Genotypic variation in growth and physiological responses of Finnish hybrid aspen (Populus tremuloides x P-tremula) to elevated tropospheric ozone concentration

Saplings of six Finnish hybrid aspen (Populus tremuloides Michx. x P. tremu la L.) clones were exposed to 0, 50, 100 and 150 ppb ozone (O-3) for 32 day s in a chamber-experiment to determine differences in O-3 sensitivity among genotypes. Based on the chamber experiment, three clones with intermediate sensitivity to O-3 were selected for a free-air O-3 enrichment experiment in which plants were exposed for 2 months to either ambient air (control) o r air containing 1.3 x the ambient O-3 concentration. We measured stem heig ht and radial growth, number of leaves, dry mass and relative growth rate o f leaves, stem and roots, visible leaf injuries, net photosynthesis and sto matal conductance of the clones. There was high clonal variation in suscept ibility to O-3 in the chamber experiment, indicated by foliar injuries and differential reductions in growth and net photosynthesis. In the free-air O -3 enrichment experiment, ozone caused a shift in resource allocation towar d stem height growth, thereby altering the shoot to root balance. In both e xperiments, low O-3 concentrations tended to stimulate growth of most clone s, whereas 100 and 150 ppb O-3 in the chamber experiment impaired growth of most clones. However, growth of the Most O-3-tolerant clone was not signif icantly affected by any O-3 treatment.