Abstract
Scots pine (Pinus sylvestris L.) and Norway spruce [Picea abies (L.) Karst.] seedlings were exposed to high phosphorus (HP) or low phosphorus (LP) availability for one growing season in the open field, and to combined P availability and elevated ozone (O3) concentrations (0, 55, 110 and 210 ppb for Scots pine and 0, 40, 75 and 150 ppb for Norway spruce, respectively) for 28 days in controlled laboratory chambers. Compared with HP, the LP treatment reduced Scots pine current-year (C) shoot and root dry masses and Norway spruce total dry mass, whereas the highest O3 concentrations increased the magnesium concentration of Scots pine C needles and P concentrations of the C needles of both tree species. Chlorophyll a, a+b and carotenoid concentrations of Scots pine C needles were significantly higher in the LP treatment compared with HP under the highest O3 concentration (210 ppb). In the mesophyll tissue of C needles of both tree species, LP treatment increased the size of mitochondria and elevated O3-induced granulation of chloroplast stroma and disintegration of cytoplasm. Exposure to elevated O3 concentrations increased swelling of chloroplast thylakoids and reduced the amount of vacuolar tannin in the LP Scots pine C needles. The results suggest disturbances in needle photosynthetic machinery due to acute exposure to the combination of elevated O3 and low P availability. However, clear additive effects were found only in needle P concentrations < 1 mg g-1 in short-term O3 exposure.
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