Changes in cytokinins and cytokinin oxidase activity in developing maize kernels and the effects of exogenous cytokinin on kernel development

Temporal changes in cytokinin levels, mitotic activity and cytokinin oxidase activity were determined within kernels at the same stage of physiological development in single ears of field-grown maize (Zea mays L.). Cytokinins were qualitatively and quantitatively characterized by immunoaffinity chromatography, high-performance liquid chromatography (HPLC) and radioimmunoassay (RIA). Zeatin (Z), zeatin riboside (ZR) and isopentenyladenosine (iPA) all reached their maximum concentrations 9 days after pollination (DAP). The mitotic activity within the endosperm also peaked at 9 DAP. Cytokinin oxidase was present in kernels at basal levels from 3-6 DAP, then increased substantially through 10 DAP. Comparison of oxidase activity in kernels which are maturing normally and those which will abort, revealed major differences. In aborting apical kernels, the enzyme activity remained at basal levels from 4-10 DAP and only increased slightly trough 15 DAP. In median kernels, which develop normally, oxidase activity increased significantly by 5 DAP and reached a peak 4-fold higher than the basal level by 9 DAP. The differences in cytokinin oxidase activity between kernels which are maturing normally and those which will abort was so pronounced that cytokinin oxidase levels can be considered an indicator of normal kernel development. Stem infusion of benzylaminopurine (BA), but not Z or ZR, into intact plants at pollination increased the kernel number per ear at maturity by up to 30% and in some cases the total kernel weight per ear. The increase was due to a reduction in apical kernel abortion