Anatomical studies on Myrtaceae roots

Wax apple [Syzygium samarangense (Blume) Merr. & Perry] and guava (Psidium guajava L.) belong to the Myrtaceae family. It has been recorded that polyderm, a special protective tissue that forms beyond the endodermis and is constituted of alternating suberized and non-suberized cell layers, is present only in certain plant families, such as Hypericaceae, Onagraceae, Rosaceae and Myrtaceae. We recently found that the non-suberized cell layers of polyderm consist of lignin-like substances. Since information about polyderm in woody root of perennial fruit trees is barely available, free-hand cross-sections of wax apple and guava roots grown under normal, drought and water-logged conditions were observed under fluorescent microscopy after staining with berberine, anniline blue and saffranin, or with phloroglucinol under optical microscope. Polyderm layers in roots increased significantly under drought stress condition indicating that polyderm formation was affected by environmental stress. Similar to lignin dramatically accumulating in cortex cell walls in the roots of Myrica rubra Sieb. et Zucc. under drought condition, crescent-like lignin thickenings were also observed in roots of wax apple and guava. Under drought stress, accumulation of lignin was first observed in cortex cell wall in young new roots and later, as roots matured, in polyderm layers.