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

Abstract

CO2 assimilation, xanthophyll cycle pigments and PSII efficiency in pumpkin plants as affected by ozone fumigation. -Physiol. Plant. 101: 881-889.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 moF of ozone (5 days, 5 h day"'). 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, Fo remained unchanged, while F^ and the Fv/Fjn 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/F^ and the de-epoxidation state of dark-adapted leaves. Decreases in the Fv/F^ ratio are generally attributed to damage to the PSII reaction centre, apart from the down-regulation ofthe capacity of PSII electron transport. While in young ozonated leaves the decrease in the Fv/Fn, ratio was not associated with damage to the Dl protein, in mature ozonated pumpkin leaves, the decrease in the Fy/Fn, was accompanied by a significant decline in the Dl 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 Dl protein. On the other hand, the loss of Dl 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.

Estimating photosynthetic light-use efficiency using the photochemical reflectance index: variations among species

“…2003). The precise mechanisms whereby the photoprotective pathways dissipate excess light energy are yet to be fully elucidated (Adams and Demmig-Adams 1995;Ciompi et al . 1997;Gilmore 1997;Niyogi 1999;Matsubara et al .…”