Silica as a Defense against Herbivory and a Growth Promotor in African Grasses

Abstract

Grasses and dung were collected in the Serengeti National Park and analyzed for silica content by wet ashing. Grasses from grasslands differing in the grazing intensities experienced were grown in the laboratory in a factorial experiment to determine factors controlling tissue silicification. Concentrations of silica in tissues of plants collected in the field were higher than have been reported for any other plants abundant in grazing ecosystems. Silica contents in the field were higher in more heavily grazed grasslands and in tissue produced earlier in the growing season. Animal dung contained substantial quantities of silica. Laboratory experiments indicated that tissue silicification was increased by defoliation, was higher in plants from more heavily grazed grasslands, varied in different organs and species in patterns confirming current hypotheses about plant defense, and was affected by the availability of soluble silica in the nutrient medium. Silica in the nutrient medium promoted the yield of unclipped plants substantially. Total yield was 18% higher than that of control plants, although hydroponic experiments with solutions prepared and handled in plastic indicated that silica was not a growth requirement, except, perhaps, at the ecologically unrealistic concentrations that might result from reagent contamination. Yield stimulation by silica was differentially distributed among organs, tending to promote photosynthetically active tissues and crowns. Flowering of one species was promoted by silica. Leaves of silica-fed plants were larger. Leaf blade chlorophyll concentrations were 15% higher in silica-fed plants from the more heavily grazed grasslands. The results suggest that complex patterns of grass silicification had a role in the radiation of grazing animals and grasses and may contribute to maintaining the biotic diversity of contemporary grassland-savanna ecosystems by influencing the partitioning of forage species and organs among grazers. Growth promotion by silica may be due to the substitution of mineral support for carbon-based support associated with the deposition of silica in the intercellular spaces of aerial tissues. Since soils of the Serengeti region commonly have pH levels above neutrality, where the availability of silica is low, silica supply could influence primary productivity and resultant energy and nutrient flow through the trophic web in the native environments of the plants.

The Effect of Seasonal Variations, Covariations with Minerals and Forage Value on Itchgrass’ Foliar Silicification from Sudanian Benin

“…Phenological stage is well known to influence silica accumulation [48]. The higher leaf SiO 2 content of R. cochinchinensis' is consistent with the hypothesis that silica concentration increases with the plant's age [10,15]. Our study indicates that R. cochinchinensis' increased its SiO 2 concentrations from May to September.…”

“…Leaf silica deposits may result from evaporation, because silica content increases where transpiration is high [10,11]. Silica is the largest mineral component of perennial grasses.…”

“…Several studies have already shown that fodder species differ in silica content and this may affect their palatability and digestibility [10,12,13]. Silica may reduce livestock preference or palatability for certain plants [14].…”

Opal Phytoliths in Southeast Asian Flora

“…Although the diversity and abundance of silica present in a plant varies in relation to soil characteristics and plant senescence (Jones and Handreck, 1967;Iler, 1979;McNaughton et al, 1985), this rarely affects the presence of diagnostic phytolith forms. Even without considering the Poaceae, phytoliths from diagnostic monocotyledon genera can be used to distinguish a diverse set of specific habitats, as shown in Table 2.…”

Leaf anatomy and its contribution to the systematics of Aechmea subgenus Macrochordion (de Vriese) Baker (Bromeliaceae)

“…epidermal cells with lignified thick walls and silica body inclusions, mechanical hypodermis, water parenchyma, sunken stomata and lignified fibers in the mesophyll). Silica bodies have been associated with resistance against herbivory due to their inedibility (MacNaughton et al 1985, Prychid et al 2004), but are also important for refracting excess of light, helping individuals to establish in sun-exposed environments (Krauss 1949). These inclusions are also commonly found in other monocots families within Poales, suggesting that this is a plesiomorphic character for the order (Pereira et al 2011).…”