CO2 enrichment predisposes foliage of an eucalypt to freezing injury and reduces spring growth

Seedlings of Eucalyptus pauciflora, were grown in open-top chambers fumigated with ambient and elevated [CO2], and were divided into two populations using 10% light transmittance screens. The aim was to separate the effects of timing of light interception, temperature and [CO2] on plant growth. The orientation of the screens exposed plants to a similar total irradiance, but incident during either cold mornings (east-facing) or warm afternoons (west-facing). Following the first autumn freezing event elevated CO2-grown plants had 10 times more necrotic leaf area than ambient CO2 plants. West-facing plants had significantly greater (25% more) leaf damage and lower photochemical efficiency (Fv/Fm) in comparison with east-facing plants. Following a late spring freezing event east-facing elevated CO2 plants suffered a greater sustained loss in Fv/Fm than west-facing elevated CO2- and ambient CO2-grown plants. Stomatal conductance was lower under elevated CO2 than ambient CO2 except during late spring, with the highest leaf temperatures occurring in west-facing plants under elevated CO2. These higher leaf temperatures apparently interfered with cold acclimation thereby enhancing frost damage and reducing the ability to take advantage of optimal growing conditions under elevated CO2