Robyn Joubert
Silicon, plants’ anti-depressant Fertilising crops with silicon helps them deal with environmental stresses such as drought, heat and cold, and enhances resistance to pests and diseases, according to growing evidence. Scientists in the Department of Botany at Laval University in Quebec, Canada, investigated the effects of silicon uptake in plants using transcriptomic analyses, which studied gene regulation to reveal which genes responded to silicon. More than 30 000 genes were analysed in Arabidopsis plants (a weed similar to shepherd’s purse), half of which were infected with powdery mildew to provide stress.
Team leader Prof Richard Belanger said their results revealed that silicon plays no role if a plant is not under any stress. “In unstressed plants it appeared that only two genes were switched on,” he said. However, when plants were stressed by powdery mildew infections, a huge number of genes were affected by the presence or absence of silicon. “This indicates that the benefits of silicon fertilisation are seen primarily under stress,” said Prof Belanger. Prof Mark Laing, head of the Plant Pathology Department at the University of KZN, explains, “The genes of silicon-treated plants lit up like a Christmas tree, with a huge number of genes switched on to defend the plant.
Fascinatingly, an equally huge number of genes were switched off to avoid stress, which can also be damaging to the plant. Silicon plays a damping-down role much like an anti-depressant does in humans.” Prof Laing says in the early days of silicon research it was thought silicon only played a physical role in plants, for example by strengthening cell walls. “There’s now no question it also plays a biochemical role, switching on or off thousands of genes in the plant’s response to stress.” Prof Laing says the findings are much like Columbus discovering America – just the start of a much greater process of discovery, mapping the whole continent.
“Now we have to make sense of the amazing complexity Prof Belanger has discovered. We’re struggling to understand how silicon can affect thousands of genes simultaneously.” While this study was based on Arabidopsis, Prof Belanger says advances in genome sequencing make it possible to study the genetic responses of agricultural plants such as wheat and rice to silicon fertilisation. “This should find many applications in the effort to optimise the use of silicon in agriculture,” he concluded.
Recycling Silicone in trees
Findings on how forest trees recycle silicon were presented by Dr JT Cornélius from the Soil Science Unit at the Catholique de Louvain University in Belgium. The study quantified the annual uptake of silicon in five different forest stands – European beech, oak, Norway spruce, Douglas fir and black pine. The stands were grown on an acid brown soil within a unique 32-year-old planting site with a humid, temperate climate. The researchers found the trees’ leaves and needles accumulated the most silicon.
“When they drop and decompose, the tree reabsorbs and recycles the silicon, making silicon mobile in the soil-tree cycle,” explains Prof Mark Laing. Researchers found some trees took up a lot of silicon but others only a little. “European beech takes up 35 times more silicon than black pine. Silicon makes up 1% of European beech needles but only 200ths of a percent of black pine ones, and 1kg of European beech leaves contain 7,4g pure silicon while 1kg of black pine needles contain only 0,2g silicon.” In oak, 1kg of leaves would contain 5,5g silicon, in Norway spruce 4,6g and in Douglas fir 5,3g. If, for example, the evergreen Douglas fir takes up 30kg/year of silicon, it becomes tied up in its needles until decomposition.
Given that trees in temperate forests often live 100 years, they tie up a lot of silicon. “These findings show how important it is to keep silicon in the agricultural cycle,” says Prof Laing. “In 1998 FAO estimated that 224 million tons of silicon were removed from farmlands per year in the world. Farmers replaced little of it. It’s crucial that when we crop maize or sugarcane or cut grasslands, for example, we recycle the trash to release its silicon, and start applying silicon fertiliser to replace what was removed with the crop. Silicon should be regarded as an essential fertiliser on par with potassium. Practices such as no-till and trashing and trashing ensure it’s naturally recycled.”
