Photosynthesis and crop growth of spring oilseed rape and broccoli under elevated tropospheric ozone
De Bock, M. | Ceulemans, R. | Horemans, N. | Guisez, Y. | Vandermeiren, K.
The impact of a season-long exposure to moderately elevated tropospheric O₃ concentrations on the canopy growth and photosynthetic capacity of two important Brassica crops, spring oilseed rape and broccoli, was studied during three consecutive growing seasons (2007–2009). Brassica napus L. cv. Ability and Brassica oleracea L. cv. Monaco were exposed to non-filtered ambient air (NF) and non-filtered air with addition of 20 (NF+) and/or 40ppb O₃ (NF++) in open-top chambers. Light saturated CO₂ assimilation (Aₛₐₜ), stomatal conductance (gₛₜ), maximum and actual quantum yield of photosystem II (Fᵥ/Fₘ, Fᵥ′/Fₘ′), performance index (PI) and leaf area index (LAI) were monitored on a weekly basis from emergence or planting until harvest. Before flowering, elevated O₃ did not have an influence on LAI nor on the photosynthetic capacity of the upper canopy leaves of either crops. This corresponded with the absence of a reduction of aboveground biomass of oilseed rape at maximum leaf area (MLA) and of broccoli plants harvested before flowering. After flowering, which coincided with MLA, the oilseed rape canopy showed a faster decline of LAI and of the chlorophyll content in NF+ compared to NF. In the NF++ treatment, this effect was intensified with an additional decrease of Aₛₐₜ, gₛₜ, Fᵥ/Fₘ, Fᵥ′/Fₘ′ and PI. In broccoli these detrimental O₃ effects were only detected in the lower canopy leaves. The changes in canopy development and photosynthetic performance of the upper canopy leaves can unravel the underlying mechanisms leading to the contrasting yield effects of O₃ on broccoli and spring oilseed rape that were previously reported (De Bock et al., 2011).Show more [+] Less [-]