Abstract: We examined the effects of humic acid (HA) and D-arginine (D-Arg) on root growth of foxtail millet seedlings of cultivars Jingu 21 and Zhangza 10 under drought stress. There were four treatments, including control (CK), drought stress (Dr), humic acid sprayed on the leaves under drought stress (Dr + HA), and humic acid and D-arginine sprayed on the leaves under drought stress (Dr + HA + D-Arg). We measured root growth, endogenous arginine (Arg) content, and arginine decarboxylase (ADC) activity of millet seedlings. The results showed that humic acid significantly increased the total root length, total root surface area, average root diameter, total root volume and root biomass under drought stress. Root dry biomass of Jingu 21 and Zhangza 10 was 40.41% and 55.57% higher than that of the Dr treatment, respectively (P<0.05). Under the treatment of Dr + HA, root activity, ADC activity, and endogenous Arg content were 18.58%, 22.43%, 31.71% (for Jingu 21), and 54.77%, 18.71%, 37.89% (for Zhangza 10) higher than those of the Dr treatment, respectively (P<0.05). Humic acid significantly improved root activity, promoted the synthesis of endogenous Arg, and enhanced ADC activity, thereby promoting root growth and alleviating the damage caused by drought stress on root systems. The effect of D-Arg was opposite to that of humic acid, suggesting that humic acid could improve the drought resistance of millet by promoting the synthesis of polyamines. There were significant positive correlations among root morphological indices and between ADC activity and endogenous Arg content under humic acid treatment (P<0.01). This study provides a new way to analyze the action mechanism of humic acid, which plays an important role in the application of humic acid in millet production and the development of green agriculture.

Key words: humic acid, D-arginine, foxtail millet, root, drought resistance