NO 3 − /NH 4 + ratios affect nutritional homeostasis and production of Tanzania guinea grass under Cu toxicity

Abstract

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.