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

Abstract

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.