ABSTRACT
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 / Ci analysis showed limited effects on photosynthetic efficiency of 700 µL L-1 CO2 but in contrast, Vcmax 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.
ABSTRACT
Leaf extension was stimulated following exposure of three interamerican hybrid poplar clones (Populus trichocarpa P. deltoides); 'Unal', 'Boelare', and 'Beaupre' and a euramerican clone 'Primo' (Populus nigra×P. deltoides) to elevated CO2 , in controlled environment chambers. For all three interamerican clones the evidence suggests that this was the result of increased leaf cell expansion associated with enhanced cell wall extensibility (WEx), measured as tensiomerric increases in cell wall plasticity. For the interameriean clone 'Boelare', there was also a significant increase in cell wall elasticity following exposure to elevated CO2 (P⩽ 0.001). The effect of elevated CO2 in stimulating cell wall extensibility was confirmed in a detailed spatial analysis of extensibility made across the lamina of expanding leaves of the clone 'Boelare'. For two of the interamerican hybrids, 'Unal' and 'Beaupre', both leaf cell water potential Ψ and turgor pressure (P) were lower in elevated than in ambient CO2 . By contrast, no significant effects on the cell wall properties or leaf water relations for the euramerican hybrid 'Primo' were observed following exposure to elevated CO2 . suggesting that the mechanism for increased leaf extension in elevated CO2 , differed, depending on clone. The cumulative total length of leaves of 'Boelare' grown in elevated CO2 , was significantly increased (P≤ 0.05) and since leaf number was not significantly increased in any inter-american clone it is hypothesized that final leaf size was stimulated in elevated CO2 for these clones. By contrast, there was no significant effect of CO2 on cumulative total leaf length for the euramerican clone 'Primo', but leaf number was significantly increased by elevated CO2 . The measurements suggest that total tree leaf area was stimulated for a range of poplar hybrids exposed to elevated CO2 . Given the short rotation of a coppiced crop, it is likely that increased leaf areas will result in enhanced stemwood production when hybrid poplars are grown in the CO, concentrations predicted for the next century.
