Photosynthetic parameters as early indicators of ozone injury in apple leaves

Ozone may affect leaf photosynthesis even before visible symptoms become ap parent. This study had the objective to test several parameters of chloroph yll fluorescence and leaf gas exchange for their usefulness as indicators o f latent ozone injury in the field. Container-grown apple trees (Malus dome stica Borkh. cv. Golden Delicious) were exposed to four different ozone lev els in open-top chambers. Identical leaves were analyzed in fixed-time inte rvals for the characteristics of fast fluorescence induction kinetics in vi vo. By using high-time resolution, characteristic parameters describing the early photochemical events could be calculated according to the JIP-test. Parameters responsive to the different ozone treatments showed clear depend ence on the accumulated ozone dose. Ozone exposure immediately preceding th e measurements was more important for the extent of the physiological effec ts than the total accumulated ozone dose. The most sensitive parameters wer e the turnover number N (indicating how many times Q(A) has to be reduced f or full reduction of all accepters; positively correlated to ozone dose) an d D-o, the density of reaction centres per leaf area (negatively correlated to ozone dose). Most parameters analyzed showed clearer responses to ozone on the adaxial than on the abaxial surface of the leaf. Changes in the par ameter N were better correlated to ozone doses with low cut-offs (AOT00 and AOT20), whereas changes in D-o and in the specific electron fluxes per rea ction centre were mainly influenced by ozone doses with high cut-offs (AOT8 0 and AOT100). Leaf gas exchange analyses revealed a higher ozone sensitivi ty in carboxylation efficiency than in light utilization efficiency and in the rate of light-saturated net photosynthesis. All ozone-induced photosynt hetic effects were observed in leaves showing no sign of visible leaf injur y. This study identified fluorescence parameters that could be useful for r apid monitoring and early detection of latent leaf injury by ozone.