Changes in cytokinin form and concentration in developing kernels correspond with variation in yield among field-grown barley cultivars

► Cytokinin concentrations in kernels of field-grown barley were evaluated. ► Levels of trans-zeatin varied by developmental stage and cultivar. ► Cis-zeatin and trans-zeatin switched in predominance from stages one to four. ► The high yield cultivar had the greatest cytokinins during starch accumulation. ► The roles of isopentenyl transferase and cytokinin oxidase were also explored.