EFFECTS OF AMBIENT CO2 CONCENTRATION ON GROWTH AND NITROGEN USE IN TOBACCO (NICOTIANA-TABACUM) PLANTS TRANSFORMED WITH AN ANTISENSE GENE TOTHE SMALL-SUBUNIT OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE

Growth of the R1 progeny of a tobacco plant (Nicotiana tabacum) transf ormed with an antisense gene to the small subunit of ribulose-1,5-carb oxylase/oxygenase (Rubisco) was analyzed under 330 and 930 mubar of CO 2, at an irradiance of 1000 mumol quanta m-2 s-1. Rubisco activity was reduced to 30 to 50% and 13 to 18% of that in the wild type when one and two copies of the antisense gene, respectively, were present in th e genome, whereas null plants and wild-type plants had similar phenoty pes. At 330 mubar of CO2 all antisense plants were smaller than the wi ld type. There was no indication that Rubisco is present in excess in the wild type with respect to growth under high light. Raising ambient CO2 pressure to 930 mubar caused plants with one copy of the DNA tran sferred from plasmid to plant genome to achieve the same size as the w ild type at 330 mubar, but plants with two copies remained smaller. Di fferences in final size were due mostly to early differences in relati ve rate of leaf area expansion (m2 m-2 d-1) or of biomass accumulation (g g-1 d-1): within less than 2 weeks after germination relative grow th rates reached a steady-state value similar for all plants. Plants w ith greater carboxylation rates were characterized by a higher ratio o f leaf carbon to leaf area, and at later stages, they were characteriz ed also by a relatively greater allocation of structural and nonstruct ural carbon to roots versus leaves. However, these changes per se did not appear to be causing the long-term insensitivity of relative growt h rates to variations in carboxylation rate. Nor was this insensitivit y due to feedback inhibition of photosynthesis in leaves grown at high partial pressure of CO2 in the air (p(a)) or with high Rubisco activi ty, even when the amount of starch approached 40% of leaf dry weight. We propose that other intrinsic rate-limiting processes that are indep endent of carbohydrate supply were involved. Under plentiful nitrogen supply, reduction in the amount of nitrogen invested in Rubisco was mo re than compensated for by an increase in leaf nitrate. Nitrogen conte nt of organic matter, excluding Rubisco, was unaffected by the antisen se gene. In contrast, it was systematically lower at elevated p(a) tha n at normal p(a). Combined with the positive effects of p(a) on growth , this resulted in the single-dose antisense plants growing as fast at 930 mubar of CO2 as the wild-type plants at 330 mubar of CO2 but at a lower organic nitrogen cost.