• Background and Aims Silicon (Si) in plants provides structural support and improves tolerance to diseases, drought and metal toxicity. Shoot Si concentrations are generally considered to be greater in monocotyledonous than in non-monocot plant species. The phylogenetic variation in the shoot Si concentration of plants reported in the primary literature has been quantified. • Methods Studies were identified which reported Si concentrations in leaf or non-woody shoot tissues from at least two plant species growing in the same environment. Each study contained at least one species in common with another study. • Key Results Meta-analysis of the data revealed that, in general, ferns, gymnosperms and angiosperms accumulated less Si in their shoots than non-vascular plant species and horsetails. Within angiosperms and ferns, differences in shoot Si concentration between species grouped by their higher-level phylogenetic position were identified. Within the angiosperms, species from the commelinoid monocot orders Poales and Arecales accumulated substantially more Si in their shoots than species from other monocot clades. • Conclusions A high shoot Si concentration is not a general feature of monocot species. Information on the phylogenetic variation in shoot Si concentration may provide useful palaeoecological and archaeological information, and inform studies of the biogeochemical cycling of Si and those of the molecular genetics of Si uptake and transport in plants.

Lacustrine sediments from southeastern Arabia reveal variations in lake level corresponding to changes in the strength and duration of Indian Ocean Monsoon (IOM) summer rainfall and winter cyclonic rainfall. The late glacial/Holocene transition of the region was characterised by the development of mega-linear dunes. These dunes became stabilised and vegetated during the early Holocene and interdunal lakes formed in response to the incursion of the IOM at approximately 8500 cal yr BP with the development of C3 dominated savanna grasslands. The IOM weakened ca. 6000 cal yr BP with the onset of regional aridity, aeolian sedimentation and dune reactivation and accretion. Despite this reduction in precipitation, the lake was maintained by winter dominated rainfall. There was a shift to drier adapted C4 grasslands across the dune field. Lake sediment geochemical analyses record precipitation minima at 8200, 5000 and 4200 cal yr BP that coincide with Bond events in the North Atlantic. A number of these events correspond with changes in cultural periods, suggesting that climate was a key mechanism affecting human occupation and exploitation of this region.

Aluminium and silicon are usually abundant in soil mineral matter, but their availability for plant uptake is limited by low solubility and, in the case of Al, high soil pH causes precipitation of the element in insoluble forms. Al toxicity is a major problem in naturally occurring acid soils and in soils affected by acidic precipitation. Al has no known role in higher plants, and is generally known as a toxic element, whereas Si is generally regarded as a beneficial element. Recently, it has been suggested that Al toxicity can be ameliorated by Si in a variety of animal systems. In this review the evidence that amelioration of Al toxicity by Si can also occur in plants is assessed. At present such amelioration has been shown in sorghum, barley, teosinte, and soybean, but not in rice, wheat, cotton, and pea. Plant species vary considerably in the amounts of Al and Si that they transport into their tissues, and it seems that very high Si accumulation and very high Al accumulation are mutually exclusive. The mechanisms considered for amelioration are: solution effects; codeposition of Al and Si within the plant; effects in the cytoplasm and on enzyme activity; and indirect effects.