Diversity of chemical phenotypes of Persea americana Mill cv. Hass: A vertical multibarrier against avocado stem weevil (Copturus aguacatae Kissinger)
Plant chemical defense (PCD) has largely been considered as a single barrier (SB), according to the "screening hypothesis", which postulated that only a few compounds participate in the defense against all plant enemies (herbivores and pathogens) and the rest of plant metabolites do not intervene in defense. However the multi-functionality, the additive and synergistic effects, and the intraspecific variation of plant phytochemicals (for example terpenoids) suggest that PCD could function as a horizontal multi-barrier (MB), where a specific combination of metabolites is effective against one herbivore and other combination is effective against another herbivore. PCD could function also as a vertical MB, where multiple phytochemicals or combinations of them are effective against one herbivore species. We tested the vertical MB and the screening hypotheses comparing chemical profiles of Persea americana cv. Hass (leaves and branches) from trees infested and non-infested by Copturus aguacatae, the avocado stem weevil (ASW), one of major pests of avocado orchards in Mexico. According to SB hypothesis, chemical differences between infested and non-infested trees must be of few compounds; whereas according to the vertical MB hypothesis, several chemical phenotypes related with susceptibility and several ones related with resistance should be expected.
We sampled leaves and branches from 180 trees (93 infested and 87 non-infested) from 9 orchards and obtained their chemical profiles by GC-MS. In branches we detected 32 compounds (mainly mono- and sesqui-terpenes, some alkanols and unidentified compounds); in univariate comparison, the concentrations of three uncorrelated compounds (β-phelandrene, benzaldehyde and persin) were different between infested and non-infested trees (U-test p ≤ 0.001). In multivariate analysis, supervised clustering with 32 phytochemicals revealed some clusters with mostly non-infested trees (resistant), other clusters with mostly infested trees (susceptible), and mixed groups (intermediate susceptibility). Multiple comparisons with Kruskal-Wallis test shown highly significant differences among clusters mainly in monoterpenes (p ≤ 0.0001) but no evidence of single compounds associated with the resistance/susceptibility. Therefore, ASW incidence in orchards is best explained by the presence of several avocado chemical phenotypes with differential susceptibility. These results gave us correlative support for the vertical MB hypothesis. Further experiments assaying the effect of identified phenotypes in preference or performance of ASW will allow test the causality.