Grapevine & Wine 01/04/2013

Water status afflicts grape quality and wine phenolic composition

In semi-arid areas, which are rapidly expanding, check the relationship between alcohol, acidity but also anthocyanins, flavonols and polymeric pigments. Irrigation and thinning the key factors

Two studies, one Spanish and one Australian, deepen the delicate issue between thinning and irrigation with the quality of the grapes.

In a climate that is gradually changing, with the arid areas that are spreading, achieve a good balance inside the berries is increasingly difficult.

Late cluster thinning is a practice frequently used in the vineyards of semiarid regions, as it is claimed to increase total soluble solids and anthocyanin concentration.
However, when performed in field conditions, it often leads to relatively inconclusive results: under some circumstances, it results in a noteworthy quality gain, whereas under other circumstances, it does not convey the improvement in quality desired.
The aim of this work was to evaluate the influence of thinning intensity and vine water status on the impact cluster thinning has on grape quality. Late cluster thinning (CT) was performed at four ‘Tempranillo’ vineyards during four consecutive years. The major
effects of CT were an increase in berry soluble solid, anthocyanin and phenolics concentration. The impact CT had on these parameters was related to thinning intensity, although it was more closely related to vine water status of from veraison to harvest, particularly when compared to water status during the two weeks with the lowest water availability.
An estimation of the degree of water deficit that is likely to occur between veraison and harvest (considering water management guidelines of each winegrower and water reserves available in the soil or for later irrigation) has therefore to be considered in order to make proper decisions on cluster thinning, at least in the range of yield and water conditions included in this study. In rain fed or deficit irrigated vineyards water deficit alleviation might be one of the main mechanisms that make cluster thinning an effective technique to improve grape quality.

Grape and wine phenolic composition was monitored over two consecutive seasons in Vitis vinifera cv. Merlot following application of irrigation treatments that produced seasonal average stem water potentials ranging between -0.7 MPa and -1.4 MPa. Fresh weight of berries was significantly reduced in response to water deficit, primarily due to decreases in pericarp weight. Increases in the concentration of grape anthocyanins and flavonols in response to water deficit were found when expressed per unit grape berry fresh weight. Skin-derived tannin concentration in grape berries was not affected by the irrigation treatments. The concentration of grapederived phenolics was monitored during five days of fermentation in a small-lot winemaking experiment. During fermentation, the concentration of anthocyanins and flavonols in wine were highest in the non-irrigated and lowfrequency- irrigated treatments, which was reflected in changes in the wine colour of ferments. Finished wines from non-irrigated and low frequency irrigated grapevines showed increases in bisulphite-resistant pigments when compared with those irrigated at a high frequency, but differences in phenolic composition were minor.
Increases in bisulphite-resistant pigments were associated with increases in vitisin A and polymeric pigment in the first and second seasons of the study respectively. Ageing of wines for an 18-month period increased bisulphite-resistant pigments, and treatment differences in wine colour density were enhanced, such that increases in both parameters were associated with the non-irrigated and low-frequency-irrigated treatments.

di R. T.