The primary aim of the study was to quantify the effects of photosynthetically-active radiation (PAR) on extension of leaves of silver birch (Betula pendula Roth.) and sycamore (Acer pseudoplatanus L.). Plants grown at 250 ?mol m?2 s?1 were exposed to a range of fluence rates (0·680 ?mol m? s?1) whilst enclosed in Perspex chambers in which temperature, vapour pressure deficit and photoenvironment were all controlled. Measurements of leaf extension, water relations, and cell wall extensibility (VVEX) were made at the end of a 3 h exposure period.<br/><br/>For leaves of birch, reducing the fluence rate resulted in reduced leaf extension and a lowering of WEX. Exposure of sycamore seedlings for 3 h to different values of PAR had little effect on the growth of leaves and WEX remained constant. The characteristics of net photosynthesis also differed for the two species and photosynthesis and wall loosening may be linked.<br/><br/>Since natural woodland shadelight, in addition to reduced PAR, also has a reduced R/FR ratio, a second group of seedlings were placed in a photoenvironment which simulated shadelight (low PAR, R/FR) for 28 d. This treatment reduced extension rate and final leaf size for both species. Measurements of extension, water relations and WEX made when seedlings were exposed to shadelight whilst held in the Perspex chambers revealed differences in the control of leaf growth for the two species.<br/><br/>It is confirmed that leaves of birch are highly responsive to changes in PAR with cell wall loosening the most likely process controlling this response
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