title: Comparative anatomy of intervessel pits in two mangrove species growing along a natural salinity gradient in Gazi bay, Kenya.
abstract: OBJECTIVE
  According to the air-seeding hypothesis, embolism vulnerability in xylem elements is linked directly to bordered pit structure and functioning. To elucidate the adaptive potential of intervessel pits towards fluctuating environmental conditions, two mangrove species with a distinct ecological distribution growing along a natural salinity gradient were investigated.
  
  
  METHODS
  Scanning and transmission electron microscopic observations were conducted to obtain qualitative and quantitative characteristics of alternate intervessel pits in A. marina and scalariform intervessel pits in Rhizophora mucronata. Wood samples from three to six trees were collected at seven and five sites for A. marina and R. mucronata, respectively, with considerable differences between sites in soil water salinity.
  
  
  RESULTS
  Vestured pits without visible pores in the pit membrane were observed in A. marina, the mangrove species with the widest geographical distribution on global as well as local scale. Their thick pit membranes (on average 370 nm) and minute pit apertures may contribute to reduced vulnerability to cavitation of this highly salt-tolerant species. The smaller ecological distribution of R. mucronata was in accordance with wide pit apertures and a slightly higher pitfield fraction (67 % vs. 60 % in A. marina). Nonetheless, its outer pit apertures were observed to be funnel-shaped shielding non-porous pit membranes. No trends in intervessel pit size were observed with increasing soil water salinity of the site.
  
  
  CONCLUSIONS
  The contrasting ecological distribution of two mangrove species was reflected in the geometry and pit membrane characteristics of their intervessel pits. Within species, intervessel pit size seemed to be independent of spatial variations in environmental conditions and was only weakly correlated with vessel diameter. Further research on pit formation and function has to clarify the large variations in intervessel pit size within trees and even within single vessels.
authors.affiliation: Vrije Universiteit Brussel, Laboratory for General Botany and Nature Management (APNA), Pleinlaan 2, 1050 Brussels, Belgium. nschmitz@vub.ac.be
authors.forename: Nele
authors.initials: N
authors.lastname: Schmitz
authors.forename: Steven
authors.initials: S
authors.lastname: Jansen
authors.forename: Anouk
authors.initials: A
authors.lastname: Verheyden
authors.forename: James Gitundu
authors.initials: JG
authors.lastname: Kairo
authors.forename: Hans
authors.initials: H
authors.lastname: Beeckman
authors.forename: Nico
authors.initials: N
authors.lastname: Koedam
mesh_terms: D000222:Adaptation, Physiological; D032407:Avicennia / Q000502:physiology / Q000648:ultrastructure*; D017753:Ecosystem; D007630:Kenya; D008855:Microscopy, Electron, Scanning; D046529:Microscopy, Electron, Transmission; D043982:Rhizophoraceae / Q000502:physiology / Q000648:ultrastructure*; D012965:Sodium Chloride; D052584:Xylem / Q000502:physiology / Q000648:ultrastructure*
chemical_list: D012965:Sodium Chloride