Microscopic Investigationsof Silicification and Lignification Suggest Their Coexistence in Tracheary Phytoliths in Date Fruits (Phoenix dactylifera L.).

Navomy George,Asha Antony,Fathalla Hamed,Afaf Kamal-Eldin,Tholkappiyan Ramachandran

doi:10.3389/fpls.2020.00977

Abstract

Date fruits are special representative of hard fruits and one of the richest sources of dietary silica and edible lignin, which are believed to have several health benefits. In this study, we used optical and scanning electron microscopy (SEM) to investigate the presence of associations between silicification and lignification in date fruits (Phoenix dactylifera, L.). Phloroglucinol staining was employed to observe lignification in date fruits, while silicification was studied by SEM of whole fruits and their acid digesta. This work revealed the presence of heterogeneity and complexity in the silica phytoliths and the lignified structures in date fruits. It was found that lignin exists independently of silica in the secondary cell walls of parenchymal and sclereid cells and that silica exists independently of lignin in the spheroid phytoliths that surround the sclereid cells. Interestingly, a small proportion of lignin and silica seemed to co-exist as partners in the spiral coils of the tracheid phytoliths.

Highlights

Date fruits (Phoenix dactylifera, Family: Palmae) are relatively dry fruits with a high content of soluble sugars (60%–75%), accompanied by dietary fiber (5%–15%) and phenolic compounds (Ghnimi et al, 2017)
The lignin deposited in the sclereid cells and xylem vessels is stained with a pinkish-red color, while the tannins deposited in the vacuoles of the tanniferous layer are stained purple
Removal of the sugars from the fruit parenchymal cells leaves behind mainly the cell walls made of dietary fiber components, which are stained with a bluishgreen color

Summary

Introduction

Date fruits (Phoenix dactylifera, Family: Palmae) are relatively dry fruits (moisture content, 10%– 30%) with a high content of soluble sugars (60%–75%), accompanied by dietary fiber (5%–15%) and phenolic compounds (up to 5%) (Ghnimi et al, 2017). The dietary fiber composition of dates consists mostly of lignin, together with cellulose, hemicelluloses, and pectins (George et al, submitted). Lignin is a class of high-molecular-weight phenolic polymers that provide rigidity to xylem vessels (Michael, 2013). Dietary soluble silicon is believed to provide several health benefits to consumers, including, inter alia, bone homeostasis and regeneration, stimulation of collagen synthesis, and skin, nail, and hair health (Martin, 2007; Zargar et al, 2012)

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Conclusion