Impact of crop load management on terpene content in gewürztraminer grapes

Abstract

Context and purpose of the study ‐ Crop load management by cluster thinning can improve ripening and the concentration of key metabolites for grape and wine quality. However, little work has been done on testing the impact of crop load management on terpene content of white grapes. The goal of the study was to assess if by reducing crop load via cluster thinning growers can increase terpene concentration of grapes, as well as to test if the timing of thinning application affects terpene concentration.

Material and methods ‐ This study was performed in 2016, 2017, and 2018 in Oliver, British Columbia. Field‐grown Gewürztraminer vines were cluster‐thinned at two developmental stages, just after fruit‐set (Early Thinning) and at veraison (Late Thinning), in order to target three crop levels: Light Crop (7 tons/ha), Moderate Crop (10.5 tons/ha), and High Crop (14 tons/ha). Treatments were replicated on five plots arranged in a randomized block design. The effect of treatments on leaf gas exchanges, vine leaf area, and berry sugar (total soluble solid, TSS), acid (titratable acidity, TA), and terpene concentration was analyzed during ripening and at harvest. Free and glycosylated terpenes were identified and quantified using a SPME‐GC‐MS and a LI‐GC‐MS, respectively.

Results ‐ Crop level treatmentsdid not affect leaf gas exchanges and vine leaf area. TSS concentration during ripening and at harvest was higher in Light Crop and Moderate Crop treatments than in High Crop, particularly for Early Thinning treatments. High Crop and Light Crop‐Early Thinning determined the highest free terpene concentration at harvest; however, a significant interaction between treatment and year effects was observed. Total glycosylated terpenes at harvest were marginally affected by treatments (P = 0.063), and Light Crop‐Early Thinning determined the highest total glycosylated terpene concentration. Interestingly, total free terpenes were significantly affected by the treatments at the sampling before harvest (20‐21 Brix), when Light Crop‐Early Thinning determined a higher concentration of total free terpenes than High Crop. This result was consistently among the three years. Our study suggests that crop load management can be used as a tool to improve grape terpenes in scenarios (regions and/or seasons) where ripening is impaired and grapes cannot reach relatively high sugar levels.