Fluorescence optical method for heterogeneity evaluation of leaf nitrogen and grape technological maturity of Vermentino cv.

Abstract

Non-destructive mapping of the leaf nitrogen index (NBI₁) at bunch closure was carried out at the ‘Fattoria di Magliano’ winery (South Tuscany, Italy) on two consecutive years, 2013 and 2014, on Vermentino cv. using the Multiplex fluorescence optical sensor. Measurements were performed manually on a 1 ha vineyard using a 15m x 15m grid, detecting leaves with similar age of three adjacent vines on two contiguous rows. The spatial heterogeneity of leaf nitrogen (N) content, based on the calibration of NBI₁ against destructive analyses, was reported as a colorimetric map with 1 x 1 m grid resolution, after kriging. The 2013 map showed lower N levels compared to the 2014 map with a range of leaf N between 1.64% and 2.14%. The effect of a winter N-fertilization was nicely evidenced by the 2014 N map, showing the recovery in leaf N up to 2.64%. Just before the 2014 harvest, the heterogeneity of grape maturity on the same vineyard was evaluated by mapping the fluorescence-based Chl index, using a 15m x 15m grid. For each point, six bunches from three adjacent vines on two contiguous rows were detected. The grape soluble solids content (SSC), estimated by means of a calibration of the Chl index, ranged between 13.8 and 21.2°Bx. Two areas with lower and higher SSC were estimated by segmentation based on the median (16.9°Bx), and confirmed by the destructive SSC determination on the related grape samplings, which resulted to be 16.3°Bx and 19.4°Bx, respectively. Our work was aimed to show the utility of a non-destructive optical sensor in precision viticulture to identify distribution and heterogeneity in leaf nitrogen and in grape berries technological maturity. The sensor could be use to split the vineyard in subzones managed differently, to correct N deficiencies only where needed reducing fertilizer leakage and to perform selective harvest either as single pick or as separate picks.