Response of gas exchange of rape to ozone concentration and exposure regime

Gas exchange characteristics, Pn-PAR and/or Pn-CO2 response parameters of rape (Brassica napus L) were studied in the field under different O3 concentrations (CF, 50 nl•L−1 and 100 nl•L−1) and fumigation regimes (constant concentration and dynamic varying concentrations) with a portable photosynthetic system (CIRAS-1). Results indicated: (1) Relatively to CF, higher ozone concentration decreased in Tr and increased in WUE under a constant concentration fumigation regime. In addition, an increase in Tr and decreases in Pn, Gs and WUE were found in the dynamic ozone exposure regime; (2) In the constant concentration exposure regimes, AQY, LSP and Pmax were markedly reduced and Rp and Γ were enhanced with an increase in ozone concentration (100 nl•L−1). There were significant differences in the parameters such as Rd, LCP, LSP, Pmax and CE between O3 exposure regimes; (3) Regardless of the exposure regimes which were imposed, the increase of O3 concentration induced significant decreases in Fv/Fo and Fv/Fm of the eighth leaves from the top canopy, but it had no effect on the fifth full-spread leaves. It can be concluded that the dynamic ozone exposure regime has greater detrimental effects on the photosynthesis of rape in spite of equal exposure dose, and thus is unfavorable for plant growth and the accumulation of dry matter.