Early detection of N deficiency in a wheat crop using physiological and radiometric methods

A winter wheat (cv Soissons) crop was made N deficient during stem elongation. The effects of the N stress were quantified by measuring the nitrogen nutrition index (NNI) (used as reference), proteolytic activity in the leaves, the rate of carbon dioxide (CO(2)) assimilation by the leaves, green leaf area, chlorophyll concentration of the leaves and crop reflectance. A reduction in available soil N 1 week after the onset of N stress resulted in decreases in NNI and CO(2) assimilation rate, a change in leaf chlorophyll concentration affecting the lower leaves most severely, and in changes in crop reflectance. Green leaf area and leaf proteolytic activity were affected only after 2 weeks of N stress. At the onset of N deficiency, NNI was correlated with the relative leaf CO(2) assimilation rate, green leaf area, the near infrared to red reflectance ratio (XS3/XS2) and the chlorophyll concentration in the lower leaves, estimated by a SPAD value. The CO(2) assimilation rate was assessed by measuring chlorophyll concentration irrespective of N status. The non-destructive measurements of crop reflectance and leaf chlorophyll concentration appear to be appropriate indicators of the current N status of the crop. ((C) Inra/Elsevier, Paris.).