Increase in lipid-dependent carotene destruction as compared to ethylene formation and chlorophyllase activity following mixed infection of sugar beet (Beta vulgaris L.) with beet yellows virus and beet mild yellowing virus

Sugar beet (Beta vulgaris L.) leaves show no changes in ethylene formation or chlorophyllase (EC 3.1.1.14) activity during the process of yellowing following mixed infection with beet yellows virus and beet mild yellowing virus. "Particles" or "solubilized protein" obtained by differential centrifugation (30 000 g) of leaf homogenates exhibit faster rates of β-carotene destruction than particles from healthy leaves. Results obtained with model reactions (e.g. crocin, α-linolenic acid and a soluble protein from the "light particles") support the view that singlet oxygen and/or the perhydroxyl radical, produced by an "unsaturated fat oxidase" at acidic pH, might be responsible for the observed carotene destruction. This enzymic activity correlates with malondialdehyde formation from α-linolenic acid, which is also increased in beet yellows virus+ beet mild yellowing virus infected leaves. Ethane formation from α-linolenic acid at pH 8·0 is decreased in preparations from infected leaves.