Journal Article

Shifts in leaf vein density through accelerated vein formation in C<sub>4</sub> <i>Flaveria</i> (Asteraceae)

Athena D. McKown and Nancy G. Dengler

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 104, issue 6, pages 1085-1098
Published in print November 2009 | ISSN: 0305-7364
Published online September 2009 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcp210
Shifts in leaf vein density through accelerated vein formation in C4 Flaveria (Asteraceae)

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  • Ecology and Conservation
  • Evolutionary Biology
  • Plant Sciences and Forestry

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Background and Aims

Leaf venation in many C4 species is characterized by high vein density, essential in facilitating rapid intercellular diffusion of C4 photosynthetic metabolites between different tissues (mesophyll, bundle sheath). Greater vein density has been hypothesized to be an early step in C4 photosynthesis evolution. Development of C4 vein patterning is thought to occur from either accelerated or prolonged procambium formation, relative to ground tissue development.

Methods

Cleared and sectioned tissues of phylogenetically basal C3 Flaveria robusta and more derived C4 Flaveria bidentis were compared for vein pattern in mature leaves and vein pattern formation in developing leaves.

Key Results

In mature leaves, major vein density did not differ between C3 and C4 Flaveria species, whereas minor veins were denser in C4 species than in C3 species. The developmental study showed that both major and minor vein patterning in leaves of C3 and C4 species were initiated at comparable stages (based on leaf length). An additional vein order in the C4 species was observed during initiation of the higher order minor veins compared with the C3 species. In the two species, expansion of bundle sheath and mesophyll cells occurred after vein pattern was complete and xylem differentiation was continuous in minor veins. In addition, mesophyll cells ceased dividing sooner and enlarged less in C4 species than in C3 species.

Conclusions

Leaf vein pattern characteristic to C4 Flaveria was achieved primarily through accelerated and earlier offset of higher order vein formation, rather than other modifications in the timing of vein pattern formation, as compared with C3 species. Earlier cessation of mesophyll cell division and reduced expansion also contributed to greater vein density in the C4 species. The relatively late expansion of bundle sheath and mesophyll cells shows that vein patterning precedes ground tissue development in C4 species.

Keywords: Bundle sheath; C4 photosynthesis evolution; Flaveria; heterochrony; leaf development; mesophyll; vein density; vein pattern formation

Journal Article.  9206 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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