THE MULTIPLE ROLES OF SOLUBLE SILICON FOR PLANTS AND GROWSTONES AS A SOURCE What is Silicon? Silicon is the second most abundant element in soils where it is found in concentrations similar to that of potassium, calcium, and other major plant nutrients, and well in excess of those of phosphate. Many plants, particularly monocotyledons, contain large amounts of Silicon, up to 10% of dry mass. Other plant species are able to accumulate Silicon to different degrees, with beneficial responses to Silicon supplementation. In spite of Silicon (Si) prominence as a mineral constituent of soils, its high concentration in plant tissues, in some cases as high as the concentration of macronutrients, and proven benefits to plants, it is still not considered an ‘essential’ nutrient for plant growth. For that reason it is not present in the formulation of commonly used nutrient solutions or fertilizers. Things have started to change and in February 2012 the Association of American Plant Food Control Officials (AAPFCO) elevated the status of Si from a fringe element to a plant-­‐beneficial element. This has allowed more research resources invested into studying the multiple roles Silicon play in plants defense mechanisms, and researchers to design standard protocols to validate Silicon fertilizers which will allow for marketing of these products to consumers. What form of Silicon do plants uptake? Silicon rarely occurs as the pure free element in nature. It is mostly found in soils, dust and sands, in various forms of silica (SiO2) or silicates and in this form it is not available to plants. To be 1 available to plants, silicate minerals need to dissolve part of its Si into the soil water, soluble Silicon, in a form available to plants, mainly as monosilicic acid (H4SiO4). What plants benefit from Silicon supplementation? Short answer -­‐ all plants can benefit from Silicon supplementation! Until recently the assumption was that high rates of Si absorption and tissue concentration is a prerequisite for Si benefits. Today, the literature is filled with studies conducted in low-­‐Si accumulators, which show enhanced tolerance to both disease and environmental stresses with Si supplementation. This validates that all plants can respond to Silicon regardless of accumulation status. What are Silicon benefits to plants? While Silicon should not be considered a panacea for stress resistance in plants, it has long been associated with a wide variety of beneficial plant responses related to biotic and abiotic stresses and morphology. Research shows Silicon increased plants resistance to fungi, insects and weeds as well as mineral toxicity and water stress (drought, salinity). Resistance to water stress was in part thought to be associated with the precipitate as amorphous silica in cell walls and development of a thicker waxy layer in leaves cuticle. However, present data shows that the beneficial role of Si goes beyond the formation of physical or mechanical barriers – it also modulates plants metabolism and modify physiological activities particularly in plants under stress conditions. For example, Si supplementation has been reported to reduce incidence of powdery mildew of miniature potted roses and cucumbers; decrease black spot disease in shrub roses; decrease Botrytis infection in sunflowers; and reduce Pythium in roots of greenhouse bitter gourd. More recent studies have focused on low-­‐accumulators response to viruses in tobacco and tomato. Si also affects plant morphology in a positive way, such as increasing stem diameters of chrysanthemum, spray rose ‘Pinocchio’, and gerbera, and increasing both stem and flower diameter of greenhouse grown sunflower and zinnias. Vegetative growth and net photosynthetic rates were improved by Si supplementation in hydroponic cucumber plants and zucchini, respectively. Leaf miners were significantly reduced in chrysanthemum supplemented with soluble Silicon up to a Si accumulation level of 200ppm in leaves in 6 weeks. Other studies show Si-­‐supplementation led to an extended shelf life compared with control plants. 2 All the different plant Silicon-­‐enhanced responses provide strong evidence that soluble Silicon induces plants physiological responses related to different defense mechanisms against natural enemies and/or induced stresses. As such, it is only logical that Silicon should be added to the overall IPM arsenal. Unless Silicon is an integral part of the growing media like in Growstones. Growstones as a source of soluble Silicon Growstones are made from recycled glass, the matrix of which is silica. Therefore Growstones have its own intrinsic source of soluble Silicon. When in contact with water, Growstones surface leaches out soluble Silicon over time as monosilicic acid (H4SiO4). Under normal growing conditions and irrigation frequency, Growstones leach out soluble Silicon at an average rate of 5 -­‐ 6 ppm per week, accumulating about 45 ppm1 of soluble Silicon in the nutrient solution recirculating system in 8 weeks. During the same period, the average accumulation of soluble Silicon from rockwool is 0.8 ppm1. Previous Silicon research showed an increased waxy layer in cucumber fruit skin with Silicon application rates between 7 to 28 ppm. So it was no surprise to observe Growstones grown cucumber fruits with waxier and thicker skin after 8 weeks, while no effect was observed in rockwool fruits grown side-­‐by-­‐side. Thicker fruit skin increases shelf life of cucumber fruits, which might affect prices favorably in North American markets. Cucumber plants grown in Growstones have also shown to be basically insensitive to powdery mildew and Pythium compared to rockwool plants. While it is well known that high silicon levels improve plants resistance against mildew, the lower incidence of Pythium in Growstones might be due to a combination of drier substrate and high Silicon concentration available for plants in the nutrient solution. Also in ornamental species, peat based potting mix with Growstones had a significantly higher level of Silicon accumulated in their shoots and roots then those grown in equivalent peat mixes with perlite. It is worth noting that as the irrigation frequency increases so does the leaching rate of Silicon from the surface of Growstones. Not surprisingly, when Growstones are continuously irrigated Silicon leaching rate is considerably increased and when soaked and shaken, the average soluble Silicon level can reach between 62 and 88 ppm1 in one week, in municipal and de-­‐ionized water respectively. This is in line with the levels found for normal irrigation frequencies with no night irrigation, as with a continuous contact between water and Growstones surface, a much higher Si dissolution rate is expected. 3 This means that with Growstones products, growers can benefit not only from more frequent irrigations with increased growth potential without the risk of root rot, and have tremendous aeration and drainage – growers can also increase soluble Silicon availability to plants with the associated benefits already included with no extra cost for external Silicon fertilizers. All this while recycling glass from landfills…. 1 filtered and acidified with HNO3 followed by ICP-­‐OES 4