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 M ountains in Southern California is a well-known example for ozone-phytotoxi c effects in forests. In October 1997, needles were collected at the severe ly impacted Dogwood plot, where pines showing severe crown thinning ('sympt omatic') 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+1) needles at the time of collection. In sun exposed needles of symptom atic trees, but not in needles which were dark adapted overnight, significa ntly 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 ne edles of symptomatic trees a significantly higher proportion of oxidised gl utathione was observed (> 20%). The redox state of the ascorbate pool remai ned unaffected, but needles of asymptomatic trees showed a significant decr ease of total ascorbate upon darkening. Tocopherol contents were not signif icantly different between symptomatic and asymptomatic individuals. The res ults indicate oxidative alterations in needles of symptomatic trees. Since these changes are detectable before severe visual symptoms appear on the ne edles, they represent initial physiological damage and might be useful for an early risk assessment of ozone phytotoxic potential on pine trees in the field.