Study finds ABA triggers ripening in grapes delayed by more than 65 days

A study published this month in OENO One reports that a slow-ripening grape genotype delayed berry ripening by more than 65 days compared with a full sibling, and that applications of abscisic acid, or ABA, were able to trigger sugar accumulation in the fruit. The findings point to a problem in the signaling process that starts ripening, rather than a simple shortage of sugars or a failure of the vine to photosynthesize.

The research was carried out by scientists working with plant material from the breeding and genetics program at E. & J. Gallo Winery in California. The team compared two proprietary white-fruited seedlings from the same 2013 cross. One ripened normally and reached more than 20° Brix before Sept. 1, which the authors said is standard for white varieties in that area. The other remained green and firm at that point and was classified as slow ripening.

The vines were grown near Madera, Calif., under the same field conditions, with full irrigation and standardized management. Over three years, and in some measurements four years, the researchers tracked total soluble solids, berry growth and berry softening to understand how the two siblings differed and whether the unusual delay was stable over time.

According to the paper, the slow-ripening genotype showed what the authors described as an unprecedented delay of more than 65 days. That gap matters because sugar accumulation is one of the main markers used to decide harvest timing in wine grapes. In warmer conditions, grapes often reach sugar targets earlier, while acidity, color compounds, aroma precursors and other quality-related components may not keep pace. A genotype that naturally slows or delays ripening could therefore become useful in breeding programs aimed at reducing those imbalances as vineyards adapt to climate change.

The study also addressed a practical problem for wineries. Earlier ripening can compress harvest windows across varieties and regions, creating logistical pressure at crush. While this work focused on a very unusual slow-ripening seedling rather than a commercial cultivar, it helps clarify which biological controls might eventually be used to select grapes better suited to future growing conditions.

To test why the fruit lagged so far behind, the researchers examined several possible causes. They measured leaf water status and gas exchange in 2022 to see whether the slow-ripening vines were limited in carbon assimilation. They also manipulated crop load in 2023 through severe cluster thinning to test whether too much fruit demand was delaying maturity. In separate experiments from 2022 through 2025, they treated clusters with compounds linked either to hormone signaling or to solute-driven softening.

Those treatments included ABA, ACC, sucrose and polyethylene glycol, along with a surfactant control. ABA is a plant hormone long associated with the onset of grape ripening. ACC is a precursor involved in ethylene production. Sucrose and polyethylene glycol were used to test whether changing solute or osmotic conditions around the berry could mimic the start of ripening.

The results did not support the idea that the vines were simply weak or unable to supply enough carbon to the fruit. The authors reported that leaf physiology was not impaired in the slow-ripening genotype. Severe cluster thinning also failed to change either ripening time or ripening rate in a meaningful way, suggesting that source-sink balance was not the main explanation for the delay.

By contrast, ABA had a clear effect. In the experiments described in the paper, ABA treatments promptly triggered sugar accumulation in slow-ripening berries. The strength of that response depended on both timing and dose. The researchers tested 400 mg/L and 2000 mg/L applications at different developmental stages based on the normal-ripening sibling, including pea size, veraison and post-veraison timing. Their results showed that treatment efficacy varied according to when it was applied and how much was used.

That pattern led the authors to conclude that the slow-ripening fruit likely suffers from a failure in signaling at ripening initiation. In grapes, ripening normally begins around veraison with berry softening and ABA-related signaling, followed by rapid sugar accumulation and, in colored varieties, pigment development. If that first signal is disrupted, berries may remain hard and green even when environmental conditions would otherwise support maturation.

The paper is careful not to present this genotype as an immediate commercial solution. It is a proprietary breeding selection studied under experimental conditions, not a released variety for growers. But its extreme behavior gives researchers a rare model for studying how ripening starts at the physiological level and which genes or regulatory networks may control it.

That question has become more urgent as climate patterns shift grape development worldwide. The authors note that warmer and drier conditions tend to advance phenological stages and shorten intervals between them. In many wine regions, this means grapes can accumulate sugars too quickly relative to acids and flavor compounds. Vineyard practices such as delayed pruning, antitranspirant sprays or late source limitation can help slow maturity, but previous work cited by the authors suggests those methods generally plateau at about a three-week delay and can carry trade-offs if poorly timed.

For that reason, breeders have been looking more closely at natural genetic variation as a longer-term adaptation strategy. The new study adds evidence that traits linked specifically to ripening onset and sugar accumulation rate can vary sharply even among full siblings. It also separates delayed ripening from broader differences across an entire growing season by focusing on berry softening and sugar accumulation once fruit development approaches veraison.

The researchers used repeated berry sampling from 2022 through 2025 and non-destructive firmness measurements from 2022 through 2024. They collected berries weekly and increased sampling near veraison when possible. Berry weight was measured directly, while sugar content per berry was estimated from berry weight and total soluble solids. Firmness was assessed with an instrument called GrapeGrabber, which estimates berry elasticity during compression.

The work also included a water-stress comparison in 2025. Based on earlier results, researchers applied ABA at 2000 mg/L after veraison-like timing in the normal sibling and compared those clusters with untreated fruit and with vines subjected to severe dry-down by stopping drip irrigation. That experiment was designed to compare externally applied ABA with stress conditions known to alter vine physiology. The source text provided does not include all numerical outcomes from that trial in its abstracted portion, but it places water stress within the broader effort to determine whether hormonal cues rather than solute accumulation are central to initiating ripening in this genotype.

The study was received on Dec. 11, 2025, accepted on May 12 and published on June 4 in Volume 60, Issue 2 of OENO One. Its authors are Pietro Previtali, Oscar Bellon, Elizabeth Green, Kenneth Shackel, Marianna Fasoli, Sara Zenoni, Peter Cousins, Megan Bartlett and Nick Dokoozlian.

For grape growers and wine producers, the immediate takeaway is not that ABA sprays will solve climate-related ripening problems across vineyards. The stronger message is that ripening timing may be controlled by specific signaling steps that can fail or be delayed independently of vine vigor or crop load. If breeders can identify those mechanisms more precisely, they may be able to develop grape genotypes that reach harvest with better alignment between sugars and other quality traits under hotter conditions.