Soil proximal sensing provides direction in delineating plant water status of ‘crimson seedless’ (Vitis vinifera L.) vineyards

Abstract

Context and Purpose of the Study – ‘Crimson Seedless’ (Vitis vinifera L.) is a late-ripening, red seedless table grape cultivar with inadequate anthocyanin accumulation and less than ideal berry size issues. It was necessary to understand the natural variations in the vineyard as well as the application of proximal sensing to monitor, and estimate these variations to get desirable attributes in this cultivar. The objective of this study was to use of proximal and remote sensing tools, specifically soil electrical conductivity (EC), canopy normalized difference vegetation index (NDVI), and carbon isotope discrimination in a precision agriculture context, to assess the water status variability, and determine the effect of inferred variability on skin anthocyanin and flavonol concentration at harvest.

Material and Methods – A ‘Crimson Seedless’ (V. vinifera L.) grafted on to ‘Freedom’ (27% vinifera hybrid) rootstock vineyard was studied for two years with contrasting precipitation amounts. Soil electrical conductivity (EC) was proximally sensed with electromagnetic induction and canopy reflectance was sensed remotely to calculate normalized difference vegetation index (NDVI). Random and equi-distant (30 m × 30 m) sampling grids were utilized in 2016 and 2017 to ground truth proximally sensed data. Grape primary metabolites, including total soluble solids, total acidity, isotopic discrimination of berry sugars (δ13C) and pH were measured, and secondary metabolites were characterized with a C18 reversed-phase HPLC.

Results – Soil EC was related to the variation of season-long plant water status in 2016 (Deep EC: r = -0.71; Surface EC: r = -0.53). There was not a significant relationship between NDVI and plant water status in either year.  The vineyard was separated and delineated into two water status zones based on stem water potential (􀀁stem) in each year, and the water status between two zones were significantly and consistently different. The juice pH showed significant differences between two zones. The δ13C was directly and significantly related to 􀀁stem integrals and the differences between the two water status zones were confirmed by either method in 2016. There were no differences in total anthocyanins in 2016. However, anthocyanin derivatives were greater in the low water status zone in the following year. Flavonol amounts were not consistently different between the two zones in either year. Our results indicated deep soil EC, season-long water status or δ13C can be used interchangeably to spatialize and cluster management zones in commercial table grape vineyards.