Reduction of ribulose-1,5-bisphosphate carboxylase/oxygenase by antisense RNA in the C₄ plant Flaveria bidentis leads to reduced assimilation rates and increased carbon isotope discrimination

Transgenic Flaveria bidentis (a C₄ species) plants with an antisense gene directed against the mRNA of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) were used to examine the relationship between the CO₂ assimilation rate, Rubisco content, and carbon isotope discrimination. Reduction in the amount of Rubisco in the transgenic plants resulted in reduced CO₂ assimilation rates and increased carbon isotope discrimination of leaf dry matter. The H₂O exchange was similar in transgenic and wild-type plants, resulting in higher ratios of intercellular to ambient CO₂ partial pressures. Carbon isotope discrimination was measured concurrently with CO₂ and H₂O exchange on leaves of the control plants and T₁ progeny with a 40% reduction in Rubisco. From the theory of carbon isotope discrimination in the C₄ species, we conclude that the reduction in the Rubisco content in the transgenic plants has led to an increase in bundle-sheath CO₂ concentration and CO₂ leakage from the bundle sheath; however, some down-regulation of the C₄ cycle also occurred.