Corn belt of USA 2021 weather has featured a cool April and May and hot June and early July. Most corn planting was done in April, as usual. Those temperatures and genetics interact to affect the timing of the apical meristem switching from producing more leaves to those specialized cells of the tassel.
Corn ancestor Teosinte, originally growing in southern Mexico, was stimulated to flower only when exposed to short days (long nights). This was an apparent advantage because it matched the wet season of that location. People selected and moved those early corn-teosinte mutants out of that environment increasingly further from the equator, changing the flowering to be less dependent upon long nights but more related to temperature. Once again, we benefit from the genetic diversity among the corn genetics and the practical selection by corn breeders over 8000 years.
Photoperiodism in plants is evident as we see many species that bloom at the same time every year. A protein called florigen is produced in leaves and moved through the phloem to the meristems that were producing stem and leaves and stimulates changes to cause it to produce flowers. It is the regulation of the gene, that is causing the gene to be active and therefore produce the RNA, and, ultimately the protein, that is more complex. There are at least 4 genes involved in the photoperiod response by corn. Adapting corn to the temperate zone summers done long before anyone acknowledged presence of genetics was done by farmers over centuries. A recent reference on genetics involved in corn flowering can be found at Genetics. 2010 Mar; 184(3): 799–812. Now we know that the photoperiod aspect is controlled by genes, but also that heat is a factor in the plant’s switch to producing reproductive structures at apex and at least on nodal bud. Tropical corns do eventually flower in the US Midwest but in some cases only close to the fall frost date. Our company does some breeding projects with tropical material. Those planted in April in our greenhouse will reach the ceiling before forming tassels after 4-5 months but those planted in December, with our short winter days, will flower by in 2-3 months and only reach a height of 5-6 feet. Tropical hybrids grown in Brazil have plant heights and flowering times very similar to US corn belt hybrids growing in Midwest summers.
One study that I did many years ago compared the heat units to time of apical meristem showing a tassel to the maturity rating for many commercial hybrids. Timing of that differentiation, occurring in June correlated very closely with our final maturity ratings for those hybrids. This supported the hypothesis that it is the heat units beginning immediately after planting that is most significant in determining the maturity of a corn crop. Heat after switching the growing points from producing stem and leaf tissue to tassel and ear tissue has an influence, but the earlier season affect is greater. Maturity in most corn belt corn is controlled by several genes affecting response to accumulating heat soon after planting. Tropical corns are also influenced by heat but other genes affecting response to number of hours of continuous darkness have a greater affect on time to flowering.
Those tassels that we see emerging in corn fields now, in mid-July, were initiated several weeks ago as determined by genetics and heat of the early corn season.
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.