Four-year field study reveals variable effects of phytohormone- and natural-based elicitors on anthocyanin metabolism in Tempranillo grapes.

Climate change has raised concerns about the imbalance between anthocyanin and sugar levels in grapevines. High temperatures tend to reduce anthocyanin content by inhibiting its synthesis and increasing degradation, while simultaneously enhancing sugar accumulation. The application of elicitors, which promote the anthocyanin biosynthetic pathway, may help alleviate this imbalance. To investigate this, a four-year field trial was conducted on Vitis vinifera cv. Tempranillo to assess the effects of four commercial products: two natural-based (an antioxidant-mannitol and an alginic acid-mannitol products) and two phytohormone-based (an abscisic acid and an ethephon-based products). The study evaluated the effect of these treatments on the phenyl propanoid pathway gene expression, various yield parameters, and the content of primary and secondary metabolites. Although none of the treatments consistently increased anthocyanin levels across seasons, the results varied depending on the year. Two products influenced some parameters in specific seasons: the ethephon-based product, which influenced the anthocyanin-to-sugar ratio through the reduction of TSS and induced the expression of key anthocyanin genes, and the natural-based antioxidant-mannitol product, which transiently increased the expression of some phenylpropanoid genes. None of the treatments increased anthocyanin levels across seasons. Ethephon (Fruitel) showed greater effectiveness under temperature stress and reduced sugar accumulation, which may be advantageous in a warming climate. Natural elicitors like antioxidant-mannitol elicitor (Vitalfit) had short-term effects on gene expression but no lasting impact on anthocyanins. Further research is needed to assess their influence on other polyphenols and their potential for commercial use.