Chloroplasts in cells within mesophyll and vascular bundle cells convert the light energy into chemical energy locked up in sugar molecules. These cell walls, although ridged giving strength, have pores (plasmadesmata) through which the proteins and sugars can be moved from the chloroplast laden cells. Most of the sugars move to the phloem portion of the vascular system. Phloem cells are living cells that are surrounded by companion cells that are the first site of sugar-protein molecules entrance into the phloem tissue. The remaining phloem is called a sieve-tube in which they have reduced cytoplasmic contents and sieve tube openings that connect them with sieve-tubes on above and below. This openings allow the transport of sugars in either direction.
As the concentration of sugars in these sieve tube portions of the phloem increases, water from the xylem moves, from the high concentration of water to the lower concentration because of the high concentration of the sugars. This adds pressure for for movement of the solution from that cell. The diffusion principle continues to guide the movement of the sugars within the phloem. This is basically a source-sink model. Pre-flowering, sugar movement to the growing points at the apical meristem and the root tips is caused by tying up by cellular respiration as well as tying up the sugars into more complex molecules such as cellulose for new cell walls. These sites become the ‘sink’ for diffusion of sugars from the ‘source’, the leaves.
This translocation process from leaves to vascular tissue continues as a source-sink phenomenon after flowering, as the growing point in the kernels become new sinks. These new sinks now compete with the root sinks. Genetics affect the strength of the sugar demand of each kernel and the number of kernels affect the total draw of sugars. Strength of the pull of sugars per kernel also involves the efficiency of the kernel removing sugars to more complex molecules such as starch. The movement of the sugars in phloem is directed by the pressure-driven bulk flow principle of moving from high concentration to lower concentration.
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.