Structure and chemistry of the xylem of arborescent species of Blechnum from South America

Abstract

This contribution provides a detailed study of the xylem in three arborescent species of Blechnum section Lomariocycas using SEM, TEM, FT-IR spectroscopy and sugar composition analysis. The overall structure of root and rhizome metaxylem tracheids was similar in all species analyzed, with various facets and mostly scalariform pitting. Pit membrane thickness and porosity varied according to the tracheid stage of maturation. Approximately rounded deposits resembling vestures were observed in the outer pit apertures of some tracheids. Under TEM, thickenings like one-sided tori appeared on the tracheid side of tracheid to parenchyma blind pits; some parenchyma cells showed, in addition, features of transfer cells. As the increase in stature creates new constraints in terms of biomechanical support and water transport in plants, the characteristics found in Blechnum xylem might be related to an improvement in the flow of water and to the prevention of embolism. Chemical analyses of roots and rhizomes of B. yungense revealed similar levels of G-type lignin deposited in the xylem cell walls. Such lignin is the most common in ferns, including other arborescent genera. Preliminary analysis of cell wall polysaccharide composition suggested that both root and rhizome xylem, consists of cellulose, xyloglucans and xylans with low amounts of mannans and pectins. Higher amount of cellulose was detected in the xylem of rhizomes compared to that of the roots. This information is discussed in the context of functional and evolutionary aspects of xylem tissue in ferns.