Why is latex usually white and only sometimes yellow, orange or red? Simultaneous visual and chemical plant defense

Abstract

Latex is a toxic and sticky defense substance produced by about 20,000 plant species and secreted following wounding. It defends the wounded plants by both chemical and physical components. A striking fact about latex is that the majority of plants produce it white, although yellow, orange, and red latex can sometimes be found. Theoretically, it is possible that there is no function or importance to latex’s color, but it seems unlikely. It is also possible that there are chemical constraints that dictate the production of white latex, but the various colors indicate that this is not a valid explanation. However, since white latex evolved independently many times in various plant taxa, there should have been a strong selection for latex being white, a possibility that has never been addressed, to my knowledge. I propose that latex is generally white because white is the best solution for visual aposematism under the lighted conditions within and under plant canopies, on the background of typical leaf and bark color, even towards color-blind animals. Yellow, orange, and red latex types should naturally also be considered as aposematic because these are typical colors of toxic and aposematic organisms. In addition, latex-exuding plants simultaneously use a chemically based, non-visual olfactory aposematism because wounding perse also causes the emission of non-latex defensive volatiles and because latex contains various but little-studied defensive volatiles. The volatile defensive aspect of latex exudation may also operate against nocturnal or blind herbivores.