Nitrate fertilization enhances manganese phytoextraction in Tanzania guinea grass: a novel hyperaccumulator plant?

Manganese (Mn) is essential for plants but very toxic at high rates. However, hyperaccumulators can tolerate high Mn concentrations in plant tissue, especially when properly fertilized with N. Tanzania guinea grass (Megathyrsus maximus Jacq.) has been indicated as metal tolerant and a good candidate for Mn phytoextraction due to its fast growth and high biomass. The objective was to evaluate the Mn hyperaccumulator potential of Tanzania guinea grass grown as affected by proportions of nitrate/ammonium (NO3-/NH4+). An experiment in a growth chamber with nutrient solution, combining NO3-/NH4+ proportions (100/0 and 70/30) and Mn rates (10, 500, 1500, and 3000 µmol L-1), was carried out. The highest Mn concentration was verified in plants grown with 100/0 NO3-/NH4+ and Mn at 3000 µmol L-1, reaching up to 5500 and 21,187 mg kg-1 in shoots and roots, respectively, an overall concentration of 13,345 mg kg-1. These numbers are typically seen in hyperaccumulators. At that combination, Mn accumulation in shoots was also the highest, reaching up to 76.2 mg per pot, a phytoextraction rate of 23.1%. Excess Mn increased both H2O2 concentration in roots and non-photochemical quenching and therefore decreased net photosynthesis, stomatal conductance, electron transport rate, and photochemical quenching. Nevertheless, proline concentration in roots affected by excess Mn was high and indicates its important role for mitigating stress since Mn rates did not even affect the dry biomass. Tanzania guinea grass is highly tolerant to excess Mn as much as a hyperaccumulator. However, to show all its potential, the grass needs to be supplied with N as NO3-. We indicate Tanzania guinea grass as a Mn hyperaccumulator plant.

NO3-/NH4+ ratios affect nutritional homeostasis and production of Tanzania guinea grass under Cu toxicity.

Nitrogen (N) can alleviate metal toxicity. However, as of yet, there have been no studies showing the efficacy of NO3-/NH4+ in mitigating Cu toxicity. The objective of this study was to evaluate the Cu toxicity on the nutritional and productive attributes of Panicum maximum cv. Tanzania as well as the role of NO3- and NH4+ ratios in nutritional homeostasis. The experiment was conducted using 3 × 4 factorial treatments arranged in a randomized complete block design with three replicates. The treatments were three NO3-/NH4+ ratios (100/0, 70/30, and 50/50) and four Cu rates (0.3, 250, 500, and 1000 µmol L-1) in nutrient solution. Copper concentrations in the diagnostic leaves (DL) were highest in plants grown under 70/30 NO3-/NH4+ ratios and a Cu rate of 1000 µmol L-1. In this combination, it was observed that DL had higher concentrations of NH4+, greater glutamine synthetase activity, lower chlorophyll concentration (SPAD value), and lower shoot dry mass, suggesting high disorders of nutritional homeostasis. Plants receiving N in the form of NO3- and 1000 Cu µmol L-1 showed that DL had lower concentrations of Cu, higher concentration of chlorophyll, higher NO3- concentration, higher nitrate reductase activity, and higher NO3- accumulation in the roots, suggesting a reduction in disorders of nutritional homeostasis. The disorders on mineral uptake, N assimilation, and biomass production caused by Cu toxicity are shown to be affected by NO3-/NH4+ ratios, and N supply via NO3- allowed for better homeostasis of the forage grass.

NO3-/NH4+ proportions affect cadmium bioaccumulation and tolerance of tomato.

With the growth of the world population, cadmium (Cd) concentration in the environment has increased considerably as a result of human activities such as foundry, battery disposal, mining, application of fertilizers containing toxic elements as impurities, and disposal of metal-containing waste. Higher plants uptake N as ammonium (NH4+), nitrate (NO3- ), and many other water-soluble compounds such as urea and amino acids, and nourishing plants with N, providing part of it as NH4+, is an interesting alternative to the supply of this nutrient in the exclusive form of NO3- under Cd toxicity. The objective was to evaluate the influence of NO3- /NH4+ proportions on the development and tolerance of tomato plants grown under the presence of Cd in the culture medium. The experiment was conducted in a completely randomized block design in a 3 × 3 factorial arrangement consisting of three Cd rates (0, 50, and 100 µmol L-1) and three NO3-/NH4+ proportions (100/0, 70/30, and 50/50) in the nutrient solution. To this end, we quantified the responses of the antioxidant enzymatic system and productive and functional changes in Solanum lycopersicum var. esculentum (Calabash Rouge). Shoot biomass production decreased with the maximum Cd rate (100 µmol L-1) tested in the growth medium, whereas the NO3- /NH4+ proportions and other Cd rates did not significantly influence this variable. The lowest SPAD values were observed at the 100/0 NO3- /NH4+ proportion and in plants exposed to Cd. The largest accumulation of the metal occurred in the shoots at the NO3- /NH4+ proportion of 70/30 and at 100 µmol L-1 Cd and in the roots at 100/0 NO3-/NH4+ and with 50 and 100 µmol L-1 Cd. The concentration and accumulation of NO3- were highest at the NO3-/NH4+ proportion of 100/0 in the shoots and at 50/50 NO3-/NH4+ in the roots, whereas for NH4+, values were higher as the proportion of N supplied in the form of NH4+ was increased. The nitrate reductase enzyme activity decreased with the Cd supply in the nutrient solution. The antioxidant system enzymes were activated as we increased the NO3-/NH4+ proportion and/or Cd rates added to the nutrient solution in both shoots and roots of the tomato plant, except for ascorbate peroxidase. Based on the results obtained, if the plant is to be used as a food source as is the case of tomato, the 100/0 NO3-/NH4+ proportion is the better alternative because it resulted in higher Cd accumulation in the root system over the translocation to the shoots and consequently to the fruit.