Uptake of Silicon by Sugarcane from Applied Sources May Not Reflect Plant-Available Soil Silicon and Total Silicon Content of Sources.

Soils of the tropics and sub-tropics are typically acid and depleted of soluble sources of silicon (Si) due to weathering and leaching associated with high rainfall and temperatures. Together with intensive cropping, this leads to marginal or deficient plant Si levels in Si-accumulating crops such as rice and sugarcane. Although such deficiencies can be corrected with exogenous application of Si sources, there is controversy over the effectiveness of sources in relation to their total Si content, and their capacity to raise soil and plant Si concentrations. This study tested the hypothesis that the total Si content and provision of plant-available Si from six sources directly affects subsequent plant Si uptake as reflected in leaf Si concentration. Two trials with potted cane plants were established with the following Si sources as treatments: calcium silicate slag, fused magnesium (thermo) phosphate, volcanic rock dust, magnesium silicate, and granular potassium silicate. Silicon sources were applied at rates intended to achieve equivalent elemental soil Si concentrations; controls were untreated or lime-treated. Analyses were conducted to determine soil and leaf elemental concentrations. Among the sources, calcium silicate produced the highest leaf Si concentrations, yet lower plant-available soil Si concentrations than the thermophosphate. The latter, with slightly higher total Si than the slag, produced substantially greater increases in soil Si than all other products, yet did not significantly raise leaf Si above the controls. All other sources did not significantly increase soil or leaf Si concentrations, despite their high Si content. Hence, the total Si content of sources does not necessarily concur with a product's provision of soluble soil Si and subsequent plant uptake. Furthermore, even where soil pH was raised, plant uptake from thermophosphate was well below expectation, possibly due to its limited liming capacity. The ability of the calcium silicate to provide Si while simultaneously and significantly increasing soil pH, and thereby reducing reaction of Si with exchangeable Al3+, is proposed as a potential explanation for the greater Si uptake into the shoot from this source.

Provision of nitrogen as ammonium rather than nitrate increases silicon uptake in sugarcane.

Silicon (Si) is important in mitigating abiotic and biotic plant stresses, yet many agricultural soils, such as those of the rainfed production areas of the South African sugar industry, are deficient in plant-available Si, making Si supplementation necessary. However, Si uptake by sugarcane (Saccharum spp. hybrids) is limited even where silicate amendments improve soil Si status. Rhizosphere pH, which can affect Si uptake, can be manipulated using different N-form fertilizers. We tested whether (i) fertilization with [Formula: see text] (rhizosphere acidification) increased Si uptake compared with [Formula: see text] (rhizosphere alkalinization); and (ii) uptake differed between an N-efficient, more acid-tolerant cultivar (N12) and an N-inefficient, less acid-tolerant cultivar (N14). Two pot trials with low-Si soil were fertilized with calcium silicate (Ca2SiO4) slag, plus N from ammonium sulphate [(NH4)2SO4], ammonium thiosulphate [(NH4)2S2O3] and calcium nitrate [Ca(NO3)2] (Trial 1) or N from (NH4)2S2O3 and Ca(NO3)2 only (Trial 2). Trial 2 included cultivars N12 and N14. Nitrate treatments significantly increased soil pH and soil Si compared with [Formula: see text] However, [Formula: see text] treatments significantly increased leaf and stalk Si content compared with [Formula: see text] reflected in a significant negative relationship between soil pH and leaf Si. Acid-extracted soil Si was negatively related to leaf and stalk Si, likely due to adsorption of silicic acid to soil surfaces under higher pH of the [Formula: see text] treatment and its reduced availability for plant uptake. We conclude that [Formula: see text] increased Si uptake into leaf and stalk, and propose that reduced rhizosphere pH solubilized Si from Ca2SiO4 and increased silicic acid availability for plant uptake. By contrast, [Formula: see text] may have reduced Si uptake due to adsorption of Si to soil surfaces at higher pH. Our results indicate that ammoniacal fertilizers, such as (NH4)2SO4 and urea, have potential for promoting dissolution of applied Ca2SiO4 and subsequent uptake of Si by sugarcane.

Silicon reduces impact of plant nitrogen in promoting stalk borer (Eldana saccharina) but not sugarcane thrips (Fulmekiola serrata) infestations in sugarcane.

The stalk borer Eldana saccharina Walker (Lepidoptera: Pyralidae) is a major limiting factor in South African sugarcane production, while yield is also reduced by sugarcane thrips Fulmekiola serrata Kobus (Thysanoptera: Thripidae). Borer management options include appropriate nitrogen (N) and enhanced silicon (Si) nutrition; the effect of N on sugarcane thrips is unknown. We tested the effects of these nutrients, in combination with resistant (N33) and susceptible (N27) sugarcane cultivars, on E. saccharina and F. serrata infestation. Two pot trials with three levels of N (60, 120, and 180 kg ha(-1)) and two levels each of calcium silicate and dolomitic lime (5 and 10 t ha(-1)) were naturally infested with thrips, then artificially water stressed and infested with borer. Higher N levels increased borer survival and stalk damage, while Si reduced these compared with controls. Silicon significantly reduced stalk damage in N27 but not in N33; hence, Si provided relatively greater protection for susceptible cultivars than for resistant ones. High N treatments were associated with greater thrips numbers, while Si treatments did not significantly influence thrips infestation. The reduction in borer survival and stalk damage by Si application at all N rates indicates that under field conditions, the opportunity exists for optimizing sugarcane yields through maintaining adequate N nutrition, while reducing populations of E. saccharina using integrated pest management (IPM) tactics that include improved Si nutrition of the crop and reduced plant water stress. Improved management of N nutrition may also provide an option for thrips IPM. The contrasting effects of Si on stalk borer and thrips indicate that Si-mediated resistance to insect herbivores in sugarcane has mechanical and biochemical components that are well developed in the stalk tissues targeted by E. saccharina but poorly developed in the young leaf spindles where F. serrata occurs.

Silicon as a plant defence against insect herbivory: response to Massey, Ennos and Hartley.

1. Silicon (Si) has received increased attention as a nutrient capable of providing some measure of defence for plants against fungal pathogens, and insect and mammalian herbivores. 2. On the basis of a study including two generalist insect folivores and a phloem feeder, Massey, Ennos & Hartley (2006; Journal of Animal Ecology, 75, 595-603) have drawn attention to a putative distinction between the effects of plant Si in defending against folivorous and phloem-feeding insects. On the basis of their results they imply that phloem feeders are less likely to be adversely affected by increased plant Si than folivores. 3. However, in making this suggestion, Massey et al. have ignored many previous studies demonstrating a clear effect of plant Si on a range of phloem-feeding and some xylem-feeding insects, and that this effect stems not only from leaf mechanical properties based on opaline silica, but also from induced chemical defences seemingly mediated by soluble Si. 4. Furthermore, Massey et al. cannot claim that their study was the first demonstration of a direct effect of Si on insect herbivore preference and performance; there have been numerous earlier studies demonstrating this from folivores, stem borers, and phloem and xylem feeders. 5. We contend that current evidence indicates that Si is likely to be involved to a similar extent in enhancing resistance to all four insect feeding guilds and that any conclusion to the contrary is, at this stage, premature.