Changes in source–sink relations during development influence photosynthetic acclimation of rice to free air CO2 enrichment (FACE)

Relationships between photosynthetic acclimation and changes in the balance between source-sink supply and demand of carbon (C) and nitrogen (N) were tested using rice (Oryza sativa L. cv. Akitakomachi). Plants were field-grown in northern Japan at ambient CO2 partial pressure [p(CO2)] or free air CO2 enrichment (FACE; p(CO2) ~ 26-32 Pa above ambient) with low, medium or high N supplies. Leaf CO2 assimilation rates (A) and biochemical parameters were measured at 32-36 (eighth leaf) and 76-80 (flag leaf) d after transplanting, representing stages with a contrasting balance between C and N supply and demand in sources and sinks. Acclimation due to FACE was pronounced in flag leaves at each N supply. This was not fully accounted for by reductions in leaf N concentrations, because A/N and Vcmax/N were lower in FACE-grown flag leaves. Acclimation did not occur in the eighth leaf, and A/N and Vcmax/N was not significantly increased in FACE-grown leaves. Soluble protein / sucrose and amino acid / sucrose concentrations decreased under FACE, whereas sucrose phosphate synthase protein levels increased. At flag leaf stage, there was a discrepancy between the demand and supply of N, which was resolved by enhanced leaf N remobilization, associated with the lower Rubisco concentrations under FACE. In contrast to the early growth stage, enhanced growth of rice plants was accompanied by increased plant N uptake in FACE. We conclude that photosynthetic acclimation in flag leaves occurs under FACE because there is a large demand for N for reproductive development, relative to supply of N from root uptake and remobilization from leaves.