KEYWORDS
Rocky desertification, Akebia trifoliata , Secondary metabolites, Medicinal ingredients
1  | INTRODUCTION
Karst rocky desertification is a process of land degradation caused by human activities, which is widely distributed in Southwest China, especially in Guizhou, and has become one of the three major ecological disasters in China (Wang et al., 2010; Stas et al., 2017). With the aggravation of rocky desertification, a large amount of soil loss leads to insufficient water accumulation, which leads to the decrease of soil water holding capacity and the stability of surface soil aggregates. Therefore, the nutrient content of rocky desertification soil is significantly different from that of normal soil (Sheng et al., 2018; Wang et al., 2018; Ma et al., 2020). In recent years, many scholars have tried to use plants to ecological restoration of rocky desertification, and the effect has only been restored, and its economic value is very small. Therefore, it is very important to find the economic value of vegetation repaired in rocky desertification habitat.
Plant secondary metabolism is the result of interaction between plant and environment in the long-term evolutionary process. Secondary metabolites and related enzymes play an important role in protecting and coordinating plants to adapt to external stress environment (Theis and Lerdau 2003; Ajmal et al., 2011). Secondary metabolites are not only involved in many kinds of plant resistance processes, but also have antioxidant, anti-inflammatory and anticancer effects. They can reduce the incidence of cancer, breast cancer and colon cancer (Romagnolo and Selmin 2012; Bhatia et al., 2011; Nourimand and Mohsenzadeh 2012), For example, polyphenols are a kind of secondary metabolites produced by plant phenylpropane and flavonoid metabolic pathways. They not only participate in plant growth and development, but also give plants the ability to resist ultraviolet, antioxidant and free radicals (Dixon et al., 2005; Skerget et al., 2005). They also have functions of preventing hypertension, hyperlipidemia (Prasain et al., 2010), diabetes (Rodrigo et al., 2011) and anticancer (Araùjo et al., 2011). Flavonoids are natural antioxidants that can directly scavenge oxygen free radicals and reactive oxygen species (Terao 2009), but also prevent osteoporosis and reduce the risk of colon cancer, prostate cancer and breast cancer (Michihara et al., 2012; Priya and Sharma 2013). Tannin is a good free radical scavenger and lipid peroxidation inhibitor (Dicko et al., 2005), with good physiological functions such as astringency, anti diarrhea, antibacterial, antioxidant and antiviral (Frasca et al., 2012). Therefore, the potential economic value of secondary metabolites in plants is very great. The synthesis of these substances is regulated by the phenylpropane pathway in plants, and the main enzymes regulated by it include PAL (Phenylalanine ammonialyase), C4H (Cinnamate-4-Hydroxylase) and 4CL (4-Coumarate: Coenzyme A Ligase) (Hahlbrock and Scheel 2003; Singh et al., 2009).
Akebia trifoliata is one of the economic species selected to control rocky desertification habitat in recent years, and has achieved good results. Its fruit is not only edible, but also can be used to make oil (Jiang et al., 2020). At the same time, it has the functions of promoting blood circulation, anti-inflammatory and diuretic (Jiang et al., 2012). Its extract can significantly inhibit the survival and proliferation of liver cancer cells (Lu et al., 2019). The main medicinal components of A. trifoliata are oleanolic acid, α-hederagenins and some secondary metabolites, such as polyphenols, tannins and flavonoids, which affect the synthesis of A. trifoliata . For example, drought can increase the content of artemisinin in Artemisia carvifolia (Yadav et al., 2014), saikosaponin a and saikosaponin b in Bupleurum chinense will increase under moderate water stress (Zhu et al., 2009), and Himalayan vegetation will secrete a lot of polyphenols and alkaloids due to long-term exposure to natural factors such as ultraviolet, drought and strong wind (Bhatia et al., 2011). However, these changes are different in different plants and different stress intensities (Zahir et al., 2014). But so far, there have been no reports on the effects of rocky desertification habitat on the content of secondary metabolites and related enzymes of A. trifoliata .
In this study, we compared the changes in the secondary metabolites and related enzymes of A. trifoliata in rocky desertification habitats and normal habitats, and attempted to clarify the response mechanism of the secondary metabolites and related enzymes of A. trifoliata in rocky desertification habitats. Meantime, it also provides a theoretical basis for the development of A. trifoliatamedicinal resources.
2  | MATERIALS AND METHODS
2.1  | Experimental site
Ludi village, Shiban Town, Huaxi District, Guiyang City, Guizhou Province China (Huaxi base) and Zhongzhai village, Huajiang Town, Guanling County, Anshun City (Guanling base) were used as experimental sites (one of them was used as a repetition). The annual rainfall is mainly concentrated in April to October. One rocky desertification habitat (> 1000 m2) is selected in each area, and one normal habitat (non rocky desertification area) is selected as the control (> 1000 m2). The geographic and meteorological data of each base were from the archives of Guizhou Meteorological Bureau (Table 1). It can be seen from table 1 that Huaxi base and Guanling base had similar environmental factors, which were the two repeated treatments of the experiment. This study focuses on the effects of rocky desertification habitat on secondary metabolites of A. trifoliata , and the different performance of plants came from the effects of rocky desertification habitat rather than the different conditions of different bases.
TABLE 1 The geographical and meteorological data of different bases