Nitric acid vapor effects on forest trees — deposition and cuticular changes

Nitric acid (HNO3) vapor is an important component of photochemical smog and occurs in high concentrations in forests of the San Bernardino and San Gabriel Mountains of southern California. Ponderosa pine (Pinus ponderosa Dougl. ex. Laws.) and California black oak (Quercus kelloggii Newb.) seedlings were exposed to H15NO3 in a series of short-term experiments performed in a Teflon cuvette system. The highest H15NO3 deposition occurred on foliar surfaces of both species. Substantial transcuticular transport of the pollutant into the leaf interior and stems and roots of two species was determined. Exposures of pines for 12 h in light to 50 ppb H15NO3 caused deterioration of needle cuticle (lesions and collapsed cells). After 12 h of dark exposures to 200 ppb H15NO3 epicuticular wax structure of oak started to disintegrate and trichomes showed a wilting appearance. Exposures to H15NO3 changed chemistry of epicuticular waxes of pines — content of fatty acids decreased and alkyl esters increased. Results of this study showed a potential for HNO3 phytotoxic effects in southern California forests in addition to the observed damage caused by ozone.