Morphology of extrafloral nectaries of Ailanthus altissima (Mill.) Swingle (Simaroubaceae)

Background and Purpose: Ailanthus altissima (Mill.) Swingle (Tree of Heaven) is a highly invasive, widespread, and widely investigated plant species native to the broader area of China. Extrafloral nectaries are major components of the A. altissima secretory system, but the knowledge of their morphology and role in the tree's physiology is limited. This research aims to explore the morphology of extrafloral nectaries, compare it to previous findings, and discuss their possible role and function. Materials and Methods: Extrafloral nectaries on leaves have been monitored through different phases of leaf development, from June to August 2015 and 2022. The nectaries' morphology was investigated using Zeiss Axioscope 5 and BOECO BSZ-405 light microscopes, and FEG QUANTA 250 FEI scanning electron microscope, operating at 7 kV and pressure of 60–100 Pa, without samples pretreatment. Results: Our investigation revealed the absence of earlier reported pores or ducts on the top of the glands. However, it supports one of the first, systematical investigations of A. altissima extrafloral nectaries conducted in Croatia a century ago, which was until recently forgotten by history. We evaluated our findings against prior theories and assessed the nectaries' potential role and purpose in disposing surplus sugars. Conclusions: The nectar in A. altissima is not secreted via an opening on the leaves but rather through epidermal tissue tearing.