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 intercell ular 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, indica ting that ozone may induce an alteration in the capability of photosys tem II (PSII) to reduce the primary acceptor Q(A). In the mature leave s the photochemical quenching (q(p)) was significantly lowered by the pollutant, but this was not the case in the young leaves where q(p) di d not change. In both mature and young ozonated pumpkin leaves, the de velopment of non-photochemical quenching caused a decrease in the PSII photochemical rate, as shown by the correlation between F-v/F-m and t he de-epoxidation state of dark adapted leaves. Decreases in the F-v/F -m ratio are generally attributed to damage to the PSII reaction centr e, apart from the down-regulation of the capacity of PSII electron tra nsport. While in young ozonated leaves the decrease in the F-v/F-m rat io was not associated with damage to the D1 protein, in mature ozonate d pumpkin leaves, the decrease in the F-v/F-m was accompanied by a sig nificant decline in the D1 content. In conclusion, ozone exposure indu ces alterations in the Light reactions of photosynthesis in both young and mature leaves. However, in young leaves the engagement of the xan thophyll cycle appears to counteract ozone effects against the photosy nthetic apparatus as demonstrated by the absence of damage to the D1 p rotein. On the other hand, the loss of D1 protein in mature fumigated leaves suggests that the activation of the xanthophyll cycle is not su fficient to prevent photoinhibition, probably because a physiological state of senescence adds to the oxidative stress.