Excess copper inhibits the growth of rice seedlings by decreasing uptake of nitrate.

Copper (Cu) toxicity has a deleterious effect on plant growth, and different nitrogen (N) forms have significantly different impacts on the uptake and accumulation of heavy metals by plants. However, it remains unclear how excess Cu inhibits the growth of rice seedlings under different N forms. Here, we examined the mechanism of Cu toxicity inhibiting the growth of rice supplied with different N forms. Rice seedlings were grown in a nutrient solution with 0.81 mmol L N, as ammonium (NH), nitrate (NO) and NH + NO, or without N (0 N) in the presence of 0.2 μmol L CuSO or 10 μmol L CuSO. The inhibition of shoot growth under excess Cu was more pronounced in plants that were supplied with NO than NH; such inhibition was not induced by higher Cu concentration in shoots. Applied with NO alone increased solution pH value up to 6.2, but supplied with NH alone and NH + NO decreased solution pH value to 4.0 and 4.2, respectively. The increment of solution pH reduced Cu concentration in shoots of rice supplied with NO alone. Copper toxicity decreased NO concentrations in rice seedlings that were supplied with NO alone but increased the NH concentrations in plants that were supplied with NH or NH + NO. High Cu levels reduced the uptake of NO in roots by the analysis of net NO flux and NO assimilation enzymes activity. Under excess Cu, the transcript levels of OsNPF6.5, OsNPF2.2 and OsNPF2.4 genes were suppressed, while OsNRT2.1, OsNRT2.2 and OsNAR2.1 were raised in roots. In conclusion, Cu toxicity inhibits NO uptake and upward translocation by modulating the expression level of NO transporter genes. The reduction in the concentrations of NO and total N decreased shoot growth of rice seedlings when N was supplied as NO. Hence, rice seedlings supplied with NO had lower shoot biomass than those with NH under Cu stress.