Delaying Riesling grape berry ripening with a synthetic auxin affects malic acid metabolism and sugar accumulation, and alters wine sensory characters

An improved understanding of the hormonal control of grape (Vitis vinifera L.) berry ripening and the ability to manipulate it are of interest scientifically and commercially. Grapes are nonclimacteric fruit with ethylene unlikely to have a principal role in berry ripening but there are several other hormones thought to be involved. In this work, a significant delay in Riesling berry ripening was achieved through preripening treatments with the synthetic auxin 1-naphthaleneacetic acid (NAA). The initiation of sugar accumulation was delayed and the rate of sugar accumulation was lower in NAA-treated fruit, resulting in a 15-day delay in harvest. NAA treatments also reduced the rate of decline in malic acid levels that occurs during ripening, and increased the synchronicity of malic acid and berry sugar accumulation. Sensory panel assessment revealed a significant difference between wine made from control and NAA-treated fruit. Analysis of the volatile composition of the wines' headspace showed that the concentration of several compounds was altered significantly by the NAA treatment. These data provide further support for the involvement of auxins in inhibiting ripening and suggest that auxin treatments may be useful in controlling both winery intake, and fruit and wine composition.