Effects of Planting Density—Potassium Interaction on the Coordination among the Lignin Synthesis, Stem Lodging Resistance, and Grain Yield in Oil Flax

To clarify the effects of planting density (PD) and potassium (K) application on the lignin synthesis of oil flax stalks and their relationship with lodging resistance, a two-year field experiment was conducted in the 2020 and 2021 growing seasons. The planting densities were 600 grains·m⁻² (D1), 750 grains·m⁻² (D2), and 900 grains·m⁻² (D3); the potassium application levels were 0 kg K·hm⁻² (K0), 60 kg K·hm⁻² (K1,) and 90 kg K·hm⁻² (K2). Then, the effects of PD and K on the stalk agronomic traits, lignin synthesis, lodging resistance, and grain yield (GY) of oil flax were studied. The results show that D3 increased the oil flax plant height and stem fresh weight, and K1 raised the height of the center of gravity. The enzyme activities of phenylalanine aminolyase (PAL), cinnamyl alcohol dehydrogenase (CAD), and peroxidase (POD) increased with an increase in planting density, whereby K1 promoted and K2 inhibited the lignin metabolic enzymes, respectively. The lignin and cellulose were significantly positively correlated with PAL. The combination of K with the D2 treatment increased the lignin and cellulose content in stems of oil flax at the kernel and maturity stages, and the effect of K1 on lignin metabolism and lodging resistance was better than that of K2. The increased GY was significantly correlated with the number of effective oil flax capsules per plant and the lodging resistance index, which were directly affected by K application. In conclusion, under the same ecological conditions as the experiment, the field management strategy of medium PD (750 grains·m⁻²) and low K (60 kg K·hm⁻²) was the best for improving the lodging resistance and GY formation of oil flax.