Diversity of lobate phytoliths in grass leaves from the Sahel region, West Tropical Africa: Tribe Paniceae

Phytoliths are microscopic amorphous silicon dioxide (SiO2.H2O) particles occurring in leaves, internodes, glumes and inflorescence within all members of the grass family Poaceae. Phytoliths of grasses are of particular interest, as they possess morphological features which have encouraged many investigators to identify these plants from which fossil phytoliths might have originated. The present study is a step towards preparing a systematic inventory of grass phytoliths in western tropical Africa. Morphology and dimensions of phytoliths from 66 species belonging to the tribe Paniceae have been studied. Four shape categories of lobate phytoliths have been determined in leaf blade spodograms: bilobate, nodular bilobate, polylobate, quadra-lobate. Bilobate shaped phytoliths are frequently represented in all genera of Paniceae. 13 groups of lobate phytoliths have been distinguished based on significant morphological criteria like shape of outer margins, shape of the shank and number of lobes. A size category system of lobate phytolith dimensions (length, width; length and width of shanks) has been developed by the analysis of average, minimum and maximum values of these dimensions. Application of the size category system results on classifying the major groups into 25 subgroups. The study proves that size and shape can be used to assign some of the lobate phytoliths to their respective genera. Some rarely produced lobate shapes like nodular bilobate and polylobate types could be used together on assemblage basis as markers for definite genera in the tribe Paniceae, e.g. Brachiaria, Panicum, Pennisetum and Setaria. Also, bilobate phytoliths with concave margins have been recorded in five species. Bilobate phytoliths with flattened and convex margins and quadra-lobate shapes are produced by almost all species which therefore resulted in an inconsistent and indefinite relationship with the taxa that produce them. The study shows a correlation between width dimensions of bilobate shapes and their shanks. Greater width dimensions usually connected to thick shanks while short ones are attached to thin shanks. A spectrum on percentages of species producing each type of lobate phytolith has been designed. It is recommended that such spectrum should be carried out for all tribes of Poaceae on phyto-geographical basis which might eliminate the effect of redundancy and multiplicity on the classification of grass phytoliths.     

Phytoliths of Indian grasses and their potential use in identification

Phytoliths are amorphous silicon dioxide (SiO₂.nH₂O) inclusions abundant in leaves, in-ternodes and glumes in members of Poaceae. They may occur as inclusions filling the entire lumen of the silica cells, bulliform cells and trichomes or may be part of the outer epidermal cell walls. Since phytoliths are resistant to fungal or animal digestive juices, a large quantity of phytoliths accumulate in the soil where grasses grow. Compared with the pollen grains of grasses which tend to be uniform, phytoliths vary in sue and morphology and can be of value in identification at different taxonomic levels and in the dating of past vegetation. The size and shape of phytoliths of about 100 species of grasses from Tamil Nadu, India, have been determined. Silica bodies were observed either after isolation or in cleared leaf blades. Size and shape of phytoliths were determined under a microscope or from micrographs of the specimens. Size and shape can be used to assign the phytoliths to their respective subfamilies and to distinguish some of the grasses at the generic level. Drawings of silica cells and an identification key are provided for 80 species.     

Identifying inflorescence phytoliths from selected species of wheat (Triticum monococcum, T. dicoccon, T. dicoccoides, and T. aestivum) and barley (Hordeum vulgare and H. spontaneum) (Gramineae)

Analysis of microfossil silica phytoliths is becoming an increasingly important research tool for taxonomists, archaeobotanists, and paleoecologists. Expanded use of phytolith analysis by researchers is dependent upon development of phytolith systematics. In this study phytoliths produced by the inflorescence bracts from four species of wheat, Triticum monoccocum, T. dicoccon, T. dicoccoides, and T. aestivum, and two species of barley, Hordeum vulgare, and H. spontaneum, were analyzed using computer-assisted image and statistical analysis with the intent to develop taxonomic tools to distinguish among the taxa. A classification key based on significant differences among the mean morphometries of the inflorescence phytoliths produced by each species was created and tested. Discriminant analysis of the morphometries of several morphotypes of phytoliths was also conducted to determine whether this computer-assisted statistical procedure could be used as another method to classify the taxa and to determine which morphotypes have measurements that can best be used in discriminant functions. Test results indicated that, at the genus level, both the classification key and discriminant analysis of certain morphotypes of phytoliths were relatively reliable tools for distinguishing among phytoliths produced in the inflorescence bracts of the taxa considered. For distinguishing among the taxa at the species level, the classification key was most reliable. Of the discriminant analyses tested, that based on all the phytolith morphotypes combined was more reliable than those based on only one morphotype.     

A morphometric study of variance in articulated dendritic phytolith wave lobes within selected species of Triticeae and Aveneae

Morphometric analysis has proven to be an effective tool for distinguishing among phytolith assemblages produced by closely related plant taxa. Elongate dendritic epidermal phytoliths are produced in the inflorescence bracts of many cereal species. Under light microscopy, these articulated dendritic phytoliths produce wave patterns between the margins of the cells that are reported to have taxonomic significance. In this study we explore morphometric variance among the lobes of the wave patterns formed by the articulated dendritic phytoliths within selected species of cereals as a first step towards understanding the variance between species. We found that there is often significant variance in dendritic wave lobes among different accessions of a species, among the different types of inflorescence bracts of the species (glumes, lemmas and paleae), and among each bract type’s location on the inflorescence (upper, middle and lower third of inflorescence spike or panicle). We observed that shape morphometries are typically more reliable and require a smaller sample size for statistical confidence than size morphometries. We further observed that adequate samples sizes for analysis of several shape morphometries of articulated dendritic wave lobes are considerably smaller than those reported to be required for analysis of the same morphometries of individual or isolated dendritic phytoliths. To gain a preliminary sense whether there is potential for discriminating between taxa in light of the significant variance within species, we compared our data to archaeological material from the historical center of Brussels. We demonstrate that while there is considerable variance in the morphometries among accessions, bract types and inflorescence locations within each species, there may yet be potential for discriminating between cereal species in archaeological samples by the morphometries of their dendritic phytolith wave lobes. We present one possible paradigm for conducting such analysis on archaeological material.     

Phytoliths in woody plants from the northern slope of the Changbai Mountain (Northeast China), and their implication

Phytoliths were extracted from 14 woody plants collected on the northern slope of the Changbai Mountain, including 10 broad-leaved species and 4 conifers. A total of 14 morphotypes of phytoliths were identified, including 3 types first examined in this study. Phytoliths in broad-leaved species were mostly silicified epidermal cells, cell walls, and vascular tissues; phytoliths in conifers were mainly silicified epidermal cells, cell walls, hypodermal cells, and parenchyma cells. Phytoliths produced by broad-leaved species in this region were usually not well silicified, and were fragile, whereas those produced by conifers were better silicified; this might be because of the different lengths of the growth periods. Phytoliths were found have potential in studies of fluctuations of the tree line in this region, and this study also provided a reference for further study of phytoliths in this region and the regional contrast of phytolith assemblages.     

Accumulation of germanium (Ge) in plant tissues of grasses is not solely driven by its incorporation in phytoliths

Ge/Si ratios of plant phytoliths have been widely used to trace biogeochemical cycling of Si. However, until recently, information on how much of the Ge and Si transferred from soil to plants is actually stored in phytoliths was lacking. The aim of the present study is to (i) compare the uptake of Si and Ge in three grass species, (ii) localize Ge and Si stored in above-ground plant parts and (iii) evaluate the amounts of Ge and Si sequestrated in phytoliths and plant tissues. Mays (Zea mays), oat (Avena sativa) and reed canary grass (Phalaris arundinacea) were cultivated in the greenhouse on soil and sand to control element supply. Leaf phytoliths were extracted by dry ashing. Total elemental composition of leaves, phytoliths, stems and roots were measured by ICP-MS. For the localization of phytoliths and the determination of Ge and Si within leaf tissues and phytoliths scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) was used. The amounts of Si and Ge taken up by the species corresponded with biomass formation and decreased in the order Z. mays > P. arundinacea, A. sativa. Results from LA-ICP-MS revealed that Si was mostly localized in phytoliths, while Ge was disorderly distributed within the leaf tissue. In fact, from the total amounts of Ge accumulated in leaves only 10% was present in phytoliths highlighting the role of organic matter on biogeochemical cycling of Ge and the necessity for using bulk Ge/Si instead of Ge/Si in phytoliths to trace biogeochemical cycling of Si.

     

Morphological variations of lobate phytoliths from grasses in China and the south-eastern United States

Abstract. Phytolith analysis of grasses is a useful tool in palaeoenvironmental and archaeobotanical research. Lobate phytolith is one of the most important morphotypes of grass phytoliths. This study describes morphological variations of diagnostic lobate phytoliths and produces a tentative classification scheme based on 250 modern grass species from China and the south‐eastern U.S.A. Eighty‐five grass species were found to contain lobate phytoliths. They are derived mainly from Panicoideae, but also include the Chloridoideae, Oryzoideae and Arundinoideae subfamilies. Twenty‐five lobate morphological types were observed from different subfamilies, genera or tribes of grasses, based on two important parameters: (1) the length of the lobate shank and (2) the shape of the outer margin of the two lobes. The identification of grass tribe or even genus is possible based on the differences in lobate shape parameters or the composition of assemblages. However, not all of the lobate assemblages have a definite relationship with the genera that produce them, because grasses can only produce a limited range of lobate shapes that often overlap from one genus to another. Several C₃ grasses and Chloridoideae subfamily grasses also produce characteristic lobate phytoliths. The variations of lobate morphologies can be related to environmental factors, especially moisture. Typical hygrophytic grasses tend to yield lobate phytoliths with very short shank, whereas typical xerophytic grasses tend to produce lobate phytoliths with a very long shank. The potential link between phytolith morphology, grass taxonomy and environmental conditions opens the possibility that phytolith morphology may be used as a proxy in palaeoclimatic reconstruction.     

Phytoliths as indicators of prehistoric maize (Zea mays subsp.mays,Poaceae) cultivation

Maize (Zea mays L. subsp.mays) has been identified in archaeological contexts by a high proportion of large cross-shaped phytoliths. Given the numerous races of maize, this study was undertaken to determine if differences below the species level could be noted. It was also designed to see if phytoliths differed in various plant parts at various stages of growth. Several races were grown under experimental conditions. No significant differences were found. Furthermore, few phytoliths alleged to be diagnostic of maize were discovered. Systemic studies of maize and analyses of prehistoric cultivation by means of phytoliths seem not to be as promising as some researchers have argued.     

Morphometric analysis of inflorescence phytoliths produced byAvena sativa L. andAvena strigos schreb

Morphometric analysis, the study of measurements of size and shape, has the potential to be an effective tool for phytolith analysis. This study reports the first attempt to apply the methodology to oats. In particular, this study was designed to determine if morphometric analysis could adequately discriminate between phytoliths produced in the inflorescence bracts of two species of oats, Avena sativa L. and Avena strigosa Schreb. Results indicate that while the taxa produce the same types of phytoliths, those phytoliths have significantly different measurements of size and shape. This suggests the technique has the potential to become a powerful research tool for investigators working in the wide variety of disciplines that utilize phytolith analysis.     

Prairie grass phytolith hardness and the evolution of ungulate hypsodonty

Silica phytoliths in grasses are thought to serve as a defence mechanism against grazing ungulates by causing excessive tooth wear. It is posited that they contributed to the evolution of hypsodonty in these animals. However, some have questioned whether grass phytoliths can abrade enamel. Here Mohs hardness testing was conducted on Blue Grama grass (Bouteloua gracilis) to determine phytolith hardness. Microindentation was performed on horse and American bison molars to establish dental constituent hardness values. To infer the phytoliths' abrasion capacity, the hardness values were contrasted. Phytolith hardness ranged from 18.0 to 191.5 HV. This is considerably softer than the values obtained for ungulate enamel, which range from 332.6 to 363.4 HV, but harder than the other dental constituents. Although Blue Grama phytoliths are incapable of directly abrading enamel, when viewed in conjunction with other data on phytolith hardness, there is considerable variation across grass species and some phytoliths are actually harder than ungulate enamel. Blue Grama grass phytoliths may even promote enamel wear due to pressure accentuation caused by the recession of softer tissues. Given these findings and considerations, it is plausible phytoliths served an integral role in the co-evolution of grasses and herbivorous ungulates, although more testing is needed to bear this out.     

Taxonomic assessment of opal phytoliths from grasses of deltaic West Bengal,India

In the present study, taxonomic potential of opal phytoliths was examined in grasses from lower Gangetic delta, West Bengal, India. The study revealed that finer classification of phytoliths can increase the efficacy of opal silica as taxonomic proxy by minimizing the influences of multiplicity and redundancy. We isolated 187 phytolith sub‐morphotypes, categorized under 10 major groups, from 110 grass species belonging to 45 genera, 21 sub‐tribes, 13 tribes and 7 subfamilies. Cluster and correspondence analyses showed that all the significantly represented subfamilies can be clearly distinguished on the basis of either principal morphotypes or sub‐morphotypes. However, genus/species level discrimination may only be possible by deploying phytolith based identification key developed by utilizing detailed grass phytolith micro‐morphometry and frequency attributes. We conclude that grass phytolith characteristics provide useful and significant information for distinguishing grass taxa of deltaic West Bengal.     

Phytolith assemblages in grasses native to central Argentina

BACKGROUND AND AIMS: Phytolith reference collections are a prerequisite for accurate interpretation of soil phytolith assemblages aimed at reconstructing past vegetation. In this study a phytolith reference collection has been developed for several grasses native to central Argentina: Poa ligularis, Piptochaetium napostaense, Stipa clarazii, Stipa tenuis, Stipa tenuissima, Stipa eriostachya, Stipa ambigua, Stipa brachychaeta, Pappophorum subbulbosum, Digitaria californica, Bothriochloa edwardsiana and Aristida subulata. METHODS: For each species, phytoliths present in the leaf blades were classified into 47 morphotypes, and their relative frequency determined by observing 300-400 phytoliths per sample (n = 5). Data were analyzed by complete linkage cluster analysis, using the Morisita Index as measure of association. KEY RESULTS: The results showed differentiation among phytolith assemblages at species level or at plant functional type level. Cluster analysis separated C3 from C4 species and palatable from non-palatable species. CONCLUSIONS: This study highlights the possibility of reconstructing past vegetation in central Argentina grasslands through the analysis of soil phytolith assemblages.     

Desert grassland dynamics estimated from carbon isotopes in grass phytoliths and soil organic matter

Abstract. We document the potential for using carbon isotopes in both soil organic matter (SOM) and grass phytoliths in soil to increase the temporal and taxonomic resolutions of long term vegetation dynamics. Carbon isotope values from both SOM and phytoliths are expected to describe both the age of material through ¹⁴C dating, and the photosynthetic pathway of the source plant material through ratios of ¹²C/¹³C. Taxonomic resolution is increased because the phytoliths examined are specific to grasses, whereas the SOM reflects the contribution of all the vegetation. Temporal resolution is increased because phytoliths are less mobile in the soil profile than SOM, and can therefore provide older dates from the same soil depth. Our results, from a desert grassland site in southwestern North America, largely confirm these expectations, and show that C₄ species have dominated the grass composition for the last 8000 yr, C₃ non‐grass vegetation increased about 100–350 yrBP, and no significant C₃ grass or non‐grass vegetation existed between 350–2000 yr BP.     

Assessing the importance of environmental factors to phytoliths of Phragmites communis in north-eastern China

Phytoliths have proved to be reliable indicators of environmental conditions both at present and in the past, and can provide evidence for the distribution of taxa or vegetation. Understanding the environmental significance of phytoliths within plants helps in drawing more reliable inferences about palaeovegetation and in reconstructing the palaeo-environment. The present study examined the relationship between phytoliths and environmental factors to assess the environmental significance of different types of phytoliths. Phytoliths were extracted from Phragmites communis growing in xerophytic and aquatic habitats at twelve sampling sites in north-eastern China and their implications for the environment were assessed using quantitative data mainly including phytolith concentration and environmental factors and statistical analyses. Principal component analysis (PCA) of several environmental factors (including the climate, micro terrain, and the physicochemical properties of soils) revealed that other factors being constant, P. communis phytolith concentrations were influenced largely by the average annual temperature and precipitation. Orthogonal experiment analysis and three-way analyses of variance of the concentrations confirmed the reliability of the results of the PCA. More specifically, the concentrations of the saddle and rondel types of phytoliths (the short-cell phytoliths) were closely linked to the average annual temperature whereas those of the elongate, lanceolate, and bulliform phytoliths (the non-short-cell phytoliths) were more sensitive to the variations in water status. The results contribute to further confirming the major environmental implications of phytoliths and, in turn, providing a reference for growth and development of wetland plants. Our findings also provide critical information that could help managers and policymakers assess and modify ecological restoration practices.     

Abrasive, silica phytoliths and the evolution of thick molar enamel in primates, with implications for the diet of Paranthropus boisei

Background

Primates—including fossil species of apes and hominins—show variation in their degree of molar enamel thickness, a trait long thought to reflect a diet of hard or tough foods. The early hominins demonstrated molar enamel thickness of moderate to extreme degrees, which suggested to most researchers that they ate hard foods obtained on or near the ground, such as nuts, seeds, tubers, and roots. We propose an alternative hypothesis—that the amount of phytoliths in foods correlates with the evolution of thick molar enamel in primates, although this effect is constrained by a species'' degree of folivory.

Methodology/Principal Findings

From a combination of dietary data and evidence for the levels of phytoliths in plant families in the literature, we calculated the percentage of plant foods rich in phytoliths in the diets of twelve extant primates with wide variation in their molar enamel thickness. Additional dietary data from the literature provided the percentage of each primate''s diet made up of plants and of leaves. A statistical analysis of these variables showed that the amount of abrasive silica phytoliths in the diets of our sample primates correlated positively with the thickness of their molar enamel, constrained by the amount of leaves in their diet (R² = 0.875; p<.0006).

Conclusions/Significance

The need to resist abrasion from phytoliths appears to be a key selective force behind the evolution of thick molar enamel in primates. The extreme molar enamel thickness of the teeth of the East African hominin Paranthropus boisei, long thought to suggest a diet comprising predominantly hard objects, instead appears to indicate a diet with plants high in abrasive silica phytoliths.     

Morphological characteristics of phytoliths from representative conifers in China

Coniferous phytoliths in sediments are an effective tool for detecting the historical appearance of conifers. However, at the timberline in mountainous areas, such coniferous phytoliths are easily confused with grass phytoliths. This study analyses modern phytoliths from 17 conifer plants. Six common types and six rare types were identified. The conifers studied produce abundant blocky polyhedral and cubic (in the average 30–40 μm size range), blocky scrobiculate (average 30–40 μm), tabular elongate unsculpted (length 50–100 μm, width 10–20 μm), tabular elongate cavate (length 50–150 μm, width ∼10 μm), tabular elongate dendritic (50–100 μm × 10–20 μm), and irregular oblong (20–40 μm) phytoliths. This paper aims to show morphological characteristics of coniferous phytoliths in China, and to show how the common coniferous phytoliths differ from similar grass phytolith types, such as blocky polyhedral coniferous phytoliths from silicified parallelepipedal bulliform cells produced by grass. Blocky polyhedral and cubic phytoliths are the commonest coniferous phytoliths found in the sediments, but need to be carefully distinguished from grass parallelepipedal bulliform cells. This study indicates that clearly protruding ridges and irregular inward edges are essential features of cubic and polyhedral morphotypes produced by conifers. Results of this paper might provide important material for the study of paleovegetation and paleoecology of mountainous areas, especially at the timberline.     

Phytoliths in Pottery Reveal the Use of Spice in European Prehistoric Cuisine

Here we present evidence of phytoliths preserved in carbonised food deposits on prehistoric pottery from the western Baltic dating from 6,100 cal BP to 5750 cal BP. Based on comparisons to over 120 European and Asian species, our observations are consistent with phytolith morphologies observed in modern garlic mustard seed (Alliaria petiolata (M. Bieb) Cavara & Grande). As this seed has a strong flavour, little nutritional value, and the phytoliths are found in pots along with terrestrial and marine animal residues, these findings are the first direct evidence for the spicing of food in European prehistoric cuisine. Our evidence suggests a much greater antiquity to the spicing of foods than is evident from the macrofossil record, and challenges the view that plants were exploited by hunter-gatherers and early agriculturalists solely for energy requirements, rather than taste.     

Do soil phytoliths accurately represent plant communities in a temperate region? A case study of Northeast China

Modern soil phytoliths can potentially provide analogues for phytolith assemblages from archaeological and palaeoecological contexts. To assess the reliability of soil phytoliths for representing different plant communities, we analysed phytoliths in surface soils and parent plants at 65 sites representing five types of regional vegetation in Northeast China. The results demonstrated that surface soil phytolith assemblages could clearly differentiate samples from herbaceous and woody communities, and samples from Poaceae and non-Poaceae communities could be separated statistically. In addition, woody communities could be differentiated into a broadleaf-Poaceae community, a broadleaf-non-Poaceae community and a conifer and broadleaf-non-Poaceae community, except for some overlapping samples. Soil phytolith assemblages are thus able to differentiate regional vegetation types into different plant community types. In the present study, soil phytoliths represented about 30% of the phytoliths present in the aboveground vegetation. In addition, soil phytoliths from different communities reflected the aboveground vegetation with slightly different degrees of accuracy, and in addition different morphotypes exhibited different degrees of representational bias. Some morphotypes (e.g. rondel, elongate psilate, lanceolate) overrepresented the abundance of the associated plant taxa; morphotypes such as tracheid, conical epidermal, stomata and others under-represented the original plant richness; and other morphotypes, e.g. saddle, trapeziform sinuate, scutiform, were in good agreement with the numbers of plant taxa in the plot inventory. Thus, any quantitative palaeovegetation reconstruction using phytoliths should begin with the calibration of soil phytolith assemblages. We conclude that our findings provide improved phytolith analogues for different plant communities, with applications in palaeoenvironmental reconstruction, and they also provide additional insights into the mechanisms of phytolith production and deposition.     

Bulliform Phytolith Research in Wild and Domesticated Rice Paddy Soil in South China

Bulliform phytoliths play an important role in researching rice origins as they can be used to distinguish between wild and domesticated rice. Rice bulliform phytoliths are characterized by numerous small shallow fish-scale decorations on the lateral side. Previous studies have shown that domesticated rice has a larger number of these decorations than wild rice and that the number of decorations ≥9 is a useful feature for identifying domesticated rice. However, this standard was established based on limited samples of modern rice plants. In this study, we analyzed soil samples from both wild and domesticated rice paddies. Results showed that, in wild rice soil samples, the proportion of bulliform phytoliths with ≥9 decorations was 17.46% ± 8.29%, while in domesticated rice soil samples, the corresponding proportion was 63.70% ± 9.22%. This suggests that the proportion of phytoliths with ≥9 decorations can be adopted as a criterion for discriminating between wild and domesticated rice in prehistoric soil. This indicator will be of significance in improving the application of fish-scale decorations to research into rice origins and the rice domestication process.