Re: What is the purpose of purple plum tree leaves being purple?

Green-leaved cherry plums (Prunus cerasifera) mutated to produce large amounts of anthocyanins in their leaves. They are called Prunus cerasifera var. atropurpurea to distinguish them from green-leaved members of the species. Most wild Prunus cerasifera have green leaves. The anthocyanins make the leaf appear reddish-purple even though the leaf contains green chlorophyll.

Mutations are random events. Reddish-purple leaves could provide a benefit to the plant, a detriment or be neither of benefit or harm to the plant. If the reddish-purple leaves were of great benefit, then you would expect that eventually most wild Prunus cerasifera would have reddish-purple leaves due to evolution.

Purpleleaf plums are plentiful because people find them attractive and have propagated, selected and cultivated them for over a century. A Mr. Pissard introduced the first purpleleaf plum into France in 1880 from Iran. Even the famous horticulturist Luther Burbank introduced the 'Thundercloud' purpleleaf plum in 1919 which the first website cited below considers the most widely grown purpleleaf plum cultivar in the USA. Purpleleaf plum is valued mainly for its colorful leaves and attractive spring flowers, not for its fruit.

You could do an experiment to determine if purpleleaf and green-leaved Prunus cerasifera differed in fruit predation. However, your hypothesis that the leaf color may protect the fruit from predators would be detrimental to the tree. The fruit's purpose is to attract animals to eat the fruit and disperse the seeds. According to the second website, purpleleaf plum fruit are eaten by a variety of animals.

References

Purpleleaf Plum cultivars

'Thundercloud' Purpleleaf Plum