Invisible red in young leaves: Anthocyanin likely plays a defensive role in some other way beyond visual warning

Abstract

Leaves of many plants are red when they are young/old, which was suggested as an adaptive strategy to reduce insect herbivory. The “coevolution hypothesis” proposes that red leaves function as a warning signal, while most herbivores in tropical and subtropical areas are known for their insensitivity to red. To examine whether young-leaf redness functioned as an effective defense to common herbivores and determine the way it works, we conducted a comparative study of leaf color and protective tissue across five forest communities in south-central China, combined with leaf surface reflective spectrum analyses. Mature/young leaves of 151 species were examined for their phylogenetic/non-phylogenetic correlation between leaf redness and mechanical protection. Reflective spectra of both adaxial/abaxial surfaces of young/mature leaves in one community were examined using a portable spectrometer. The data were transformed into a bee vision model and compared in different color groups (red/green-young-leaf species) to examine whether they could be discriminated by common herbivores in subtropical area. In our study, about half species of each community (43.3% ~ 61.1%) present red young leaves, much more than present among mature/senescent leaves. Green-young-leaf species presented a significantly higher proportion of protective tissue than red-young-leaf species in all communities. Phylogenetic analyses using Pagel's correlated evolution test found a correlation between young-leaf color and mechanical structure, indicating leaf redness might be complementary to mechanical protection. Alternatively, reflective spectrum analyses suggested that the most common insect herbivores (Lepidoptera larvae and Coleoptera) could hardly distinguish red from green leaves. Our results suggest the accumulation of anthocyanin is a common defense in spring young leaves, which likely functions in some other way beyond being a visual warning. Given that anthocyanin also plays an important role in physiological functions, it is important to consider and understand the ecological significance of anthocyanin from multiple aspects.