Evidence that inducible C₄‐type photosynthesis is a chloroplastic CO₂‐concentrating mechanism in Hydrilla, a submersed monocot

Abstract

Hydrilla verticillata (L.f.) Royle exhibits an inducible C4-type photosynthetic cycle, but lacks Kranz anatomy. Leaves in the C4-type state (but not C3-type) contained up to 5-fold higher internal dissolved inorganic carbon (DIC) concentrations than the medium, indicating that they possessed a CO2-concentrating mechanism (CCM). Several lines of evidence indicated that the chloroplast was the likely site of CO2 generation. From C4-type leaf [DIC] measurements, the estimated chloroplastic free [CO2] was 400 mmol m~^. This gave a calculated 2% O2 inhibition of photosynthesis, which was identical to the measured value, and provided independent evidence that the estimated [CO2] was close to the true value. A homogeneous distribution of DIC in the C4-type leaf could not account for such a high [CO2], or the resultant low O2 inhibition. For C3-type leaves the estimated chloroplastic [CO2] was only 7 mmol m~^, which gave high, and similar, calculated and measured O2 inhibition values of 22 and 26%, respectively. The CCM did not appear to be located at the plasma membrane, as it operated at low and high pH, indicating that it was independent of use of HCO3~ from the medium. Also, both C3-and C4-type Hydrilla leaves showed pH polarity in the light, with abaxial and adaxial boundary layer values of about pH 4*0 and 10-5, respectively. Thus, pH polarity was not a direct component of the CCM, though it probably improved access to HCO3. Additionally, iodoacetamide and methyl viologen greatly reduced abaxial acidification, but not the steady-state CCM. Inhibitor studies suggested that the CCM required photosynthetically generated ATP, but Calvin cycle activity was not essential. Both leaf types accumulated DIC in the dark by an ATP-requiring process, possibly respiration, and C4-type leaves fixed CO2 at 11-8% of the light rate. The operation of a CCM to minimize photorespiration, and the ability to recapture respiratory CO2 at night, would conserve DIC in a densely vegetated lake environ-

Biochemical and biophysical CO2 concentrating mechanisms in two species of freshwater macrophyte within the genus Ottelia (Hydrocharitaceae)

“…However, NAD-ME actitvity was not measured and the activity of NADP-ME was about half that of PEPC so it is not impossible that NAD-ME is also involved in decarboxyation in this species. H. verticillata is also assumed to belong to the NADP-ME sub-group (Bowes 2011;Bowes et al 2002) but much more evidence is available to support this contention since physiological characteristics changed in parallel to NADP-ME activity and oxygen inhibition measurements are consistent with high concentrations of CO 2 being generated in the chloroplast where NADP-ME is located (Magnin et al 1997;Reiskind et al 1997). In H. verticillata, the ratio of NAD-ME to NADP-ME is about five, much less than that found in the two species of Ottelia studied here (Table 4).…”

The role of carbonic anhydrase in photosynthesis and the activity of the carbon‐concentrating‐mechanism in bryophytes of the class Anthocerotae

“…In C % and CAM plants, primary fixation of carbon as HCO $ − by PEP carboxylase and subsequent decarboxylation of the organic acid product generates high values of p i which maximize the ratio of carboxylase : oxygenase activity of Rubisco. Furthermore, a range of C $ -C % intermediate strategies exists in higher plants (Griffiths 1989 ;Reiskind et al, 1997) and the possibility cannot be excluded that the Anthocerotae utilize a primitive form of these pathways.…”

Individual Maize Chromosomes in the C3 Plant Oat Can Increase Bundle Sheath Cell Size and Vein Density

“…As well as this diversity within the biochemistry of C 4 photosynthesis, there is also variation in terms of the leaf anatomy associated with the C 4 pathway. Although several examples of C 4 photosynthesis operating in single cells have been described (Reiskind et al, 1997;Casati et al, 2000;Reinfelder et al, 2000;Voznesenskaya et al, 2002Voznesenskaya et al, , 2003, most C 4 species compartmentalize carboxylation and decarboxylation activities into two distinct cell types. At least 22 variations of two-celled "Kranz" anatomy have been documented (Dengler and Nelson, 1999;Edwards and Voznesenskaya, 2011;Sage et al, 2012) whereby different cell types have been coopted as the photosynthetic carbon assimilation and photosynthetic carbon reduction tissue.…”