The response of mesophyll conductance to ozone-induced oxidative stress is genotype-dependent in poplar

Under O(3)stress, decreased leaf mesophyll conductance to CO(2)of Populusx canadensisis genotype-dependent and limits net assimilation rate, related to leaf mass per area and chlorophyll content. The CO2 diffusion conductance within the leaf mesophyll (g(m)) is considered a major limiting factor of photosynthesis. However, the effects of the major secondary air pollutant ozone (O-3) on g(m) have been poorly investigated. Eight genotypes of the economically important tree species Populus x canadensis Moench were exposed to 120 ppb O-3 for 21 d. g(m) showed a genotype-dependent response to O-3-induced oxidative stress and was a major limiting factor of net assimilation rate (A(net)), ahead of stomatal conductance to CO2 (g(sc)) and of the maximum carboxylation capacity of the Rubisco enzyme (V-cmax) in half of the tested genotypes. Increased leaf dry mass per area (LMA) and decreased chlorophyll content were linked to the observed g(m) decrease, but this relationship did not entirely explain the different genotypic g(m) responses. Moreover, the oxidative stress defence metabolites ascorbate and glutathione were not related to O-3 tolerance of g(m). However, malondialdehyde probably mitigated the observed g(m) decrease in some genotypes due to its oxidative stress signalling function. The large variation of g(m) suggests different regulation mechanisms amongst poplar genotypes under oxidative stress.