Changes in Free-Radical Scavengers Describe the Susceptibility of Pinus Ponderosa to Ozone in Southern Californian Forests

Damage of Pinus ponderosa Dougl. Ex P. and C. Laws, in the San Bernardino Mountains in Southern California is a well-known example for ozone-phytotoxic effects in forests. In October 1997, needles were collected at the severely impacted Dogwood plot, where pines showing severe crown thinning (‘symptomatic’) and asymptomatic ones grow together. Visible chlorosis was not or only mildly developed on the sampled current year's (c) and previous year's (c+l) needles at the time of collection. In sun exposed needles of symptomatic trees, but not in needles which were dark adapted overnight, significantly lower glutathione contents were found compared to asymptomatic trees. In asymptomatic trees sun-exposed and dark adapted needles contained 85 to 90% of the glutathione pool in the reduced state, whereas in sun exposed needles of symptomatic trees a significantly higher proportion of oxidised glutathione was observed (>20%). The redox state of the ascorbate pool remained unaffected, but needles of asymptomatic trees showed a significant decrease of total ascorbate upon darkening. Tocopherol contents were not significantly different between symptomatic and asymptomatic individuals. The results indicate oxidative alterations in needles of symptomatic trees. Since these changes are detectable before severe visual symptoms appear on the needles, they represent initial physiological damage and might be useful for an early risk assessment of ozone phytotoxic potential on pine trees in the field.