PHOTOSYNTHETIC ACCLIMATION TO ELEVATED CO2 OCCURS IN TRANSFORMED TOBACCO WITH DECREASED RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE/OXYGENASE CONTENT

Inhibition of net carbon assimilation rates during growth at elevated CO2 was studied in transgenic tobacco (Nicotiana tabacum L.) plants containing zero to two copies of antisense DNA sequences to the small subunit polypeptide (rbcS) gene of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). High- and low-Rubisco tobacco plants were obtained from the selfed progeny of the original line 3 transformant (S.R. Rodermel, M.S. Abbott, L. Bogorad [1988] Cell 55: 673-681). Assimilation rates of high- and low-Rubisco tobacco plants increased 22 and 71%, respectively, when transferred from 35- to 70-Pa CO2 chamber air at 900 mu mol m(-2) s(-1) photon flux density. However, CO2-dependent increases of net carbon assimilation rates of high- and low-Rubisco plants virtually disappeared after 9 d of growth in elevated CO2 chamber air. Total above-ground dry matter production of high- and low-Rubisco plants was 28 and 53% greater, respectively, after 9 d of growth at 70 Pa compared with 35 Pa CO2. Most of this dry weight gain was due to increased specific leaf weight. Rubisco activity, Rubisco protein, and total chlorophyll were lower in both high- and low-Rubisco plants grown in enriched compared with ambient CO2 chamber air. Soluble leaf protein also decreased in response to CO2 enrichment in high- but not in low-Rubisco tobacco plants. Decreased Rubisco activities in CO2-adapted high- and low-Rubisco plants were not attributable to changes in activation state of the enzyme. Carbonic anhydrase activities and subunit levers measured with specific antibodies were similar in high- and low-Rubisco tobacco plants and were unchanged by CO2 enrichment. Collectively, these findings suggested that photosynthetic acclimation to enriched CO2 occurred in tobacco plants either with or without transgenically decreased Rubisco levels and also indicated that the down-regulation of Rubisco in CO2-adapted tobacco plants was related to decreased specific activity of this enzyme.