Growth responses of three ornamental plants to Cd and Cd–Pb stress and their metal accumulation characteristics

Abstract

Up to now, there was no document on ornamental plants that had been applied to phytoremediation, which can remedy contaminated environment and beautify it at the same time. Thus, the growth responses and possible phytoremediation ability of three ornamental plants selected from the previous preliminary experiments were further examined under single Cd or combined Cd–Pb stress. The results showed that these tested plants had higher tolerance to Cd and Pb contamination and could effectively accumulate the metals, especially for Calendula officinalis and Althaea rosea. For C. officinalis, it grew normally in soils containing 100 mg kg⁻¹ Cd without suffering phytotoxicity, and the Cd concentration in the roots was up to 1084 mg kg⁻¹ while the Cd concentration in the shoots was 284 mg kg⁻¹. For A. rosea, the Cd accumulation in the shoots was higher than that in the roots when the Cd concentration in soils was <100 mg kg⁻¹, and reached 100 mg kg⁻¹ as the criteria of a Cd hyperaccumulator when the Cd concentration in soils was 100 mg kg⁻¹. Their accumulation and tolerance to Cd and Pb were further demonstrated through the hydroponic-culture method. And A. rosea had a great potential as a possible Cd hyperaccumulator under favorable or induced conditions. Furthermore, the interactive effects of Cd and Pb in the three ornamentals were complicated, not only additive, antagonistic or synergistic, but also related to many factors including concentration combinations of heavy metals, plant species and various parts of plants. Thus, it can be forecasted that this work will provide a new way for phytoremediation of contaminated soils.

Introduction

Large areas of soils have been contaminated by heavy metals, which are deleterious to the existence, reproduction and development of living organisms including plants, animals and microorganisms. This phenomenon has even threatened the health of ecosystems and human beings themselves [1], [2]. Because soils contaminated by toxic heavy metals have important characteristics such as concealment, delay, accumulation, regionalism, and irreversibility [3], [4], [5], soil remediation has not only received more attention in environmental science and engineering, but also becomes global problems to be solved urgently [6], [7].

Phytoextraction, which makes use of the harvestable parts of plants to remove pollutants, represents a green and environmental-friendly tool for cleaning metal-polluted soils and waters compared with conventional chemical and physical remediation technologies, which are generally costly and often harmful to soil ecosystems [8], [9], [10]. In phytoremediation, screening out effective hyperaccumulators has become important, however, limited hyperaccumulators have been reported [11], [12], [13]. Thus, it is necessary to search for more hyperaccumulators to remedy contaminated soils effectively [3].

Ornamental plants are an important type of higher plants apart from those in the food chain, and are quite crucial if they have hyperaccumulation properties and can be applied to remediation of contaminated soils [14]. It can be inferred from available data that if we can find hyperaccumulative ornamentals which can be used to remedy contaminated soils from abundant plant species and types, they may bring economic benefits because they can beautify the environment at the same time. This is the special advantage that ornamental plants are different from other hyperaccumulators. Up to now, there is no systematic identification of ornamental plants applicable to remediation of contaminated soils. In fact, in populous urban areas, ornamentals have many practical applications in indication and prevention of pollution produced by atmospheric precipitation and sewage discharge while ornamentals can beautify environment [15], [16]. Thus, using ornamentals for remediation of contaminated environment has a significant and realistic purpose [17]. According to an elementary screen from herbaceous ornamental plants [18], Impatiens Balsamina, Calendula officinalis and Althaea rosea had higher tolerance and accumulation ability to cadmium (Cd) and lead (Pb). Therefore, this work investigated the growing responses and possible hyperaccumulation ability of the three ornamentals under single Cd or combined Cd–Pb contamination by the further experiment. Both the soil-culture and hydroponic-culture methods were employed. The results can provide scientific methods for generating a new way of phytoremediation.