Spatial Variability of Grape Berry Maturation Program at the Molecular Level

The application of sensors in viticulture is a fast and efficient method to monitor grapevine vegetative, yield, and quality parameters and determine spatial intra-vineyard variability. Molecular analysis at the gene expression level can further contribute to the understanding of the observed variability by elucidating how pathways contributing to different grape quality traits behave in zones diverging on any of these parameters. The intra-vineyard variability of a Cabernet Sauvignon vineyard was evaluated through a Normalized Difference Vegetation Index (NDVI) map calculated from a multispectral image and detailed ground-truthing (e.g., vegetative, yield, and berry ripening compositional parameters). The relationships between NDVI and ground measurements were explored by correlation analysis. Moreover, berries were investigated by microarray gene expression analysis performed at five time points from fruit set to full ripening. Comparison between the transcriptomes of samples taken from locations with the highest and lowest NDVI values identified 968 differentially expressed genes. Spatial variability maps of the expression level of key berry ripening genes showed consistent patterns aligned with the vineyard vigor map. These insights indicate that berries from different vigor zones present distinct molecular maturation programs and suggest that transcriptome analysis may be a valuable tool for the management of vineyard variability. The experiment was conducted in 2017 growing season in a commercial vineyard of approximately 12 hectares of Vitis vinifera cv. Cabernet Sauvignon (FPS clone 8 grafted onto 110R rootstock). The vineyard was planted in 1992 and located in the Alexander Valley American Viticultural Area of California (38°42'50N, 122°55'4W). The vines were trained as a bilateral cordon, established at 1.5 m above the ground, in a split canopy configuration, with the two cordons installed on two separate wires and pointing in opposite directions. Vine and row spacings were 1.8 and 3.35 m respectively. At each vineyard spot, four fully developed inflorescences (pre-bloom) per vine were labeled (therefore, a total of 36 clusters per spot). At each time point (from fruit set to full maturity), one berry per marked cluster was selected determining a sampling size of 36 berries per spot. Prior flash freezing in liquid nitrogen, the berries were weighed to calculate the spot average berry weight and represent berry growing profiles by vineyard spot. Eighteen frozen berries were ground in liquid nitrogen: 1 g of frozen powder was thawed to measure TSS over time, whereas the remaining material was stored and later used for gene expression analysis.