Silicon: Plant Nutrient and Biocontrol Agent
Editor’s note: Wendy Zellner made an in-depth presentation on silicon and its use as a biocontrol agent in vegetable crops at the Biocontrols USA West 2017 Conference & Expo. To earn more about the Biocontrols Conference & Expo Series and its upcoming events, visit BiocontrolsConference.com.
Silicon is used in a number of physiological processes within plants. The most visible and well-studied is its role in stress alleviation to both abiotic and biotic stress. While the molecular mechanisms are not yet known, this protection has been demonstrated in a number of plants against a variety of stressors. In addition to stress, Si has also been shown to interact with heavy metals and thought to play roles in both nutrient toxicity and deficiency. Si is known to be involved in the reactive oxygen species (ROS) pathway and is responsible for the priming phenomenon where plants are more sensitive to stress, allowing them to respond more quickly. Si also has been implicated in developmental pathways, but this is even less understood.
Si has reduced adverse growth effects experienced by abiotic stress such as temperature, drought, salinity and nutrient imbalance. In Figure 1 is an example of drought stress where New Guinea Impatiens treated with Si (in red) recovered from a severe dry down sooner than control plants as seen below in blue. In addition, Si is known to protect plants from a number of pathogens, reducing both the onset and spread of these organisms.
Figure 1 also shows an example of tobacco plants grown with low or moderate amounts of Si and infected with Tobacco ringspot virus. Si-treated plants had a reduction in the spread of viral symptoms as well as a delay in the onset of symptoms, compared to the controls.
Si has the ability to protect plants through both physical and chemical means. Physically, Si helps strengthen cell walls and is present in protective layers, such as scales on the surface of epidermal tissue. This thickening protects the roots and allows for plants to better deal with adverse environmental conditions.
Chemical responses enhanced by Si include both the induction of the reactive oxygen species pathway and production of phenolic compounds. ROS is responsible for the hyper-sensitive response that leads to necrosis within plants and is part of the innate defense pathway, giving rise to protection against a wide range of infections. The Phenolic pathway protects plants against both environmental stress and pathogens through the production of defense compounds.
Silicon as Insect Control …
Other researchers also have some interesting examples of how Si works as a biocontrol agent, reducing both insect herbivory and pathogen infection. Many studies are beginning to emerge showing how Si reduces insect pressure. Studies conducted by Goussain and colleagues (Neotropical Entomology, 2005) in wheat have shown that treating plants with Si reduces the ability of aphids to produce eggs resulting in a lower number of offspring. They have observed that these insects also have a reduction in honeydew droplets and believe that Si has an effect on the sap ingestion as opposed to a simple physical barrier to stylet penetration. On cucumber, Correa and colleagues (Neotropical Entomology, 2005) found that foliar Si treatment has also been shown to reduce the incidence of whiteflies. And there are a number of studies that have been performed by Certis USA with Sil-Matrix, a foliar applied potassium silicate, showing reductions in various insect pests, including spider mites on strawberries, apple European red mites, and lettuce green peach aphids.
… and as Disease Control
Si not only reduces insect pressure, but it also has an even more pronounced effect on fungal pathogens. A majority of the research focuses on Si reduction in fungal pathogens, as it is easily observed. It is Important to note that Si does NOT protect against all stress at all times. The response can be highly variable and depends on both environmental conditions and plant species. This is why Si has shown protection and no effect against some of these pathogens, including powdery mildew, coffee leaf rust, and Asian soybean rust.
Foliar-applied Si may reduce the incidence of fungal pathogens through both a physical and chemical response. When foliar Si is applied, as it dries, it polymerizes to form an amorphous Si layer on top of the cuticle. As the Si is sprayed, some of the material may also run-off leaves into the media and be taken up into the plant via the roots and transported to shoots, where it can play a role in internal plant defense pathways. As fungal spores move through the air and land on the surface of the leaf, its structures are thought to more easily penetrate a non-treated vs. Si treated plant. Since it takes these structures longer to penetrate, this helps reduce disease pressure on the plant. In addition, once the fungus is able to penetrate, in untreated plants, it can quickly grow and spread in the nutrient-rich environment, while the Si-treated plant has the ability to quickly produce defense compounds, such as phenolics, further reducing the growth and spread of the fungus.
This quicker response time is known as priming. Figure 2 shows an example of priming in tomatoes infected with Xanthomonas gardnerii.
Both plants were inoculated with the same concentration of the bacteria and the Si-treated plants on the right responded more quickly to the bacteria, showing enhanced necrosis and chlorosis, compared to the control plant. By responding more rapidly to infections, plants can reduce the spread of the disease both within its tissue as well as to neighboring plants.
Product Forms to Consider
By reducing disease and stress, Si amendment leads to healthier plants with better yields, compared to their Si-deprived counterparts. So what products are available to suit your current needs?
It may be surprising to know that there are a number of products on the market that provide Si to plants through both soil-amendment and foliar applications. Solid products include recycled steel slag, mined wollastonite and ignimbrite, recycled glass and plant-based material such as rice hulls and biochar produced from gasified rice hulls. These solid products tend to be incorporated into media and allow for a slower release of silicic acid to the plants. The liquid products are used for foliar application methods or as drenches or additives to hydroponic or aeroponic type systems and include potassium, calcium and sodium silicates. Both the liquid and solid products are labeled for a wide variety of uses, including alternative liming agents, fertilizers, biostimulants, and biocontrol agents. Since we now know that Si has many roles within plants, it is not surprising that it fits into all these categories. Products are labeled based on listed claims. This means that Si products labeled as a biocontrol agent have been tested to reduce insect and/or other pathogens. Liquid applied products tend to have a more rapid response compared to solid amendments, which tend to vary in response time depending on material composition and surface area.
Silicon Handling and Application Tips
With the wide variability in these products, as with any material, it is extremely important to read and follow all labeled instructions.
While precautions are needed for many of these products, they tend to pose less risk than alternative chemical reagents. Silicon products labeled for biocontrol have a risk for eye, skin and respiratory irritation and proper PPE is required for application, including gloves, goggles and dust mask (for solid products that may be inhaled). The concentrated potassium silicates have a high pH and as with many pesticides, diluting the concentrated material poses the highest risk of exposure.
When mixing the liquid silicates into solution, there may be some adverse reactions. To begin, these concentrated silicon solutions have the ability to gel at pH 6.0. If your target pH range is lower than 6 then it is suggested to move through pH 6 as quickly as possible to avoid this reaction. Some water sources are more sensitive to gelling and those containing higher Ca or Mg may see this effect occur more rapidly. Once the material gels, it is impossible to re-suspend back into solution. Another possible problem is with the addition of the soluble silicate to a premixed nutrient solution where high pH may precipitate out nutrients within the solution. Diluting the material and lowering the pH prior to mixing may help avoid this problem. As with all products, make sure to talk with your sales rep. to determine how to avoid these potential issues.
Another effect you might see when beginning to use these products are unintended responses in the plants. Foliar-applied, liquid silicates are recommended to be sprayed at concentrations of 600-1500 ppm. My research works with Si applied as media drenches at concentrations below 50 ppm, since silicic acid begins to polymerize around 58 ppm (or 2.7 mM), reducing the amount of available Si for uptake. This graph is representative of the amount of available Si for plant uptake as Si concentrations increase. We see that as concentrations rise above 58 ppm, less Si is available to the plant, so the priming effect and other internal Si defenses we discussed earlier are inhibited, as you are reducing the concentration of Si within the plant. In addition, these high concentrations of polymerized Si in the growth media can also begin to chelate out other micronutrients such as copper, manganese, molybdenum and zinc.
This chelation of micronutrients can lead to deficiency symptoms in sensitive plants or cropping systems. In a study by Kamenidou and Cavins (HortScience, 2008), they reported that Si caused foliar malformations in sunflower and warned of its toxicity to certain plants. The researchers were adding 100-200 mM Si to the media which likely caused a zinc deficiency in the sunflowers leading to their findings. Reports of foliar burn on plants have also been reported and this could be due to salt effects on the leaves. As with any new amendment or biologic, plants should be tested to determine application doses. For sensitive plants, reducing the concentration of the biocontrol or increasing the addition of micronutrients could help counteract these effects.
In conclusion, Si is an important plant nutrient for all plants. Si has a specific role in stress responses and plants deficient in Si will not respond as well to stress as Si-fed plants. Both foliar and soil-applied Si show benefits to stress in a wide range of plant species and it’s important to understand your cropping system and how to best incorporate the products to get the most benefits.