Zinc is a component of many structural substances in plants, and also an activator of many enzymes, which plays an important role in plant photosynthesis. In this study, it was found that the photosynthesis of leaves could be affected by the application of zinc. With the increase of exogenous zinc concentration, the photosynthesis of Dendrobium nobile was promoted by zinc concentration below 400μmol/L. The net photosynthetic rate and stomatal conductance, and chlorophyll a and b reached the maximum when the concentration of zinc was 400 μmol/L. The water use efficiency of leaves decreased gradually with the increase of concentration and treatment time. The results also showed that proper concentration of zinc could help Dendrobium nobile produce chlorophyll and promote plant photosynthesis. It may be that exogenous application of zinc increased the zinc content in chloroplasts, inhibited the activity of RNA hydrolase, promoted the production of RNA, and then promoted the synthesis of chloroplast protein, increased the content of chlorophyll a and chlorophyll b. The increase of photosynthesis increased the synthesis of soluble sugar in Dendrobium. The results showed that the soluble sugar content reached the maximum at the concentration of zinc with 400 μmol/L on the 21st day. Polysaccharide is a kind of complex and huge carbohydrate which is formed by condensation and dehydration of many soluble sugar (monosaccharide) molecules. Therefore, soluble monosaccharide molecules are the raw materials for the biosynthesis of polysaccharides, and the concentration of 400 μmol/L zinc can promote the photosynthesis of Dendrobium nobile and increase the synthesis of soluble monosaccharide, which providing the necessary raw materials for the synthesis of Dendrobium nobile polysaccharides.

When plants are in adversity environment, hydrogen peroxide and free radical ions will produce toxicity to plants. However, plants have resistance mechanism to adversity. In order to avoid injury, plants can eliminate the damage caused by reactive oxygen species by adjusting themselves, such as improving the activity of antioxidant enzymes and increasing the content of antioxidant substances. This study showed that when exogenous zinc was applied to Dendrobium nobile, the production rate of hydrogen peroxide and superoxide radicals in the leaves of Dendrobium nobile increased gradually with the increase of zinc treatment concentration, indicating that exogenous zinc had different degrees of stress on Dendrobium nobile, and high zinc stress was the most obvious.

SOD is an important enzyme for scavenging reactive oxygen species in plants. It is responsible for transforming superoxide anion radicals into hydrogen peroxide. In this study, SOD activity increased first and then decreased with the increase of zinc concentration, indicating thatDendrobium nobile can resist external stress through self-regulation within a certain concentration range. But when zinc concentration was more than 1000 μmol/L, SOD activity was lower than that of control.

Ascorbic acid (AsA) and glutathione (GSH), as important non-enzymatic antioxidants in plants, can directly act and scavenge reactive oxygen species. GSH increased in response to biological stresses such as reactive oxygen species accumulation. Our study shows that with the increase of zinc concentration, GSH in Dendrobium nobileincreases first and then decreases. The results showed that high concentration of zinc increased the oxidative stress of Dendrobium nobile . AsA can scavenge many kinds of reactive oxygen radicals such as hydrogen peroxide directly or indirectly through the AsA-GSH cycle. After applying exogenous zinc, AsA first increases and then decreases with increasing concentration. APX is an important antioxidant enzyme in plants. Its activity determines the role of AsA-GSH cycle in stress resistance physiology. The results show that APX activity increases first and then decreases with the increase of zinc concentration. Therefore, the activities of antioxidant enzymes were also destroyed when zinc concentration was too high.

In conclusion, low concentration of zinc can improve the stress resistance of Dendrobium nobile , but high concentration of zinc can cause the degree of peroxidation of Dendrobium nobile , and even affect its normal growth.

Polyphenols and flavonoids are also main active ingredients in Dendrobium nobile . The results showed that zinc had a low promoting and high inhibiting effect on the polyphenols and flavonoids contents of Dendrobium nobile. This may be because zinc, as a heavy metal ion, is a kind of inducer, which can induce and stimulate plants to produce secondary metabolites, which is similar to the research of Jin (Jin et al., 2015), which found that exogenous zinc can promote the production of vinblastine in Changchun flower. The results showed that after the application of zinc in medicinal plants, it was beneficial to the accumulation of medicinal components and increase the medicinal value of Dendrobium nobile.