Effects of different nitrogen forms on nutrient uptake and distribution of Cunninghamia lanceolata plantlets under drought stress

Objectives Drought stress is one of important abiotic factors limiting plant growth, adding nitrogen exerts important alleviating effects on improving plant drought resistance. This study was aimed to investigate the effects of different nitrogen sources on nutrient uptake and distribution of Cunninghamia lanceolata plantlets under drought stress.

Methods A hydroponic culture experiment was conducted with two superior Chinese fir clones (No. 7–14 and No. 8–8) as study materials. The drought stress was made by adding 10% (w/v) PEG-6000 in the nutrient solution. Three nitrogen forms (NO₃^–-N, NH₄⁺-N and the combination of both) were setup in the same concentration of 4.571 mmol/L. Nutrient content and biomass of root, stem and leaves were measured at 20 days since the beginning of culture.

Results 1) The clones, drought stress and N forms had significant effects on root N, P and K contents and leaf P content. Compared to normal water supplying, adding NH₄⁺-N enhanced total N and K uptake in leaves and total P and K uptake in stems and leaves; adding both N sources promoted total K absorption in roots under drought stress. Adding NH₄⁺-N increased Ca uptake in roots and stems under drought stress, but the Ca uptake in leaves was not changed. Drought stress had significant effects on Fe, Mn, Cu, and Zn uptake in roots, and N forms had significant effects on Cu, Zn uptake in leaves. All the three N source treatments showed inhibitory effect on the Mg, Fe, Mn and Cu uptake in organs, but NH₄⁺-N showed less inhibition than NO₃^–-N did. Compared to normal control, all the three N source treatments reduced Zn uptake in roots but increased those in stems and leaves, implying that N supply could adjust the distribution of Zn in organs and alleviate the occurrence of Zn deficiency under drought stress. The nutrient uptake varied significantly among organs. Total N and P uptake was in the order of leaf > root > stem, K and Ca uptake in the order of leaf > stem > root, Fe and Cu uptake in the order of root > leaf > stem, Mg, Mn and Zn uptake was not in consistent orders. NH₄⁺-N uptake was in the order of leaf > root > stem, and it was significantly higher than NO₃^–-N uptake in all organs, which indicated that Chinese fir tended to absorb NH₄⁺-N.

Conclusions Under drought stress, nitrogen supplying sources affect nutrient uptake and distribution of Chinese fir plantlets, but the effect varied among nutrients and clones. NH₄⁺-N performs better than NO₃^–-N in improving nutrient uptake for Chinese fir plantlets under drought stress, so Chinese fir is thought prefer NH₄⁺-N nutrition.