AOBPreview published online on June 11, 2007
Annals of Botany, doi:10.1093/aob/mcm103
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Comparative Anatomy of Intervessel Pits in Two Mangrove Species Growing Along a Natural Salinity Gradient in Gazi Bay, Kenya
1 Vrije Universiteit Brussel (VUB), Laboratory for General Botany and Nature Management (APNA), Pleinlaan 2, 1050 Brussels, Belgium
2 Royal Museum for Central Africa (RMCA), Laboratory for Wood Biology and Wood Collection, Leuvensesteenweg 13, 3080 Tervuren, Belgium
3 Jodrell Laboratory, Royal Botanic Gardens Kew, Richmond, Surrey TW9 3DS, UK
4 SUNY Orange County Community College, Department of Biology, 115 South Street, Middletown, NY 10940, USA
5 Kenya Marine and Fisheries Research Institute (KMFRI), PO Box 81651, Mombasa, Kenya
* For correspondence. E-mail nschmitz{at}vub.ac.be
Received: 6 February 2007 Returned for revision: 5 March 2007 Accepted: 4 April 2007
Background and Aims: 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.
Key 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.
Key words: Rhizophora mucronata, Avicennia marina, intervessel pits, salinity, Kenya, pit membrane, vestures, ecological wood anatomy, cavitation vulnerability, xylem, field-emission SEM, TEM