The SPAD 502 Chlorophyll Meter can estimate leaf chlorophyll content as a surrogate measure of the nitrogen (N) status of plants, and therefore, it can be used to assess the N requirements of crops. In this work, we show that irradiance, leaf water status and time of measurement (i.e., morning vs. afternoon), may interfere with the SPAD 502 measurements. SPAD values increased by 2-3 units as relative leaf water content decreased from 94 to 87.5% 57492768n wheat leaves. A change in irradiance (from 1100 to 600-650 mmol m⁻² s⁻¹) caused an increase of about 2 SPAD units in potted maize plants. Throughout the day, SPAD values varied by up to 4 units in well-watered plants of maize growing under field conditions. The slope of the response of the SPAD meter to chlorophyll content was steeper in the morning than in the afternoon. Since the range of SPAD values that may separate N deficiency from N surplus in a crop are often relatively narrow, time of measurement, irradiance and plant water status must be taken into account to precisely monitor crop N needs with the SPAD 502.La mesure de la quantité de chlorophylle permet d’estimer la quantité d’azote des plantes, et donc la quantité d’azote à apporter aux cultures. Ici, nous montrons que l’irradiance, l’état hydrique des feuilles ainsi que le moment de la mesure (matin, après-midi) interfèrent avec la mesure de la quantité de chlorophylle.Instituto de Fisiología Vegeta
