Metabolic responses of Carlina acaulis L. to chronic and acute cadmium stress: insights into chelation mechanisms, non-enzymatic antioxidants, and specialized metabolism

BackgroundChronic and acute stress can lead to completely different metabolic responses of plants exposed to the same abiotic factor. The effect of long-term chronic cadmium stress (ChS, 0.1 µM Cd, 85 days) or short-term acute cadmium stress (AS, 10 µM Cd, 4 days) on the physiology of Carlina acaulis L. (Asteraceae) and selected secondary metabolites was compared to identify specific physiological and biochemical reactions.ResultsThe bioconcentration of Cd in all analyzed organs was higher under AS in comparison with ChS (130 vs. 16 µg g-1 DW, 7.9 vs. 3.2 µg g-1 DW, and 11.5 vs. 2.4 µg g-1 DW in roots, leaves, and trichomes, respectively). The high concentration of Cd in the trichomes in the AS treatment might be an anatomical adaptation mechanism. ChS evoked an increase in the root biomass, whereas its impact on shoot biomass was not significant in any treatment. The amounts of ascorbic acid and the sum of phytochelatins were higher in the shoots, whilst organic acids (malic and citric) reached higher levels in the roots of plants from the ChS treatment. Glutathione depletion occurred in the shoots, but there was no change in its root level in both treatments. The amount of chlorogenic acid, but not that of ursolic and oleanolic acids, was increased under ChS. On the other hand, AS exposure elevated the level of ursolic and oleanolic acids, but not chlorogenic acid in the shoots. ConclusionsThese data indicate that ChS and AS induce different physiological and biochemical defense mechanisms. Both chelation and enhancement of the antioxidative machinery contribute to protection of C. acaulis exposed to long-term (chronic) Cd exposure and alleviate Cd toxicity effectively. However, triterpene acids were elevated only under AS treatment, which may suggest an instantaneous action of these metabolites under short-term acute Cd stress.