Premium
Evidence that inducible C 4 ‐type photosynthesis is a chloroplastic CO 2 ‐concentrating mechanism in Hydrilla , a submersed monocot
Author(s) -
REISKIND J. B.,
MADSEN T. V.,
GINKEL L. C.,
BOWES G.
Publication year - 1997
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1046/j.1365-3040.1997.d01-68.x
Subject(s) - hydrilla , photosynthesis , chloroplast , botany , carbon dioxide , chemistry , polarity (international relations) , photosynthetic efficiency , biology , aquatic plant , biochemistry , ecology , macrophyte , organic chemistry , gene , cell
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 CO 2 generation. From C 4 ‐type leaf [DIC] measurements, the estimated chloroplastic free [CO 2 ] was 400 mmol m −3 . This gave a calculated 2% O 2 inhibition of photosynthesis, which was identical to the measured value, and provided independent evidence that the estimated [CO 2 ] was close to the true value. A homogeneous distribution of DIC in the C 4 ‐type leaf could not account for such a high [CO 2 ], or the resultant low O 2 inhibition. For C 3 ‐type leaves the estimated chloroplastic [CO 2 ] was only 7 mmol m −3 , which gave high, and similar, calculated and measured O 2 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 HCO 3 − from the medium. Also, both C 3 − and C 4 ‐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 HCO 3 . 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 C 4 ‐type leaves fixed CO 2 at 11·8% of the light rate. The operation of a CCM to minimize photorespiration, and the ability to recapture respiratory CO 2 at night, would conserve DIC in a densely vegetated lake environment where daytime [CO 2 ] is severely limiting, while [O 2 ] and temperatures are high.