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|a Leaf growth of first year seedlings of silver birch (Betula pendula Roth.) and sycamore (Acer pseudoplatanus L.) was measured continuously using linear variable transducers (LVTDs) when shoots were enclosed in small Perspex chambers, in which temperature, vapour pressure deficit (VPD) and photon fluence rate were all controlled. A new method was used to measure the yield turgor (Y) of growing leaves. The VPD of air in the chamber was altered so that transpiration rate was changed. Turgor pressures, measured using thermocouple psychrometers, and growth rates varied as a result of this treatment, and the relationship between leaf growth and turgor pressure (P) and the turgor pressure at which growth ceased (Y) could be measured within a very short time period using well-watered plants. The value of Y was constant for illuminated and darkened leaves of both species, but Y was higher for sycamore leaves (Y= 0.250 MPa) compared with the value for birch leaves (K = 0.071 MPa). Cell wall extensibility (w, MPa<sup>-1</sup> h<sup>-1</sup>) was calculated from the slope of the line relating turgor pressure to growth. For leaves of birch, there was a significant increase in m when leaves were illuminated (m, dark = 1.3 × 10<sup>-2</sup> MPa<sup>-1</sup> h<sup>-1</sup>; m, light = 46 × 10<sup>-2</sup> MPa<sup>-1</sup>), similar in magnitude to the increase in wall extensibility measured previously by mechanical analysis of killed leaf tissue (WEX, the plasticity of tissue subjected to a uniaxial stress). No change in m was detected when darkened sycamore leaves were illuminated (m, dark = 4.2 × 10<sup>-2</sup> MPa<sup>-1</sup> h<sup>-1</sup>; m, light = 4.4 × 10<sup>-2</sup> MPa<sup>-1</sup> h<sup>-1</sup>). The results confirm earlier work on the factors controlling the diurnal patterns of leaf growth of these two species and suggest that leaf growth of birch is primarily limited by cell wall loosening but that the growth of sycamore leaves may be limited by leaf turgor.
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