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 co ntaining zero to two copies of antisense DNA sequences to the small su bunit polypeptide (rbcS) gene of ribulose-1,5-bisphosphate carboxylase /oxygenase (Rubisco). High- and low-Rubisco tobacco plants were obtain ed from the selfed progeny of the original line 3 transformant (S.R. R odermel, M.S. Abbott, L. Bogorad [1988] Cell 55: 673-681). Assimilatio n rates of high- and low-Rubisco tobacco plants increased 22 and 71%, respectively, when transferred from 35- to 70-Pa CO2 chamber air at 90 0 mu mol m(-2) s(-1) photon flux density. However, CO2-dependent incre ases 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 pla nts 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 incr eased specific leaf weight. Rubisco activity, Rubisco protein, and tot al chlorophyll were lower in both high- and low-Rubisco plants grown i n 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 hi gh- and low-Rubisco plants were not attributable to changes in activat ion state of the enzyme. Carbonic anhydrase activities and subunit lev ers measured with specific antibodies were similar in high- and low-Ru bisco tobacco plants and were unchanged by CO2 enrichment. Collectivel y, these findings suggested that photosynthetic acclimation to enriche d 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 spec ific activity of this enzyme.