The utilization of pre‐anthesis reserves in grain filling of wheat. Assessment by steady‐state 13CO2/12CO2 labelling

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Abstract
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.