Competitive Uptake Dynamics of Iron and Nickel in Rice Plants (Oryza sativa L.)

Nickel (Ni) is an emerging contaminant in paddy soils, but its uptake by rice and competitive interactions with other elements like iron (Fe) have been largely overlooked. Rice plants (Selenium variety) were grown hydroponically under controlled conditions. After 44 days, plants were exposed for 7 days to modified Yoshida solutions with varying Ni:Fe ratios, fixing Ni concentration at 5 or 20 uM, and Fe concentration from 0 to 100 uM. The plant biomass was measured, and the metal accumulation analyzed via ICP-MS. Additionally, plants grown for two weeks at Ni:Fe ratios of 10:1 and 1:2 in agar were examined for Ni and Fe distribution using uXRF. The Ni concentration did not affect the dry biomass of rice plants. At low Ni levels, biomass remained unaffected by Ni:Fe variations, though a rising trend with Fe concentration was noted. However, at 50 uM Ni, a significant biomass upward trend emerged with increasing Fe. Ni and Fe accumulation was higher in roots than in shoots, especially at high Ni concentrations. The decline in root Ni levels at 5 uM Ni with increasing Fe suggests Fe inhibits Ni uptake, likely due to competition at Fe transporters (IRT), which have a higher Fe affinity. The stable Fe concentration in roots indicates a steady state, where plants maintain adequate Fe levels without further IRT induction. At high Ni concentrations, root Ni decreased, while Fe significantly increased, reinforcing Fe-Ni competition and suggesting plants enhance Fe uptake to counter Ni toxicity. The decrease in root Ni concentration at 5 uM Ni with increasing Fe also led to a progressive Ni reduction in shoot. However, at 50 uM Ni, shoot Ni levels were significantly higher than at 5 uM and remained largely unaffected by increasing Fe, except for a slight decrease at 100 uM Fe. This suggests that at high Ni concentrations, plants maintain a stable Ni accumulation in shoots, while increased Fe may be required for antioxidant enzyme synthesis to counteract Ni toxicity. The rise in Fe under high Ni could also indicate distinct translocation mechanisms for these metals. The Ni:Fe ratio decreased significantly in plants at both Ni concentrations. At low Ni, the decline is due to reduced Ni accumulation, while at high Ni, it results from increased Fe uptake. uXRF analysis confirms that Ni accumulation in roots is affected by Fe, whereas Fe accumulation in roots is not apparently affected by high Ni.