Ozone sensitivity and ethylenediurea protection in ash trees assessed by JIP chlorophyll a fluorescence transient analysis

The effect of ethylenediurea (EDU) was tested using the chlorophyll (Chl) a fluorescence transient analysis, performed with JIP-test, to assess ambient ozone (O-3) effects on photosynthesis of adult trees under natural conditions. Twelve adult European ash (Fraxinus excelsior L.) trees, known to be sensitive or tolerant to O-3, determined by presence symptomatic (S) or absence asymptomatic (AS) trees of foliar symptoms in previous years, were treated either with distilled water containing 450 g m(-3) EDU or with distilled water. Once a month across the growing season [the accumulated exposure over a threshold of 40 nmol(O-3) mol(-1) was 32.49 A mu mol mol(-1) h(-1)], Chl a fluorescence transients were measured in vivo on dark-adapted leaves of 1-year-old labeled shoots, from the lower crown part. Twenty-five parameters were calculated. The maximum quantum yield of primary photochemistry (phi(Po) or F-v/F-m) did not differentiate between S- and AS-trees, while increased Chl content and de-excitation rates suggested compensation of O-3 injury in S-trees. Seasonal reductions in absorbing fluxes and increase in heat and fluorescence dissipation processes was due to leaf ageing and drought, the latter suggesting water deficit influenced Chl a fluorescence stronger than ambient O-3 exposure. AS-trees showed elevated probability of connectivity among photosystem 2 units, a mechanism to stimulate energy dissipation and reduce photo-oxidative injury. EDU prevented the inactivation of reaction centers. This slight effect does not warrant EDU as a tool to assess O-3 effects on photosynthesis, while the JIP-test is suggested for a quantitative assessment in adult trees.