An anatomical study of developmental changes in maturing root tissues of pomegranate (Punica granatum L.) and formation of a unique type of periderm

Roots of pomegranate (Punica granatum L.), belonging to Punicaceae, were investigated anatomically to record changes in tissue development from growth to maturity. When roots start secondary growth, a protective tissue called polyderm composed of alternating suberized and non-suberized cell layers, develop beyond the endodermis in certain families of plants including Myrtaceae. Punicaceae family is not known to develop a polyderm. However, new layers formed beyond the endodermis during secondary growth and biopolymer deposition was observed in their cell walls. The present study aims to gather more knowledge on this tissue discovered in pomegranate roots and cross check whether it is a polyderm or a unique type of periderm. Root specimens were sectioned freehand or with an ultramicrotome after embedding in Technovit 7100 resin. After staining with berberine hemisulfate-aniline blue-safranin O, the root sections were observed under fluorescent or optical microscopes. Unlike the polyderm in Myrtaceae roots, in pomegranate roots, ligno-suberic material accumulated in every cell layer beyond the endodermis. The alternating suberized and non-suberized layers that define the polyderm were absent. Lignin accumulation in the cell wall was pronounced in every cell layer of this outermost tissue and suberin-like autofluorescence was also observed in the same layer. We considered this to be a unique feature typical in pomegranate periderm. It is possible that accumulating both lignin and suberin in the same cell layer instead of alternative layers is more efficient because metabolic energy is not spent in forming a separate cell layer. Further experiments are underway to acknowledge changes in such biopolymer accumulation in the outermost tissue in abiotic stress conditions.