Recent and future increases in atmospheric [CO2] are predicted to increase plant growth while also reducing resource investment in photosynthetic machinery (i.e. Rubisco). However, many C3 plants like tobacco continue to make “excess” Rubisco relative to the current “high CO2” environment. The continued large investment in Rubisco may be an enduring legacy of plant adaptation to low [CO2] conditions during the not so distant past. If this is the case, then bioengineering efforts to enhance plant growth under future [CO2] conditions by improving the carboxylation efficiency of Rubisco may be ineffective. Here, we investigated potential physiological and enzymatic constraints on C3 plant responses to rising [CO2] by growing wild type (WT) and mutant tobacco genotypes (where Rubisco carboxylation efficiency has been improved) under past, present, and future [CO2]. We measured plant growth at a constant day post emergence (dpe) and at a constant developmental stage. Leaf gas exchange rates, Rubisco content, and Rubisco activation were measured at a constant developmental stage.
Results/Conclusions
Low [CO2] greatly reduced plant growth such that WT plants exhibited a 94% and 51% reduction in leaf biomass at 180 and 270 ppm [CO2], respectively (measured at 20 dpe). Biomass reductions under low [CO2] were significant for all genotypes even when plants were allowed to reach a constant developmental stage. Elevated [CO2] did not significantly increase plant biomass. In fact, two of the mutant genotypes were smaller under 700 ppm compared to 400 ppm [CO2]. Furthermore, none of the mutant genotypes were significantly larger than WT plants grown at the same [CO2]. Rubisco content was largely unaffected by [CO2] for all genotypes except the NL25 mutant genotype. NL25 mutants exhibited reduced Rubisco content and the greatest biomass reductions under low [CO2]. Our results support the hypothesis that high Rubisco content likely benefited C3 plant growth under low [CO2], and that continued investment in Rubisco may be a legacy of past conditions. Furthermore, we show that efforts to increase plant growth via improved Rubisco carboxylation efficiency are largely unsuccessful in tobacco across a broad [CO2] gradient. We hypothesize that the benefits of carboxylation efficiency improvements in tobacco are veiled by changes in the amount of active Rubisco.