CARBON-ISOTOPE DISCRIMINATION DURING C-4 PHOTOSYNTHESIS - INSIGHTS FROM TRANSGENIC PLANTS

We have measured the discrimination against C-13 during CO2 assimilati on in Flaveria bidentis wild type plants and in transgenic Flaveria bi dentis plants transformed (1) with an antisense RNA construct targeted to the nuclear encoded gene for the small subunit of Rubisco-these pl ants had reduced amounts of Rubisco, decreased CO2 assimilation rates and increased carbon isotope discrimination, which was also evident in the carbon isotope discrimination of leaf dry matter; and (2) transfo rmed with the mature coding region of carbonic anhydrase, CA, from tob acco (Nicotiana tabacum) in the sense direction under the control of t he cauliflower mosaic virus 35S promoter-these plants had slightly inc reased CA activity in the mesophyll as well as a 2-4-fold increase in CA activity in the bundle-sheath cells,The introduction of tobacco CA manifested itself by a reduction in CO2 assimilation rate and an incre ase in carbon isotope discrimination. We suggest that the increased ca rbon isotope discrimination is a result of increased bicarbonate leaka ge out of the bundle sheath.