Silicon is a plant beneficial element. It is associated with several positive physiological responses in plants (Ma, 2004), including reduced lodging (Ma and Yamaji, 2006; Savant et al., 1999), increased stem diameter (Kamenidou et al., 2008), higher rates of photosynthesis (Liang et al., 1996; Romero-Aranda et al., 2006), increased dry weight (Liang et al., 1996; Romero-Aranda et al., 2006), increased yield (Savant et al., 1999), increased flower size (Kamenidou et al., 2010), and earlier flowering (Boldt et al., 2015). Silicon has been shown to mitigate the impacts of many biotic and abiotic stresses, including powdery mildews (e.g., Blumeria graminis f. sp. tritici, Erysiphe cichoracearum, and Sphaerotheca fulginea) (Chain et al., 2009; Fauteux et al., 2006; Guével et al., 2007; Menzies et al., 1992), rice blast (Magnaporthe grisea) (Datnoff et al., 1997; Ishiguro, 2001), herbivory (Massey et al., 2006; Reynolds et al., 2009), drought (Hattori et al., 2005; Zhu and Gong, 2014), salt stress (Liang et al., 1996; Romero-Aranda et al., 2006), heat stress (Agarie et al., 1998), chilling injury (He et al., 2010; Liang et al., 2008), nutrient deficiencies (Ma, 2004), and heavy metal toxicity (Frantz et al., 2011; Liang et al., 2007).
Foliar Si concentration generally ranges from 0.1% to 10% plant dry weight (Liang et al., 2007; Ma and Takahashi, 2002). Plants are typically classified as Si accumulators or nonaccumulators, with a lower threshold of ≥10,000 mg·kg−1 foliar Si (i.e., 1% dry weight) considered the baseline for Si accumulators (Epstein, 1999; Ma et al., 2001). Many species within Poaceae are classified as Si accumulators, including important agronomic crops like wheat (Triticum aestivum L.), rice (Oryza sativa L.), corn (Zea mays L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), and sugarcane (Saccharum officinarum L.). In addition, some species of the horticulturally important Cucurbitaceae [cucumber (Cucumis sativus L.), squash, and pumpkin (Cucurbita spp.)], and Asteraceae [sunflower and zinnia (Zinnia elegans L.)] are Si accumulators (Frantz et al., 2010). However, most greenhouse-grown ornamentals are considered low Si accumulators or nonaccumulators. Frantz et al. (2010) grew 48 horticultural crops hydroponically in a modified Hoagland’s solution amended with 1 mm Si and quantified foliar Si concentration. It ranged between 102 and 12,682 mg·kg−1 Si [ornamental tobacco (Nicotiana sylvestris Speg. & Comes.) and zinnia, respectively], and more than half the species accumulated less than 1000 mg·kg−1 Si.
For many years, it has been debated as to whether Si supplementation would be beneficial to nonaccumulators, because they do not accumulate high foliar concentrations (Ma et al., 2001; Mitani and Ma, 2005). The role of Si in plants may be both in a structural capacity and as a signaling compound (Fauteux et al., 2006), and therefore, nonaccumulators could still benefit from Si supplementation and increased Si accumulation following imposition of a stress. For example, the addition of Si to the hydroponic nutrient solution delayed Tobacco ringspot virus symptom formation and reduced symptomatic leaf area in tobacco (Nicotiana tabacum L.) (Zellner et al., 2011), and it ameliorated copper (Cu) toxicity in arabidopsis [Arabidopsis thaliana L. (Heynh.)] (Li et al., 2008) and snapdragon (Antirrhinum majus L.) (Frantz et al., 2011), all Si nonaccumulators.
Soluble, plant-available Si can be supplied through substrate components, substrate amendments, liquid fertilization, or foliar sprays. Silicon is absorbed by plants primarily via the roots through passive or active uptake (Liang et al., 2006; Mitani and Ma, 2005). Plants accumulate Si in all tissues, although some may accumulate high Si concentrations in roots, and perhaps other tissues, but not in foliage. Recent studies have shown that root Si fractions in nonaccumulators may be as high as, or higher than, in Si accumulators. For example, we detected low foliar Si concentrations (1473 mg·kg−1) but higher root Si concentrations (2000–7000 mg·kg−1) in rose (Rosa ‘Radrazz’) plants grown at low P (2.5–20 mg·L−1; J.E. Altland and J.K. Boldt, unpublished data). Differences in root and shoot Si concentrations may result from differential regulation of Si uptake, the mechanism of xylem loading (active or passive), transporter concentration, or the stress status of a plant (Liang et al., 2006; Mitani and Ma, 2005).
There is little published research on the distribution of Si in plants. For species in which it has been documented, Si is not uniformly distributed throughout the plant. Rice grown in nutrient solution with 150 ppm SiO2 (70 mg·L−1 Si) averaged 9800 mg·kg−1 Si in roots, 57,000 mg·kg−1 in leaf sheaths, and 63,000 mg·kg−1 in leaf blades, on a dry weight basis, as calculated from values reported as %SiO2 (Yoshida et al., 1962). In oat, Si ranged from 280 mg·kg−1 in caryopses to 36,000 mg·kg−1 in inflorescences, with more than 40% of total aboveground Si localized in inflorescences (Jones and Handreck, 1967). Kamenidou et al. (2008) compared the effects of different sources of Si supplementation on tissue Si concentration of sunflower ‘Ring of Fire’ grown in a peat-based soilless substrate, and although tissue concentrations were not compared within a treatment, reported that leaf Si concentrations (4900–15,300 mg·kg−1) were greater than those in flowers (3800–5100 mg·kg−1) or stems (2900–4200 mg·kg−1).
Knowledge of Si distribution in plants is critical for using Si to mitigate plant stresses that are tissue specific. For example, the common greenhouse pathogen botrytis (Botrytis cinerea) typically attacks flowers and leaves, whereas pythium (Pythium ultimum) causes root rot. If, for instance, flowers were found to contain appreciable quantities of Si, follow-up studies could investigate whether the accumulation of Si can provide protection against fungal pathogens like botrytis and offer growers a nonpesticide alternative to include in their pest management rotation.
The objective of this study was to document Si distribution in greenhouse crops to better understand how it might (or might not) be useful for mitigating stress in these species. Specifically, the experiment was designed to evaluate Si accumulation and distribution in roots, leaves, stems, and flowers of plants, using parboiled rice hulls as the source of supplemental Si. Two species were selected: petunia, classified as a nonaccumulator of Si, and sunflower, classified as a Si accumulator.
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