Grain filling lasts for about twenty days in total. We have broken the time down into three sections, 11-16, 17-21 and 21-30 days after flowering and describe what happens to the endosperm, the cell layers surrounding the embryo sac and the embryo during each of these time periods. These periods correspond to the Medium Milk, Soft Dough and Hard Dough stages used by farmers to describe the grain.
At the 'Hard Dough' stage the grain achieves a golden colour. Protein and starch have been accumulating in the endosperm at a maximum rate and this now ceases. This final accumulation of dry matter will be followed by the death of the endosperm cells in preparation for dry down. The endosperm cells toward the outside of the grain have the highest protein content and this can sometimes be detected by the naked eye as having a slightly 'glassy' appearance. Starch granules of two types are present in the endosperm .The larger, type-A, starch granules are lens shaped; the smaller, type - B, which are added after the type- A are spherical.
On the outside of the grain the pericarp has started to fuse with the maternal epidermis. On the inside the pressure exerted by the expanding endosperm during this phase results in all the layers of the seed coat becoming compressed. The cells of the aleurone are producing storage and antioxidant proteins which are associated with osmoprotection and desiccation tolerance.
The embryo is now a fully developed miniature plant and is accumulating its own storage reserves, mainly in the form of tricylglycerols, as well as osmoprotectant proteins to enable it to survive desiccation. The scutellum, which is essentially a massive storage organ for the support of the embryo during germination, has started to adhere to the endosperm.