Submitted to: Plant Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/3/2003
Publication Date: 6/1/2003
Citation: OZTURK, L., CAKMAK, I., KOCHIAN, L.V. SHOOT BIOMASS AND ZINC/CADMIUM UPTAKE FOR HYPERACCUMULATOR AND NON-ACCUMULATOR THLASPI SPECIES IN RESPONSE TO GROWTH ON A ZINC-DEFICIENT CALCAREOUS SOIL. PLANT SCIENCE. 2003. Interpretive Summary: Heavy metal contamination of soils poses serious problems to our society, and the current technologies used to remediate soils are quite costly and disruptive. There is considerable interest in the use of terrestrial plants to clean up heavy metals from the soil. A small number of metal hyperaccumulating plants have been identified that can grow in highly contaminated soils and accumulate these metals to high shoot concentrations. We have been studying the mechanisms for metal hyperaccumulation in Thlaspi caerulescens, a zinc/cadmium hyperaccumulator. We have hypothesized that the processes that contribute to metal hyperaccumulation in T. caerulescens may also make this species sensitive to Zn deficiency, as the ability to tolerate high levels of Zn in the leaves may involve the sequestering of the Zn in forms unavailable for normal cellular functioning. This was tested by growing T. caerulescens and a related non-accumulator, T. arvense, in Zn deficient soil supplemented with different levels of Zn and Cd. We did find that T. caerulescens was much more susceptible to Zn deficiency than was T. arvense, and the hyperaccumulator had an internal critical Zn level (the Zn concentration below which Zn deficiency occurs) that was 10-fold higher than normal, non-accumulator plants. On the other hand, at high levels of Zn and Cd, T. arvense showed severe toxicity symptoms and growth reduction, while T. caerulescens was asymptomatic. The findings indicate that the processes underlying the metal hyperaccumulation trait in T. caerulescens also cause this plant species to be sensitive to Zn deficiency stress. This information should assist us in understanding metal hyperaccumulation, in order to develop more effective heavy metal-remediating plant species.
Technical Abstract: In this study, the Zn/Cd hyperaccumulator, Thlaspi caerulescens and a related non-accumulator, Thlaspi arvense, were used to study shoot growth (dry matter production) and Zn and Cd uptake from a severely Zn-deficient calcareous soil supplemented with increasing amounts of Zn and Cd. Shoot dry matter production of T. caerulescens was severely depressed by Zn deficiency, while in T. arvense, Zn deficiency slightly reduced growth. At the highest Zn supply, T. caerulescens did not show any symptoms, while T. arvense exhibited leaf Zn toxicity. Supply of Cd resulted in marked decreases in shoot growth of T. arvense, particularly under low Zn supplies, but had no effect on the growth of T. caerulescens. At the low soil Zn level supplies (<0.5 mg Zn kg-1) shoot Zn concentrations were lower in T. caerulescens compared to T. arvenses, and were below 10 mg Zn kg-1 dry wt in T. caerulescens, being lower than those of T. arvense. However, at the high supplies of Zn (>5 mg Zn kg-1), shoot Zn concentrations were considerably higher in T. caerulescens than T. arvense. Increases in Zn supply from 0 to 75 mg kg-1 enhanced shoot Zn concentrations by 84-fold in T. caerulescens and only 8-fold in T. arvense. Shoot Zn concentrations of both species were not affected by Cd supply, while increases in Zn supply markedly reduced shoot Cd levels in T. arvense. The results demonstrate that T. caerulescens is extremely sensitive to Zn deficiency in soils, but tolerant to excessive accumulation of Zn and also Cd in shoot, while T. arvense is tolerant to Zn deficiency but not to accumulation of Zn and Cd in shoot. The findings indicate that the processes underlying the metal hyperaccumulation trait in T. caerulescens also cause this plant species to be sensitive to Zn deficiency stress.