Leaf volatile organic compounds profiles from two citrus genotypes differing in susceptibility to Phytophthora citrophthora infection

The leaves of two citrus genotypes, citrange 'Carrizo' and sweet orange 'Washington Navel' were artificially infected by the oomycete Phytophthora citrophthora, the causal agent of trunk gummosis and fruit brown rot of citrus, to study the profile of leaf stored terpenoids in the response to the pathogen infection. On 'Washington Navel' leaves the symptoms were more severe and manifested earlier than on 'Carrizo' leaves. Overall, 35 volatile organic compounds (VOCs) were identified by GC-MS analysis in the not-infected (control) and infected leaf extracts from the two citrus genotypes. Interesting the two volatile profiles differed not in the quality, but in the quantity of each compound. After infection, the amount of 15 out of the 35 identified VOCs varied significantly. The level of five metabolites changed in both species, while two of the compounds changed exclusively in 'Citrange' and eight exclusively in 'Washington Navel'. To understand the potential role of differentially expressed stored volatiles in the citrus response mechanisms to the infection, the inhibitory activity of ten of them volatiles was tested in vitro on P. citrophthora. Citral, linalool and trans-2-hexenal completely inhibited the mycelium growth suggesting the possible involvement of these compounds in the defence mechanisms of citrus against P. citrophthora. This study provides new insights into a potential role of stored VOCs in the response mechanisms against the oomycete pathogen P. citrophthora in citrus species. Furthermore, it paves the way for future investigations aimed at assessing the potential utilization of VOCs for priming purposes.