Effect of chronic ozone fumigation on the photosynthetic process of poplarclones showing different sensitivity

Rooted cuttings from two poplar clones (Populus x euramericana, I-214. and Populus deltoides x maximowiczii, Eridano) were exposed for 15 days to diur nal square-wave treatment with ozone (60 nL L-1 for 5 h day(-1)). Completel y fully expanded leaves exposed to ozone showed a reduction in net CO2 assi milation rate as compared to the control leaves during whole exposure perio d in both the clones. The reduction was related to a strong stomatal closur e in clone I-214, but also to an altered mesophyll activity ascribed to lim itation of the dark reactions of photosynthetic process. The results obtain ed in leaves of I-214 subjected to long-term fumigation seem to support the view that the decrease in quantum yield of electron transport may be a mec hanism to down-regulate photosynthetic electron transport so that productio n of ATP and NADPH would be in equilibrium with the decreased demand in the Calvin cycle. In Eridano the CO2 assimilation was reduced because of the e xposure and any alteration in stomatal conductance was observed. Thus, chlo rophyll fluorescence parameters showed that an inhibition of photosystem II had occurred (reduction in F-v/F-m, ratio), while no alterations in quench ing parameters were observed upon illumination. The results seem to indicat e that an alternative sink for reducing equivalent, other than carbon metab olism is present. (C) 2001 Elsevier Science Ltd. All rights reserved.