Martin J. Hodson

Temporary and permanent ponds: An assessment of the effects of drying out on the conservation value of aquatic macroinvertebrate communities

Abstract This study was designed to investigate whether temporary ponds were markedly different from more permanent ponds in terms of their species richness, species rarity and community type. Macroinvertebrates were recorded from six temporary ponds in Oxfordshire, and the resulting data were compared with species data from 33 Oxfordshire ponds of a more permanent character. Classification and ordination using TWINSPAN and DECORANA suggest that there were marked differences in the macroinvertebrate assemblages of temporary and permanent ponds. Overall, temporary ponds had similar rarity indices to permanent ponds. However, four of the five highest rarity index values were from temporary or semi-permanent sites. The number of aquatic species was significantly lower in the temporary ponds.

Aluminium/silicon interactions in barley (Hordeum vulgare L.) seedlings

The response of seedlings of the monocot Hordeum vulgare L. cv. Bronze to 0,25 and 50 μM aluminium in factorial combination with 0, 1.4, 2.0 and 2.8 mM Si was tested in hydroponic culture at pH 4.5. Nutrient solution (500 μM calcium nitrate) and Al/Si treatments were designed to avoid the precipitation of Al from solution. Silicon treatments gave significant amelioration of the toxic effects of Al on root and shoot growth and restored calcium levels in roots and shoots at harvest to levels approaching those of control plants. Aluminium uptake by roots was also significantly diminished in the presence of Si. Silicon alone gave a slight stimulation of growth, insufficient to explain its ameliorative effect on Al toxicity. The mechanism of the Si effect on Al toxicity in monocotyledons awaits further investigation.

ALUMINIUM/SILICON INTERACTIONS IN CONIFERS

Abstract It is widely accepted that acidic precipitation has been responsible for the die-back of trees in Western Europe and North America. The mechanisms responsible for the die-back are, however, not very well understood. It is known that free aluminium (Al) is mobilised in soils that are affected by acidic precipitation, and Al toxicity is probably a major factor in the die-back. Al toxicity in conifers has been attributed to the induction of calcium (Ca) or magnesium deficiency. However, the potential role of silicon (Si) in alleviating Al toxicity has received little attention. Here we review the literature concerning Al and Si in conifers, and present some new data documenting mineral deposition in their needles. Needles of Picea glauca , Pinus strobus , Larix laricina and Abies balsamea were analysed using X-ray microanalysis to determine mineral distribution in the leaf tissues. The distribution of Al, Ca and Si varied with the species, but Al was invariably codeposited with Si. This may be a mechanism for sequestering toxic Al and thus could represent a defence against Al toxicity.

The amelioration of aluminium toxicity by silicon in wheat (Triticum aestivum L.): malate exudation as evidence for an in planta mechanism

Abstract. Two wheat (Triticum aestivum L.) cultivars, one aluminium tolerant (Atlas 66) and one sensitive (Scout 66), were grown in a continuous-flow culture system (≤pH 5.0) containing aluminium (0–100 μM) and silicon (0–2000 μM) in factorial combination. Treatment with silicon resulted in a highly significant amelioration of aluminium toxicity as assessed by root growth in both cultivars. Amelioration was influenced by wheat cultivar and silicon concentration, as 2000 μM silicon significantly ameliorated the toxic effects of 100 μM aluminium in Atlas 66, and only 5 μM silicon alleviated the effect of 1.5 μM aluminium on Scout 66. Nutrient medium pH was critical, as an amelioration by silicon was apparent only at pH > 4.2 for Atlas 66, and at pH > 4.6 for Scout 66. Silicon neither reduced levels of toxic aluminium species in the growth solutions, nor the amount of aluminium taken up by roots. In experiments to assess exudation of malate by Atlas 66 roots treated with 100 μM aluminium, the presence of 2000 μM silicon (pH 4.6) was found to have a negligible effect on exudation. In contrast, citrate, a known aluminium chelator, reduced aluminium-induced exudation of malate at 5–40 μM and completely inhibited it at 100 μM citrate. The results indicate that silicon does not reduce aluminium phytotoxicity as a result of aluminium/silicon interactions in the external media, and that the mechanism of amelioration has an in planta component.

Chapter 5 Silicon deposition in higher plants

Silicification is reported in the Pteridophyta and the Spermatophyta, including gymnosperms and angiosperms. Dicotyledon families containing Si accumulators of considerable agricultural significance include the Fabaceae, Cucurbitaceae and Asteraceae. Among the monocotyledons, the Cyperaceae and Poaceae (Gramineae) are pre-eminent. Silica deposits, commonly called phytoliths, occur in cell walls, cell lumens or in extracellular locations. These deposits frequently possess a characteristic morphology revealing their tissue and taxonomic origin. Silicification occurs in roots and the shoot including leaves, culms and in grasses, most heavily in the inflorescence. Deposits occur in epidermal, strengthening, storage and vascular tissues. Biogenic silica structure is affected by ambient physico-chemical conditions mediated by tissue maturation, pH, ionic concentrations and cell wall structure, as illustrated by the results of a developmental study of silicification in wheat seedlings. Silicified tissues provide support and protection and may also sequester toxic metals, as illustrated by our recent work on the codeposition of aluminum with silicon in cereals and conifers. Some phytoliths have been implicated as carcinogens. Phytoliths are being increasingly used in archaeology as many retain their morphology in sediments.

Phytolith analysis from the archaeological site of Kush, Ras al-Khaimah, United Arab Emirates

Despite the wealth of archaeological sites and excellent conditions for preservation, few phytolith investigations have been undertaken from the Arabian Gulf region. The results from the Sasanian and Islamic archaeological tell of Kush, Ras al-Khaimah, United Arab Emirates, are presented. Kush is situated just inside the Gulf on an important trade route. The occupation sequence dates from the 4th century A.D. until the 13th century A.D., recording the development of the site in the Sasanian period, followed by the arrival of Islam in the 7th century A.D. and the final abandonment of the site in the late 13th century when the nearby site of al-Mataf (Julfar) began to develop closer to the present day coastline. All the samples analyzed contained abundant phytoliths (short cells, elongated cells, and groups of elongated cells) of various types. They included date palm (Phoenix dactylifera), papillae (possibly from barley (Hordeum)), and hair cells possibly from species of canary grass (Phalaris spp.). Some researchers have suggested that groups of elongated cells may indicate the presence of irrigation in semiarid environments. The present results for this class of phytoliths appeared to imply that intensive irrigation was unlikely to have taken place around Kush.

The use of root growth and modelling data to investigate amelioration of aluminium toxicity by silicon in Picea abies seedlings.

Three-week-old Picea abies seedlings were grown for 7 days in 100 microM aluminium (Al), combined with 1000 or 2000 microM silicon (Si). Solution pH was adjusted to 4.00, 4.25, 4.50, 4.75, or 5.00. In the absence of Si, solution pH had no effect on the decrease in root growth caused by 100 microM Al. Silicon did not ameliorate toxic effects of Al on root growth at pH 4.00, 4.25 and 4.50, whereas significant, and apparently complete, amelioration was found at pH 4.75 and 5.00. An equilibrium speciation model (EQ3NR), with a current thermodynamic database, was used to predict the behaviour of Al and Si in growth solutions. When Si was not present in the 100 microM Al solutions, Al(3+) declined from 92.4% of total Al at pH 4.00 to 54.6% at pH 5.00, and there was a concomitant increase in hydroxyaluminium species as pH increased. The addition of 1000 microM Si to the 100 microM Al solutions caused a reduction in Al(3+) content over the whole pH range: at pH 4.00 Al(3+) fell from 92.4 to 83.3% in the presence of Si; and at pH 5.00 the fall was from 54.6 to 17.7%. These falls were attributed to the formation of hydroxyaluminosilicate (HAS) species. Similar, but somewhat greater, changes were observed in solutions containing 2000 microM Si. The match between root growth observations and the modelling data was not very good. Modelling predicted that change in Al(3+) content with pH in the presence of Si was gradual, but root growth was markedly increased between pH 4.50 and 4.75. Differences between root growth and modelling data may be due to the model not correctly predicting solution chemistry or to in planta effects which override the influence of solution chemistry.

INTERACTION BETWEEN SILICON AND ALUMINUM IN TRITICUM AESTIVUM L. (CV. CELTIC)

ABSTRACT Seedlings of Triticum aestivum L. (cv. Celtic) were suspended in plastic tubs containing 500 μmol L−1 Ca(NO3)2 and 31 μmol L−1 KC1 as background solution. A1C13 (0 and 100 μmol L−1) and Na2SiO3.5H2O (0 and 2000 μmol; L−1) were added to this basal nutrient medium, and solution pH was set at 4.2 or 4.6. Tubs were aerated and supplied with a continuous flow of pH-adjusted test solution. Plants were grown for 4 d in a growth cabinet at 25 °C with a 16 h photoperiod. At pH 4.2 and 4.6 root length of the seedlings was inhibited at 100 μmol L−1 Al. An amelioration of Al-induced toxicity symptoms was observed in the 100 μmol L−1 A1/2000 μmol L−1 Si treatment at pH 4.6, but not at pH 4.2. Both the shoot (S) and root (R) dry weight of seedlings treated with 100 μmol L−1 Al were reduced when compared with controls. Treatment with Al increased S:R ratios, and this effect was ameliorated by Si, but only at pH 4.6. Al content of roots treated with 100 μmol L−1 Al or 100 μmol L−1 Al/2000 μmol L−1 Si increased s...

Silica Deposition in Subterranean Organs

Silica in roots and rhizomes has not been investigated in a comprehensive or extensive manner. Excluding the grasses, the literature indicates two monocotyledons, two dicotyledons and a conifer contain silica in subterranean organs. Grasses, although not completely investigated, have been examined in more detail; major sites of deposition include the root endodermis and rhizome epidermis. Review of our knowledge of silica function, localization and deposition mechanisms indicates that many gaps still exist. Although it appears at this time that aerial parts produce phytoliths of greater taxonomic significance, subterranean organs should be systematically surveyed.

ION LOCALIZATION AND X-RAY MICROANALYSIS

Publisher Summary Most biological X-ray microanalysis is now carried out in scanning electron microscope (SEM), transmission electron microscopy (TEM), or scanning transmission electron microscope (STEM) to which energy dispersive X-ray microanalysis (EDX) equipment has been fitted. When electrons are fired at or through the specimens in these machines they interact with the atoms of material, resulting in the emission of X-rays that are characteristic for the elements present. Nearly all equipment will detect only those elements with the atomic number of sodium or above. Microanalysis can be quantified, but cannot indicate what form or combination the element is in. This chapter focuses on the problems of producing plant material for microanalytical investigations. When considering a microanalytical study, there are now two major procedures. SEM coupled with EDX is usually employed when distinguishing ion contents between different cell types or layers within the plant tissue is required. Higher resolution is normally needed if localization at the subcellular level is the aim, and TEM (or STEM) fitted with EDX is preferred. The other main dichotomy concerns whether the elements of interest are deposited and immobile, or soluble and mobile.