Stomatal density and needle anatomy of Scots pine (Pinus sylvestris) are affected by elevated CO2

Morphological and anatomical characteristics of needles from different whor l levels were examined on Scots pines (Pinus sylvestris), grown for 4 yr un der either ambient or elevated (ambient + 400 mu mol mol(-1)) CO2 concentra tions in open-top chambers. Needle characteristics were studied using light microscopy, scanning electr on microscopy and laser scanning confocal microscopy. Under the elevated CO2 treatment stomatal density was reduced on both adaxi al and abaxial needle surfaces although the number of rows of stomata did n ot change significantly. Needle cross-sectional area increased by 10%; this was largely the result of an increase in needle thickness and, to a lesser extent, needle width. The increase in needle thickness was due to a large increase in mesophyll tissue. The relative area (i.e. proportion of the tot al area) of epidermis plus hypodermis, of resin canal, of xylem and of cent ral cylinder decreased, whereas the relative area of needle phloem signific antly increased. The results suggest that a prolonged exposure to elevated CO2 has an effect on needle structure, anatomical and stomatal characteristics of Scots pine needles.