terpenoids, an important group of plant chemicals including many essential oils, carotenoids, plant hormones, sterols, and rubber. They contribute much to the unique qualities of the vine. Chemically, they are multiples of branched, five-carbon (isoprene) units yielding a variety of compounds with diverse properties: the C10 monoterpenes make an important contribution to floral aromas (see flavour compounds); the C15 sesquiterpenoids include the hormone abscisic acid, and the C20 diterpenes include the gibberellin hormones. Carotenoids, with 40 carbon atoms, contribute to the skin colour of so-called white grapes and are metabolized to norisoprenoid flavour compounds that contribute to non-floral aroma of grapes. See also terpenes in oak flavour.