Arbutus unedo is a sclerophyllous evergreen, characteristic of Mediter
ranean coastal scrub vegetation. In Italy, trees of A. unedo have been
found close to natural CO2 vents where the mean atmospheric carbon di
oxide concentration is about 2200 mu mol mol(-1). Comparisons were mad
e between trees growing in elevated and ambient CO2 concentrations to
test for evidence of adaptation to long-term exposure to elevated CO2.
Leaves formed at elevated CO2 have a lower stomatal density and stoma
tal index and higher specific leaf area than those formed at ambient C
O2, but there was no change in carbon to nitrogen ratios of the leaf t
issue. Stomatal conductance was lower at elevated CO2 during rapid gro
wth in the spring. In mid-summer, under drought stress, stomatal closu
re of all leaves occurred and in the autumn, when stress was relieved,
the conductance of leaves at both elevated and ambient CO2 increased.
In the spring, the stomatal conductance of the new flush of leaves at
ambient CO2 was higher than the leaves at elevated CO2, increasing in
stantaneous water use efficiency at elevated CO2. Chlorophyll fluoresc
ence measurements suggested that elevated CO2 provided some protection
against photoinhibition in mid-summer. Analysis of A/C-i curves showe
d that there was no evidence of either upward or downward regulation o
f photosynthesis at elevated CO2. It is therefore anticipated that A.
unedo will have higher growth rates as the ambient CO2 concentrations
increase.