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Notes: 
cv, cultivar
δ, deviation of C isotope composition from a standard
Δ, C isotope discrimination
WSC, water soluble carbohydrates

Steady-state labelling of all post-anthesis photosynthate of wheat was performed to assess the mobilization of pre-anthesis C (C fixed prior to anthesis) in vegetative plant parts during grain filling. Results were compared with estimates obtained by indirect approaches to mobilization of pre-anthesis C: ‘classical’ growth analysis and balance sheets of water soluble carbohydrates (WSC) and protein. Experiments were performed with two spring wheat cultivars grown with differential nitrogen fertilizer supply in 1991 and 1992. The fraction of pre-anthesis C mobilized in above-ground vegetative biomass ranged between 24 and 34% of total C present at anthesis. Treatment effects on mobilization of pre-anthesis C in total above-ground vegetative biomass were closely related (r2 = 0·89) to effects on mobilization of WSC-C plus protein-C (estimated as N mobilized × 3·15). On average, 81% of pre-anthesis C mobilization was attributable to the balance of pre-anthesis WSC (48%) and protein (33%) between anthesis and maturity. In roots, WSC and protein mobilization accounted for only 29% of the loss of pre-anthesis C. Notably, mobilization of pre-anthesis C was 1·4–2·6 times larger than the net loss of C from above-ground vegetative biomass between anthesis and maturity. This discrepancy was mainly due to post-anthesis C accumulation in glumes and stem. Post-anthesis C accumulation was related to continued synthesis of structural biomass after anthesis and accounted for a mean 15% of total C contained in above-ground vegetative plant parts at maturity. A close correspondence between net loss of C and mobilization of pre-anthesis C was only apparent in leaf blades and leaf sheaths. Although balance sheets of WSC and protein also underrated the mobilization of pre-anthesis C by ≈ 19%, they gave a much better estimate of pre-anthesis C mobilization than growth analysis.

Notes: 
 δ, C isotope composition relative to Pee Dee Belemnite
WSC, water-soluble carbohydrates
N, nitrogen
C, carbon
cv, cultivar
ME, efficiency of mobilized pre-anthesis C utilization in grain filling (g C g–1C)

Significant mobilization of protein and carbohydrates in vegetative plant parts of wheat regularly occurs during grain filling. While this suggests a contribution of reserves to grain filling, the actual efficiency of mobilized assimilate conversion into grain mass (ME) is unknown. In the present study the contribution of pre-anthesis C (C fixed prior to anthesis) to grain filling in main stem ears of two spring wheat (Triticum aestivum L.) cultivars was determined by 13C/12C steady-state labelling. Mobilization of pre-anthesis C in vegetative plant parts between anthesis and maturity, and the contributions of water-soluble carbohydrates (WSC) and protein to pre-anthesis C mobilization were also assessed. Experiments were performed with two levels of N fertilizer supply in each of 2 years. Pre-anthesis reserves contributed 11–29% to the total mass of C in grains at maturity. Pre-anthesis C accumulation in grains was dependent on both the mass of pre-anthesis C mobilized in above-ground vegetative plant parts (r2 = 0·87) and ME (defined as g pre-anthesis C deposited in grains per g pre-anthesis C mobilized in above-ground vegetative plant parts; r2 = 0·40). ME varied between 0·48 and 0·75. The effects of years, N fertilizer treatments and cultivars on ME were all related to differences in the fractional contribution of WSC to pre-anthesis C mobilization. Multiple regression analysis indicated that C from mobilized pre-anthesis WSC may be used more efficiently in grain filling than C present in proteins at anthesis and mobilized during grain filling. Possible causes for variability of ME are discussed.

Notes: The contribution of pre-defoliation reserves and current assimilates to leaf and root growth was examined in Lolium perenne L. during regrowth after defoliation. Differential steady-state labelling with 13C (CO2 with δ13C = -0.0281 and -0.0088) and 15N (NO3− with 1.0 and 0.368 atom percentage, i.e. δ15N = 1.742 and 0.0052, respectively) was applied for 2 weeks after defoliation. Rapidly growing tissues were isolated, i.e. the basal elongation and maturation zones of the most rapidly expanding leaves and young root tips, with a biomass turnover rate 〉 1 d−1. C and N weights of the elongation zone showed a transient decline. The dry matter and C concentration in fresh biomass of leaf growth zones transiently decreased by up to 25% 2 d after defoliation, while the N concentration remained constant. This ‘dilution’ of growth zone C indicates a decreased net influx of carbohydrates relative to growth-related influx of water and N in expanding cells, immediately after defoliation. Recovery of the total C and N weights of the leaf elongation zone coincided with net incorporation of currently absorbed C and N, as shown by the kinetics of δ13C and atom percentage 15N in the growth zones after defoliation. C isotope discrimination (Δ13C) in leaf growth zones was about 23‰, 1–2‰ higher than the Δ in root tips. Δ15N in the leaf and root growth zones was 10±3‰. The leaf elongation zones (at 0–0.03 m from the tiller base) and the distant root tips (about 0.2 m from the base) exhibited similar kinetics of current C and N incorporation. The amount of pre-defoliation C and N in the growth zones, expressed as a fraction of total C and N, decreased from 1.0 to 0.5 at 3 (C) and 5 (N) d after defoliation, and to 0.1 at 5 (C) and 14 (N) d after defoliation. Thus, the dependence of growth zones on current assimilate supply was significant, and stronger for C than for N. The important roles of current assimilates (as compared to pre-defoliation reserves) and ‘dilution’ of dry matter in regrowth after defoliation are discussed in relation to the method of labelling and the functional and morphological heterogeneity of shoot tissues.