Effects of free-air CO2 enrichment on the growth of summer oilseed rape (Brassica napus cv. Campino)

The cultivated area of oilseed rape (Brassica napus L., OSR) is currently rapidly expanding primarily due to its growing importance as a bioenergy crop. While the increasing use of biodiesel is realised as a measure to improve the carbon balance, little is known about the yield and quality responses of the crop to climate change and the increase of atmospheric carbon dioxide (CO2). Here we report on the first field study addressing OSR performance in a free-air CO2 enrichment (FACE) system, in which plants were exposed to moderately elevated CO2 levels for a complete growing season. Three treatments were used with five replicates each, consisting of a control (CON) without a fumigation ring, an ambient treatment (AMB) with a fumigation ring but without CO2 supply and a treatment with a ring and elevated CO2 concentration (FACE). The mean seasonal daylight (08:00–20:00 CET) concentrations were 388 (±9) μmol mol−1 in the AMB and 494 (±16) μmol mol−1 CO2 in the FACE treatments. Two intermediate and a final harvest were performed to study the effects of CO2 on plant development, allocation patterns and the final seed and oil yield. By the first harvest (59 days after sowing, DAS) CO2 enrichment had significantly increased plant height and the dry weight of reproductive organs, indicating that plant development and the reallocation from vegetative to generative organs were sped up. By the second harvest (78 DAS) shoot weight was significantly increased by 40% (p = 0.013) under elevated CO2. At the same time the dry weights of senescent leaves were significantly higher in the plants from the FACE treatment, also suggesting that the plant phenology was affected by CO2. At the final harvest shoot biomass per plant still tended to be higher by 20% (p = 0.064) in the FACE as compared to the AMB treatments, but seed output per plant tended to increase to a lesser extent (+17%, p = 0.092). While seed oil content did not change significantly under elevated CO2, oil yield tended to increase by 15% (p = 0.076) on average. The only significant effect was the increase by 17% (p = 0.046) of threshed pod wall biomass per area, while the harvest index (HI) was not significantly affected by the CO2 enrichment. The results suggest that the positive effects of the CO2 fertilisation on growth of OSR diminish after flowering of the plants and will not translate into a significant increase of seed oil contents and oil yields. Testing the growth responses of further OSR cultivars to a moderate CO2 enrichment may enable plant breeders to select those lines in which CO2 fertilisation will also bring about a significantly increased seed and oil output.