Physiological and biochemical differences amongUlmus minorgenotypes showing a gradient of resistance toDutch elm disease

Abstract

SummaryDutch elm disease (DED) spread across Europe and North America in the 20th century killing most natural elm populations. Today, breeding programmes aim at identifying, propagating and studying elm clones resistant toDED. Here, we have compared the physiology and biochemistry of six genotypes ofUlmus minorof variableDEDresistance. Leaf gas exchange, water potential, stem hydraulic conductivity and biochemical status were studied in 5‐year‐old trees ofAB‐AM2.4, M‐DV2.3, M‐DV2 × M‐CC1.5 and M‐DV1 and 6‐year‐old trees ofVA‐AP38 andBU‐FL7 before and after inoculation withOphiostoma novo‐ulmi. Leaf water potential and net photosynthesis rates declined, while the percentage loss of hydraulic conductivity (PLC) increased after the inoculation in susceptible trees. By the 21st day, leaf predawn and midday water potential, stomatal conductance to water vapour and net photosynthesis rates were lower, andPLCwas higher in trees of susceptible (S) genotypes inoculated with the pathogen than in control trees inoculated with water, whereas no significant treatment effect was observed on these variables in the resistant (R) genotypes. Fourier transform infrared spectroscopy analyses revealed a different biochemical profile for branches of R and S clones. R clones showed higher absorption peaks that could be assigned to phenolic compounds, saturated hydrocarbons, cellulose and hemicellulose than S clones. The differences were more marked at the end of the experiment than at the beginning, suggesting that R and S clones responded differently to the inevitable wounding from inoculation and repeated sampling over the experimental course. We hypothesize that a weak activation of the defence system in response to experimental wounding can contribute to the susceptibility of some genotypes toO. novo‐ulmi. In turn, the decline in shoot hydraulic conductivity and leaf carbon uptake caused by the infection further exacerbates tree susceptibility to the fungus.