MECHANISM FOR INCREASED LEAF GROWTH IN ELEVATED CO2

The effect of exposure to elevated CO2 on the processes of leaf cell p roduction and leaf cell expansion was studied using primary leaves of Phaseolus vulgaris L. Cell division and expansion were separated tempo rally by exposing seedlings to dim red light for 10 d (when leaf cell division was completed) followed by exposure to bright white light for 14 d (when leaf growth was entirely dependent on cell expansion). Whe n plants were exposed to elevated CO2 during the phase of cell expansi on, epidermal cell size and leaf area development were stimulated, Thr ee pieces of evidence suggest that this occurred as a result of increa sed cell wall loosening and extensibility, (i) cell wall extensibility (WEx, measured as tensiometric extension using an Instron) was signif icantly increased, (ii) cell wall yield turgor (Y, MPa) was reduced an d (iii) xyloglucan endotransglycosylase (XET) enzyme activity was sign ificantly increased. When plants were exposed to elevated CO2 during t he phase of cell division, the number of epidermal cells was increased whilst final cell size was significantly reduced and this was associa ted with reduced final leaf area, WEx and XET activity, When plants we re exposed to elevated CO2 during both phases of cell division and exp ansion, leaf area development was not affected. For this treatment, ho wever, the number of epidermal cells was increased, but cell expansion was inhibited, despite exposure to elevated CO2 during the expansion phase. Assessments were also made of the spatial patterns of WEx acros s the expanding leaf lamina and the data suggest that exposure to elev ated CO2 during the phase of leaf expansion may lead to enhanced exten sibility particularly at basal leaf margins which may result in altere d leaf shape. The data show that both cell production and expansion we re stimulated by elevated CO2, but that leaf growth was only enhanced by exposure to elevated CO2 in the cell expansion phase of leaf develo pment. Increased leaf cell expansion is, therefore, an important mecha nism for enhanced leaf growth in elevated CO2, whilst the importance o f increased leaf cell production in elevated CO2 remains to be elucida ted.