A new curve of critical leaf nitrogen concentration based on the maximum root dry matter for diagnosing nitrogen nutritional status of sweetpotato

Context The unreasonable application of nitrogen fertilizer in sweetpotato seriously affects its yield. Rapid diagnosis of nitrogen nutrition in sweetpotato plays an important role in sweetpotato fertilizer management. Objective This study aimed to develop a new critical leaf N curve based on root dry matter (RDM) for rapid monitoring and diagnosis of root yield in sweetpotato. Methods Three field experiments were conducted using two sweetpotato cultivars, Yanshu 25 and Shang 19 with varying five N application rates (N0-N4). The biomass, nitrogen content of different parts, and leaf SPAD of sweetpotato were measured at each sampling date. The relationship between root dry matter and leaf N concentration was analyzed, and a model of critical N concentration for sweetpotato was constructed. Independent experimental data divided into N-limitation and non-N-limitation treatments were used for model validation. Results The critical leaf N (Nleaf) for sweetpotato based on RDM is determined by the equation Nleaf = 3. 98 RDM−0. 113, R2=0.9168. The critical leaf N concentration curve based on leaf dry matter (LDM) is higher than that based on RDM. With the increase of nitrogen application rates, the nitrogen nutrition index (NNI) increases continuously, while the dry matter weight of the underground part increased firstly and then decreased. The NNI of the two varieties were positively correlated with the normalized values of SPAD, and the determination coefficients R2 were 0.7416. Conclusion We found that (i) nitrogen is primarily supplied to the aboveground development during the early growth stage. And excessive nitrogen inhibits the differentiation of underground tuberous roots. (ii) The critical leaf nitrogen concentration model based on the RDM can be used to construct the NNI to determine the presence of nitrogen deficiency or excess during cultivation. (iii) The normalized SPAD value can accurately and rapidly estimate NNI, could be used for nitrogen fertilizer diagnosis in sweetpotato production. Significance The study demonstrates that the critical leaf N concentration curves based on RDM can effectively assess leaf N status and be used as reliable indicators for sweetpotato yield. In the actual production of sweetpotato, these results provided a rapid and accurate diagnosis tool for nitrogen nutrition management of sweetpotato.