Effects of root anatomy and Fe plaque on arsenic uptake by rice seedlings grown in solution culture

Abstract

Hydroponic experiments were carried out to investigate the effects of root anatomy, induced by aeration and stagnation, and Fe plaque on arsenic (III&V) uptake and translocation by rice plants. The results showed that As uptake in rice plants (Gui Chao-2) treated by aeration was decreased due to lower root specific surface area. Rice roots with larger specific surface area tended to form more Fe plaque, and Fe plaque affected As uptake kinetics by changing As influx curves from linear to hyperbolic for As(III) and from hyperbolic to S-curve for As(V). Fe plaque increased As(III&V) adsorption and minimized the effects of root anatomy characteristics on As uptake into roots and subsequently translocation to shoots. Fe plaque increased As(III) uptake rate at As(III) concentrations of 0.5∼8 mg L⁻¹, reduced As(V) uptake rate at low As(V) concentrations (<2 mg L⁻¹), but increased As uptake rate at high As(V) concentrations (>6 mg L⁻¹).

Introduction

Arsenic (As) has been the focus of environmental concern in recent years because of its toxicity and potential risk to ecosystems and to human health (Duxbury and Zavala, 2008). Arsenic levels in soil have been significantly enhanced by anthropogenic activities, such as mining, application of pesticides and herbicides, and irrigation with contaminated groundwater in many parts of the world, especially Bangladesh, India, China and Vietnam (Kim et al., 2009). Because of the high phytoavailability of As under reduced soil conditions, paddy rice (Oryza sativa L.) accumulates the highest amount of As among all grain crops (Marin et al., 1993, Williams et al., 2007). Since paddy rice is the major staple food crop in Southeast Asia, the consumption of As-contaminated rice grain will impose potential risk to human health especially in countries such as Bangladesh where water containing elevated levels of As is used as drinking and irrigation water (Meharg and Rahman, 2003). Arsenic typically exists in two oxidation states, arsenite (As(III)) and arsenate (As(V)). It is essential to investigate the mechanism related to how As(III) and As(V) are taken up by rice plants in order to gain a better understanding of As accumulation and metabolism by rice. Consequently, new strategies can be developed for reducing As accumulation in rice grains.

Most studies on the factors contributing to As accumulation in plants focused on soil conditions especially Eh (Signes-Pastor et al., 2007), pH (Carbonell-Barrachina et al., 1999), and nutrient elements, such as nitrogen (Charter et al., 1995), phosphorus (Signes-Pastor et al., 2007), silicon (Guo et al., 2007), sulphur (Hu et al., 2007), and the role of plants (Fayiga et al., 2007). However, the effects of roots have been largely ignored. A recent study showed that the oxidizing ability of roots influenced As accumulation in rice (Mei et al., 2009). Root oxidizing ability can be estimated by radial oxygen loss (ROL), manipulated by O₂ diffusion from shoot to root and then released to rhizosphere (Armstrong, 1980). When rice roots are grown in stagnant conditions, ROL barrier and aerenchyma as well as other root anatomy traits will appear and develop (Colmer et al., 2006). On the contrary, when grown in aerobic soils, rice possesses a different root structure which is characterized by longer maximum root, fewer adventitious root, lower porosity and less pronounced barrier against ROL (Colmer, 2003a).

Fe is a major soil element. In waterlogged conditions, the solubility of Fe is increased due to the influences of physical, chemical and biological processes, which dramatically increases Fe²⁺ concentration (Wang and Peverly, 1999). Due to ROL of the root system, Fe²⁺ in the rhizosphere is oxidized into oxyhydroxide plaque on the root surface of rice plants (Armstrong, 1964). The Fe hydroxides in soil and solution have a very strong binding affinity for As (Meng et al., 2002, Liu et al., 2004a, Liu et al., 2004b). Some studies have indicated that Fe plaque may enhance As(III) adsorption while decreasing As(V) absorption by rice (Chen et al., 2005). It has been widely accepted that the effect of Fe plaque on plant uptake of heavy metals or nutrients is related to the amount of Fe plaque on the roots surface (Zhang et al., 1998). Fe plaque can be formed on rice root both under natural and laboratory conditions (Greipsson, 1994, Greipsson, 1995). Therefore, it seems practical to reduce As accumulation in rice by means of the formation of Fe plaque. In addition, as a major absorption organ, the anatomic characteristics of root can affect As accumulation directly or by influencing Fe plaque formation (Hupfer and Dollan, 2003) which affects As accumulation. Nonetheless, so far there is a severe lack of information focusing on the three-way interactions between As, Fe plaque and root anatomy. Therefore, the major objectives of this study were to:

• (1) investigate the effects of root anatomy on As (III&V) uptake into roots and translocation to shoots; (2) assess the effects of root anatomy on As (III&V) uptake and translocation to roots with Fe plaque deposited; and (3) study the kinetics of As(III) and As(V) uptake by rice plants of different root anatomy characteristics with or without the deposition of Fe plaque.