Nitrogen plays a major role in leaves when source-sink relations change: Cand N metabolism in Lolium perenne growing under free air CO2 enrichment

Swards of Lolium perenne L. were grown in the field in a long-term free air CO2 enrichment (FACE) facility. The CO2 treatment was combined with two le vels of N fertilization and regular defoliation, which resulted in plants w ith a wide range of source-sink relations. C and N metabolism were investig ated to assess the role of carbohydrate and nitrogenous compounds in leaves in indicating source-sink relations. Sucrose exhibited the largest changes in contents during the day-night cycle; therefore, it was identified as th e main short-term storage compound for night-time export. Fructan accumulat ion indicated the degree of surplus C supply in the source compared to C us e in sinks. Nitrate content depended mainly on N fertilization, and was red uced under elevated p(CO2). Nitrate appeared to indicate a current surplus of available N relative to the need for growth. Amino acid content responde d strongly to N fertilization but decreased only slightly under elevated p( CO2). Protein content, however, decreased significantly under elevated p(CO 2). The patterns of diurnal changes of C or N compounds did not differ betw een CO2 treatments. Down-regulation of photosynthesis appeared to occur whe n plants were extremely N-limited as under elevated p(CO2), low N and at a late regrowth stage.