Effects of nitrogen forms on Cd uptake and tolerance in wheat seedlings

Hydroponic experiment was conducted to explore the effects of two nitrogen (N) levels with five nitrate nitrogen (NO₃^--N) and ammonium nitrogen (NH₄⁺-N) ratios on the growth status and Cd migration patterns of wheat seedlings under Cd5 and Cd30 level. Results showed that higher Cd were detrimental to the growth, absorption of K and Ca, expression of genes mediating NO₃^--N and NH₄⁺-N transport, which also increased the content of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) in shoots and roots of wheat seedlings. Higher N treatment alleviated the inhibitory effects of Cd stress on the biomass, root development, photosynthesis and increased the tolerance index of wheat seedlings. The ratio of NO₃^--N and NH₄⁺-N was the main factor driving Cd accumulation in wheat seedlings, the combined application of NH₄⁺-N and NO₃^--N was more conducive for the growth, nitrogen assimilation and Cd tolerance to the Cd stressed wheat seedlings. Increased NO₃^--N application rates significantly up-regulated the expression levels of TaNPF2.12, TaNRT2.2, increased NH₄⁺-N application rates significantly up-regulated the expression levels of TaAMT1.1. The high proportion of NO₃^--N promoted the absorption of K, Ca and Cd in the shoots and roots of wheat seedlings, while NH₄⁺-N was the opposite. Under low Cd conditions, the NO₃^--N to NH₄⁺-N ratio of 1:1 was more conducive to the growth of wheat seedlings, under high Cd stress, the optimal of NO₃^--N to NH₄⁺-N was 1:2 for inhibiting the accumulation of Cd in wheat seedlings. The results indicated that increasing NH₄⁺-N ratio appropriately could inhibit wheat Cd uptake by increasing NH₄⁺, K⁺ and Ca²⁺ for K and Ca channels, and promote wheat growth by promoting N assimilation process.