Elevated CO₂ protects poplar (Populus trichocarpa × P. deltoides) from damage induced by O₃: identification of...

CO₂ concentrations in the Earth's atmosphere will rise to between 550 and 700 μL L^-1 by 2100 (IPCC 2001). In much of the world, ozone (O₃) is the air pollutant most likely to be having adverse effects on the growth of plants. Here we describe the impacts of CO₂ and O₃ episodes (rising to 100 nL L^-1), singly and in mixtures on the growth and physiology of an interamerican hybrid poplar (Populus trichocarpa L. (Torr. & Gray ex Hook.) × P. deltoids Bartr. ex Marsh). 700 μL L^-1 CO₂ increased all growth variables relative to values in 350 μL L^-1. Mainstem dry weight showed a 38% increase in year 1 and a 32% increase in year 2. Ozone episodes reduced mainstem dry mass by 45% in 350 μL L^-1 CO₂ and by 34% in 700 μL L^-1 CO₂. A / C_i analysis showed limited effects on photosynthetic efficiency of 700 μL L^-1 CO₂ but in contrast, V_cmax was reduced by O₃ episodes. CO₂ tended to increase leaf expansion but O₃ episodes reduced expansion rates generally although a short period of increased leaf expansion in response to O₃ was also observed. O₃ reduced leaf solute potentials (Ψs) and increased turgor (P) in young leaves. Cell wall properties (elasticity and plasticity) were both stimulated by ozone and this was associated with increased leaf expansion. A new mechanism is proposed which suggests that O₃ may act directly on the cell wall, attacking polysaccharides in the wall that result in altered cell wall properties and leaf growth. O₃ episodes increased leaf loss, elevated CO₂ delayed abscission and O₃ was less effective at accelerating leaf loss in elevated CO₂. Overall CO₂ increased growth, O₃ caused decreases and the treatment combination gave intermediate effects. Thus O₃ episodes are less likely to be detrimental to P. trichocarpa × P. deltoides in the CO₂ concentrations of the future.