Reiskind, J.B., T.V. Madsen, L.C. van Ginkel, and G. Bowes, 1997. Evidence that Inducible C4-Type Photosynthesis is a Chloroplastic CO2-Concentrating Mechanism in Hydrilla, a Submersed Monocot, University of Florida.

Reprinted from Plant, Cell and Environment (1997) 20(2): 211-220

ABSTRACT
Hydrilla verticillata (L.f.) Royle exhibits an inducible C_4- type photosynthetic cycle, but lacks Kranz anatomy. Leaves in the C_4- type state (but not C_3- type) contained up to 5-fold higher internal dissolved inorganic carbon (DIC) concentrations than the medium, indicating that they possessed a CO_2-concentrating mechanism (CCM). Several lines of evidence indicated that the chloroplast was the likely site of CO₂ generation. From C_4- type leaf [DIC] measurements, the estimated chloroplastic free [CO₂] was 400 mmol m^-3 .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 C_4- type leaf could not account for such a high [CO₂], or the resultant low O₂ inhibition. For C_3- type leaves the estimated chloroplastic [CO₂] was only 7 mmol m^-3 ,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 C_3- and C_3- 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 &-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 environment where daytime [CO₂] is severely limiting, while [O₂] and temperatures are high.

Entire Paper
Contact: George Bowes, University of Florida, Department of Botany, Gainesville, FL 32611
Keywords: Hydrilla, Basic_biology
Product Type: Research, Basic_biology
User Type: General