How much citric acid powder to use when preparing fruit for drying
From Teresa Marrone's "The Beginner's Guide to Making and Using Dried Foods" - 

"plums need no pre-treatment."

The standard range, per quart of water, is 1 to 3 teaspoons of citric acid. Since they supposedly don't need it at all, I'd lean towards the lower end, if you want to use it anyway.