With the exception of sunflower, which had larger chloroplasts and chloroplast coverage of the M cell area, there was no significant variation between the species in chloroplast or mitochondrial coverage of the M cell area Supplemental Table S1. Enlarged BS cells and organelle number are often associated with high vein density in arid-zone plants such as Flaveria spp. For averages of each of the species, see Supplemental Table S2. Plant Cell Environ Data Analysis Results were analyzed with Sigmaplot version In situ immunolocalization of GDCp in leaves of F. Species previously characterized as C 3 species are indicated in blue; C 2 species and the predominantly C 2 clade B are shown in purple; the predominantly C 4 clade A is shown in red.
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Gas-exchange measurements were conducted using a LI photosynthesis system Li-Cor.
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Supplemental Data The following materials are available in the online version of this article. The sunflower chloroplasts are preferentially in the outer half of the BS cell, where they are positioned along the intercellular air space IAS ; Fig.
In C 2 photosynthesis, the photorespiratory enzyme glycine decarboxylase GDC is localized to an interior compartment, generally the inner half of bundle sheath BS cells. Abstract The evolution of C 4 photosynthesis in many taxa involves the establishment of a two-celled photorespiratory CO 2 pump, termed C 2 photosynthesis. In all species studied, the M organelles occur on the cell periphery, usually opposite the intercellular air spaces.
Plant Cell Environ 9: In the three typical C 3 species sunflower, S. Mitochondria were generally associated with M chloroplasts. Comparisons between groups indicate that photorespiratory CO 2 recycling, termed C 2 photosynthesis, is an important intermediate phase in the evolution of C 4 photosynthesis in many of the C 4 lineages Monson and Rawsthorne, ; Sage et al.
A Working Hypothesis As shown by sunflower, the typical BS cells in C 3 plants are relatively small, with a few centrifugal chloroplasts and little potential for refixation of photorespired CO 2.
These patterns were similar, although generally less in magnitude, than those observed in the C 2 species Flaveria angustifolia and Flaveria sonorensis. JohnsonPatrick C. C, Chloroplast; M, mesophyll; V, vascular tissue.
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The relative number of BS chloroplasts in the C 2 species is more than twice that in sunflower, S. Compared with less-related C 3 species, bundle sheath BS cells of F.
If the characteristics in these species reflect ancestral states, as suggested by their intermediate branching positions in the genus Flaveria phylogeny, the data support the hypothesis that the assembly of the C 2 pathway occurs gradually and in its incipient form may be a mechanism to scavenge photorespired CO 2.
Fertilizer was supplied weekly as a If the inner BS represents a large enough sink for Gly, this arrangement could also metabolize any Gly that may overflow from the M cells, such as during hot periods when photorespiration may be particularly high.
Flrp were prepared for 2. and transmission electron microscopy as described by Sage and Williams J Exp Bot No study has rigorously described the ultrastructure and GDC and Rubisco enzyme localization of these species, however. If the hypothesis is supported, the results would enhance our understanding of the earliest developments in the evolution of C 4 photosynthesis and could clarify why this complex trait has evolved so frequently in the plant kingdom.
Results were analyzed with Sigmaplot version The preparation of tissue samples for the immunolocalization of Rubisco and GDCp was performed as described by Marshall et al. This could have facilitated the formation of a glycine shuttle between mesophyll and BS cells that characterizes Dlpr 2 photosynthesis.
The following materials are available in the online version of this article. In situ fllr of the P-subunit of the glycine decarboxylase complex in BS cells of leaves from Flaveria spp. Plants were trimmed every 4 to 8 weeks to minimize canopy crowding and maintain a balance between roots and shoots.
Initial Events during the Evolution of C4 Photosynthesis in C3 Species of Flaveria
As shown by sunflower, the typical BS cells in C 3 plants are relatively small, with a few centrifugal chloroplasts and little potential for refixation of photorespired CO 2. Number, size, and distribution of M cell organelles. For ultrastructural analysis, leaf samples were collected from three to four plants.
With the exception of sunflower, which had larger chloroplasts and chloroplast coverage of the M cell area, there was no significant variation between the species in chloroplast or mitochondrial coverage of the M cell area Supplemental Table S1. High vein density increases hydraulic flux in such environments, thereby maintaining favorable leaf water status Roth-Nebelsick et al.
Representative responses of A to low C i for S.