Foliar secretory structures in Melia azedarach (Meliaceae), a widely cultivated and often invasive tree

Abstract

Melia azedarach (chinaberry; syringa), a tree with bipinnately compound leaves, is one of the most widely cultivated of all tropical trees, but has become naturalised in several parts of the world. New observations are presented on the morphology and anatomy of extrafloral nectaries (EFNs). These should serve as baseline information in ecological studies on the invasiveness of the species, as well as its potential biocontrol. We report, for the first time, extrafloral nectaries (EFNs) on the leaflet lamina, as well as capitate glands on very young leaflets. A sugar analysis of the exudate of the former confirmed it to be nectar. In addition, known EFNs occurring in paired recesses at the petiole base were studied anatomically and developmentally. These so-called paired glands were found to be groups of individual and separate EFNs, for which the new term ‘compound EFN’ is suggested. Each EFN making up a compound EFN is similar in structure to the laminar EFNs except that they tend to protrude beyond the epidermal surface to a greater degree and to be composed of cells which differ slightly in shape. The secretory tissue of both petiolar and laminar EFNs originates from protoderm and ground tissues, albeit with slightly different initial cell divisions. Capitate glands were found on very young leaflets. Ants were frequent visitors to mainly the petiolar EFNs, less often the laminar ones. Members of the Meliaceae (including M. azedarach) are well known sources of limonoids, compounds with insect antifeedant properties. It is suggested that petiolar and laminar EFNs may attract different preferential nectar feeders, thus providing a first line of indirect defence, resulting in the removal or deterring of potential herbivores even before any damage is inflicted on the plant. Additionally, nectar from the EFNs may protect against co-evolved or other herbivores tolerant of limonoids.