In our experiments, SA was spayed on the plants of the Wuyi Rock cultivar and its effect on the resistance of the tea cultivar to tea red root rot was studied. Furthermore, the influence of SA spraying on tea quality was evaluated.

Morphological indices of tea seedlings treated with root rot pathogen and SA

Morphological indices of tea plants were determined under different treatments. In the GP+ group (no spraying with SA, only fungal inoculation G. philippine), 76 out of 80 tea plants inoculated showed red root rot phenotype, with a disease index of 95% (Fig. 1A). However, pretreated with SA for 24h, tea plants showed significantly enhanced resistance to root rot (the disease index was 15%) and only 12 /80 inoculated tea seedlings showed red root-rot symptoms (Fig. 1A). These results are consistent with results published earlier (Li et al. 2017).

In the production process of Wuyi Rock tea, the new branches with "two leaves and one bud " are usually collected as raw materials for tea production. Therefore, we determined the number of new branches with " two leaves and one bud " on the shoots of tea plants under different treatments.

As shown in Fig. 1B, the lowest number of new branches with “two leaves and one bud” was observed in the tea plants treated with G. philippine, which showed highest disease incidence. Compared with untreated control (72 new branches with “two leaves and one bud” per 10 plants), the new branch number in the tea plants treated with G. philippine was only 8 per 10 plants. The reduction of new branches in the plants treated with G. philippine indicated that the growth of new leaves was inhibited by the pathogenic fungi.

Number of new branches in the SA treatment group was not significantly higher (78 new branches with “two leaves and one bud” per 10 plants) compared with the control group (72 new branches with “two leaves and one bud” per 10 plants). However, in the SA+ GP group, the number of new branches with “two leaves and one bud” per 10 plants reached 70, which was almost similar to that in the CK group. These results suggested that SA spraying could offset the negative effect of the pathogenic fungus on plant growth.

Further, we analyzed the fresh weight of new branches under different treatments. As shown in Fig. 1C, the fresh weight of new branches in the plants treated with G. philippine was the lowest. However, the pretreatment with SA could significantly increase the fresh weight of new branches in the tea plants inoculated with G. philippine, showing that spraying with SA could increase the production of tea leaves. These results are consistent with the findings of the new branch number.

Effect of SA on defense enzyme activities

From the studies above, we found that SA played a significant role in improving the resistance and production of the tea plant. Previous studies have shown that SA acts as an endogenous signal, which then enhance the plant disease resistance by inducing enzymes related to resistance to the pathogen (Chandrasekhar et al. 2017; Li et al. 2017). To verify this the activities of some enzymes, which have been reported to play essential roles in protecting plants from attacks, were determined.

PPOs have been reported to play an important role in protecting plants from pathogen attacks (Zhu et al. 2018; Teng et al. 2017). As shown in Fig. 2A, the activity of PPOs in tea leaves that had been treated by GP infestation was enhanced compared with the control. Under the treatment of SA plus GP, the activity of PPOs was much higher than that in the GP group, suggesting that SA improves the resistance of tea plants to GP attack by enhanced activity of PPOs.

LOX, is a key enzyme in the metabolic pathway of plant octadecanoic acid. The products of LOX play important roles in the growth and development of plants and the responses of plants to environmental stress (Hu et al. 2013; Park et al. 2010), Fig. 2B shows that the GP inoculation resulted in a slightly higher LOX activity than that in control. However, the LOX activity was marginally higher in the SA plus GP group. These results indicate that the pretreatment with SA improves the LOX activity, and this contributing to the enhanced resistance of tea plants to GP.

CAD, a critical enzyme in the secondary metabolism of plants, especially in the biosynthesis of lignin, has a close relationship with plant development and resistance to pathogen invasion (Li et al. 2017). As shown in Fig. 2C, CAD activity was higher in plants after being treated with GP. However, CAD activity in SA plus GP sample were down-regulated compared to that in the GP group, suggesting that the enhanced resistance of tea to GP by SA pretreatment is not through the lignin pathway. Besides, in the SA treatment, the CAD activity was down-regulated compared to control. This explains why the pretreatment of SA reduced the CAD activity in tea plants incubated with GP.

Effect on Tea quality

To check if SA treatment has any effect on tea quality while it enhances the resistance to GP, we evaluated the main ingredients of tea under different treatments.

Flavonoid is another component of tea polyphenol. The contents of flavonoid in SA+, GP+, and SA+GP were higher than that in an untreated sample (Fig. 3A), which showed similarity with the results to the content of tea polyphenols.

Tea polyphenols, the main tea quality parameter, are a mixture of several phenolic derivatives and determine the taste and color of prepared tea. Fig. 3B, shows that the contents of polyphenols showed no statistical difference between GP and SA+GP treatments, but both of them were significantly higher than in untreated tea plants. This suggests that the GP attack could induce the production of polyphenols, although it had negative effects on tea yield. Pretreatment with SA could improve the content of polyphenols while it also enhanced the resistance of tea plants.

Catechins are the main ingredients among tea polyphenols, which account for 70%-80%. Of the total tea polyphenols, Catechins provide the exceptional astringent taste to the tea soup. Fig. 3C, showed that GP infection reduced the content of catechins. Interestingly, however, the catechin level was increased in the tea plants in GP+SA treatment, indicating that SA treatment alleviated the loss of catechin production due to GP infection.

Since catechins are the main ingredients among tea polyphenols, we further analyzed the catechins, (EGC, EC, EGCG and ECG). As seen in Fig. 4, the contents of EGC and EGCG was similar under different treatment. The production of these two components were reduced in plants infected by GP. However, the reduction could be offset by SA pretreatment. EC and ECG showed similar patterns and the pretreatment with SA could not recover or increase their content. These findings indicate that SA treatment alleviates the loss of catechins resulting from GP infection by enhancing the production of several other catechins.