Effects of insect herbivory on leaf volatile organic compound emission and photosynthetic characteristics of saplings in subtropical region.

Abstract

Insect herbivory can influence tree growth, community structure and ecological processes in forest ecosystems. We investigated the effects of insect herbivory and leaf defoliation on the emission of biogenic volatile organic compounds (BVOCs), isoprene (ISO) and monoterpenes (MTs) in Cinnamomum camphora (broad-leaf tree) and Cryptomeria japonica (coniferous tree), and explored the underlying mechanisms by measuring leaf phy-siological characteristics such as photosynthetic parameters and chlorophyll fluorescence. The results showed that insect herbivory and leaf defoliation increased the emission of BVOCs from the leaves of both species compared to the control. Three days after the completion of insect herbivory treatment, the emission flux of ISO in C. camphora and C. japonica increased by 4.9 and 3.1 times, respectively, while leaf defoliation increased the ISO emission flux of C. camphora by 4.6 times. Insect herbivory increased photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, and transpiration rate in C. camphora, but did not affect the photosynthetic parameters of C. japonica. In contrast, leaf defoliation significantly reduced chlorophyll fluorescence parameters in C. japonica. The Pearson correlation analysis and structural equation modeling showed that the emission of ISO and MTs from C. camphora leaves significantly correlated with photosynthetic and chlorophyll fluorescence parameters. Compared to chlorophyll fluorescence parameters, photosynthetic parameters had a greater influence on the emission of ISO and MTs. For C. japonica, ISO emission was significantly correlated with chlorophyll fluorescence parameters, and chemical induction due to insect herbivory having a more pronounced effect on ISO emission. In conclusion, our results indicated that insect herbivory could enhance the emission of BVOCs, but the response mechanism varied with tree species. For C. camphora, the increase in BVOC emission was due to the enhanced photosynthetic rates. The chemical induction resulting from insect herbivory played a more important role in increasing BVOC release from C. japonica.