Effect of crop nitrogen status and temperature on the radiation use efficiency of winter oilseed rape

In temperate environments, the total dry matter (TDM) of a crop is closely related to the amount of photosynthetically active radiation absorbed (Sigma PARa), as long as other factors (water, nutrients,...) are non-limiting. For oilseed rape crops, many authors have shown that the radiation use efficiency (RUEa) varies within a wide range, from 1 to 4 g MJ(-1) of Sigma PARa according to developmental stage and environmental conditions. In order to explain this variability, effects of N and temperature on RUEa were investigated during a 1-year field experiment involving three N treatments (N fertilisation rates: 0, 135 and 270 kg ha(-1)). Leaf, flower, and pod surface areas, as well as the DM and N content of the various plant parts were measured every 2-4 weeks for 17 sampling dates from emergence to harvest. RUEa was calculated from total generated DM (shoot and root DM, plus that of dead leaves fallen to the ground). Daily PARa was calculated using a 3-layer model laking into account leaf and pod absorption and reflection of PAR by flowers and soil. The N nutrition index (NNI) proposed by Lemaire and Gastal (1997) was used to evaluate the N effect on RUEa. NNI was significantly higher than the critical N value (meaning that N was non-limiting) for the N270 treatment from emergence to pod formation, but N deficiency occurred with NO at the 12-leaf stage and later with N135 at flowering. The maximum possible RUEa (RUEa(max)) was assumed to be the value obtained with the treatment N270, where N was non-limiting. The N deficiencies which occurred for NO and N135 significantly reduced the green LAI and PAI, and consequently Sigma PARa. To remove any effect other than N on the RUEa, the ratio of actual to maximum RUEa (RUEa/RUEa(max)) proposed by Belanger et al. (1992) was calculated for each developmental stage of oilseed rape. A linear regression fit well (R-2=0.919; 8 d.f.) the response of RUEa/RUEa(max) versus N deficiency, for values of NNI lower than 1. The resulting equation was the following: RUEa/RUEa(max) = 0.74 x NNI + 0.23. RUEa(max) was also significantly affected by developmental stage. Whereas the corresponding changes in RUEa(max) from the 6-leaf stage to the end of flowering could be related to air temperature, there was evidence of a developmental effect in the other stages. RUEa was lower in the early stages (emergence to 5-6 leaves), and from pod formation until ripening; the latter decrease could be attributed to the high energy cost of lipid biosynthesis. (C) 2000 Elsevier Science B.V. All rights reserved.