| Abstract: | Crop growth is related to radiation‐use efficiency (RUE), which is influenced by the nitrogen (N) status of the crop, expressed at canopy level as specific leaf N (SLN) or at plant level as N nutrition index (NNI). To determine the mechanisms through which N affects dry‐matter (DM) production of forage kale, results from two experiments (N treatment range 0–500 kg ha?1) were analysed for fractional radiation interception (RI), accumulated radiation (Racc), RUE, N uptake, critical N concentration (Nc), NNI and SLN. The measured variables (DM, RI and SLN) and the calculated variables (NNI, Racc and RUE) increased with N supply. RUE increased from 0·74 and 0·89 g MJ?1 IPAR for the control treatments to 1·50 and 1·95 g MJ?1 IPAR under adequate N and water in both experiments. This represented an increase in RUE of 52–146% for the range of N treatments used in both experiments, whilst Racc increased by 9–17%, compared with the control treatments. Subsequently, the total DM yield of kale increased from 6·7 and 8 t DM ha?1 for the control treatments to ≥ 19 t DM ha?1 when ≥150 kg N ha?1 was applied. The DM yields for the 500 kg N ha?1 treatments were 25·5 and 27·6 t DM ha?1 for the two experiments. RUE increased linearly with SLN, at an average rate of 0·38 g DM MJ?1 IPAR per each additional 1 g N m?2 leaf until a maximum RUE of 1·90 g MJ?1 IPAR was reached in both experiments. There were no changes in RUE with SLN of > 2·6 g m?2 and NNI >1, implying luxury N uptake. RUE was the most dominant driver of forage kale DM yield increases in response to SLN and NNI. |
