Effects of the Structure and Molecular Weight of Alkali–Oxygen Lignin Isolated from Rice Straw on the Growth of Maize Seedlings

The abundant and low-cost features of lignin in combination with its natural activities make it a fascinating biopolymer for valorization, especially, in agriculture as an active plant growth regulator. However, the structure–activity relationship of lignin in regulating plant growth and metabolism remains unclear. In this work, rice-straw-based low-molecular-weight (LWM, 1860 Da) and high-molecular-weight (HMW, 6840 Da) alkali–oxygen lignins are structurally and comparatively investigated to understand their effects on the growth and metabolism of maize seedlings. The results indicate that LMW lignin at 150 mg·L–1 displays early growth stimulation in maize. Under the optimal concentration of LMW lignin (25 mg·L–1), the growth of maize shoot is ∼83% higher than that of the control one. Furthermore, LMW lignin also has a positive effect on the upregulation of photosynthetic pigment, carbohydrate, and protein synthesis. In contrast, HMW lignin shows an overall inhibitory effect on the above-mentioned biochemical parameters. Based on the structural characterization, LMW lignin contains a higher syringyl/guaiacyl ratio (0.78) and carboxyl content (1.64 mmol·g–1) than HMW lignin (0.43 and 1.27 mmol·g–1, respectively), which demonstrates that methoxyl and carboxyl content of lignin may play a decisive role in seedling growth.