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Wisteria Sap Stains. Further to previous correspondence, I hoped to get a scientific explanation of this. The RHS Advice service was unable to help but I received the following information from a biochemist at Cambridge University. I have not asked his permission so cannot quote his name - but be assured I appreciate his interest. He says, "I am interested, because I have had exactly the same problem. I, too, have identified Wisteria as the main source of trouble, but in principle I think almost any plant could do it, given the chance. In my case, at least, the stains are the result of the summer pruning of Wisteria, when one has to go burrowing into a mass of leaf and one gets a bit overwhelmed with the long trailers one is cutting off. I have also thought that the sticky nectar exuded by the flowers of Abutilon might have a similar effect, but I am not so certain about this.
I have assumed (in principle I am reasonably confident I am correct) that the stains are caused by the oxidation of polyphenols in the Wisteria sap. Polyphenols occur almost universally in higher plants, but there is a great variety in terms of chemical type and distribution, and in some plants the oxidation occurs much more readily than in others. In principle the oxidative browning is what you observe when cutting many plant tissues, apples for example.
In the washing machine the warm water, probably alkaline from the soap powder, would encourage oxidation, and the oxidized products are likely to be much more difficult to remove than the original polyphenols because the molecules join together during oxidation.
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