Leaf growth of Betula and Acer in simulated shadelight

Leaves of birch (Betula pendula Roth) and sycamore (Acer pseudoplatanus L.) were initiated and grown either in a simulated shadelight (80 μmol m^-2 s^-1, R/FR ratio 0.28)/dark photoenvironment or a white light (250 μmol m^-2 s^-1, R/FR>1)/dark photoenvironment. Until the leaves were more than 50% expanded, growth rates (measured every 24 h) were the same for both species in both environments. After this time, growth rate slowed and this correlated well with a decrease in wall extensibility (WEX). Birch leaves in shadelight showed reduced surface acidification and were the first to show reduced growth. WEX under these conditions was particularly low.Daily patterns of leaf growth of the two species were very different. Sycamore leaves showed a slightly higher growth rate in the dark than in shadelight, while birch leaves grew more rapidly in shadelight than in the dark. Limitation of growth of sycamore leaves in light may be explained by a very high yield threshold turgor for growth (Y). The daily pattern of leaf growth shown by birch is more difficult to explain but the importance of a possible limitation of growth by solute availability and a diurnal variation in Y are discussed.