Micromorphological study of pits in tracheary elements of woody angiosperms.

Part I: Systematic distribution of pit membrane thickenings and vesturesChapter 1: The micromorphology of pit membranes in tracheary elements of Ericales: new records of tori or pseudo-tori?Intervascular pit membranes were examined within Ericales to determine the distribution and structure of torus-like thickenings. Forty-nine species representing 12 families of the order Ericales were investigated using light, scanning and transmission electron microscopy. They were compared with four species of Oleaceae to determine the true nature of the thickenings.Pit membranes with torus-like thickenings were observed in seven species of Ericaceae and were found to be amorphous, plasmodesmata associated structures with an irregular distribution. These pseudo-tori show major differences compared to true tori with respect to their distribution, ultrastructure and chemical composition. Genuine tori, which are strongly correlated with round pit apertures in narrow tracheary elements, were found in two species of Osmanthus (Oleaceae). The pseudo-tori found in some Ericaceae are considered to be similar to pit membrane thickenings previously recorded in Rosaceae. While true tori appear to be functionally significant in terms of efficiency and safety of water transport, the possible function of pseudo-tori could be associated with the role of plasmodesmata during differentiation of tracheids, fibre-tracheids or narrow vessels.Chapter 2: Micromorphology and systematic distribution of pit membrane thickenings in Oleaceae: tori and pseudo-toriRecent studies on the functional significance of pit membranes in water conducting cells have renewed general interest in their micromorphology. At least two types of pit membrane thickenings have been described in angiosperm families, i.e. genuine tori and pseudo-tori. This study explores the distribution and morphology of pit membrane thickenings in 69 species and 23 genera within Oleaceae using light and electron microscopy. Torus-bearing pit membranes are confirmed for Osmanthus, and new records are reported for Chionanthus retusa, Picconia azorica, and P. excelsa, but not for the other species of Chionanthus studied. This intrageneric variation suggests that tori represent a plastic feature that has evolved more than once within the family as the result of functional adaptation to efficient and safe water transport. Pseudo-tori are observed in species of Abeliophyllum, Fontanesia, Forsythia, Jasminum, Ligustrum, Menodora, and Syringa. Based on structural differences, we state that tori and pseudo-tori can be distinguished as non-homologous features. Chapter 3: Torus-bearing pit membranes in species of OsmanthusTorus thickenings of pit membranes are found not only in gymnosperms, but also in certain genera of eudicots. One such genus is Osmanthus. Wood from 17 species of Osmanthus was examined for tori.Fourteen species from three of the four sections investigated showed torus thickenings. Ten of the species represent new records. Only the three New Caledonian species of section Notosmanthus lacked tori. This observation in combination with other characters suggests us to separate this section from the remainder of the genus.Chapter 4: The phylogenetic significance of vestured pits in BoraginaceaeThe bordered pit structure in tracheary elements of 105 Boraginaceae species is studied using scanning electron microscopy to examine the systematic distribution of vestured pits. Forty-three species out of 16 genera show a uniform presence of this feature throughout their secondary xylem. Most vestures are small, unbranched and associated with the outer pit aperture of bordered intervessel pits. The feature is likely to have originated independently in the distantly related subfamilies Boraginoideae (tribe Lithospermeae) and Ehretioideae. The distribution of vestures in Ehretia agrees with recent molecular phylogenies: (1) species with vestured pits characterise the Ehretia I group (incl. Rotula), and (2) species with non-vestured pits belong to the Ehretia II group (incl. Carmona). The occurrence of vestured pits in Hydrolea provides additional support for excluding this genus from Hydrophylloideae, since Hydrolea is the only genus of this subfamily with vestured pits. Functional advantages of vestured pits are suggested to promote parallel evolution of this conservative feature, although the exact hydraulic benefits of poorly developed vestures remain unknown. Part II: Development of pseudo-toriChapter 5: The ultrastructure and development of pit membranes with plasmodesmata associated thickenings in xylem elementsRecent micromorphological observations of angiosperm pit membranes have extended the range of taxa with pseudo-tori in tracheary elements. Since developmental studies on pseudo-tori have only been conducted on Pyrus and Sorbus (Rosaceae), it is unclear whether all pseudo-tori in angiosperms develop in a similar way and whether they are homologous structures. We studied the ontogeny of pseudo-tori using transmission electron microscopy in four species belonging to four different orders of flowering plants: Malus yunnanensis, Ligustrum vulgare, Pittosporum tenuifolium, and Vaccinium myrtillus. The developmental patterns observed in these four species are similar. A primary thickening is formed when the secondary cell wall formation is initiated followed by the deposition of a secondary cap. After hydrolysis of the pit membrane, the cap becomes persistent, partially covering the pit membrane. Our observations do not fully corroborate the earlier ontogenetic studies in Rosaceae. The formation of the primary thickening in Pyrus and Sorbus is completed before the secondary cell wall formation is initiated, which is not the case in our study. This developmental difference questions the homologous nature of pseudo-tori throughout the flowering plants.Part III: Structure-function relationships of bordered pits in angiosperm vesselsChapter 6: Wood anatomical features associated with cavitation resistance across 10 species of PrunusWood of 10 Prunus species that were hydraulically studied by Cochard et al. (2008) were examined to study structure-function trade-offs regarding vulnerability to drought-induced cavitation. A detailed study of hydraulically relevant characters is conducted using light microscopy, scanning and transmission electron microscopy.Vessel network characters are most important in explaining cavitation resistance (quantified as P50 value). Strong relations are demonstrated between P50 and vessel grouping indices (VG, VS), vessel contact characters (FC), and vessel dimension characters (DH, LV). The most drought resistant Prunus species in our study tend to show vessels that are less grouped and more solitary than the more vulnerable species. Our results support the theoretical model on vessel connectivity of Loepfe et al. (2007), but provide no evidence for Carlquist s vessel grouping hypothesis. At the pit level however, we found a strong negative correlation between P50 and the thickness of intervessel pit membranes, although this correlation was not found significant and was not supported in PCA. No other pit related character (including AP) was found to be related with P50 . This is surprising since the rare pit hypothesis, linking P50 with AP, is was originally formulated based on observations in Rosaceae (incl. Prunus), this hypothesis could not be fully corroborated in our study.status: publishe