Radiation‐use efficiency for forage kale crops grown under different nitrogen application rates

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 ( R acc ), RUE , N uptake, critical N concentration ( N c ), NNI and SLN . The measured variables ( DM , RI and SLN ) and the calculated variables ( NNI , R acc 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 R acc 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 .