Foliar micromorphology, ultrastructure and histochemical analyses of Tagetes minuta L. leaves

Many Tagetes species are known for producing essential oils and commercially useful bioactive compounds. This study investigated the micromorphological features of the internal and external foliar structures of Tagetes minuta that produce and store these compounds. Stereomicroscopy, light microscopy, scanning electron microscopy, transmission electron microscopy, and histochemical analyses were used to examine T. minuta leaves at three developmental stages. The development of the subdermal secretory cavities revealed that the cells undergo autolysis to form a schizolysigenous cavity in the mature leaves. The ultrastructure of the parenchymal sheath and secretory epithelium within the secretory cavity revealed that plastids change to contain lipid and osmiophilic molecules. The histochemical analyses showed that trichomes on the surface of T. minuta leaves appear to be linear and non-glandular but maintain the ability to store bioactive phytocompounds. These are new findings for T. minuta and provide a better understanding of the exudation process, which can help to optimise essential oil production for industrial applications.