Zinc, as an essential trace element for plant growth and development, plays an important role in plant physiology and secondary metabolism. Appropriate amount of zinc can promote the growth of plants and enhance their stress resistance. As a special heavy metal ion, zinc can also regulate the secondary metabolism of plants. However, excessive zinc can also affect the normal growth and development of plants, and even cause plant death. Dendrobium nobile is a traditional precious Medicinal Dendrobium, which is one of the Medicinal Dendrobium species clearly recorded in Pharmacopoeia of the people’s Republic of China. Dendrobium nobile is mainly distributed in Hejiang county and Mabian County of Sichuan Province and Chishui area of Guizhou Province. However, the contents of As, Cu, Zn, Cd, Pb and Cr in the soils of these areas are relatively high. The highest content of Zn is 320.58mg/g, which is 4.32 times of the average soil content of zinc in China( Zhang et al., 2020.). The growth and secondary metabolites of Dendrobium nobile will be affected by the soil with high zinc content.

Plant secondary metabolism is a stress response process to the stress environment ( Li et al.,2020), and the secondary metabolites produced in this process are necessary substances for plants to resist the stress environment. Studies have shown that the stress of external environment can induce the increase of plant secondary metabolites (Gao et al.,2020) . It is one of the most popular methods to induce plant secondary metabolites by elicitors. As one of the common elicitors, many studies have pointed out that the suitable concentration of heavy metals can effectively induce the biosynthesis of plant secondary metabolites, and excessive concentration will damage the plant organism (Berni et al., 2019) . Some studies show that vinblastine content in catharanthus roseus increases with the increase of exogenous zinc concentration, and the maximum zinc concentration is 150 μmol/L (Ling et al.,2015) . Therefore, it can be assumed that zinc, as a heavy metal and essential trace element, has an effect on the secondary metabolites of plants to a certain extent, but with the increase of dose, it may show inhibition effect. The specific mechanism of the effect needs further study.

Some plant polysaccharides can be combined with Zn²⁺to become more stable substances, so as to realize the transformation of inorganic zinc into organic zinc, which is more easily absorbed by the human body. Moreover, the polysaccharide with Zn²⁺often has higher biological activity, so polysaccharide-zinc has become a research hotspot in recent years. For example, the MFP4P polysaccharide isolated from Mulberry by Wang et al(Wang et al.,2019) has good antioxidant activity and hypoglycemic effect. MFP4P and zinc complex (MFP4P-Zn) have higher antioxidant and hypoglycemic activities than MFP4P at the same concentration. Polysaccharide (P1)-zinc complex (P1-Zn) from Prunella vulgaris could effectively inhibit the proliferation (98.4% inhibition rate at 500 μg/mL) of HepG2 cells through induction of apoptosis, evidenced by morphological changes, chromatin condensation and G0/G1 phase cell cycle arrest. (Li et al., 2016). A polysaccharide-zinc (FUP-Zn) complex from Fritillaria ussuriensis exhibited scavenging ability on hydroxyl radicals and superoxide anion radicals. And the effect of polysaccharide-zinc is heavily dependent on the amount of Zn(Zhang et al.,2020). Polysaccharide chelates of zinc were also found in Pholiota nameko (Lan et al.,2015), Dictyophora indusiate (Liao et al.,2015) and Lentinus edodes (Wang,et al.,2015), which showed strong biological activity in vitro and in vivo.

Recent research shows that Dendrobium nobile has many functions such as anti-aging (Nie et al.,2020), antioxidant activity(Luo et al., 2010), antitumor activity(Wang et al., 2010), immunoregulation(Fan et al.,2020). In the secondary metabolites of Dendrobium nobile, polysaccharides, flavone, alkaloid and polyphenols are abundant, which have high biological activity and important medicinal value. While how does zinc affect the physiological and medicinal composition of Dendrobium nobile？and whether it can chelate with Dendrobium polysaccharide to form zinc polysaccharide products has not been studied. Therefore, this study revealed the effects of exogenous zinc on physiological, medicinal components of Dendrobium nobile and the combination of zinc and polysaccharides.