and the plants were harvested. Leaves, petioles, crowns, and roots were separated, weighed and used for chemical analyses.

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Chlorophyll Analysis Leaf discs of 1.0 cm diameter were extracted with 80% acetone. Chlorophyll content of leaf discs was determined using the spectrophotometric method (Bruinsma, 1961). Sugar Analysis Tissues were extracted with 80% ethanol, and then centrifuged at 5,000 g for 5 min. The residue was re-extracted and washed twice by centrifugation and resuspended in 80% ethanol. The supernatants were combined and an aliquot of the extract was concentrated to dryness in vacuo in 5-mL derivatizing vials. Derivatization of the sugars was performed according to the procedures described by Wang et al. (1987). Derivatized samples were injected for gas Chromatographie separation and quantification. A Hewlett-Packard 5880A gas Chromatograph equipped with a flame ionization detector and a fused silica capillary column (methyl silicone fluid, 12.5 m x 0.2 mm) were used for separation of sugars. Initial oven temperature was set at 160°C and held isothermal for 12 min, then a temperature program of 10°C min 1 began and continued to a final oven temperature of 250°C. The final oven temperature was held isothermal for 3.5 min. The injection port and detector temperatures were set at 250°C. Helium was used as a carrier gas and the column head pressure was set at 75 kPa. A 1-uL injection was made, using a split ratio of 40:1. Flow rates were 30 and 400 mL min 1 for H2 and air, respectively. Separated sugars were compared with derivatized sugar standards for qualitative and quantitative determination. A known amount of ß-phenyl-Dglucopyranoside was included in all samples as an internal standard. Organic Acid Analysis A Baker 10 extraction system (Phillipsburg, NJ) was used for purification of organic acids in strawberry leaves, petioles, crowns and roots. Samples were extracted in pH 7.0 deionized water and the supernatants were placed onto 3-mL quaternary amine columns which were previously conditioned with hexane and methanol. Organic acids were eluted with 3 mL 0.1N HC1. The eluate was concentrated to dryness under vacuum. Derivatization and determination of organic acids were similar to those for nonstructural carbohydrates except that the initial oven temperature was reduced to 100°C and held isothermal for 3 min, the temperature program rate was 4°C min'1, and final oven temperature was 230°C. Separation of organic acids was compared with derivatized organic acid standards for qualitative and quantitative determination.

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Lipid and Fatty Acid Analysis Lipids in leaves, petioles, crowns and roots were extracted, fractionated and analyzed according to procedures described previously (Wang et al., 1988; Wang and Faust, 1990). Strawberry tissues were extracted with isopropanol [containing 4 ug 2, 6-di-/-butyl-4-methylphenol (BHT) ml/ 1 ]. The purified lipids were separated into neutral, glyco-, and phospholipid fractions by silicic acid column chromatography on 100- to 200-mesh Bio Sil A (Bio Rad Laboratories, Richmond, CA). The chloroform eluate contained neutral lipids. The acetone fraction contained glycolipids and the methanol: H2O (10:1 v/v) eluate contained phospholipids. Total glycolipid and phospholipid in the polar lipid fractions were determined by the spectrophotometric assays of Roughan and Batt (1968) and Ames (1966), respectively. Total fatty acids esterified to polar lipids from leaf, petiole, crown and root tissues were derivatized to fatty-acyl methyl esters (FAMEs) for flame ionization detection gas chromatography (FID-GC) analysis (Wang and Faust, 1990). A known amount of heptadecanoic acid was included in all samples as an internal standard, and methyl heptadecanoate was used as an external standard. Individual FAMEs were identified by comparing their retention times with authentic standards (Supelco, Bellefonte, PA). This tentative identification of major polar lipid fatty acids was corroborated by further analyses of the FAMEs by gas chromatography-mass spectrometry (GC-MS) (Wang et al., 1988). RESULTS AND DISCUSSION Effect of Silicon on Plant Growth Silicon enhanced strawberry plant growth (Table 1). Plants sprayed with Si developed shorter petioles, but significantly produced more dry matter, as measured by aerial and root weights, than the controls (containing 0 mM Si). The enhanced growth was evident even at a low Si concentration (4.25 mM). The increase in strawberry plant growth by Si may be related to enhanced tissue elasticity and

TABLE 1. Growth enhancement by Si treatment in strawberry plants. Data shown are means of four replications. Values within a column followed by different letters are significantly different at p