Maize plant responses, in terms of growth and metal uptake, to different concentrations of cadmium ions (4, 20 µM) were analyzed in a hydroponic culture, for 2 weeks. For a 4 µM cadmium-contaminated environment, the maize plant presents the highest bioaccumulation level after 192 h, with a recovery degree of 52%, meanwhile, at a 20 µM concentration, the highest bioaccumulation was registered after 366 h, with a corresponding recovery degree after 288 h (10.56%). The translocation factor presented higher values for 20 µM induced contamination than for 4 µM, which means that increasing metal concentration in the medium increased the concentration in the upper parts of the plant. Anatomical sections of a maize plant (in a 4 and 20 µM cadmium-contaminated environment) were observed to evidence the changes in plant morphological structure. The efficiency of phytoextraction is related to the metal concentration in the environment and to the plant's ability to grow on polluted soil sites, concomitantly with a high biomass yield. Keywords: heavy metal, phytoremediation, bioaccumulation, translocation factor INTRODUCTION Heavy metal contamination is a serious environmental problem that limits crop production and threatens human health through the food chain. Cadmium, one of the most toxic environmental pollutants for plants, may interfere with numerous biochemical and physiological processes -including photosynthesis, respiration, nitrogen and protein metabolism, and nutrient uptake. Phytoremediation is an in situ nondestructive technique, characterized by the utilization of hyperaccumulator plant species to remove the heavy metals from soil. The suitability of a certain plant for heavy metal remediation is determined by various plant properties, such as heavy metal tolerance, size, growth rate and rooting depth, heavy metal accumulation in aboveground plant parts and climatic adaptation and pest resistance. 1,2 The aim of this research was to evaluate the maize plant responses to cadmium stress conditions, every 48 h, for 2 weeks, and the efficiency in phytoremediation processes. EXPERIMENTAL Maize seeds (Zea mays) were sterilized in the commercial bleaching agent HOCl (1%) for 30 min and rinsed with distilled water under stirring for 10 min, the process being repeated 3 times. The seeds were placed over moist filter paper disks in Petri dishes and stored in the dark at 25 ºC, with a view to their germination. The , 5 µM Fe) with the pH adjusted 3 to 6.8. The plastic pots were covered with an aluminum foil to prevent the development of photosynthetic algae. After 5 days of germination, seedlings of maize with the same size were assembled in each hydroponic unit. The volume of nutrient solution (150 mL) was not modified throughout the experiments, to avoid the variation of metal concentrations. Every 48 h and at the end of the assay (2 weeks), the contents of cadmium in the roots, stem and leaves, as well as the growth parameters of maize plants, were determined. The plant roots were rinsed in abundant tap and distilled water ALINA STINGU et al. 288 before mineralization. Maize plants separated into roots, stems and leaves were oven-dried at 60 ºC, until constant mass was reached, and then the plant tissues were digested 4 using HNO 3 (65%) and H 2 O 2 (30%), on a hot plate at 120 ºC, for at least 5 h. The measurement of the metal content in the solution was accomplished through AAS (using a GBC Avanta 2003 Atomic Absorption Spectrophotometer). To evaluate the growth rate of the maize plant every 48 h, in a cadmium-contaminated environment, the following formula was used: growth rate, % = 100 x (growth parameters at the beginning of the experiment -growth parameters at a considered time)/growth parameters at a considered time. Spectrophotometric quantification of heavy metal concentration in maize plant tissues permitted the evaluation of cadmium bioaccumulation, translocation factor and recovery: Bioaccumulation coefficient = (cadmium concentration µg/g dry plant tissue)/(cadmium concentration µg/mL nutrient solution); 5 Translocation factor (TF) = ratio of metal concentration in shoots/ratio of metal concentration in roots; 6 Recovery, % = metal content in shoot or root/metal content in the medium. 7 At the end of the experiment, histological cross-sections were obtained for maize roots. The sections were cut manually, using microtome and elder pith as a support. The histological sections were washed in sodium hypochlorite, then in acetic acid (to eliminate the cellular content) and distilled water. The sections were coloured with iodine green (1 min), washed in 90% ethylic alcohol and distilled water, then coloured with ruthenium red (1 min) and again washed in distilled water. RESULTS AND DISCUSSION In the first hours, under 4 µM cadmium stress conditions, an increasing trend in plant growth and development was observed, the maximum growth rate being registered at 144 h. 192 h after the beginning of the experiment, the growth process seemed to stop, being resumed after 48 h. For a 20 µM cadmium-contaminated environment, the maximum growth rate was registered in the first 48 h. After 192 h, a decreasing trend in plant growth was observed. Maize plant growth rate decreased with increasing cadmium concentration in the growth medium Cadmium concentration (μg/g dry mass) and content (μg/plant) in maize plant presented different values, as depending on metal contamination level. The highest values for cadmium concentration and content (390.04 μg/plant), under 4 µM stress conditions (15430.99 μg/g), were registered at 192 h. Reported to the 20 µM cadmiumcontaminated environment, the highest metal content (395.82 μg/plant) and concentration (22443.54 μg/g) were recorded at 228 and 336 h. These results could be correlated with the plant growth rat
