AbstractDetached ears from barley plants of the same age but of different K status (0.25 or 1.0 me K/l in the nutrient solution) were fed with14C sucrose at 14,21,28 and 35 days after ear emergence. Single grain weights were always higher in plants grown at 1 mMK. To establish whether this effect was due to a direct influence of K on the storage capacity, incorporation of14C sucrose into grain starch was studied and the activity of bound ADPG‐starch synthase under in vitro conditions determined.14C sucrose incorporation into grain declined with grain development without significant differences between K treatments. The conversion of14C sucrose into grain starch was also unaffected by the K status of the plant. The activity of ADPG starch synthase was either unaffected or influenced inconsistently by the K nutrition of the crop.Results of both approaches indicate, that at K contents>0.7% of grain‐DM, the rate of starch synthesis is not limited by the K concentration in the grain. This nutrient thus appears to increase grain yield more by its effect on assimilate supply (source) than by influences on the rate of synthesis of storage products (si