Cytokinins and auxins are operative during all of the corn plants life, including the movement of sugars to the young kernels. These two kinds of hormones have different roles in origin and effect on corn growth. Cytokinins are mostly produced in root tips in root meristems and transported through the water distribution in the xylem tissue. Auxins are mostly produced in stem meristems and distributed in the phloem system. Cytokinins are associated with increasing cell division in the stem meristems whereas auxins are involved in cell elongation. Apical dominance resulting in the corn plant usually having only one upright stem is because of the interactions of the auxins produced in the apical meristem. Removing that stem tip in early corn development and thus reducing auxin production tips the balance towards more cytokinin and stimulation of cell division in the lateral buds of the corn plant, resulting in branches.
Pollination of the multiple ovules in the corn ear results in attraction of cytokinins to each developing kernel. Moisture stress during the first 10 days after pollination is known to cause early death to some kernels, perhaps because of reduction transportation of cytokinins to the most immature embryos (my conjecture!). Cell division in the new embryo meristems establishes the movement of sugars through the phloem to the kernels. Much of the sugar is deposited into the endosperm portion where it is changed to more complex carbohydrates and thus allow the osmotic pressure for more sugar movement towards the kernels.
More is known about the effect of these plant hormones on plant growth than all of the mechanisms involved with those effects. Auxins involvement in cell growth involves softening cell walls, making elongation of cells easier. Cytokinins have been shown to prevent protein breakdown and activating protein synthesis.
Cytokinins produced in root meristems are transported to and stimulate the cell division in the kernel embryos. Meristems of those embryos produce auxins. Auxins are associated with production of ethylene which has been associated with formation of abscission tissue as leaves and fruit mature. It is assumed that the auxins are associated with formation of the black layer at the base of kernels, resulting in stoppage of movement of material to the kernels.
We know that these plant hormones are associated with the growth of corn tissues including the formation of kernels but there remains lots to learn of the actual molecular interactions that allows this to happen. Meanwhile, corn breeders, agronomists and growers attempt to coordinate it all by selecting the genetics that maximize grain production.
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About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.