CO2 assimilation, xanthophyll cycle pigments and PSII efficiency in pumpkin plants as affected by ozone fumigation

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, F-0 remained unchanged, while F-m and the F-v/F-m ratio decreased after O-3 fumigation, indicating that ozone may induce an alteration in the capability of photosystem II (PSII) to reduce the primary acceptor Q(A). In the mature leaves the photochemical quenching (q(p)) was significantly lowered by the pollutant, but this was not the case in the young leaves where q(p) 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 F-v/F-m and the de-epoxidation state of dark adapted leaves. Decreases in the F-v/F-m 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 F-v/F-m ratio was not associated with damage to the D1 protein, in mature ozonated pumpkin leaves, the decrease in the F-v/F-m 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.