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Peroxyactyl nitrate (PAN) is the most abundant of the gaseous organic nitrates produced from the photochemistry of hydrocarbons and NO x (i.e. ozone and smog production). PAN is known to be toxic to plants and also as a reservoir for the transport nitrogen dioxide in the troposphere. Here, the effect of vegetation on PAN deposition was investigated in four plant species by measuring leaf fluxes of PAN in a dynamic leaf chamber using atmospheric PAN fumigations between 0.7 and 18 nmol mol ---

Phenylalanine ammonia-lyase (PAL) induction in UVBexposed plants leads to an increased synthesis of UV-absorbing phenols. As phenols, including anthocycanins, are linked to many protective mechanisms in plants, we tested the hypothesis that UVB-induced phenol accumulation, mediated by PAL, may confer freezing tolerance in jack pine (Pinus banksiana Lamb) seedlings. The hypothesis was tested by applying UVB in the presence and absence of the PAL-inhibitor, 2-aminoindan-2-phosphonic acid (AIP). Jack pine seedlings were grown for 3 weeks with and without 10 mM aqueous AIP. Each treatment was then divided into two groups. One group received near-ambient UVB (5.5 kJ m À2 day À1 of biologically effective radiation) for up to 30 h. A second, control group of seedlings received no UVB. Anthocyanin concentration declined by > 99% in PAL-inhibited seedlings and other methanolextractable UV-absorbing phenols declined by > 48%, relative to the controls. A 20-h exposure to UVB increased seedling freezing (À15 C) tolerance in the absence of the PAL-inhibitor, as shown by a 30% reduction in membrane injury, determined by electrolyte leakage measurements. In PAL-inhibited seedlings, by contrast, the same UVB pre-treatment increased freezing injury by 48%. A longer (30 h) UVB exposure was damaging to both AIP-treated and untreated seedlings. Root feeding with 10 mM AIP during a 3-week exposure of older (6-month-old) seedlings similarly reduced phenol accumulation in UVBexposed seedlings. The decline in phenol production in PALinhibited seedlings correlated with increased freezing injury. These results suggest a role for ambient UVB in seedling frost hardiness, mediated by a PAL-induced production of phenolic compounds.

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