Correlation between Pummelo Leaf Nitrogen Concentrations Determined by Combustion Method and Kjeldahl Method and their Relationship with SPAD Values from Portable Chlorophyll Meter

Abstract

Discrepancies in reports of leaf tissue nitrogen (N) concentrations are in part due to different analytical methods among laboratories. Two standard methods of N determination, the combustion method and the Kjeldahl method, were compared for pummelo [Citrus maxima (Burm. f.) Merr.] leaf N analysis. Pummelo plants were fertilized with different N rates and leaves with different ages were sampled to obtain various leaf N levels. Leaf greenness (SPAD value) of each sample was measured by a portable chlorophyll meter prior to nitrogen analysis. The results showed that N concentrations of pummelo leaf samples were in the range 1.23-2.54% using the combustion method and 1.00-2.08% for the Kjeldahl method. The combustion method yielded higher N concentration values than the Kjeldahl method in every sample. A linear relationship between the combustion and Kjeldahl N values was highly significant (Kjeldahl N (%) = 0.7829 × Combustion N (%) + 0.1150; R² = 0.86 and p-value < 0.0001). The addition of samples of pummelo fruit parts that had wider ranges of N concentrations (0.74-4.08% for the combustion method and 0.57-3.51 % for the Kjeldahl method) than leaf tissue into the analysis increased the coefficient of determination (Kjeldahl N (%) = 0.8676 × Combustion N (%) – 0.0384; R² = 0.96 and p-value < 0.0001). Therefore, comparisons and interpretation of pummelo leaf N analysis results from different laboratories should mention the analytical method used and provide a linear equation for adjusting the N values. In addition, SPAD values had a positive correlation with leaf N concentrations determined by both methods and could be explained by quadratic polynomial equations (Combustion N (%) = 0.0003 × SPAD² – 0.0313 × SPAD+2.5024; R² = 0.21 and p-value < 0.0001; Kjeldahl N (%) = 0.0004 × SPAD² – 0.0372 × SPAD+2.4086; R² = 0.16 and p-value < 0.0001). It is suggested that the SPAD value determined by a portable chlorophyll meter can be used to obtain a quick estimation of pummelo leaf N status.