Portable Chlorophyll Meter for Indirect Evaluation of Photosynthetic Pigments and Nitrogen Content in Sweet Sorghum

Portable chlorophyll meters can be a useful tool for nitrogen (N) determination in crops, particularly in those that show a high correlation between N levels in plants tissues and chlorophyll readings. However, there is a lack of information regarding these correlations in some crops, such as sweet sorghum. The aim of this study was to determine the relationship between the photosynthetic pigments extracted in dimethylsulfoxide and N content in leaves with the readings of a portable chlorophyll meter (ClorofiLog®), as a function of the application of N doses in sweet sorghum. The experimental design was a split-plot, with five N doses applied at sowing (0, 15, 30, 45 and 60 kg N ha−1) in the main plot and as subplots were considered five topdressing of N doses in order to reach 0, 100, 200, 300 and 400 kg N ha−1. At 25 and 43 days after emergence, portable chlorophyll meter readings, destructive determinations of photosynthetic pigments and N content in sorghum leaves were measured. At harvesting, biometric and yield characteristics of plants were determined. ClorofiLog® was not able to estimate N status at V5 stage, but it can assess the N status in sorghum leaves at V9 stage. At the latter stage, topdressing N promoted greater increases on dry mass, the chlorophyll b index (obtained via ClorofiLog®) and total chlorophyll (obtained by the destructive method).