Susceptibility to Melampsora leaf rust of poplar clones from diverse genetic backgrounds: effects on photochemistry and water relations

The selection of resistant genotypes is the most appropriate approach in the prevention of the reduction ofbiomass and mortality caused by rust infection in poplar plantations. Thus, it is pertinent that we improve ourunderstanding of the consequences that this fungal disease has on leaf physiology. Here, we studied thesusceptibility to Melampsora rust in three different poplar clones of commercial interest: Lux clone - Populusdeltoides Batr. (cottonwood) and Luisa Avanzo and Adige clones - both Populus × canadensis Mönch. The mostsusceptible clone to the infection was L. Avanzo whereas Lux and especially Adige were only slightly affected.The propagation of the disease was very rapid in L. Avanzo; their leaves showed a high incidence and severity ofthe disease in early and advanced stages of infection as was clearly evidenced by the degree of infection.Infected leaves of L. Avanzo were shown to have drought impaired water relations during summer as reflectedby the marked decline in the relative water content (RWC). Chlorophyll fluorescence imaging revealedheterogeneity of the effect of the pathogen in the leaves, and areas with pustules showed low maximum quantumyield (Fv/Fm) and PSII quantum yield (?PSII) values, indicative of strong photoinhibition. In L. Avanzo, with agreater pustule density, rust provoked a decline in whole leaf photochemistry as indicated by Fv/Fm andphotochemical reflectance index (PRI) results. Leaf structural parameters were not affected by the disease butresults in L. Avanzo and Lux showed higher leaf mass per area (LMA) and higher leaf density (D) indicating anadaptation to increasing summer drought. In all clones, the effect of the pathogen was reflected in lower leafchlorophyll content.