Allelopathic Activity of Ginseng-Cultivated Soil: Extracts on Seed Germination and Growth of Five Vegetables in China

Allelopathy means that one plant produces chemical substances to affect the growth of other plants. Crop rotation is considered as a potential strategy to alleviate the allelopathic inhibition. So, it is important to identify rotation crops with wide availability and low inhibitory effects. In this study, the allelopathic potential of soil extracts was investigated on the germination, seedling growth, biomass, and biochemical parameters (malondialdehyde, photosynthetic pigments, and antioxidant enzyme activities) of five crops, by a series of laboratory experiments. Firstly, both soil water extracts (SWE) and soil ethanol extracts (SEE) exhibited allelopathic inhibition on the seed germination and the root length of all seedlings in a dose-dependent relationship. The SWE significantly promoted the shoot length of bok choy and Chinese lettuce, while the SEE had no significant effect in bok choy. The application of SEE resulted in a significant increase in the dry weight of bok choy and rocket. In contrast, SWE had a negligible effect on bok choy and lettuce. Both of them caused decrease in the dry weight of the other seedlings. Then, the allelopathic synthetic effect index of water/ethanol extracts was chemo-inhibitory, and the inhibitory effect increased with increasing extract concentration. The SWE had the strongest inhibition on rocket and the SEE on lettuce. Both of them had the weakest effect on bok choy. The extracts significantly inhibited the photosynthetic capacity in five crops, manifested as decrease in photosynthetic pigments and dose-dependent effects. The malondialdehyde (MDA) content in all crops increased in a dose-dependent manner, confirming that the extracts caused lipid peroxidation. However, the defense strategies of different crops vary significantly. There is crop with active defense, such as bok choy treated with SWE. It delayed oxidative damage by continuously upregulating the activities of superoxide dismutase (SOD) and catalase (CAT). This is the key physiological mechanism for tolerance. There is also the oxidative stress failure type, as follows: CAT activity of rocket and cabbage increased, but the SOD activity did not increase by SEE. This reveals the physiological essence of their sensitivity-the lack of persistent scavenging ability for reactive oxygen species. Based on the inhibition of peroxidase (POD) and ascorbic acid peroxidase (APX), it is speculated that the extracts may inhibit the hydrogen peroxide scavenging pathway, which centered on the ascorbate-glutathione cycle. It is the fundamental reason why the continuous accumulation of MDA though SOD/CAT is up. This study confirmed the allelopathic effects of the water and ethanol extracts on five vegetable crops, and found that bok choy was less affected by them. The soil extracts affected the growth and development of seedlings by regulating their oxidative metabolism and photosynthetic capacity. These results support recommending pak choi as a rotation crop. This provides crops for subsequent field experiments and a new direction for next-step research on continuous cropping obstacles.