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Starch grains and Rubisco are abundant in the inner BS chloroplasts of F. For examples of how these values where determined, see Figure 7. Am J Bot A simplified version of the composite Flaveriinae phylogeny of McKown et al. The genus Flaveria has long served as the biochemical and physiological model for C 4 evolution, and its phylogenetic relationships have been resolved in recent years Powell, ; Monson and Rawsthorne, ; McKown et al. By contrast, BS mitochondria in sunflower, S. If this is correct, then it is possible that F.

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CO 2 compensation points of Panicum spp. The internal anatomy of leaves was assessed on sections sampled from the middle of recent, fully expanded leaves one leaf per plant; three to four plants per species.

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Under conditions promoting photorespiration, as would occur in hot environments, substantial amounts of CO 2 are released in the BSraising its CO 2 concentration two to four times above the atmospheric value Bauwe et al. An improved understanding of fplr, and how, the C 2 pathway evolved could thus be important for understanding how C 4 plants originated and why they did so with such high frequency. Oxygen response and enzyme activities of the Laxa groups of Panicum.

Why C 3 species may initially develop a larger BS is unclear, but it may be related to the greater vein density of leaves in hot environments with high potential evapotranspiration Sage, ; Osborne glpr Sack, Along with Sartwelliathe sister genus to Flaveriathese C 3 species represent an ideal opportunity to study trait changes within a C 3 clade where C 4 evolution was initiated.

The high number of C 4 obt could thus reflect the relative ease of establishing the proto-Kranz condition and initiating such cascades.

Initial Events during the Evolution of C4 Photosynthesis in C3 Species of Flaveria

Arrows mark mitochondria, and asterisks label BS cells. Am J Bot As a consequence, Rubisco efficiency and photosynthetic capacity are enhanced in environments promoting photorespiration Schuster and Monson, ; Monson and Jaeger, ; Dai et al.

Ribulose 1,5-bisphosphate regeneration capacity is reduced flor low light, which will reduce Rubisco oxygenase activity and, in turn, the rate of Gly formation von Caemmerer, All of the Flaveria species in this study have similar Rubisco specificity factors and, hence, similar oxygenation potentials Kubien et al. Mitochondria were generally associated with M chloroplasts. As indicated by S.

The classical view of BS cells in C 3 plants is that they are small with few organelles Metcalfe and Chalk, ; Sage, In recent geological time, heat and aridity would have combined with low atmospheric CO 2 to depress C i and greatly increase photorespiratory vv in the C 3 Flaveriinae species. Studies of C 4 evolution have led to conceptual models proposing a stepwise origin of the C 4 pathway Monson and Rawsthorne, ; Sage et al.

We specifically evaluated the hypothesis that F. Unlike sunflower, the BS chloroplasts in F. The net CO 2 exchange rate corresponding to the convergence point was the estimate of R d. 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. Ann Mo Bot Gard Abstract The evolution of C 4 photosynthesis in many taxa involves the establishment of vlpr two-celled photorespiratory CO 2 pump, flor C 2 photosynthesis.

To quantify BS and M ultrastructural parameters, transmission electron microscopy images from 10 BS and M cells from each sampled leaf were analyzed using ImageJ software National Vlpr of Health; Schneider et al.

Once the proto-Kranz traits are in place, benefits arising from the recycling of photorespiratory CO 2 could flrp subsequent selection for greater flux of Gly into the inner BS.

For ultrastructural analysis, leaf samples were collected from three to four plants.

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Given the diffusive barrier represented by the vacuole, photosynthetic activity by the inner BS chloroplasts would probably depend upon photo respired CO 2 released by nearby mitochondria. Compared with sunflower and S. J Exp Bot If the inner BS represents a large enough sink for Gly, this arrangement could 23.

metabolize any Gly that may overflow from the M cells, such as during hot periods when photorespiration may be particularly high. In the three typical C 3 flpd sunflower, S. As a representative of a typical C 3 phenotype, we selected sunflower, which is in the same taxonomic tribe as Flaveria spp.

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These results demonstrate that observed changes in BS organelle numbers and distribution are unique to BS cells rather than reflecting altered patterning in all photosynthetic cells. With increased organelle volume in the inner BSthere may arise a large enough sink for Gly such that a chance reduction of GDC expression in the M tissue would not be deleterious but would instead improve carbon gain in photorespiratory conditions. These cellular traits, combined with the high vein density and large BS size reported for these species McKown and Dengler,demonstrate that F.

Light A and transmission B electron micrographs of leaf cross sections of sunflower illustrating BS cells and their organelles.