Alain Razungles

Biogeneration of C13-norisoprenoid compounds: experiments supportive for an apo-carotenoid pathway in grapevines

The first step of the proposed biogenetic pathway in grapes that leads from carotenoids to C 13-norisoprenoids involves the enzymatic degradation of carotenoids by regiospecific oxygenases. Chemical, photochemical and oxidase-coupled degradation of carotenoids to norisoprenoids have been studied in vitro and enzymatic systems shown to be involved in mammals. However, no enzymatic system has been shown to be involved in the formation of C13-norisoprenoids in grapes, despite all recent studies carried out on grapevines supporting such a model. These findings include the preponderance of norisoprenoids possessing 13 carbon atoms that thereby indicates the specificity of cleavage, the configuration of the asymmetric centres and axes common to C 13-norisoprenoids and the corresponding carotenoids, the negative correlations observed between the levels of norisoprenoids and carotenoids during berry development, and the in vivo transfer of 13 C markers from carotenoids to norisoprenoids in berries between veraison and berry maturity. All of these findings are major arguments in favour of the hypothesis that glycosylated C13-norisoprenoids derive from carotenoids in grape berries. Carotenoids are mostly synthesised from the first stage of fruit formation until veraison, and then degrade between veraison and maturity to produce glycosylated C13-norisoprenoids and other compounds.

The aroma of Muscat of Frontignan grapes: effect of the light environment of vine or bunch on volatiles and glycoconjugates

Effects of the modification of whole vine or individual cluster light environment by shade cloth from berry set to maturity were studied on the volatiles and glycoconjugates in Muscat (Muscat of Frontignan; Vitis vinifera L) berries over 2 years. Whole vines were shaded with 50 and 70% shade cloth, while bunches were shaded with 90% shade cloth. The sun-exposed berries were chosen as control berries, and the berries naturally shaded under foliage were also studied. The natural shading of bunches under foliage did not decrease the levels of free and bound compounds in Muscat berries compared to sun-exposed berries. The artificially shaded bunches showed lower levels of monoterpenols and C13 norisoprenoids than sun-exposed berries and berries from naturally shaded bunches. Moreover, the effect of vine shading on the aroma composition of Muscat berries was lower compared to artificial bunch shading. In 1996 the leaf area/fruit yield ratio was modified by decreasing the bunch number per vine. This change did not influence the total amounts of glycosidically bound compounds, except for monoterpenic glycoconjugates. However, the higher monoterpenic glycoconjugate levels in these berries were likely related to their early maturity. Under our experimental conditions, berry aroma composition did not appear to be affected by foliage shade. © 2000 Society of Chemical Industry