Manganese is an essential element for plants, intervening in several metabolic processes, mainly in photosynthesis and as an enzyme antioxidant-cofactor. Nevertheless, an excess of this micronutrient is toxic for plants. Mn phytotoxicity is manifested in a reduction of biomass and photosynthesis, and biochemical disorders such as oxidative stress. Some studies on Mn toxicity and Mn translocation from soil to plant cells in Mn 2+ form have demonstrated their importance under low pH and redox potential conditions in the soil. When Mn is inside the cells, mechanisms that can tolerate this toxicity are also observed, being important the compartmentalization of this metal in different organelles of shoot and leaf plant cells. A key role of antioxidative systems in plants in relation to high Mn amounts has also been reported as a defense mechanism. The purpose of this review is to show the role of Mn as an essential micronutrient and as a toxic element to higher plants as well as to their transport and tolerance mechanisms. The forms and dynamics of this element in soils and the importance of the acidity for this dynamic and availability for plants are also given.

The effects of exposure to increasing manganese concentrations (50–1500 µM) from the start of the experiment on the functional performance of photosystem II (PSII) and photosystem I (PSI) and photosynthetic apparatus composition of Arabidopsis thaliana were compared. In agreement with earlier studies, excess Mn caused minimal changes in the PSII photochemical efficiency measured as Fv/Fm, although the characteristic peak temperature of the S2/3QB – charge recombinations was shifted to lower temperatures at the highest Mn concentration. SDS-PAGE and immunoblot analyses also did not exhibit any significant change in the relative abundance of PSII-associated polypeptides: PSII reaction centre protein D1, Lhcb1 (major light-harvesting protein of LHCII complex), and PsbO (OEC33, a 33kDa protein of the oxygen-evolving complex). In addition, the abundance of Rubisco also did not change with Mn treatments. However, plants grown under excess Mn exhibited increased susceptibility to PSII photoinhibition. In contrast, in vivo measurements of the redox transients of PSI reaction centre (P700) showed a considerable gradual decrease in the extent of P700 photooxidation (P700+) under increased Mn concentrations compared to control. This was accompanied by a slower rate of P700+ re-reduction indicating a downregulation of the PSI-dependent cyclic electron flow. The abundance of PSI reaction centre polypeptides (PsaA and PsaB) in plants under the highest Mn concentration was also significantly lower compared to the control. The results demonstrate for the first time that PSI is the major target of Mn toxicity within the photosynthetic apparatus of Arabidopsis plants. The possible involvement mechanisms of Mn toxicity targeting specifically PSI are discussed.

We compared the aluminum tolerance of two highbush blueberry (Vaccinium corymbosum) cultivars, Legacy and Bluegold, grown in a greenhouse in Hoagland's solution with increasing concentrations of Al (0, 25, 50, 100, and 200 μm) for 7 to 20 days, using root lipid peroxidation (LP), radical scavenging activity (RSA), Al uptake by roots, and relative growth rate (RGR) as criteria. Leaf physiological [photochemical and non-photochemical parameters of photosystem II (PSII)] and biochemical (pigments, LP, RSA, and total soluble carbohydrates) responses to Al stress were also analyzed and then a principal component analysis (PCA) was performed. The results indicated that ‘Bluegold’ showed the highest Al uptake and LP in roots and a lower RGR in contrast to ‘Legacy’. The photochemical parameters were more affected in ‘Bluegold’ than in ‘Legacy’, particularly at the beginning of the experiment. At this point, a sharp increase in RSA was found in ‘Legacy’. According to these parameters, ‘Legacy’ was more Al tolerant than ‘Bluegold’. PCA revealed that among the underlying processes affected by Al toxicity in the highbush blueberry, the photochemical efficiency of PSII followed by modifications of photosynthetic pigment contents are of greatest significance after long-term Al stress. Additionally, RSA plays an important role in the long-term acclimation response mechanisms to Al stress in highbush blueberry leaves.

Manganese (Mn) is an essential micronutrient for plants; however, in excess, it can have negative effects on their physiological and biochemical characteristics. We evaluate photosynthetic performance, antioxidant activity, and organic acid exudations to clarify the mechanisms involved in highbush blueberry (Vaccinium corymbosum L.) under Mn excess. One-year-old plants of V. corymbosum cultivars (Legacy, Brigitta, and Bluegold) were grown in hydroponic solution with 2, 10, 50, 250, 500, and 1000 μM of Mn treatments for 25 days. Plant growth as shoot and root biomass, Mn content, photosynthetic performance [photochemical efficiency of PSII, CO 2 assimilation, and stomatal conductance (g s)], lipid peroxidation, radical scavenging activity, superoxide dismutase activity, and organic acid exudations were analyzed. Mn excess triggered detrimental effects in terms of plant growth, photochemical efficiency, and CO 2 assimilation, as well as in biochemical features in V. corymbosum cultivars. Despite Legacy decreasing its photosynthesis, plant growth was maintained throughout the experiment; by contrast, Brigitta maintained photosynthesis and growth despite the decrease in stomatal conductance (g s). Meanwhile, Bluegold presented the lowest level in net photosynthesis and photochemical processes and an increase in lipid peroxidation. Oxalate and citrate were the most important organic acid anions in Legacy and Brigitta, gradually increasing their concentration with the enhancement of Mn doses. Our study demonstrated that Mn excess negatively and differently affects the physiological and biochemical features of V. corymbosum cultivars, with Legacy and Brigitta being Mn-resistant and Bluegold Mn-sensitive.

Manganese (Mn) is an essential micronutrient for plants, and is necessary for biochemical and physiological processes. The objective of this research was to determine the early responses to Mn excess and its relation to antioxidant performance mechanisms and organic acid exudation in commercial barley cultivars. We determined early responses to Mn excess in four barley cultivars (Barke, Tatoo, Scarlett, Sebastian), which were subjected to increasing Mn concentrations (2.4-150-350-750-1500 µM Mn), pH 4.8, under nutrient solution during seven days. Results showed that plant growth parameters: biomass, length and relative growth rate (RGR) were negatively altered with the higher Mn treatments. Antioxidant performance such as antioxidant activity (AA) and antioxidant enzymes such as superoxidase dismutase (SOD) were activated in presence of excess Mn. Oxalate was the major organic acid roots exudate, and the cultivar Sebastian had the highest oxalate exudation. In conclusion, Tatoo and Sebastian are proposed as the most Mn tolerant cultivars given that the biomass parameters were not affected by increasing Mn doses, showing major oxalate exudation. It is suggested that the mechanisms associated to Mn alleviation could be attributed to SOD, AA and organic acid production, mainly oxalate, in tolerant cultivars (Sebastian, Tatoo) together to significant decrease of total phenols (TP) in shoot of sensitive cultivars (Barke and Scarlett). Non-enzymatic barriers were not related to early responses, and an enzymatic barrier and oxalate exudation were considered as early indicators of Mn stress, projecting that the tolerance of Mn-tolerant cultivars could increase under field conditions.