Objective To study the effects of understorey intercropping and different planting densities on patchouli growth, physiological traits and herb quality, and to provide a theoretical basis for optimising ecological planting of patchouli.

Method In this experiment, a patchouli-macadamia set-planting model was established, and a monoculture control group (CK: 4 plants/m², row spacing 50 cm×50 cm), with an understorey set-planting test group. In the set-planting experimental group, gradient density experiments were designed: XT1 (4 plants/m², row spacing 50 cm×50 cm), XT2 (5 plants/m², row spacing 50 cm×50 cm, 'zigzag' planting), and XT3 (6.67 plants/m², row spacing 30 cm×50 cm). Fresh weight, dry weight, stem thickness, plant height and other growth indices of patchouli transplanted and grown for 180 d were determined; superoxide dismutase (SOD) activity, peroxidase (POD) activity, catalase (CAT) activity and malondialdehyde (MDA) content were measured; patchouli alcohol content was determined by GC-MS, patchouli ketone content was determined by HPLC and volatile oil yield was determined by steam distillation. Volatile oil yield was determined by hydrodistillation.

Result Compared to CK, the above-ground fresh weight, dry weight and number of primary branches of patchouli increased significantly by 38.03%, 23.96%, and 58.82% in XT1, XT2, and XT3, respectively. With the increase in planting density, the fresh weight and dry weight of patchouli showed a tendency to decrease and then increase. The increase in both understorey planting and planting density significantly increased CAT activity and SOD activity, while MDA content significantly decreased by 64.35%, 76.97%, and 76.47%, respectively. Understorey planting had no significant effect on patchouli alcohol content, but patchouli ketone content was significantly increased by 18.56%. Compared to XT1, patchouli ketone content and patchouli volatile oil yield were significantly reduced by XT2 and XT3 treatments. Correlation analysis showed that the fresh and dry weight of individual patchouli plants were positively correlated with the number of primary branches; patchouli alcohol content was positively correlated with plant height and stem thickness; and patchouli ketone content was positively correlated with stem thickness. Principal component analysis showed that the understorey planting pattern scores of patchouli and macadamia nuts were in the following order XT1 > CK > XT2 > XT3.

Conclusion Understorey planting and different planting density have an important influence on the growth, physiological characteristics and herb quality of patchouli. By planting macadamia nuts and maintaining a planting density of 4 plants/m², row spacing 50 cm × 50 cm, high quality and high yielding patchouli can be obtained.