No abstract

No abstract

No abstract

Summary• Physiological and biochemical responses of the ozone (O 3 )-sensitive pumpkin cultivar ( Cucurbita pepo 'Ambassador') to O 3 exposure are reported here to determine critical O 3 concentrations related to duration of pollutant exposure.• Pumpkin plants were exposed to 150 nl l − 1 of O 3 for 5 h d − 1 for 5 d. Gasexchange, chlorophyll a fluorescence and pigment-content measurements were determined on days 1, 3, 5 for young leaves and days 1, 3, 4 for the mature leaves of control and treated plants.• In both young and mature leaves, O 3 induced a significant decrease in photosynthetic rate through impacts on stomata and photochemistry. A reduction in PSII photochemistry, photochemical quenching and quantum yield, accompanied by a parallel increase in nonphotochemical quenching was detected. Following O 3 treatment the pool of xanthophyll cycle pigments and the de-epoxidation index underwent a significant increase, whereas chlorophylls and β -carotene were less affected.• The results showed that most of the parameters were affected by O 3 after 1 d of exposure and indicated an overall effect of O 3 and days of treatment, irrespective of leaf age.

CO2 assimilation, xanthophyll cycle pigments and PSII efficiency were analyzed in two different ages of pumpkin leaves (Cucurbita pepo L. cv. Ambassador) exposed to 150 nmol mol‐1 of ozone (5 days, 5 h day‐1). Gas‐exchange measurements revealed a reduction in CO2 assimilation and stomatal conductance, accompanied by an increase in the intercellular CO2 concentration both in young and in mature leaves as compared to their respective controls. In both leaves, F0 remained unchanged, while Fm and the Fv/Fm ratio decreased after O3 fumigation, indicating that ozone may induce an alteration in the capability of photosystem II (PSII) to reduce the primary acceptor QA. In the mature leaves the photochemical quenching (qp) was significantly lowered by the pollutant, but this was not the case in the young leaves where qp did not change. In both mature and young ozonated pumpkin leaves, the development of non‐photochemical quenching caused a decrease in the PSII photochemical rate, as shown by the correlation between Fv/Fm and the de‐epoxidation state of dark‐adapted leaves. Decreases in the Fv/Fm ratio are generally attributed to damage to the PSII reaction centre, apart from the down‐regulation of the capacity of PSII electron transport. While in young ozonated leaves the decrease in the Fv/Fm ratio was not associated with damage to the D1 protein, in mature ozonated pumpkin leaves, the decrease in the Fv/Fm was accompanied by a significant decline in the D1 content. In conclusion, ozone exposure induces alterations in the light reactions of photosynthesis in both young and mature leaves. However, in young leaves the engagement of the xanthophyll cycle appears to counteract ozone effects against the photosynthetic apparatus as demonstrated by the absence of damage to the D1 protein. On the other hand, the loss of D1 protein in mature fumigated leaves suggests that the activation of the xanthophyll cycle is not sufficient to prevent photoinhibition, probably because a physiological state of senescence adds to the oxidative stress.