Silicon (Si) is not considered an essential plant nutrient; however, several plant species demonstrate improved disease resistance, abiotic stress tolerance, and altered morphological traits when Si is present (Epstein, 1999). Soil contains, on average, 31% Si from silica (SiO2) (Epstein, 1999). Plants absorb Si in the form of soluble silicic acid, which is found in soils at concentrations ranging from 0.1 to 0.6 mm (2.8 to 16.9 ppm Si) (Epstein, 1994). Soilless substrates (Chen et al., 2000) and standard commercial fertilizers (Epstein, 1994) contain little soluble Si. Depending on the water source, irrigation water may contain trace amounts of Si (Voogt and Sonneveld, 2001).
Horticultural crops grown in Si-amended substrates exhibit a variety of responses related to abiotic and biotic stresses and morphology. For example, Si supplementation has been reported to reduce incidence of powdery mildew of miniature potted roses (Rosa hybrid L.) (Datnoff et al., 2006; Larsen, 2008). ‘Meipelta’ shrub rose irrigated with Si-supplemented water had decreased black spot disease occurrence (Gillman et al., 2003). Botrytis infection was significantly decreased in calcium silicate-amended sunflowers (Helianthus annuus L. ‘Ring of Fire’) and Si-supplemented plants had an extended postharvest life compared with control plants (Kamenidou et al., 2002). Pythium colonization was reduced on roots of a greenhouse-grown bitter gourd (Mormodica charantia L.) that received continuous Si supply in the irrigation water (Heine et al., 2007). However, powdery mildew [Podosphaera fusca (Fr.) U. Braun & Shishkoff (2000)] severity of gerbera (Gerbera jamesonii Bolus ex. Hook f. ‘Snow White’) was unaffected by Si treatment (Moyer et al., 2008). Si foliar sprays were effective in ameliorating bract edge burn in poinsettia (Euphorbia pulcherrima Willd. ‘Supjibi Red’) (McAvoy and Bible, 1996).
Si supplementation was reported to increase stem diameters of chrysanthemum (Chrysanthemum ×morifolium Ramat. ‘Backwang’) (Moon et al., 2008), spray rose ‘Pinocchio’ (Hwang et al., 2005), and gerbera (Savvas et al., 2002). Si additions increased both stem and flower diameter of greenhouse-grown sunflower (Kamenidou et al., 2008) and zinnia (Zinnia elegans Jacq. ‘Oklahoma Formula Mix’) (Kamenidou et al., 2009). Vegetative growth of NaCl-stressed hydroponic cucumber (Cucumis sativus L. ‘Jinlu4’ and ‘Jinyan4’) (Zhu et al., 2004) and cut-flower roses (Savvas et al., 2007) was improved by Si supplementation. Si also reportedly improves the net photosynthic rate of NaCl-stressed zucchini (Savvas et al., 2009).
Si is accumulated by a broad range of bedding and potted plant species (Frantz et al., 2008). Of the 14 species examined by Frantz et al. (2008), all 14 accumulated additional amounts of Si in their leaves when supplemented with potassium silicate. Leaf tissue concentration varied from 237 mg·kg−1 Si for petunia (Petunia ×hybrida Vilm. ‘White madness’) to 11,700 mg·kg−1 for zinnia ‘Oklahoma white’. Although Frantz et al. (2008) demonstrated that these ornamental species accumulate Si under hydroponic cultivation, it is not clear how these results would translate to substrate-based growing conditions. It has been suggested (Ma and Yamaji, 2006), and many researchers operate under the assumption, that high rates of Si absorption and tissue concentration are a prerequisite for Si benefits; hence, it is desirable to determine which floriculture species accumulate this element.
Previous researchers who have examined the effects of Si supplementation on greenhouse crops have either focused on a limited number of species or have grown plants using hydroponic methods. It is unproven in the literature whether the majority of bedding or potted crops grown using substrates and tap water would respond positively to Si supplementation. Therefore, the objectives of this experiment were to examine whether weekly potassium silicate drenches would increase leaf Si concentration and affect morphological traits of several floriculture species.
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