No abstract

Despite having the ability to bioaccumulate trace elements such as cadmium (Cd), many species also present morphophysiological disorders that can hamper their use as phytoremediation plants. Since it can lead to alterations in biomass accumulation. The employment of elements that mitigate stress, such as silicon (Si), can diminish the deleterious effects caused by trace elements. The objective of this study was to analyze the anatomical and physiological modulations induced by the synergy between Cd and Si in Alternanthera tenella plants, as well as to elucidate whether Si can mitigate the harmful effects caused by Cd under in vitro conditions. Nodal segments were cultured in media containing a concentration gradient of Cd (0, 50, 100, or 200 μM) combined with two levels of Si (0 or 40 μM) for a total of eight treatments. After 34 days, the plants' anatomy, physiology, and tolerance index were analyzed. The plants presented anatomical adjustments (such as lower stomatal index and number of vessel elements), suggesting lower translocation of Cd to the aerial part. When cultured with 200 μM Cd, the plants presented the lowest Chl a/b ratio. In the presence of Si, the decline of this ratio was smaller. Plants exposed to Cd concentrations of 50 μM without Si presented a signi cant decrease in the performance of the photosynthetic apparatus and tolerance index. The presence of Si in the medium reduced the damages caused by cadmium to the plants' physiology, resulting in greater growth and higher tolerance to this element.

at time t after start of actinic illumination; FV/F0 -ratio of the de-excitation rate constants for photochemical and nonphotochemical events; KP -photochemical de-excitation rate constant; PItotal -total performance index, which measures the performance up until the final electron acceptors of PSI; RC/CSm -total number of active reaction centers; SFI(ABS) -PSII structure and functioning index; VI -relative variable fluorescence at 30 ms (step I); VJ -relative variable fluorescence at 2 ms (step J); VK -relative variable fluorescence at 0.3 ms (step K); WK -represents the damage to oxygen-evolving complex; WL -indicates disturbance in the thylakoid membranes, reducing the energetic connectivity between the PSII units; ∆VIP -relative variable fluorescence amplitude of the increase from I to P = relative contribution of the increase from I to P to the increase in OJIP; φD0 -quantum yield of energy dissipation (at t = 0); φE0 -quantum yield of electron transport (at t = 0); φP0 -maximum quantum yield of primary photochemistry (at t = 0); φR0 -quantum yield of reduction of end electron acceptors at the PSI acceptor side; ψR0 -efficiency/probability by which electrons move from PSII to PSI acceptor side.

Ft -fluorescence at time t after the beginning of actinic illumination; FV/F0 -ratio of the de-excitation rate constants for photochemical and nonphotochemical events; OEC -oxygen-evolving complex; PItotal -total performance index, which measures the performance up until the final electron acceptors of PSI; RC/CSm -total number of PSII active reaction centers; Sm/tFmax -average fraction of open RC in the period of 0 to tFmax (time of maximum fluorescence production); VI -relative variable fluorescence at 30 ms (step I); VJ -relative variable fluorescence at 2 ms (step J); VK -relative variable fluorescence at 0.3 ms (step K); WK -the damage to OEC; WL -indicates disturbance in the thylakoid membranes, reducing the energetic connectivity between the PSII units; δR0 -efficiency/probability with which an electron from the intersystem electron carriers moves to reduce end electron acceptors at the PSI acceptor side (RE); φD0quantum yield of energy dissipation (at t = 0); φE0 -quantum yield of electron transport (at t = 0); φP0 = FV/Fm -maximum quantum yield of primary photochemistry of PSII reaction center (at t = 0); φR0 -quantum yield of reduction of end electron acceptors at the PSI acceptor side (RE); ψR0 -efficiency/probability by which electrons move from PSII to the PSI acceptor side.

The use of plants that present clear physiological responses to heavy metals such as cadmium (Cd) can assist in environmental monitoring. The aim was to evaluate physiological responses of Cd in Alternanthera tenella plants in micro-controlled conditions and to assess the its tolerance level and bioindicator potential. Nodal segments of A. tenella were grown in flasks containing semi-solid MS medium supplemented with (0, 50, 100, and 150 μM Cd). The total fresh and dry weight of the plants, in addition to physiological analyses, were determined after 30 days of cultivation. Plants submitted to Cd showed a reduction in biomass and photosynthetic pigments content. The decline in the Chl a/b ratio indicated the deleterious effects of Cd, as evidenced by the decrease in the density of active reaction centers (RC/CSM). The levels of deterioration in the functionality or structural integrity of the thylakoid membranes (WL) and the activity of the oxygen evolution complex (WK) increased as a function of Cd concentrations. The decreases in photochemical and non-photochemical quenching (qP and qN, respectively) observed in plants exposed to Cd indicated photochemical dysfunctions. Additionally, the increase in the quantum yield of non-regulated energy dissipation (ΦNO) indicated the presence of photodamage. Plants had a low/intermediate TI level (≥21.7) in the analyzed concentration range. A. tenella plants can bioaccumulate high amounts of Cd. The level of physiological responses of A. tenella is dependent on the concentration of Cd, which makes the use of this species even more promising as a bioindicator for this metal.