The diagnosis of nitrogen deficiency in wheat by means of a critical nitrate concentration in stem bases

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Abstract
Stem bases from wheat plants in a glasshouse pot experiment conducted under varying nitrogen and two water regimes, were analysed for nitrate (NO3) concentration. The concentration of NO3 at three stages of growth i.e. tillering, jointing and anthesis were related both to rates of applied nitrogen and to shoot dry matter yield at time of sampling. Plotted against rate of increasing nitrogen application, the response curve of NO3 concentration in wheat stem bases was sigmoidal. The level of nitrogen application at which NO3 began to accumulate in the plants was the supply at which plants reached maximum dry matter yield. The concentration of NO3 at which plant yield was 90% of maximum dry matter was taken as the critical level. This concentration was around 1000 ppm NO3‐N at all stages. Compared with plants supplied with unlimited water, plants under moderate water stress accumulated relatively more NO3 but had a similar critical NO3 concentration. Maximum grain yield was obtained from plants which stayed above the critical level throughout the growing season.