First-year results on the effects of elevated atmospheric CO sub(2) and O sub(3) concentrations on needle ultrastructure and gas exchange responses of Scots pine saplings

The effects of realistically elevated O sub(3) and CO sub(2) concentrations on the needle ultrastructure and photosynthesis of ca. 20 year-old Scots pine (Pinus sylvestris L.) saplings were studied during one growth period in open-top field chambers situated on a natural pine heath at Mekrijaervi, in eastern Finland. The experiment included six different treatments: chamberless control, filtered air, ambient air and elevated O sub(3), CO sub(2) and O sub(3) + CO sub(2). Significant increases in the size of chloroplast and starch grains were recorded in the current-year needles of the saplings exposed to elevated CO sub(2). These treatments also delayed the winter hardening process in cells. In the shoots treated with O sub(3), CO sub(2) and combined O sub(3) + CO sub(2) the P sub(max) was decreased on average by 50 % (ambient CO sub(2)) and 40 % (700 ppm CO sub(2)). Photosynthetic efficiency was decreased by 60 % in all the treated shoots measured under ambient conditions and by 30 % in the CO sub(2) and O sub(3) + CO sub(2) treated shoots under 700 ppm. The effect of all the treatments on photosynthesis was depressive which was probably related to evident accumulation of starch in the chloroplasts of the pines treated with CO sub(2) and combined O sub(3) + CO sub(2). But in O sub(3) treated pines, which did not accumulate starch in comparison to pines subjected to ambient air conditions, some injuries may be already present in the photosynthetic machinery.