A tillering inhibition gene influences root-shoot carbon partitioning and pattern of water use to improve wheat productivity in rainfed environments

Genetic modification of shoot and root morphology has potential to improve water and nutrient uptake of wheat crops in rainfed environments. Near-isogenic lines (NILs) varying for a tillering inhibition (tin) gene and representing multiple genetic backgrounds were phenotyped in contrasting, controlled environments for shoot and root growth. Leaf area, shoot and root biomass were similar until tillering, whereupon reduced tillering in tin-containing NILs produced reductions of up to 60% in total leaf area and biomass, and increases in total root length of up to 120% and root biomass to 145%. Together, the root-to-shoot ratio increased two-fold with the tin gene. The influence of tin on shoot and root growth was greatest in the cv. Banks genetic background, particularly in the biculm-selected NIL, and was typically strongest in cooler environments. A separate de-tillering study confirmed greater root-to-shoot ratios with regular tiller removal in non-tin-containing genotypes. In validating these observations in a rainfed field study, the tin allele had a negligible effect on seedling growth but was associated with significantly (P<0.05) reduced tiller number (-37%), leaf area index (-26%), and spike number (-35%) to reduce plant biomass (-19%) at anthesis. Root biomass, root-to-shoot ratio at early stem elongation, and root depth at maturity were all increased in tin-containing NILs. Soil water use was slowed in tin-containing NILs, resulting in greater water availability, greater stomatal conductance, cooler canopy temperatures, and maintenance of green leaf area during grain-filling. Together these effects contributed to increases in harvest index and grain yield. In both the controlled and field environments, the tin gene was commonly associated with increased root length and biomass, but the significant influence of genetic background and environment suggests careful assessment of tin-containing progeny in selection for genotypic increases in root growth.

Movement of newly assimilated 13C carbon in the grass Lolium perenne and its incorporation into rhizosphere microbial DNA

One of the key processes that drives rhizosphere microbial activity is the exudation of soluble organic carbon (C) by plant roots. We describe an experiment designed to determine the impact of defoliation on the partitioning and movement of C in grass (Lolium perenne L.), soil and grass-sterile sand microcosms, using a (13)CO(2) pulse-labelling method. The pulse-derived (13)C in the shoots declined over time, but that of the roots remained stable throughout the experiment. There were peaks in the atom% (13)C of rhizosphere CO(2) in the first few hours after labelling probably due to root respiration, and again at around 100 h. The second peak was only seen in the soil microcosms and not in those with sterilised sand as the growth medium, indicating possible microbial activity. Incorporation of the (13)C label into the microbial biomass increased at 100 h when incorporation into replicating cells, as indicated by the amounts of the label in the microbial DNA, started to increase. These results indicate that the rhizosphere environment is conducive to bacterial growth and replication. The results also show that defoliation had no impact on the pattern of movement of (13)C from plant roots into the microbial population in the rhizosphere.

The effects of dwarfing genes on seedling root growth of wheat

Most modern wheat cultivars contain major dwarfing genes, but their effects on root growth are unclear. Near-isogenic lines (NILs) containing Rht-B1b, Rht-D1b, Rht-B1c, Rht8c, Rht-D1c, and Rht12 were used to characterize the effects of semi-dwarfing and dwarfing alleles on root growth of 'Mercia' and 'Maris Widgeon' wheat cultivars. Wheat seedlings were grown in gel chambers, soil-filled columns, and in the field. Roots were extracted and length and dry mass measured. No significant differences in root length were found between semi-dwarfing lines and the control lines in any experiment, nor was there a significant difference between the root lengths of the two cultivars grown in the field. Total root length of the dwarf lines (Rht-B1c, Rht-D1c, and Rht12) was significantly different from that of the control although the effect was dependent on the experimental methodology; in gel chambers root length of dwarfing lines was increased by approximately 40% while in both soil media it was decreased (by 24-33%). Root dry mass was 22-30% of the total dry mass in the soil-filled column and field experiments. Root length increased proportionally with grain mass, which varied between NILs, so grain mass was a covariate for the analysis of variance. Although total root length was altered by dwarf lines, root architecture (average root diameter, lateral root:total root ratio) was not affected by reduced height alleles. A direct effect of dwarfing alleles on root growth during seedling establishment, rather than a secondary partitioning effect, was suggested by the present experiments.

Effects of carbon dioxide on the searching behaviour of the root-feeding clover weevil Sitona lepidus (Coleoptera: Curculionidae)

The respiratory emission of CO2 from roots is frequently proposed as an attractant that allows soil-dwelling insects to locate host plant roots, but this role has recently become less certain. CO2 is emitted from many sources other than roots, so does not necessarily indicate the presence of host plants, and because of the high density of roots in the upper soil layers, spatial gradients may not always be perceptible by soil-dwelling insects. The role of CO2 in host location was investigated using the clover root weevil Sitona lepidus Gyllenhall and its host plant white clover (Trifolium repens L.) as a model system. Rhizochamber experiments showed that CO2 concentrations were approximately 1000 ppm around the roots of white clover, but significantly decreased with increasing distance from roots. In behavioural experiments, no evidence was found for any attraction by S. lepidus larvae to point emissions of CO2, regardless of emission rates. Fewer than 15% of larvae were attracted to point emissions of CO2, compared with a control response of 17%. However, fractal analysis of movement paths in constant CO2 concentrations demonstrated that searching by S. lepidus larvae significantly intensified when they experienced CO2 concentrations similar to those found around the roots of white clover (i.e. 1000 ppm). It is suggested that respiratory emissions of CO2 may act as a 'search trigger' for S. lepidus, whereby it induces larvae to search a smaller area more intensively, in order to detect location cues that are more specific to their host plant.

Climate change and food security

Dynamic interactions between and within the biogeophysical and human environments lead to the production, processing, distribution, preparation and consumption of food, resulting in food systems that underpin food security. Food systems encompass food availability (production, distribution and exchange), food access (affordability, allocation and preference) and food utilization (nutritional and societal values and safety), so that food security is, therefore, diminished when food systems are stressed. Such stresses may be induced by a range of factors in addition to climate change and/or other agents of environmental change (e.g. conflict, HIV/AIDS) and may be particularly severe when these factors act in combination. Urbanization and globalization are causing rapid changes to food systems. Climate change may affect food systems in several ways ranging from direct effects on crop production (e.g. changes in rainfall leading to drought or flooding, or warmer or cooler temperatures leading to changes in the length of growing season), to changes in markets, food prices and supply chain infrastructure. The relative importance of climate change for food security differs between regions. For example, in southern Africa, climate is among the most frequently cited drivers of food insecurity because it acts both as an underlying, ongoing issue and as a short-lived shock. The low ability to cope with shocks and to mitigate long-term stresses means that coping strategies that might be available in other regions are unavailable or inappropriate. In other regions, though, such as parts of the Indo-Gangetic Plain of India, other drivers, such as labour issues and the availability and quality of ground water for irrigation, rank higher than the direct effects of climate change as factors influencing food security. Because of the multiple socio-economic and bio-physical factors affecting food systems and hence food security, the capacity to adapt food systems to reduce their vulnerability to climate change is not uniform. Improved systems of food production, food distribution and economic access may all contribute to food systems adapted to cope with climate change, but in adopting such changes it will be important to ensure that they contribute to sustainability. Agriculture is a major contributor of the greenhouse gases methane (CH4) and nitrous oxide (N2O), so that regionally derived policies promoting adapted food systems need to mitigate further climate change.

Sources of sulphur in rain collected below a wheat canopy

Vegetation plays an important role in the cycle of sulphur between the atmosphere and the soil. We have measured the quantity of sulphur in rain collected below a maturing wheat canopy. This sulphur has three sources: first, the atmosphere, from which falling rain gains SO2 and sulphate; second, leaf surfaces, from which rain washes sulphur which was previously deposited by turbulent transfer ('dry deposition'), and third, leaf tissue, from which rain leaches sulphur. We have now deduced from field and laboratory measurements that leaching supplied nearly 90% of the sulphur gained by rain as it fell through the wheat canopy. Only a small fraction of sulphur which had been dry-deposited on the surface of leaves could be washed off.

Host plant recognition by the root feeding clover weevil, Sitona lepidus (Coleoptera: Curculionidae)

This study investigated the ability of neonatal larvae of the root-feeding weevil, Sitona lepidus Gyllenhal, to locate white clover Trifolium repens L. (Fabaceae) roots growing in soil and to distinguish them from the roots of other species of clover and a co-occurring grass species. Choice experiments used a combination of invasive techniques and the novel technique of high resolution X-ray microtomography to non-invasively track larval movement in the soil towards plant roots. Burrowing distances towards roots of different plant species were also examined. Newly hatched S. lepidus recognized T. repens roots and moved preferentially towards them when given a choice of roots of subterranean clover, Trifolium subterraneum L. (Fabaceae), strawberry clover Trifolium fragiferum L. (Fabaceae), or perennial ryegrass Lolium perenneL. (Poaceae). Larvae recognized T. repens roots, whether released in groups of five or singly, when released 25 mm (meso-scale recognition) or 60 mm (macro-scale recognition) away from plant roots. There was no statistically significant difference in movement rates of larvae.

Plant roots release phospholipid surfactants that modify the physical and chemical properties of soil

• Plant root mucilages contain powerful surfactants that will alter the interaction of soil solids with water and ions, and the rates of microbial processes. • The lipid composition of maize, lupin and wheat root mucilages was analysed by thin layer chromatography and gas chromatography-mass spectrometry. A commercially available phosphatidylcholine (lecithin), chemically similar to the phospholipid surfactants identified in the mucilages, was then used to evaluate its effects on selected soil properties. • The lipids found in the mucilages were principally phosphatidylcholines, composed mainly of saturated fatty acids, in contrast to the lipids extracted from root tissues. In soil at low tension, lecithin reduced the water content at any particular tension by as much as 10 and 50% in soil and acid-washed sand, respectively. Lecithin decreased the amount of phosphate adsorption in soil and increased the phosphate concentration in solution by 10%. The surfactant also reduced net rates of ammonium consumption and nitrate production in soil. • These experiments provide the first evidence we are aware of that plant-released surfactants will significantly modify the biophysical environment of the rhizosphere.

An X-ray micro-tomography system optimised for the low-dose study of living organisms

An X-ray micro-tomography system has been designed that is dedicated to the low-dose imaging of radiation sensitive living organisms and has been used to image the early development of the first few days of plant development immediately after germination. The system is based on third-generation X-ray micro-tomography system and consists of an X-ray tube, two-dimensional X-ray detector and a mechanical sample manipulation stage. The X-ray source is a 50kVp X-ray tube with a silver target with a filter to centre the X-ray spectrum on 22keV.A 100mm diameter X-ray image intensifier (XRII) is used to collect the two-dimensional projection images. The rotation tomography table incorporates a linear translation mechanism to eliminate ring artefact that is commonly associated with third-generation tomography systems. Developing maize seeds (Triticum aestivum) have been imaged using the system with a cubic voxel linear dimension of 100 microm, over a diameter of 25mm and the root lengths and volumes measured. The X-ray dose to the plants was also assessed and found to have no effect on the plant root development.

Deriving empirical models of evaporation from soil beneath crops in a Mediterranean climate using microlysimetry

Microlysimeters were used to measure evaporation from bare soil and from soil under wheat and lupin canopies. The data were used to develop empirical relationships between evaporation, potential evaporation, soil water content, and leaf area index to allow estimation of evaporation losses in seasons when evaporation was not measured directly. Canopy cover was found to decrease evaporation losses from soil in both Stage 1 and 2 evaporation. This finding has significance to applications of the Ritchie model (1972) in Mediterranean climates since the model assumes Stage 2 evaporation to be independent of canopy cover. The empirical models derived during one season were successful in predicting evaporation from soil beneath crops during a second season. The model parameters were derived over the complete range in water content, leaf area index, and potential evaporation expected for crops at the site, suggesting that the models may be reliably applied to predict evaporation from soil at the site for other seasons. The technique may be useful for general application in estimating evaporation fromsoil beneath crops in field studies, although the model parameters may be site and/or soil specific. Further research into the generality of the relationships is warranted.

Complicated crown fracture of an unerupted permanent tooth--a case report

Trauma to primary teeth may result in direct damage to underlying developing permanent teeth because of the close relationship that exists between the apices of the primary teeth and their permanent successors. Injuries to developing teeth have been classified into ten different categories, using a classification that is largely based on developmental alterations to the permanent teeth. However, this classification does not include other types of trauma that may occur to developing teeth, such as crown fractures. Although apparently rare, such injuries can occur by direct contact of the impacting object with a developing tooth, as illustrated by the following case report.

Alternative crops for duplex soils: growth and water use of some cereal, legume, and oilseed crops, and pastures

Lupin is the major break crop used by farmers in Western Australia but neither lupin nor wheat uses much water from the B horizon of the widespread duplex soils. This study investigated the productivity and water use of a range of crops and pastures during 2 seasons on a shallow duplex soil, with a sandy layer 30-40 cm deep, at East Beverley, WA. The aims of the work were to evaluate the crops as alternative break crops to lupin on these soils, and to establish whether roots could proliferate in the clay layer, promoting both water extraction from the subsoil by that crop and improving yields of subsequent wheat crops. During the winter of the first season, a perched watertable developed for almost 3 months and some crops (especially lentil) grew poorly. Yields in the second season were generally good (lupin was close to the calculated potential yield and canola and Indian mustard were >2 t/ha), establishing that successful crops of oilseeds and grain legumes can be grown on this soil provided that there is adequate water without topsoil waterlogging. Yields of subsequent wheat crops were largest when following legume crops (40% in one season and 135% in the second compared with wheat following wheat or barley) but were also significantly greater following oilseeds (22% and 102%). Roots of cereals and pastures reached 80 cm in both seasons, whereas those of the oilseeds reached 60-80 cm depending on crop and season. Rooting depth of legumes varied from 70-80 cm for field pea to 30-50 cm for chickpea and faba bean, with lupin extending to 60 cm in both seasons. As with shoot mass, root mass differed between seasons, although on average, in mid September cereals and oilseeds had a smaller proportion (0·12 and 0·14) of total mass below ground than the legumes (0·24) and pasture species (0·18). Only a few millimetres of water was extracted from the subsoil by any crop in either season and there was no evidence that tap-rooted legumes or oilseeds were better able than other crops either to exploit subsoil water for their own use or to create pores that subsequent wheat crops might exploit.

Wheat response to alternative crops on a duplex soil

Summary. Rates of crop growth (including the root system), water extraction from specific soil layers, leaf water potential and crop nitrogen content were measured in a field experiment of wheat (Triticum aestivum L.) grown after alternative crops. The previous crops of lupin (Lupinus angustifolius L.), faba bean (Vicia faba L.), chickpea (Cicer arietinum L.), vetch (Vicia sativa L.), canola (Brassica napus L.), and Indian mustard (Brassica juncea L.) all showed a ‘break crop’ effect compared with wheat after wheat. In addition, the legumes lupin, faba bean, chickpea and vetch stimulated biomass production of successive wheat crops via increased nitrogen supply. When the previous crop was lupin, wheat root length density at depth in a duplex soil tended to be increased and post-anthesis water uptake by wheat was enhanced. Wheat after lupin resulted in a 42–76% higher yield than wheat after the other legumes, a 37–68% higher yield than wheat after the non-legumes and a 131% higher yield than wheat after wheat. Wheat yields did not relate to previous crop root depth, but correlated well with residue nitrogen from the various alternative crops.

Growth of shoots and roots, and interception of radiation by wheat and lupin crops on a shallow, duplex soil in response to time of sowing

Crops of lupin (Lupinus angustifolius L. cv. Gungurru) and wheat (Triticum aestivum cv. Kulin or Spear) sown close to the break of the season and 3-6 weeks later were grown on a duplex soil at East Beverley, WA, over 3 seasons. The overall aim of the work was to examine the influence of time of sowing on growth and water use of the crops, and this paper reports their growth and yield. Early sowing resulted in greater shoot weight of all crops (up to 2.8 t/ha for lupin and 1.7 t/ha for wheat at maturity) and grain yield of lupin, but grain yield of wheat was increased in only 1 of the 3 seasons. The principal effect of the delayed sowing was to reduce the duration of linear growth; the rate of the initial exponential phase was slightly reduced by later sowing as was the rate of growth during the linear phase in lupin (by about 1.5 g/m2day) but not in wheat. Late sowing generally reduced both the number of pod/ears per unit area and the number of grains per pod/ear. Doubling the density of sowing in one of the seasons had no effects on the shoot weight and grain yield of lupin with early or late sowing but decreased those of wheat. Downward root growth of early-sown crops averaged 5.2 mm/day for lupin and 8.7 mm/day for wheat in the 3 seasons and ceased at about 0.8 m; time of sowing had no effect on these measures. Root weight at flowering was greater in lupin than in wheat crops, and root weight of lupin was about 0.5 of total plant weight during vegetative growth compared with 0.25-0.3 in wheat. Typically, only 5-6% of the root length of both crop species was present in the clay layer at flowering irrespective of sowing time. The proportion of radiation intercepted reached a higher maximum value for early-sown crops (about 0.75 in 1991 and 0.90 in 1992) than late-sown crops (about 0.60 in 1991 and 0.8 in 1992). The conversion coefficients of radiation to dry matter were very similar (about 1.8 g/MJ) for both species, but the greater partitioning of dry matter to roots in lupin than wheat meant that conversion coefficients for shoot dry matter were greater in wheat (1.43-1.68 g/MJ) than in lupin (0.93-1.16 g/MJ). The results demonstrate that early sowing produced larger crops of both lupin and wheat; this resulted in larger lupin yields, but yield of wheat was affected by disease and drought during grain filling.

Restricted growth of lupin and wheat roots in the sandy A horizon of a yellow duplex soil

Yellow duplex soils are the dominant soil type in the cropping region of Western Australia, but crop yields on these soils are often variable and below potential. We are seeking to understand the causes of the spatially variable crop growth, and a preliminary study conducted in 1988 at a site east of Beverley indicated that variable shoot growth was associated with variable early (< 6 weeks after sowing) root growth in the sandy A horizon. The present study aimed to identify the constraints to early root growth by locating the position in the A horizon where root growth becomes restricted and measuring the local soil properties. In poor growth areas, root penetration slowed markedly at about 15 cm (about 2 weeks after sowing), while in the good growth areas roots continued to grow downwards at about 8 mm day-1 for lupin and 4 mm day-' for wheat. The soil was a particularly difficult environment for root growth, with generally low pH, low K and B, low porosity and aeration and high strength. Roots in poor growth areas appeared to experience greater mechanical impedance than in the good areas, most likely due to much lower clay contents leading to higher frictional resistance to particle movement or less stability of soil pores created by past roots or fauna.

Constraints to root growth of wheat and lupin crops in duplex soils

Duplex soils constitute about 60% of the cropping region of Western Australia and are usually cropped with wheat or lupins. Extensive and deep root growth is particularly important to crop production on these soils, because the nutrient- and water-holding capacities of the A horizon are frequently low. However, properties of the soils and the Mediterranean-type climate impose several constraints to root growth. Physical and chemical properties of duplex soils are spatially variable, leading to pronounced variation (from metres to tens of metres) in the growth of roots and shoots. Both the A and B horizons often impede root growth mechanically, with bulk density and penetrometer resistance frequently exceeding 1.8 Mg/m3 and 2 MPa, respectively. Also, saturated conductivities of the B horizon are often <0.01 m/day, leading to waterlogging. Topsoil acidity is often a problem in lighttextured A horizons, with pH declining about 0.1 unit each decade in yellow duplex soils near Beverley, Western Australia, where pH is already usually <4.8. Conversely, in the B horizon of red-brown earths and, sometimes, yellow duplex soils, pH >7 restricts growth of roots of Lupinus angustifolius. Major constraints to root growth often occur together (e.g. waterlogging with acidity, salinity, or mechanical impedance), and this exacerbates problems of root growth and necessitates identification and amelioration of the particular combination of constraints to improve root growth. Although L. angustifolius is often grown on duplex soils, its roots are not suited to these soils. Rooting depth is restricted, and unlike wheat roots, those of L. angustifolius are poorly adapted to ramifying through the soil for efficient water and nutrient extraction. Lupinus angustifolius is also particularly sensitive to high pH, salinity, and, probably, waterlogging. Other species of lupin which are more tolerant of high pH (e.g. L. pilosus) and waterlogging (e.g. L. luteus) may be more appropriate on duplex soils.

Components of the water balance on duplex soils in Western Australia

The water balance of soils is of interest both in terms of crop productivity and of sustainability, because water that is not used by crops contributes not only to loss of yield but also to the processes of land degradation. Land clearance for agriculture in Western Australia has changed the hydrological balance resulting in greater drainage and increased secondary salinisation. The relative importance of components of the water balance changes with rainfall. In the drier, eastern wheatbelt, evapotranspiration dominates the balance and there is little drainage, except through narrow, preferred pathways. In contrast, in the wetter region of the Darling Peneplain up to 40% of rainfall may be lost by pathways other than evaporation. Movement of water at the interface of the sand and clay layers (throughflow) has been poorly quantified. The dominant direction of flow is vertical, but substantial lateral flow may occur during winter storms on sloping sites. For example, measurements at a site near Narrogin showed up to 15 mm of throughflow during the cropping season. Drainage using interceptor drains provides a practical means of removing the throughflow and reducing the incidence of waterlogging. Yields of wheat on duplex soils are poorly related to the amount of evapotranspiration. At low rainfall, the relationship is poor, because rainfall is rarely uniformly distributed throughout the season, and lack of water during grain-filling may depress yields. In high rainfall areas, there are few detailed studies of water use, but yields may be lower than anticipated because of waterlogging, low inputs of fertiliser, and disease.

Root and shoot growth, and water and light use efficiency of barley and wheat crops grown on a shallow duplex soil in a mediterranean-type environment

Growth, interception of radiation and water use of three genotypes of barley (Beecher, O'Connor and Syrian) and one of wheat (Gutha) were measured on a duplex soil at East Beverley, W.A. All crops received 11 kg P ha-1 with the seed but no nitrogen fertilizer. Growth was initially slow until 69 days after sowing (das) with only small and inconsistent differences between crops. Thereafter, Beecher and O'Connor grew faster than Syrian and Gutha, maintained growth for longer, and at harvest weighed about 7.5 t ha-1 shoot dry matter, while Syrian and Gutha were about 5.0 t ha-1. Total root weight and length of Gutha increased until anthesis but reached their maxima at 83 das in all barley crops. Root weight as a percentage of total plant weight was about 40% during the winter decreasing to about 15% by anthesis. Root distributions of Beecher and O'Connor were similar but different from those of Syrian and Gutha; the former had up to 25% of the total root length at anthesis below 40 cm, but the latter had only about 5%. Dry matter production was linearly related to the amount of photosynthetically active radiation (PAR) intercepted, although the relations showed a break at about the time of flag leaf emergence, and efficiencies of conversion of PAR to dry matter were higher prior to this than after. Efficiencies for the barley crops were similar (about 2 g total dry weight MJ-1 before flag leaf emergence) and about 20% greater than for Gutha. Despite the large differences in dry matter production between crops, evapotranspiration (ET) was similar. For Beecher and O'Connor, evaporation E was about 40% of seasonal ET but for Syrian and Gutha it was about 50%. Values of water use efficiency (about 30 kg shoot ha-1 mm-1 for Beecher and OIConnor and 20 kg ha-1 mm-1 for Syrian and Gutha) were similar to other cereal crops and the amount of shoot dry matter per unit of water transpired for barley crops grown in W.A., U.K. and Syria was nearly constant; the value of the 'crop specific constant' was 3.0 Pa.

Parental awareness of the emergency management of avulsed teeth in children

Although public awareness campaigns on avulsed teeth have been mounted in the past, no study has investigated the knowledge parents possess of the protocol for the management of this dental emergency. The purpose of this investigation was to evaluate, by means of a questionnaire, the parental awareness of the emergency management of avulsed teeth in children. Over 2000 parents were surveyed during a four-week period at 20 suburban vacation swimming centres. The results indicated that almost two-thirds of respondents would attempt replantation of an avulsed tooth but further questioning showed they did not know the correct procedures. Thirty-three per cent of respondents were unaware of any after-hours emergency dental services. Ninety-two per cent felt they should seek professional help urgently following an avulsion injury, but their knowledge of transport media for the tooth was poor. Only 5 per cent knew that milk was the medium of choice for both washing and transporting an avulsed tooth. Ninety per cent of parents surveyed had never received advice on what to do in the event of an accident where a permanent tooth was avulsed. This study revealed the need for educational campaigns aimed at parents to increase their knowledge of the emergency procedures required when the tooth is avulsed.

Prenatal detection of D trisomy

The second instance of a prenatally diagnosed fetus of D trisomy is reported in a 45-year-old woman. The fetus had bilateral hare lip and cleft palate, arrhinencephaly, and numerous other malformations.

Antenatal diagnosis of fetal abnormality with special reference to amniocentesis

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