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Abstract
Standardization of sampling methodology is essential when using plant analysis as an indicator of the nutrition status of a crop. Time of sampling during the day is a factor which was shown to affect tissue nitrate (NO3) concentration of irrigated crops. The diurnal variation of NO3 concentration in small grain cereal plants, grown under rainfed mediterranean conditions, was tested by an in situ sap test and by analysing dried tissue. Variation in NO3 concentration was shown to exist among plants grown under field conditions. Variation was also shown to exist at different hours of sampling within the day. However, these variations did not follow any diurnal trend and the magnitude of the difference in NO3 concentration did not affect the diagnosis of the nitrogen (N) nutrition status of the crop. The results suggest that diurnal variation of NO3 concentration of cereals grown under rainfed Mediterranean conditions is not a major concern in using NO3 tissue tests to predict N availability provided that sampling is done during the daylight hours.