Alin Song

Silicon-enhanced resistance to cadmium toxicity in Brassica chinensis L. is attributed to Si-suppressed cadmium uptake and transport and Si-enhanced antioxidant defense capacity

A series of hydroponics experiments were performed to investigate roles of silicon (Si) in enhancing cadmium (Cd) tolerance in two pakchoi (Brassica chinensis L.) cultivars: i.e. cv. Shanghaiqing, a Cd-sensitive cultivar, and cv. Hangyoudong, a Cd-tolerant cultivar. Plants were grown under 0.5 and 5 mg Cd L(-1) Cd stress without or with 1.5 mM Si. Plant growth of the Cd-tolerant cultivar was stimulated at the lower Cd level, but was decreased at the higher Cd level when plants were treated with Cd for one week. However, Plant growth was severely inhibited at both Cd levels as stress duration lasted for up to three weeks. Plant growth of the Cd-sensitive cultivar was severely inhibited at both Cd levels irrespective of Cd stress duration. Addition of Si increased shoot and root biomass of both cultivars at both Cd levels and decreased Cd uptake and root-to-shoot transport. Superoxide dismutase, catalase and ascorbate peroxidase activities decreased, but malondialdehyde and H2O2 concentrations increased at the higher Cd level, which were counteracted by Si added. Ascorbic acid, glutathione and non-protein thiols concentrations increased at the higher Cd level, which were further intensified by addition of Si. The effects of Si and Cd on the antioxidant enzyme activity were further verified by isoenzyme analysis. Silicon was more effective in enhancing Cd tolerance in the Cd-tolerant cultivar than in the Cd-sensitive cultivar. It can be concluded that Si-enhanced Cd tolerance in B. chinensis is attributed mainly to Si-suppressed Cd uptake and root-to-shoot Cd transport and Si-enhanced antioxidant defense activity.

Silicon in Agriculture

Although silicon (Si) is not yet listed among the essential elements for the growth of higher plants, it has been well documented to play an important role in providing benefi cial effects on growth and yield, especially in plants under stressful environments. From a practical perspective, the use of slag-based silicate fertilizers in agriculture can be dated back to the Middle Ages in Europe. Over the last decade, the discovery of specifi c Si transporters in rice roots has allowed great progress in the understanding of Si uptake by plants at the molecular level. In the same manner, important advancements have been made in dissecting the molecular mechanisms by which Si enhances plant resistance to fungal and bacterial diseases and insect pest damage. In contrast, more efforts are needed to explain at the molecular level the numerous reports showing Si benefi ts against abiotic stresses. In this chapter, a brief review is presented focusing on the most important historical points and general introduction of worldwide Si research.

Denitrification potential under different fertilization regimes is closely coupled with changes in the denitrifying community in a black soil

Preferable inorganic fertilization over the last decades has led to fertility degradation of black soil in Northeast China. However, how fertilization regimes impact denitrification and its related bacterial community in this soil type is still unclear. Here, taking advantage of a suit of molecular ecological tools in combination of assaying the potential denitrification (DP), we explored the variation of activity, community structure, and abundance of nirS and nirK denitrifiers under four different fertilization regimes, namely no fertilization control (N0M0), organic pig manure (N0M1), inorganic fertilization (N1M0), and combination of inorganic fertilizer and pig manure (N1M1). The results indicated that organic fertilization increased DP, but inorganic fertilization had no impacts. The increase of DP was mirrored by the shift of nirS denitrifiers’ community structure but not by that of nirK denitrifiers’. Furthermore, the change of DP coincided with the variation of abundances of both denitrifiers. Shifts of community structure and abundance of nirS and nirK denitrifiers were correlated with the change of soil pH, total nitrogen (TN), organic matter (OM), C:P, total phosphorus (TP), and available phosphorus (Olsen P). Our results suggest that the change of DP under these four fertilization regimes was closely related to the shift of denitrifying bacteria communities resulting from the variation of properties in the black soil tested.

The effect of Silicon on photosynthesis and expression of its relevant genes in rice (Oryza sativa L.) under high-zinc stress.

The main objectives of this study were to elucidate the roles of silicon (Si) in alleviating the effects of 2 mM zinc (high Zn) stress on photosynthesis and its related gene expression levels in leaves of rice (Oryza sativa L.) grown hydroponically with high-Zn stress. The results showed that photosynthetic parameters, including net photosynthetic rate, transpiration rate, stomatal conductance, intercellular CO2 concentration, chlorophyll concentration and the chlorophyll fluorescence, were decreased in rice exposed to high-Zn treatment. The leaf chloroplast structure was disordered under high-Zn stress, including uneven swelling, disintegrated and missing thylakoid membranes, and decreased starch granule size and number, which, however, were all counteracted by the addition of 1.5 mM Si. Furthermore, the expression levels of Os08g02630 (PsbY), Os05g48630 (PsaH), Os07g37030 (PetC), Os03g57120 (PetH), Os09g26810 and Os04g38410 decreased in Si-deprived plants under high-Zn stress. Nevertheless, the addition of 1.5 mM Si increased the expression levels of these genes in plants under high-Zn stress at 72 h, and the expression levels were higher in Si-treated plants than in Si-deprived plants. Therefore, we conclude that Si alleviates the Zn-induced damage to photosynthesis in rice. The decline of photosynthesis in Zn-stressed rice was attributed to stomatal limitation, and Si activated and regulated some photosynthesis-related genes in response to high-Zn stress, consequently increasing photosynthesis.

Effects of slag-based silicon fertilizer on rice growth and brown-spot resistance.

It is well documented that slag-based silicon fertilizers have beneficial effects on the growth and disease resistance of rice. However, their effects vary greatly with sources of slag and are closely related to availability of silicon (Si) in these materials. To date, few researches have been done to compare the differences in plant performance and disease resistance between different slag-based silicon fertilizers applied at the same rate of plant-available Si. In the present study both steel and iron slags were chosen to investigate their effects on rice growth and disease resistance under greenhouse conditions. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the effects of slags on ultrastructural changes in leaves of rice naturally infected by Bipolaris oryaze, the causal agent of brown spot. The results showed that both slag-based Si fertilizers tested significantly increased rice growth and yield, but decreased brown spot incidence, with steel slag showing a stronger effect than iron slag. The results of SEM analysis showed that application of slags led to more pronounced cell silicification in rice leaves, more silica cells, and more pronounced and larger papilla as well. The results of TEM analysis showed that mesophyll cells of slag-untreated rice leaf were disorganized, with colonization of the fungus (Bipolaris oryzae), including chloroplast degradation and cell wall alterations. The application of slag maintained mesophyll cells relatively intact and increased the thickness of silicon layer. It can be concluded that applying slag-based fertilizer to Si-deficient paddy soil is necessary for improving both rice productivity and brown spot resistance. The immobile silicon deposited in host cell walls and papillae sites is the first physical barrier for fungal penetration, while the soluble Si in the cytoplasm enhances physiological or induced resistance to fungal colonization.

The role of silicon in enhancing resistance to bacterial blight of hydroponic- and soil-cultured rice.

Here we report for the first time that bacterial blight of rice can be alleviated by silicon (Si) added. In both inoculated and uninoculated plants, shoot dry weight was significantly higher in the +Si plants than in the −Si plants. A soil-cultured trial showed that disease severity was 24.3% lower in the Si-amended plants than in the non-Si-amended plants. Plants that were switched from −Si to +Si nutrient solution and simultaneously inoculated with Xoo also exhibited the same high resistance to bacterial blight as the plants that were treated continuously with Si, with control efficiencies of 52.8 and 62.9%, respectively. Moreover, total concentrations of soluble phenolics and lignin in rice leaves were significantly higher in the +Si plants than in the −Si plants. Polyphenoloxidase (PPO) and phenylalanine ammonia-lyase (PAL) activities in rice leaves were observed to be higher in the +Si plants than in the −Si plants. The expression levels of Os03g0109600, Prla, Rcht2 and Lox2osPil, were also higher in +Si plants than in −Si plants post-inoculation during the experimental time. Addition of Si resulted in increased Pal transcription, and inhibited CatA and Os03g0126000 expression in the earlier and later stages of bacterial inoculation, respectively.

Cadmium fate and tolerance in rice cultivars.

Cadmium (Cd) is present in all soils, usually as a trace constituent, but it can reach higher levels in agricultural soils. Cd can then be absorbed by plants and become a potential risk to human health. Once taken up by a plant, there are mechanisms for heavy metal detoxification in the plant. Here, a cadmium-tolerant and a cadmium-sensitive rice cultivars were grown hydroponically to investigate the effects of cadmium (Cd) applied at low levels on uptake and transport, subcellular distribution and binding forms of Cd in rice plants. Our results showed that increasing the Cd treatment from 1.0 μM to 5.0 μM Cd increased the shoot Cd content by 55% in the cadmium-tolerant cultivar, and by 108% in the cadmium-sensitive cultivar. For the cadmium-tolerant cultivar, increasing Cd treatment from 1.0 μM to 5.0 μM increased the root Cd content by 116%, whereas for the cadmium-sensitive cultivar, increasing Cd treatment from 1.0 μM to 5.0 μM increased the root Cd content by 80%. Further, the ratio of Cd accumulation in shoots over roots decreased from 0.19 to 0.14 in the cadmium-tolerant cultivar, while it increased from 0.20 to 0.26 in the cadmium-sensitive cultivar, showing that the transportation ability for Cd was different between the two tested rice cultivars. At the higher Cd level of 5.0 μM, most of the Cd in the plants was localized in cell walls and vacuoles in both cultivars, whereas small portions of Cd were distributed in the cytoplasm, suggesting that the important metabolic and physiological processes were not impaired under Cd stress. Furthermore, the major portions of Cd in the cells were combined with organic acids, proteins and polysaccharide, and were consequently detoxified. The difference in the distribution of cadmium in rice plants resulted in the difference in Cd tolerance between the two rice cultivars used. It can be concluded that the retention of Cd in root cell walls, compartmentation of Cd into vacuoles and the suppressed transportation of Cd from roots to shoots are the most important mechanisms involved in the detoxification of Cd in rice plants.

The potential for carbon bio-sequestration in China’s paddy rice (Oryza sativa L.) as impacted by slag-based silicate fertilizer

Rice is a typical silicon-accumulating plant. Silicon (Si), deposited as phytoliths during plant growth, has been shown to occlude organic carbon, which may prove to have significant effects on the biogeochemical sequestration of atmospheric CO2. This study evaluated the effects of silicate fertilization on plant Si uptake and carbon bio-sequestration in field trials on China’s paddy soils. The results showed (1) Increased Si concentrations in rice straw with increasing application rates of silicate fertilizer; (2) Strong positive correlations between phytolith contents and straw SiO2 contents and between phytolith contents and phytolith-occluded carbon (PhytOC) contents in rice straw; (3) Positive correlations between the phytolith production flux and either the above-ground net primary productivity (ANPP) or the PhytOC production rates; (4) Increased plant PhytOC storage with increasing application rates of silicate fertilizer. The average above-ground PhytOC production rates during China’s rice production are estimated at 0.94 × 106 tonnes CO2 yr−1 without silicate fertilizer additions. However, the potential exists to increase PhytOC levels to 1.16–2.17 × 106 tonnes CO2 yr−1 with silicate fertilizer additions. Therefore, providing silicate fertilizer during rice production may serve as an effective tool in improving atmospheric CO2 sequestration in global rice production areas.

Silicon Biogeochemistry and Bioavailability in Soil

Whether silicon (Si) deficiency occurs in plants depends largely on plant-available Si concentration in soils but not on total Si content. Si bioavailability in soils is related closely to soil Si biogeochemistry including biogeochemical cycling of Si, forms and solubility of Si in soils and solubility of Si in soils. This chapter deals in detail with soil Si biogeochemistry and bioavailability, solid forms of Si in soils, Si in natural waters, soluble and plant-available Si in soils, and Si-supplying power.

Effect of Silicon on Crop Growth, Yield and Quality

Silicon (Si) has widely been reported to increase the growth and biomass, yield and quality of a broad range of crops including monocotyledonous crops such as rice, wheat, maize, barley, millet, sorghum and sugarcane that actively take up and accumulate high amounts of Si in their organs and some dicotyledonous crops such as cotton and some vegetable and fruit crops. The yield increment, however, may be attributable not only to the beneficial effects of Si including growth promotion, lodging resistance and biotic and abiotic stress resistance but also to some indirect effects such as pH adjustment and acquisition of macro- and micronutrients contained in the silicate fertilizers, especially when slags or Si-containing mineral ores are used as sources of silicate fertilizer.