Translocation in corn

​Sugar is the product of photosynthesis, a process at which corn is especially good. The sucrose form of sugar is moved (translocated) from the photosynthetically-active leaf (source) to sinks such as growing leaves, roots and, eventually, seeds. Hormones, mostly cytokinins, direct direction of the flow. Translocation occurs through the phloem portion of vascular bundles through cell membranes at the cost of some energy.  Cytokinins are mostly produced by the newly developing cells at growing points such as tips of root branches, leaf buds, growing leaf tips and embryos in newly formed kernels. We humans selected from the Teosinte ancestor, plants that not only met the minimal needs of producing seeds to assure a future generation but also those with extra storage of carbohydrate in the fruit (grain) for our own consumption.  To do this we selected for excessive photosynthesis, temporary storage of excess carbohydrates in the pith of the stalk and eventual movement of it to the grain. This was not done cheaply.  We had to get more leaf area and more root tissue to not only support the plants but also to uptake the water and nutrients to grow the bigger plant and to initiate the larger grains.  All of this required more energy.  After pollination, the newly formed embryo in each kernel begins to produce the cytokinins directing the flow of sugar towards it.   This is occurring at the same time that root tips are not as prolific and consequently producing less cytokinin.  
 
It takes about 10 days after pollination for the flow to each kernel to gain full speed. Varieties, and environments, differ in the flow rate per kernel but from day 11 to about day 40 the flow per kernel appears to be constant. Production of sugars per day may be affected by cloudy days, or leaf damage but the power of the individual kernel sinks remains strong during that time.  Any shortage of new sugar is replaced by sugars stored in the stalk pith tissue. After the 50thday, the draw per day is reduced until finally an abscission layer is formed at the base of the kernel in which the phloem tissue no longer can move the sugars.  However during that 60-day period the root is competing with the kernels for sugars and our attempt to capture the maximum carbohydrate in the grain.