Pre-Anthesis Reserve Utilization for Protein and Carbohydrate Synthesis in Grains of Wheat

We assessed the contribution of pre-anthesis reserve C to protein and carbohydrate deposition in grains of wheat (Triticum aestivum L.) using a new approach comprised of steady-state (13)C/(12)C labeling and separation of the protein and carbohydrate fractions of mature grains. Experiments were performed with two spring wheat cultivars (Kadett and Star) grown with differential N fertilizer supply over 2 years. Pre-anthesis reserves contributed between 30% and 47% of the C in protein and 8% to 27% of the C in carbohydrates of grains. Partitioning of pre-anthesis C among the grain fractions was strongly dependent on the C/N (w/w) ratio in mobilized pre-anthesis biomass (r(2) = 0.92). There appeared to be no significant exchange of pre-anthesis C between amino acids and carbohydrates during redistribution. The mean apparent efficiency of mobilized carbohydrate-C use in grain filling (ME(CHO), estimated as the mass of pre-anthesis C deposited in grain carbohydrates per gram of pre-anthesis C mobilized from carbohydrates in vegetative plant parts) was 0.72, whereas that of protein-C (ME(P)) was 0.56. However, ME(P) and ME(CHO) varied among treatments. ME(CHO) increased with increasing contributions of water-soluble carbohydrates to total pre-anthesis carbohydrate mobilization. ME(P) decreased with increasing residence time of protein in vegetative biomass. Possible causes for variability of ME(P) and ME(CHO) are discussed.