Interactive effects of drought and infection by a xylem-limited bacterial pathogen on the ecophysiology of a liana and on subsequent attack by an insect herbivore

Two stresses of particular importance to plants are drought and pathogens. One major plant pathogen is Xylella fastidiosa, a xylem-limited bacterial pathogen. This organism causes leaf scorch symptoms across its extensive host range. Previous observations suggested that leaf scorch symptoms were more severe if plants were experiencing other environmental stresses simultaneously. However, no experimental studies have tested this hypothesis. Parthenocissus quinquefolia, Virginia creeper, is a widespread liana throughout the eastern half of the United States. This species is an important component of natural forest communities, as well as ornamental plantings. It is susceptible to infection by X. fastidiosa, and displays prominent leaf scorch symptoms. My objectives in this study were to test the interaction between X. fastidiosa-infection and drought using P. quinquefolia as the host plant. I also wanted to assess how pathogen and water stress induced changes in P. quinquefolia affected subsequent susceptibility to attack by a non-vector insect herbivore. In two experiments in 1999 and 2000, I manipulated soil moisture levels and pathogen infection. Leaf scorch symptoms of infected plants were more severe and progressed further along the stem in low water compared to high water treatments. P. quinquefolia growth was additively reduced by low water and infection. Whole shoot hydraulic conductance and xylem vessel lengths were reduced by both low water and infection, while only low water increased embolism and reduced vessel diameter. Thus, the major effect of infection on hydraulic conductance was caused by clogging of vessels. P. quinquefolia responded initially to low water and infection by reducing stomatal conductance. This response maintains relatively high leaf water potentials during drought. Chlorophyll fluorescence measurements showed that P. quinquefolia has an efficient means of dissipating excess light energy, avoiding irreversible photoinhibitory damage. However, severe stress induced by infection and low water eventually led to decreases in photosynthesis and accelerated leaf senescence. Low water and infection treatments reduced relative water content and leaf nitrogen contents of leaves, while increasing the carbon:nitrogen ratio. In whole leaf assays, Japanese Beetles (Popillia japonica) preferred and consumed the most leaf tissue in high water, non-infected treatments compared to stressed treatments.