Evaluation of the sensitivity of a15N test for nitrogen deficiency in plants

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Abstract
A test for nitrogen (N) deficiency in plants showed that rate of uptake of 15N by excised roots was inversely related to the amount of N supplied to the growing plant. Deficiency was successfully identified from the rate of uptake of 15NH4 + from solution in a two hour period, and this was adopted in the early development of the method. In this study, modifications were tested in a sensitivity analysis, to compare 15NH4 + and 15NO3 − uptake for different times of immersion and with different washing treatments, to determine the most sensitive method. Common bentgrass (Agrostis capillaris L.), grown in sand culture at a range of N concentrations from deficient to optimum, was the test plant. Although uptake by roots from N deficient treatments was always significantly higher for both ammonium (NH4) and nitrate (NO3), the uptake of NH4 was two to five times higher than that of NO3. The rate of uptake of both ions was approximately linear over 20 mins, one hour, or two hours. Although a two hour period of immersion would be necessary in some cases to obtain sufficient 15N in the roots for analysis, it should therefore be possible to interpolate, for direct comparison of rates of uptake by different species, and also with results from similar phosphorus (P) and potassium (K) bioassays, where radioisotope uptake is measured over a shorter time (15 min). The results suggest that the method initially adopted was the most sensitive to identify N deficiency.