New insight into photosynthetic acclimation to elevated CO₂: The role of leaf nitrogen and ribulose-1,5-bisphosphate carboxylase/oxygenase content in rice leaves

We tested the hypothesis that photosynthetic (A) acclimation to elevated CO₂ partial pressure (p[CO₂]) is associated with the inhibition of protein synthesis, inhibition of nitrogen (N) partitioning into the leaf blade and/or accelerated leaf senescence in rice (Oryza sativa L. cv. Notohikari). Plants were grown for 70 days hydroponically in artificially illuminated growth chambers at a p[CO₂] of either 39 or 100Pa at N 2mM. Leaf A, V_c.max, J_max, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, E.C.4.1.1.39), mRNA for associated genes rbcS and rbcL, total N and carbohydrate concentrations in leaves at different positions in the canopy were measured. Spatial allocation of N and Rubisco synthesis of expanding leaf blade was also measured from the leaf ligule to tip of the leaf blade. Growth at elevated p[CO₂] suppressed light saturated A, V_c.max and J_max in leaf blades at all positions in the canopy. The suppression of A was 15% for the upper leaf blades compared to 37% in the lower leaf blades. Similar reductions in the amount of Rubisco, Chlorophyll, and total N were observed in the leaves of the plants grown in 100 p[CO₂] compared to the 39 p[CO₂]. Sucrose and starch concentration concentrations increased at elevated p[CO₂] but we found no relationship between A, Rubisco or the amount of transcript abundance of rbcS and rbcL. Elevated p[CO₂] substantially reduced N allocation into expanding leaf blades and this was well correlated with Rubisco synthesis. These results suggest that A acclimation to elevated p[CO₂] occurs during all phases of the leaf development, is initiated during the cell maturation process and linked with spatial N allocation into the leaf blade. In addition, elevated p[CO₂] accelerated lower leaf blade senescence which compounded the effect on A acclimation.