Anatomical adjustment of mature leaves of sycamore maple (Acer pseudoplatanus L.) to increased irradiance.

Trees regenerating in the understory respond to increased availability of light caused by gap formation by undergoing a range of morphological and physiological adjustments. These adjustments include the production of thick, sun-type leaves containing thicker mesophyll and longer palisade cells than in shade-type leaves. We asked whether in the shade-regenerating tree Acer pseudoplatanus, the increase in leaf thickness and expansion of leaf tissues are possible also in leaves that had been fully formed prior to the increase in irradiance, a response reported so far only for a handful of species. We acclimated potted seedlings to eight levels (from 1 to 100%) of solar irradiance and, in late summer, transferred a subset of them to full sunlight. Within 30 days, the shaded leaves increased leaf mass per area and became thicker mostly due to elongation of palisade cells, except for the most shaded individuals which suffered irreversible photo-oxidative damage. This anatomical acclimation was accompanied by partial degradation of chlorophyll and a transient decline in photosynthetic efficiency of PSII (F