Growth, senescence and water use efficiency of spring oilseed rape (Brassica napus L. cv. Mozart) grown in a factorial combination of nitrogen supply and elevated CO2

Atmospheric CO2 enrichment is expected to affect the resource use efficiency of C3 plants with respect to water, nutrients and light in an interactive manner. The responses of oilseed rape (OSR) to elevated CO2 have not much been addressed. Since the crop has low nitrogen use efficiency, the interactive effects of CO2 enrichment and nitrogen supply deserve particular attention.Spring OSR was grown in climate chambers simulating the seasonal increments of day length and temperature in South-Western Germany. Three levels of N fertilisation representing 75, 150 and 225 kg ha−1 and two CO2 concentrations (380 and 550 μmol mol−1) were used to investigate changes in source–sink relationships, plant development and senescence, water use efficiency of the dry matter production (WUEprod.), allocation patterns to different fractions, growth, yield and seed oil contents. Seven harvests were performed between 72 and 142 days after sowing (DAS).Overall, plant performance in the chambers was comparable to the development under field conditions. While CO2 responses were small in the plants receiving lowest N-levels, several significant N × CO2 interactions were observed in the other treatments. Increasing the N availability resulted in longer flowering windows, which were furthermore extended at elevated CO2 concentrations. Nevertheless, significantly less biomass was allocated to reproductive structures under elevated CO2, while the vegetative C-storing organs continued to grow. At the final harvest shoot mass of the CO2 exposed plants had increased by 9, 8 and 15% in the low, medium and high N treatments. Root growth was increased even more by 17, 43 and 33%, respectively and WUEprod. increased by 23, 42 and 35%. At the same time, seed oil contents were significantly reduced by CO2 enrichment in the treatments with ample N supply.Obviously, under high N-supply, the CO2 fertilisation induced exaggerated growth of vegetative tissues at the expense of reproductive structures. The interruption of source–sink relationships stimulated the formation of side shoots and flowers (branching out). While direct effects of elevated CO2 on flowering can be excluded, we assume that the increased growth under high N and CO2 supply created nutrient imbalances which hence affected flowering and seed set.Nevertheless, the final seed macronutrient concentrations were slightly increased by elevated CO2, indicating that remobilisation of nutrients from the sources (leaves) to the sinks (seeds) remained effective. These findings were supported by the lower nitrogen concentrations in senescing leaves and probably increased N remobilisation to other plant parts under elevated concentrations of CO2. All the same, CO2 enrichment caused a decline in seed oil contents, which may translate into a reduced crop quality.

► Plant performance of chamber grown spring oilseed rape was comparable to the development under field conditions.
► While CO2 responses were small in plants receiving lowest N-levels, several significant N × CO2 interactions were observed in the other treatments.
► Increasing the N availability resulted in longer flowering windows, which were furthermore extended at elevated CO2 concentrations.
► Despite the strong stimulation of shoot and root growth, significantly less biomass was allocated to reproductive structures under elevated CO2 and seed oil contents were significantly reduced by CO2 enrichment in the treatments with ample N supply.