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Summary • In grasslands, the loss of structural carbon (C) from nonharvested plant parts is a primary C source for the soil. The amount of input depends not only on the size of structural C pools but also on their loss rates. • In the field, we examined the effects of elevated atmospheric partial pressures of CO2 (pCO2) and nitrogen (N) supply on pool size and rates of structural C loss in stubble and roots of perennial ryegrass (Lolium perenne) by using multiple-pulse labelling and steady-state labelling. • Stubble retained structural C for roughly half the time it was retained in roots. Elevated pCO2 combined with low N supply enlarged the pools of roots and stubble. These conditions also stimulated the rate of structural C loss from stubble and, thus, the amounts available for further transformation. • The potential of multiple-pulse labelling as a field technique is highlighted. The stimulation of structural C loss from stubble by elevated pCO2 at low N provides a missing link between increased C assimilation and low yield response and indicates a potentially higher input of structural C into the soil.

○ Plant response to elevated atmospheric CO 2 may depend on the carbon sink strength, determined by the availability of resources other than CO 2 , and the developmental stage. ○ In a 2-yr field experiment with model swards of Lolium perenne, the effect of CO 2 enrichment (FACE) on yield and allocation of dry mass (DM) and N were examined under three N fertilization treatments during vegetative and reproductive growth. ○ During vegetative growth, in the highest N treatment, the greatest increase in DM yield occurred at elevated CO 2 ; there was no change in DM allocation. By contrast, at low N, residual biomass, but not yield, increased under CO 2 enrichment, and the tillers were shorter. During reproductive growth, under CO 2 enrichment DM yield increased similarly across all N treatments; there was no change in DM and N partitioning. The mean weight and height of the reproductive tillers increased. ○ At high N availability, or during reproductive growth, L. perenne swards overcome carbon-sink limitation and show a strong yield response to elevated CO 2 . Biomass allocation and the height of the plants, in response to elevated CO 2 , clearly depend on N fertilization and developmental stage.

SUMMARY The fructan-β-fructosidase activity (1-FEH; EC 3.2.1.80) that degrades inulin in tubers of Helianthus tuberosus L. appears to be developmentally regulated; it was low in growing tubers but increased during dormancy and sprouting. In spite of relatively high 1-FEH activity in vitro, fructose concentration was very low in developing and dormant tubers and increased markedly only during sprouting. A fructan-β-fructosidase from such sprouting tubers was purified 41-fold to a single protein band on one-dimensional sodium dodecylsulphate-polyacrylamide gels. The purification procedure included ammonium sulphate precipitation, lectin-affinity chromatography on concanavalin A, anion-exchange and cation-exchange chromatography. The enzyme had an apparent molecular mass of 75000 measured by size-exclusion chromatography, and 79000 measured by one-dimensional sodium dodecylsulphate-polyacrylamide gel electrophoresis. It exhibited a high substrate specificity, hydrolysing terminal β-(2-1)-fructosyl-fructose-linkages in linear and branched fructan oligomers; β-(2-6)-linkages were hardly hydrolysed. Hydrolysis of inulin oligomers followed normal saturation kinetics: Km values for 1,1-kestotetraose and 1,1,1-kestopentaose were 8-3 mM and 12 mM, respectively. Fructosyl residues were hydrolysed from inulin oligomers by a multi-chain mechanism. The fructan-β-fructosidase showed optimal enzyme activity at pH 5–2, and it retained its full activity after pre-incubation for 1 h at up to 40°C. The release of fructose from 5 mm 1,1-kestotetraose was reduced by 25 % when 1-FEH was assayed in the presence of 10 mM sucrose. It is proposed that the inhibition of 1-FEH activity by sucrose is a mechanism for controlling fructan degradation in planta.

SUMMARY A fructan: fructan fructosyl transferase (FFT, EC 2.4.1.100) was purified 61-4-fold from roots of Taraxacum officinale Weber. The enzyme is a glycoprotein with an apparent molecular weight of 49000 as determined by SDS-polyacrylamide gel electrophoresis. FFT activity was detected by 1-kestose-dependent nystose production. The enzyme was most active at pH 6·5 and was stable at 30°C (1 h). Separation by preparative iso-electric focusing yielded four different forms with iso-electric points around pH 4·8. The purified enzyme was active on different oligofructans of the inulin series, but not on melezitose, 6-kestose, neokestose, maltopentaose or sucrose as the sole substrate.

SUMMARY Fructan: fructan fructosyltransferase activity (FFT, EC 2.4.1.100) from Helianthus tuberosus L. was purified 221-fold by a basic procedure involving ammonium sulphate precipitation, lectin chromatography and ion-exchange chromatography. The resulting FFT preparation was separated into three protein bands, of apparent molecular weight 72800, 60 500 and 56 200, by denaturing polyacrylamide gel electrophoresis. These proteins showed affinity to sucrose-Eupergit. FFT proteins with a molecular weight of 72800 were isolated by preparative native gel electrophoresis, and yielded six distinguishable forms on separation by analytical isoelectric focusing. Proteins from the basic purification were separated by preparative isoelectric focusing into several forms with isoelectric points between pH 4.3 and 4.5. Samples from the gel with FFT activity were analyzed by denaturing polyacrylamide gel electrophoresis. Three samples contained only protein with the molecular weight 72800, and one sample contained only protein of apparent molecular weight 60500. The remaining samples contained a mixture of proteins with molecular weights of 72800, 60500 and 56 200. FFT was detected by 1-kestose-dependent nystose production. The enzyme was most active at pH 65, and up to 80 % of the activity was retained on pre-incubation (1 h) at temperatures of up to 40 °C. FFT transferred fructosyl groups from oligofructans [degree of polymerization (DP) 3–8] of the inulin series. No glycosyl transfer occurred with 6-kestose, neokestose, maltose, raffinose and maltotriose as the sole substrate. Sucrose efficiently accepted fructosyl units from oligofructans with a Km of approximately 0.2 mM. The rate of fructosyl transfer increased with degree of polymerization (from DP 4). 1-kestose was shown to be an efficient donor of fructosyl units to sucrose.