LEAF GROWTH OF HYBRID POPLAR FOLLOWING EXPOSURE TO ELEVATED CO2

Leaf extension was stimulated following exposure of three interamerica n hybrid poplar clones (Populus trichocarpa x P. deltoides); 'Unal', ' Boelare', and 'Beaupre' and a euramerican clone 'Primo' (Populus nigra x P. deltoides) to elevated CO2 in controlled environment chambers. F or all three interamerican clones the evidence suggests that this was the result of increased leaf cell expansion associated with enhanced c ell wall extensibility (WEx), measured as tensiometric increases in ce ll wall plasticity. For the interamerican clone 'Boelare', there was a lso a significant increase in cell wall elasticity following exposure to elevated CO2 (P less than or equal to 0.001). The effect of elevate d CO2 in stimulating cell wall extensibility was confirmed in a detail ed spatial analysis of extensibility made across the lamina of expandi ng leaves of the clone 'Boelare'. For two of the interamerican hybrids , 'Unal' and 'Beaupre', both leaf cell water potential (psi) and turgo r pressure (P) were lower in elevated than in ambient CO2 By contrast, no significant effects on the cell wall properties or leaf water rela tions for the euramerican hybrid 'Primo' were observed following expos ure 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 signifi cantly increased (P less than or equal to 0.05) and since leaf number was not significantly increased in any inter-american clone it is hypo thesized that final leaf size was stimulated in elevated CO2 for these clones. By contrast, there was no significant effect of CO2 on cumula tive total leaf length for the euramerican clone 'Primo', but leaf num ber was significantly increased by elevated CO2. The measurements sugg est that total tree leaf area was stimulated for a range of poplar hyb rids exposed to elevated CO2. Given the short rotation of a coppiced c rop, it is likely that increased leaf areas will result in enhanced st emwood production when hybrid poplars are grown in the CO2 concentrati ons predicted for the next century.