Relationship between Mg, B and Mn status and tomato tolerance against Cd toxicity.

Distinct tomato genotypes possess different tolerance degree to cadmium (Cd), but the mechanisms behind this phenomenon are scarcely understood. To this end, the physiological, biochemical, anatomical, nutritional and molecular mechanisms associated to the plant tolerance against Cd toxicity were investigated in five tomato accessions with contrasting sensitivity to Cd exposure. Firstly, the data revealed that larger biomass loss was not always coupled to higher Cd concentration, indicating that other events, in addition to the internal Cd accumulation, impact tomato performance at early stages of Cd exposure. Secondly, the results indicated that the fine regulation of nutrient status, particularly magnesium (Mg), boron (B) and manganese (Mn), is associated to the mitigation of Cd toxicity. Magnesium status was coupled to the modulation of root development, resulting in changes in root hair formation and biomass allocation. Boron accumulation in leaves was linked to Cd toxicity, suggesting that tolerance mechanisms involved strategies to decrease or even avoid B excess in photosynthetic tissues. Disturbances in Mn status, i.e. Mn excess in leaves and Mn deficiency in roots, were also related to tomato sensitivity to Cd exposure. Thirdly, plant capacity to maintain leaf blade expansion is a relevant strategy for a better tomato development after short-term Cd exposure. Fourthly, tomato tolerance to Cd-induced stress does not depend on CAT activity enhancements in such conditions. In conclusion, tomato ability to quickly manage its nutritional status is necessary for alleviation of the Cd effects at early stages of exposure to this metal. The better understanding about tolerance mechanisms and mode of action of Cd toxicity in plants can help in the establishment of strategies to mitigate its impacts on crops.

Investigation into the relationship among Cd bioaccumulation, nutrient composition, ultrastructural changes and antioxidative metabolism in lettuce genotypes under Cd stress.

Lettuce (Lactuca sativa L.) is known to have high cadmium (Cd) concentrations in its shoots, which makes it necessary to protect against Cd toxicity. Understanding Cd-induced physiological responses in lettuce plants can contribute to the definition of useful strategies to decrease Cd uptake. This study aimed to gain new insights into Cd-induced stress by measuring Cd bioaccumulation, nutrient composition, anatomical and ultrastructural changes, and antioxidative metabolism in three lettuce genotypes characterized as having different degrees of Cd tolerance (Vanda = low, Lidia = medium and Stela = high). Plants were grown hydroponically with Cd concentrations of 0.0 and 0.1 or 0.5 µmol L-1, for 30 days. Cadmium uptake in the lettuce genotypes assayed is controlled by the root/shoot ratio, higher root/shoot ratios allowing greater Cd uptake. The Fe and Ni content increased in shoots of the genotype Lidia, which could be associated with a decrease in oxidative stress in chloroplasts due to superoxide dismutase (SOD) isozyme activity. Cadmium-induced oxidative stress is associated with de-structuring of the phloem and xylem in roots, and starch grain and plastoglobule accumulation in chloroplasts. Lettuce genotypes that presented higher SOD and ascorbate peroxidase (APX) activity presented better preserved anatomical structures. These results suggest that genotypes with less efficient antioxidant defence in the roots tend to take up more Cd, increasing root-to-shoot Cd translocation.

Cadmium toxicity degree on tomato development is associated with disbalances in B and Mn status at early stages of plant exposure.

Cadmium (Cd) toxicity is frequently coupled to its accumulation in plants, but not always the highest Cd concentration triggers the worst damages, indicating that additional events influence the magnitude of Cd side-effects. We investigated the early mechanisms behind the differential Cd-induced impacts on plant development of four tomato accessions with contrasting tolerance to Cd toxicity. At organ level, the highest Cd concentration was not associated with the largest biomass losses. In leaves, changes in superoxide dismutase and catalase activities were not related to differences in Cd concentration, which was unable to provoke H2O2 overproduction on the sixth day of plant exposure to this metal. Further investigation in the mineral profile revealed that magnitude of Cd toxicity depends probably on synergic effects from increased B status, in addition to the own Cd accumulation. Furthermore, disbalances in Mn status (i.e., excess in leaves and deficiency in roots) may enhance Cd toxicity degree. According to data, however, the low magnesium (Mg) status can be linked to tomato tolerance against Cd toxicity. In conclusion, the tomato tolerance degree under short-Cd exposure depends on actively, finely regulation of mineral homeostasis that results in different development of plant organs. The better understanding on the mode of action of Cd toxicity in plants can help in the establishment of strategies to mitigate its impacts on crop yield.

Estimating tomato tolerance to heavy metal toxicity: cadmium as study case.

This work aimed to develop a reliable and fast approach to estimate the plant tolerance degree to heavy metal (HM) phytotoxicity. Two independent experiments were carried out using tomato accessions, with contrasting morphological features, that were grown in a hydroponic solution containing different CdCl2 concentrations for 7 days. Plant dry weight and chlorophyll content (SPAD units) were evaluated, and tolerance degree to Cd toxicity was estimated according to the tolerance index (TI), which is a new mathematical formula based on plant biomass proposed in this study. Although with different magnitudes, tomato exhibited reductions in their dry weight concurrently with the increasing CdCl2 concentration. By contrast, chlorophyll content presented no standard response, decreasing and even increasing according to CdCl2 concentrations, indicating that only under certain conditions (particularly, at CdCl2 50 µM), this parameter can be used to estimate plant tolerance to Cd toxicity. TI was efficiently able to segregate tomato cultivars with similar performance (based on the total dry weight of plants), and such segregation was optimized when the hydroponic solution contained from 25 to 50 µM CdCl2. Within this range, data pointed at 35 µM CdCl2 as the best concentration to be employed in studies related to the tomato tolerance/sensitivity to Cd toxicity. In conclusion, TI proved to be a reliable estimator of tolerance degree to Cd exposure in genetically distinct tomato accessions. Moreover, TI can be used for this same purpose in plants under other HM-induced stresses.