Comparative response of wheat and oilseed rape to nitrogen supply: absorption and utilisation efficiency of radiation and nitrogen during the reproductive stages determining yield

We investigated the response of spring wheat and oilseed rape to nitrogen ( N) supply, focusing on the critical period for grain number definition and grain filling. Crops were grown in containers under a shelter and treated w ith five combinations of applied N. Wheat and oilseed rape produced compara ble amounts of biomass and yield when corrected for the costs of biomass sy nthesis (SC). From the responses of biomass and yield to late N application s and the apparent contribution of mobilised biomass to yield, it seems tha t the yield of oilseed rape was more source-limited during grain filling th an that of wheat, particularly at the medium and high N levels. Both specie s recovered equal amounts of N from the total available N in the soil and h ad similar N use efficiencies, expressed as yield per unit of N absorbed. H owever, oilseed rape had higher efficiency to convert absorbed N in biomass , but lower harvest index of N than wheat. Oilseed rape had similar or lowe r root biomass than wheat, depending on N level, but higher root length per unit soil volume and specific root length. The specific uptake rate of N p er unit root dry weight during the critical period for grain number determi nation was higher in oilseed rape than in wheat. In wheat, N limitation aff ected growth through a similar or lower reduction in radiation use efficien cy corrected for synthesis costs (RUESC) than in the cumulative amount of i ntercepted photosynthetically active radiation (IPARc). In oilseed rape, lo wer growth due to N shortage was associated more with RUESC than IPARc, dur ing flowering while during grain filling both components contributed simila rly to decreased growth. RUESC and the concentration of N in leaves and inf lorescence (LIN%) decreased from flowering to maturity and were curvilinear ly related. Oilseed rape tended to have higher RUESC than wheat at high N s upply during the critical period for grain number determination, and genera lly lower during grain filling. The reasons for these differences and possi bilities to increase yield potential are discussed in terms of the photosyn thetic efficiency of the different organs and changes in source-sink ratio during reproductive stages.