Cumulative response of Tempranillo vines to the crop forcing technique and pre-forcing and post-veraison water stress in terms of yield and grape and wine quality

Elevated temperatures during berry ripening are detrimental to grape quality. The crop forcing technique (summer pruning that 'forces' the vine to start a new cycle) increases must acidity and malic acid concentration at harvest by delaying the date of veraison. However, little information is available on the sensitivity to water stress of forced vines. A 3-year trial was conducted to test three irrigation strategies in forced vines: a minimum threshold of mid-day stem water potential (psi(s)) of -0.75 MPa before forcing (DI), a minimum psi(s) threshold of -1.2 MPa only after veraison (RDI), and the combination of both treatments (DI + RDI). Results were compared to a non-forced treatment with a minimum psi(s) threshold of -1.2 MPa after veraison (C-RDI). Must acidity increased, and pH decreased in the forced treatments. However, yield was reduced by 35% and irrigation requirements increased by 20% when comparing forced and unforced treatments. As a result, water use efficiency was reduced in forced treatments. Only after a dry spring did the, DI (11%) and DI + RDI (30%) treatments, save water compared to the C-RDI treatment. Moreover, although psi(s) before forcing never fell below -0.75 MPa, a significant negative correlation (R-2 = 0.76) was found between the integral of water stress before the vines were forced and the number of forced bunches per vine. Post-veraison water stress in forced vines reduced the polyphenol content of the wine. Our findings suggest that forced vines are extremely sensitive to even mild water stress.

Automated summary

Elevated temperatures during grape ripening reduce quality. The crop forcing technique increases acidity and malic acid concentration by delaying veraison, but sensitivity to water stress is not well understood. A 3-year trial tested different irrigation strategies and found that forced vines had increased acidity but reduced yield and water use efficiency compared to unforced vines. Mild water stress before forcing and post-veraison reduced bunches per vine and polyphenol content of the wine, indicating high sensitivity of forced vines to water stress.