SENSORES PARA ESTIMATIVA DE NITROGÊNIO E PIGMENTOS FOTOSSINTETIZANTES EM SORGO SACARINO

Abstract

In Brazil, among the various renewable raw materials available for ethanol production, special emphasis has been given to the culture of sweet sorghum. However, the fertilization recommendation for crop still lacks much information, especially for nitrogen. Thus, the objective of this work is to evaluate the indirect estimation of nitrogen “status” in leaves of sweet sorghum through readings with portable chlorophyll sensors. The first experiment was carried out in the 2017/18 season in an experimental area belonging to Nexsteppe Sementes do Brasil LTDA, in the municipality of Rio Verde – GO, under a dystrophic Red Latosol with constructed soil fertility. The experimental design was randomized blocks, with 5 x 5 split plots and four replications. The plots consisted of five N doses (urea source) applied at sowing, with: 0; 15; 30; 45 and 60 kg ha-1 of N, and in topdressing in the subplots with: 0; 100; 200; 300 and 400 kg ha-1 of N, applied at sowing at stage V5 of crop development. The second experiment was carried out in greenhouse at the UFG agronomy school, campus Goiânia. The experimental design was completely randomized blocks, with a 3 x 6 factorial scheme and five replications. The treatments consisted of three types of soil, with: LVw – Acrustox, RQo – Quartzpsammment and LVd – Haplustox and six N doses (urea source) with: 0; 50; 100; 150; 200 and 300 kg ha-1 of N, applied in topdressing at stage V3 of plant development. The readings were taken at stages V5 and V9, in the first trial, and at stages V6, V8 and V10 in the second trial, where the portable devices ClorofiLog® (Falker, Brazil), SPAD-502® (Minolta, Japan) and Dualex® (Force-A, France) were used. After the readings, leaf disc samples were collected for pigment analysis by the destructive method in DMSO and leaf samples to determine the total N content in plant tissues. The readings taken with portable chlorophyll meters, to verify the responses to fertilization with N doses, are not very accurate in the initial stages of sorghum development. The portable chlorophyll meters ClorofiLog®, SPAD-502® and Dualex® can be useful and reliable tools for the indirect determination of N contents in sorghum leaves since the total chlorophyll results were very strongly correlated with the N contents and for photosynthetic pigments obtained by the destructive method from the V8 stage of plant development. In sandy textured soil, the readings with the portable chlorophyll meters SPAD® and Dualex® estimated with high precision the dry mass production indices in plants with six fully developed leaves. In stage V10, the best performance of sweet sorghum, considering the dry mass production, the N content of leaf and total chlorophyll obtained by the destructive method were reached with the average application in topdressing of 200 kg ha-1 of N in different types of soil.