Sensitivity of the Chlorophyll Indirect Measurement In Fertigated Sweet Orange Trees.

As fertigation allows the application of nutrients more frequently, there is need for more dynamic methods to assess the nutritional status of the plants. With a portable equipment (SPAD-502/Minolta, in this work) that measures the green intensity of the leaves, it is possible to indirectly measure ​​the chlorophyll in the field, in a fast and simply way. Given that from 50% to 70% of the nitrogen (N) in the leaves are associated with chloroplasts enzymes, chlorophyll and leaf N concentrations are usually correlated. This study aimed to evaluate the sensitivity of the chlorophyll indirect measurement in citrics plants when applied different rates of N by fertigation. The work was conducted in the city of Reginópolis-SP/Brazil with Hamlin and Valencia orange trees, both grafted on citrumelo Swingle rootstock. The treatments consisted of five rates of N applied by fertigation: 0, 35, 70, 140 and 280 kg ha^-1 of ammonium nitrate. The chlorophyll indirect measurement was monthly evaluated in recent mature leaves (3^th and 4^th leaves), in the four quadrants, at the medium of the tree height, and during two seasons. The same leaves were used to assess N by leaf analysis. As both varieties showed similar behavior, the statistical analysis was done together. The SPAD responses and the leaf N concentration showed a quadratic behavior with respect to the N rates. The responses between 70 and 75 were related to plants with adequate concentrations of N. In the fitted curve, the maximum SPAD response was 78, which corresponded to 245 kg ha^-1 of N. A correlation of 0.95 (p <0.05) was observed between the SPAD measurement and the leaf N concentration. The results suggest that the chlorophyll indirect measurement can be used as a fast and non-destructive tool to monitor N in citrus plants.