Abstract:

In order to study the resistance mechanism of Syringa spp. against Lepidosaphes salicina, 13 species/cultivars of Syringa were categorized into four groups: highly-resistant group (including 7 species, such as Syringa chinensis), moderately-resistant group (3 species such as S. emodi), susceptible group (2 species such as S. oblata), highly-susceptible group (only 1 species, e.g. S. villosa), on the basis of the population density of L. salicina on the Syringa spp. by cluster analysis. In the present study, the leaves of the 13 species/cultivars were sampled separately in pre-infesting period (by the end of May), peak-infesting period (by the end of June), weakly-infesting period (by the end of July) and late-infesting period (by the end of August), the contents of various tannins and phenolic acids of the samples were measured by Vanillin-HCl and HPLC methods, and the relationships between the leaf extracts and the susceptibility to L. salicina were analyzed. The results showed that in the highly-resistant group, a full range/types of phenolic acids were detected in the uninfected leaves, and the contents of both total phenolic acids and tannins in the uninfected leaves was significantly higher than those species/cultivars in other groups (P<0.05), the total phenolic acids and tannins in the infected leaves of the highly-resistant group increased by 81.75%-888.43% and the increments were higher than other groups. In the moderately-resistant group, the increment in total phenolic acids contents at the peak-infesting period was significantly higher than that in the highly-susceptible and susceptible groups, and the tannins content was significantly higher than that of susceptible group. The susceptible and highly-susceptible groups had relatively low contents of tannin and total phenolic acids during the pre-infesting period, and delayed induction of tannins and total phenolic acids during the peak-infesting period, and the types of phenolic acids were only a half of the highly- and moderately resistant groups. In addition, the presence or absence of some specific phenolic acids and their contents also had correlation with the resistance of Syringa spp. For example, from May to August, the cinnamic acid was detected in all highly- and moderately-resistant groups, whereas it wasn't detected in the susceptible and the highly susceptible groups. In short, the resistance of Syringa spp. against L. salicina was related to the constitutive and induced types and contents of tannins and total phenolic acids. The species containing more types of phenolic acids and high contents of total phenolic acids and tannin, or quickly producing or increasing the contents of tannin or phenolic acids in response to insect attacks, might have a strong resistance potential against L. salicina, and vice versa.

Key words: insect attack, secondary plant metabolites, Syringa, Lepidosaphes salicina, tannin, phenolic acids