Mechanism for increased leaf growth in elevated CO2

Abstract

The effect of exposure to elevated CO2 on the processes of leaf cell production and leaf cell expansion was studied using primary leaves of Phaseolus vulgaris L. Cell division and expansion were separated temporally 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). When plants were exposed to elevated CO2 during the phase of cell expansion, epidermal cell size and leaf area development were stimulated. Three pieces of evidence suggest that this occurred as a result of increased cell wall loosening and extensibility, (i) cell wall extensibility (WEx, measured as tensiometric extension using an Instron) was significantly increased, (ii) cell wall yield turgor (V, MPa) was reduced and (iii) xyloglucan endotransglycosylase (XET) enzyme activity was significantly increased. When plants were exposed to elevated CO2 during the phase of cell division, the number of epidermal cells was increased whilst final cell size was significantly reduced and this was associated with reduced final leaf area, WEx and XET activity. When plants were exposed to elevated CO2 during both phases of cell division and expansion, leaf area development was not affected. For this treatment, however, 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 across the expanding leaf lamina and the data suggest that exposure to elevated CO2 during the phase of leaf expansion may lead to enhanced extensibility particularly at basal leaf margins which may result in altered leaf shape. The data show that both cell production and expansion were stimulated by elevated CO2, but that leaf growth was only enhanced by exposure to elevated CO2 in the cell expansion phase of leaf development. Increased leaf cell expansion is, therefore, an important mechanism for enhanced leaf growth in elevated CO2, whilst the importance of increased leaf cell production in elevated CO2 remains to be elucidated.