Benzoquinones, such as 2,6-dimethoxybenzoquinone (1.62), are present in root exudates of maize and stimulate parasitic plants to form haustoria (Matvienko et al., 2001). Ubiquinones, such as ubiquinone(3) (1.63), where (3) indicates the number of isoprenoid sidechains, is also known as Coenzyme Q and has a role in electron transport in the mitochondria.
Naphthaquinones are rare. Among the naphthaquinones juglone (1.64) is relatively common. It is found in walnuts. Anthraquinone is the most widely distributed of the quinones in higher plants and fungi. There are numerous compounds. The anthtraquinone emodin (1.65) occurs as a rhamnoside in rhubarb roots.
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