Micro-PIXE studies of elemental distribution in Cd-accumulating Brassica juncea L.

doi:10.1016/S0168-583X(99)00356-0

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 158, Issues 1–4, 2 September 1999, Pages 329-334

https://doi.org/10.1016/S0168-583X(99)00356-0 Get rights and content

Abstract

Brassica juncea L. is a high biomass producing crop plant, being able to accumulate Cd and other heavy metals in their roots and shoots. It is a good candidate for efficient phytoextraction of heavy metals – such as Cd – from polluted soils.

PIXE and STIM analyses were applied to investigate Cd-uptake in roots and the resulting effects on the elemental distribution of Cd stressed plants. The axial distribution of trace elements as a function of distance from the root tip as well as the radial distribution within cross-sections were analysed. The results are compared with the elemental distribution in control plants.

Introduction

In most soils Cd occurs at concentrations of 0.1–1 mg/kg, but can reach levels of above 100 mg/kg in zinc-and lead-polluted soils [1]. Geochemically, Cd is strongly associated with other calcophile elements and is highly soluble in soils [2]. The ability of plants to (hyper-) accumulate and to concentrate heavy metals in the shoot biomass is responsible for the spreading of this element in the food chain. According to Wagner [3], at least 70% of the Cd taken up by humans originate from plant food. On the other hand accumulation of heavy metals in vascular plants has recently been investigated as a novel strategy for environmental cleanup technologies, e.g. phytoremediation.

Important for phytoremediation is that high biomass crop plants are chosen which are able to transfer the heavy metal efficiently and rapidly from soil via the root system to the shoot that thereafter will be harvested and disposed. Root to shoot translocation of the incorporated heavy metals is thought to be one of the major limiting factors at the plant level for reaching promising cleanup results. Mechanisms for Cd uptake and translocation are not yet well understood. However, three major sites regulating root to shoot translocation can be identified as possible bottle necks: (i) transport from the extracellular space (apoplast) to intracellular continuum (symplast) within the root cortex, (ii) translocation from root cortex to central cylinder across the endodermis, and (iii) further transport into the long distance transport system (xylem).

To further understand uptake and translocation of heavy metals by accumulating plant species, we investigated how elemental distributions are influenced by the Cd exposure and how Cd itself is distributed within the root tissue and along the root axis. For our experiments we chose the Cd accumulating species Brassica juncea L.

In the present study we used proton induced X-ray emission (PIXE) [4] and scanning transmission ion microscopy (STIM) [5] to determine two different aspects of elemental distribution within the root: the elemental concentrations as function of the distance from the root tip, and additionally, the radial distribution of the elements within root cross-sections with special emphasis on the most active zone of elemental uptake, the root hair zone (RHZ) [6].

Section snippets

Plant material

Seeds of Brassica juncea L. var. Vitasso were germinated on sand and grown in a greenhouse with supplementary light to provide a 16 h light period. After four weeks plants were selected for uniformity and transferred to a hydroponic system. The plants were fixed on a polystyrol-plate in a pot containing 3 l of nutrient solution each. The nutrient solution (pH 5.2) contained 0.5 mM NH₄H₂PO₄, 0.5 mM MgSO₄, 1.5 mM KNO₃, 1.0 mM Ca(NO₃)₂, 20 μM Fe–EDTA (ethylenediaminetetraacetic acid), 11.5 μM H₃BO₃

Longitudinal elemental distribution

First, the flattened root samples were analysed by STIM to determine the area density of the regions of interest (ROIs); the coordinates of the ROIs were stored on hard disk. Thereafter the X-ray measurements of every single region were executed automatically with the help of PC-controlled stepping motors.

The scan direction was orientated parallel to the roots longitudinal axis; the dimensions of the ROIs for each spectrum were set to 100 × 500 μm² to ensure that the flattened root was totally

Discussion

This study presents preliminary data on elemental distributions along the root axis and within root cross-sections of control and Cd treated Brassica juncea L. It can be emphasised that the elemental distribution within the roots shows clear and interesting alterations after Cd stress.

A higher S content of the RHZ in Cd stressed roots as compared to that of the control root and a radial gradient in the cross-section between the outer regions and the central cylinder are found. This possibly

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Micro-PIXE studies of elemental distribution in Cd-accumulating Brassica juncea L.

Abstract

Introduction

Section snippets

Plant material

Longitudinal elemental distribution

Discussion

Adv. Agronomy

Nucl. Instr. and Meth.

Nucl. Instr. and Meth. B

Nucl. Instr. and Meth. B

Nucl. Instr. and Meth. B

Nucl. Instr. and Meth. B

Nucl. Instr. and Meth. B