Effect of elevated [CO₂] on photosynthesis and growth of snow gum (Eucalyptus pauciflora) seedlings during winter and spring

Abstract

Snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) seedlings were grown from autumn through spring in open top chambers located in a pasture naturally subject to freezing temperatures in either ambient or elevated (350 µL L^-1 above ambient) CO₂ concentrations. Sustained reduction in quantum efficiency, as measured by chlorophyll fluorescence (F_v/F_m), in over-wintering leaves may be related to seasonal down-regulation of photosynthesis, combined with cumulative effects of freeze- induced damage to the photosynthetic apparatus, with the effect being greater in leaves grown under elevated [CO₂]. Down-regulation of photosynthesis apparently occurred in response to seasonal limitations to growth which were not overcome by elevation of [CO₂] despite temperatures being favorable for photosynthesis during most of the photoperiod. Elevated [CO₂] had no effect on growth of over-wintering seedlings, but enhanced growth in spring when minimum temperatures rose consistently above freezing. As there were no effects of elevated [CO₂] on allocation, the stimulation of growth in spring was attributable to increase in net assimilation rates. Thus seasonal differences in photoinhibition were consistent with seasonal differences in the capacity for growth, with plants grown under elevated [CO₂] having to dissipate more excess excitation energy over-winter.