Adaptations to foliar absorption of faeces: a pathway in plant carnivory

BACKGROUND AND AIMS: Roridula plants capture insects but have no digestive enzymes. It has been hypothesized that Roridula leaves absorb nitrogen from the faeces of obligately associated, carnivorous hemipterans. But rapid movement across the leaf surfaces of most plant leaves is prevented by the presence of an impermeable cuticle. However, in carnivorous plants, cuticular gaps or pores in digestive/absorptive cells allow rapid movement across the leaf surface. Recently, it was suggested that the hemipteran-plant interaction constituted a new pathway for plant carnivory. Here, a further adaptation to this pathway is described by demonstrating how Roridula plants probably absorb hemipteran faeces rapidly through their leaf cuticles. METHODS: The dye neutral red was used to document the rapidity of foliar absorption and TEM to determine the nature of cuticular discontinuities in the leaf of Roridula. KEY RESULTS: Aqueous compounds diffuse rapidly across the cuticle of Roridula's leaves but not across the cuticles of co-occurring, non-carnivorous plant leaves. Furthermore, immature Roridula leaves were unable to absorb neutral red whereas mature leaves could. Using TEM, cuticular gaps and pores similar to those in other carnivorous plants were found in the epidermal cells of mature Roridula leaves. CONCLUSIONS: The leaf cuticle of Roridula is very thin (0-120 nm) and cell wall elements project close to the leaf surface, possibly enhancing foliar absorption. In addition to these, cuticular gaps were frequently seen and probably perform a function similar to those found in other carnivorous plants: namely the absorption of aqueous compounds. The cuticular gaps of Roridula are probably an adaptation to plant carnivory, supporting the newly described pathway.