Phytoextraction and phytoexcretion of Cd by the leaves of Tamarix smyrnensis growing on contaminated non-saline and saline soils

Phytoremediation and more specifically phytoextraction, is an alternative restoration strategy for the clean up of heavy metal contaminated soils. Phytoextraction can only be successful if suitable plant species colonize the contaminated area, extract the toxic substances and accumulate them in their above ground tissues. In this study, the salt cedar Tamarix smyrnensis that is a widespread salt-tolerant plant in the Mediterranean region has been investigated. A pot experiment is conducted with T smyrnensis grown in polluted soil with 16 ppm of cadmium and at three different salt concentrations (0.0, 0.5, 3.0% NaCl) for a 10-week period. It took place in an open-air area with natural light, at ambient temperature and humidity in an effort to keep the plants under conditions as similar as possible to those in the field. However, care was taken not to let them be rained on. Temperature ranged from 19 to 50 degrees C with 33 and 21 degrees C being the average day and night temperature, respectively. Humidity ranged from 28% to 87% with a 13-14h photoperiod. The specific aims of this work are to investigate the accumulation of cadmium via root uptake at different saline conditions and cadmium excretion through salt glands on. the surface of the leaves as a probable detoxification mechanism of the plant. Furthermore, measurements of chlorophyll content, biomass, and shoot length are used to evaluate the potential of the plant for the removal of cadmium from contaminated saline and non-saline soils. The experimental data suggest that increased soil salinity results in an increase of the cadmium uptake by T smyrnensis. Analysis of white salt crystals taken from glandular tissue confirmed the fact that this plant excretes cadmium through its salt glands on the surface of the leaves as a possible detoxification mechanism in order to resist metal toxicity. Excreted cadmium is again released into the environment and it is redeposited on the top soil. Furthermore, increased salinity results in an increased excretion of the metal on Tamarix leaf surface. The presence of metals usually affects negatively the plant health, but T smyrnensis developed no visible signs of metal toxicity, only salt toxicity symptoms were observed. Cadmium usually decreases the chlorophyll content in plants; however, the amount of photosynthetic pigments of T smyrnensis was found not to be affected. All the above points to the potential of T smyrnensis for use in phytoremediation with the metal secretion from the leaves being a unique advantage that may change current phytoextraction practices. (C) 2007 Elsevier Inc. All rights reserved.