Effects of fulvic acid addition rate on wheat growth and root morphology under low nitrogen stress

Objectives Fulvic acid (FA), the most active component of humic acid with low molecular weight, contains biostimulants that improve plant growth and stress tolerance. Here, we studied the effects of different FA addition rates on wheat root morphology under low nitrogen stress to provide practical and theoretical references for wheat nitrogen fertilizer reduction and improved efficiency.

Methods Wheat cultivar Xinhuamai 818 was used in a hydroponic experiment. We set up low N stress by adjusting the N level in the nutrient solution to 0.1 mmol/L NO₃^– (LN), and FA was added into the culture solution at the rate of 0, 30, 60, 90, and 120 mg/L, namely LN-FA0, LN-FA30, LN-FA60, LN-FA90 and LN-FA120, respectively. A normal culture solution (4 mmol/L NO₃^–) was used as the control (NN). Wheat root morphology, plant biomass, plant N concentration, and the activities of key nitrogen metabolism enzymes in the leaf were analyzed.

Results Compared with LN-FA0 treatment, LN-FA30, LN-FA60, LN-FA90, and LN-FA120 treatments increased shoot biomass by 16.31%, 23.18%, 26.75%, and 35.16%. There was no significant difference in the shoot biomass of FA120 and the control. Compared with LN-FA0 treatment, LN-FA30, LN-FA60 and LN-FA90 treatments significantly increased total root length by 35.00%–44.67%, root surface area by 39.93%–95.42%, root volume by 55.38%–144.61%, root tip number by 62.88%–90.12%, and primary root length by 26.29%–31.71%. Among the four diameter root classes, FA promoted wheat root growth of class Ⅰ–Ⅲ. Compared with LN-FA0, the four FA treatments (P<0.05) increased shoot N accumulation, root N uptake efficiency, and N distribution in shoots. Moreover, the four FA treatments significantly increased the activities of glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) in wheat leaves under low N stress. Except for the shoot biomass, the stimulant effect peaked when the FA addition rate reached 90 mg/L, and LN-FA120 offset the stimulant effects to a different extent.

Conclusions Under low nitrogen stress, FA addition promoted wheat root growth of class Ⅰ–Ⅲ and the total root length, primary root length, root surface area, root volume, and root tip numbers. FA treatment also improved root nitrogen uptake efficiency, nitrogen distribution in the shoot, and the activities of enzymes related to leaf nitrogen metabolism. All the promotion effects peaked at the FA addition rate of 90 mg/L. The FA addition rate of 120 mg/L did not show the strongest effect on the measured indexes but achieved the highest shoot biomass, which merits further research.