Assimilated carbon during vegetative and early reproductive growth in wheat is temporarily stored in stem internodes and leaf sheaths (LSs), and can later be remobilized and transported to developing grain. The present study was undertaken to determine the effect of several peduncle (the uppermost internode) morpho-anatomical and biochemical traits on grain weight, and to assess the contribution of the peduncle water-soluble carbohydrate (WSC) reserves shortly after anthesis to its variation. In 2-year field trials, 61 wheat genotypes were used (27 F4:5 families, 17 parents used for the crosses and the 17 current best standards), comparing intact control plants (CP) with plants that were defoliated (DP) by cutting off all leaf blades 10 days after anthesis. Estimated contributions of peduncle (culm (C) and flag LS) assimilate reserves to grain weight/spike were from 0·06 to 0·31 and from 0·11 to 0·45 in CP and DP plants, respectively. In both CP and DP plants, a higher contribution was from the LS than from the C. High peduncle reserve mobilization efficiency, a longer exposed part of the peduncle and larger C storage capacity (through higher parenchyma and/or lower lignified area) were of specific benefit for maintaining grain weight in defoliated plants. F4:5 families had higher transport capacity in the peduncle, but without any improvement in WSC-related traits compared with the best standards.