REDUCTION OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE BY ANTISENSE RNA IN THE C-4 PLANT FLAVERIA-BIDENTIS LEADS TO REDUCED ASSIMILATION RATES AND INCREASED CARBON-ISOTOPE DISCRIMINATION/

Transgenic Flaveria bidentis (a C-4 species) plants with an antisense gene directed against the mRNA of ribulose-1,5-bisphosphate carboxylas e/oxygenase (Rubisco) were used to examine the relationship between th e CO2 assimilation rate, Rubisco content, and carbon isotope discrimin ation. Reduction in the amount of Rubisco in the transgenic plants res ulted in reduced CO2 assimilation rates and increased carbon isotope d iscrimination of leaf dry matter. The H2O exchange was similar in tran sgenic and wild-type plants, resulting in higher ratios of intercellul ar to ambient CO2 partial pressures. Carbon isotope discrimination was measured concurrently with CO2 and H2O exchange on leaves of the cont rol plants and T-1 progeny with a 40% reduction in Rubisco. From the t heory of carbon isotope discrimination in the C-4 species, we conclude that the reduction in the Rubisco content in the transgenic plants ha s led to an increase in bundle-sheath CO2 concentration and CO2 leakag e from the bundle sheath; however, some down-regulation of the C-4 cyc le also occurred.