Elevated CO2 protects poplar (Populus trichocarpa x P-deltoides) from damage induced by O3:: identification of mechanisms

CO2 concentrations in the Earth's atmosphere will rise to between 550 and 700 mu L L-1 by 2100 (IPCC 2001). In much of the world, ozone (O-3) is the air pollutant most likely to be having adverse effects on the growth of plants. Here we describe the impacts of CO2 and O-3 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.) x P. deltoids Bartr. ex Marsh). 700 mu L L-1 CO2 increased all growth variables relative to values in 350 mu L 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 mu L L-1 CO2 and by 34% in 700 mu L L-1 CO2. A/C-i analysis showed limited effects on photosynthetic efficiency of 700 mu L L-1 CO2 but in contrast, V-cmax was reduced by O-3 episodes. CO2 tended to increase leaf expansion but O-3 episodes reduced expansion rates generally although a short period of increased leaf expansion in response to O-3 was also observed. O-3 reduced leaf solute potentials (Psi s) 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 O-3 may act directly on the cell wall, attacking polysaccharides in the wall that result in altered cell wall properties and leaf growth. O-3 episodes increased leaf loss, elevated CO2 delayed abscission and O-3 was less effective at accelerating leaf loss in elevated CO2. Overall CO2 increased growth, O-3 caused decreases and the treatment combination gave intermediate effects. Thus O-3 episodes are less likely to be detrimental to P. trichocarpa x P. deltoides in the CO2 concentrations of the future.