Project title
Influence of light and temperature at the vine and bunch level on growth and ripening of Sauvignon blanc (Vitis vinifera L.) berries.

Project leader
Howell, C L

Institution
University of Stellenbosch. Department of Viticulture and Enology

Team members
Deloire, A
Hunter, K
Nieuwoudt, H H
Strever, A

Project description
The effect of light and temperature on the evolution of berry growth, composition and colour has been studied world wide for a number of white and red cultivars. Despite these studies, there is still a great deal of uncertainty regarding the effect of these climatic parameters on berry ripening and colour evolution.

The complexity of the influence of both light and temperature on Sauvignon berry ripening has been observed in an industry related project conducted at the Department of Viticulture and Oenology, Stellenbosch University in collaboration with Distell collegues. Sauvignon blanc berry ripening and in particular, colour evolution has been monitored for the 2008/09 growing season for a network of plots in the Western Cape Coastal region of South Africa. Berry colour was measured using the Dyostem instrument which measures berry tint angle. This instrument has been used world wide in the wine industry for the last three years to measure berry colour as an indicator of berry ripeness level and harvest potentiality for white cultivars. Results from the research at Stellenbosch have raised a number of valid scientific questions relating to berry colour evolution (green to yellow) for the white cultivars. In addition, recent results obtained on white cultivars using the evolution of the berry colour to determine the level of ripening in relation to the style of wine (pyrazine and/or thiols aromatic character of wines) are promising.

The loss of green colour in senescent leaves and ripening fruit is a natural phenomenon and is mainly due to chlorophyll breakdown. Chlorophyll degradation is a symptom of transition of chloroplasts to gerontoplasts (Parthier, 1988) which are a senescence specific form of plastids. According to literature, the yellowing of senescente leaves and fruits is due to unmasking and partial retention of carotenoïds rather than to the new synthesis of yellow pigments that occurs when chloroplasts differenciate into chromoplasts (Matile et al., 1999).

On the other hand, the presence of carotenoïds in grape berries has been confirmed by numerous authors. The predominant carotenoids present in berries have been identified as -carotene and xanthophylls. There are many parameters that can affect the concentration and the biosynthesis of carotenoïds and xanthophylls. These parameters include: the cultivars themselves, the climate and the canopy management.

Of all the questions, the most important and relevant concerns light and temperature thresholds related to the limit of white cultivar functioning and ripening. The aim of this project is therefore to generate fundamental knowledge on the effect of temperature and light on Sauvignon blanc berry ripening and colour evolution. This project will therefore determine thresholds in terms of optimum light and temperature levels for Sauvignon blanc berry ripening and colour evolution. In addition, a comprehensive overview of all the pertinent chemical parameters playing a role in berry ripening will be obtained. Essentially, this project therefore addresses the physiology and biochemistry of berry colour evolution.

The main objectives of the first year of the project are to identify a suitable vineyard, apply the appropriate light and temperature treatments and take bunch samples and perform chemical analyses.

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