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

Abstract

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 levels of N fertilization and regular defoliation, which resulted in plants with a wide range of source–sink relations. C and N metabolism were investigated 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 the main short-term storage compound for night-time export. Fructan accumulation indicated the degree of surplus C supply in the source compared to C use in sinks. Nitrate content depended mainly on N fertilization, and was reduced under elevated p_CO2. Nitrate appeared to indicate a current surplus of available N relative to the need for growth. Amino acid content responded strongly to N fertilization but decreased only slightly under elevated p_CO2. Protein content, however, decreased significantly under elevated p_CO2. The patterns of diurnal changes of C or N compounds did not differ between CO₂ treatments. Down-regulation of photosynthesis appeared to occur when plants were extremely N-limited as under elevated p_CO2, low N and at a late regrowth stage.