Carbon mobilization in shoot parts and roots of wheat during grain filling: assessment by <sup>13</sup>C/<sup>12</sup>C steady‐state labelling, growth analysis and balance sheets of reserves

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<jats:p>
cv, cultivar
<jats:italic>δ</jats:italic>, deviation of C isotope composition from a standard
<jats:italic>Δ</jats:italic>, 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 (<jats:italic>r</jats:italic><jats:sup>2</jats:sup> = 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.</jats:p>

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