Photosynthesis and crop growth of spring oilseed rape and broccoli under elevated tropospheric ozone

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 40 ppb 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).

► After flowering, chlorophyll content and leaf area of oilseed rape were reduced by O3.
► At the highest O3 exposure photosynthetic capacity of oilseed rape was also reduced.
► Photosynthesis of upper canopy leaves and leaf area of broccoli were not affected by O3.
► Photosynthetic performance of upper canopy supported O3 effects on biomass and yield.