Nitrogen availability modifies the ozone responses of Scots pine seedlingsexposed in an open-field system

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were exposed to e ither ambient or elevated (1.5-1.6 x ambient) ozone concentration ([O-3]) f or three growing seasons in an open-field fumigation facility where they we re irrigated during the growing season with a nutrient solution providing n itrogen (N) at 70 (LN treatment), 100 (control) or 150% (HN treatment) of t he optimum supply rate. Treatment effects were most evident during the thir d year of exposure, when the ambient [O-3] + HN treatment enhanced whole-pl ant biomass, root/shoot dry weight ratio, needle pigment concentrations and the number of chloroplast plastoglobuli in the mesophyll cells in current- year (C) needles, whereas it reduced starch accumulation in C needles and a bscission of 2-year-old (C+2) needles. In the control fertilization, 3 year s of exposure to elevated [O-3] decreased stem-base diameter and increased K concentration and electron density of chloroplast stroma in C needles. Pl ants in the HN treatment exposed for 3 years to elevated [O-3] had signific antly lower heights, current-year main shoot length and root/shoot dry mass ratio than control plants, and increased abscission of C+2 needles. In con trast, O-3-induced changes in the ultrastructure of mesophyll cells were mo st evident in seedlings grown for 3 years in the LN treatment. We conclude that, in Scots pine, a relatively O-3-tolerant species, chronic O-3 exposur e leads to cumulative growth reduction, increased needle abscission and cha nges in carbon allocation that are strongly influenced by plant N availabil ity.