Roots down, stems up

​At this time of the year in northern temperate zones, as growers are waiting for soil temperatures to be high enough for planting, one can contemplate why, no matter the position of the seed, does the root go down and the shoot go up.  This has been studied by many, including Charles Darwin in 1880.   The total mechanism is still not completely understood.
 
Geotropism is the response to gravity and phototropism is response to light.  Root’s downward growth is affected by the tissue in the root tip meristem area.  The root cap cells (outside the meristematic cells) effect the tropism.  Removal of the cap appears to disorient the growth direction.  Removal of cells on one side of the cap, causes the root to grow towards the remaining cells.  It is possible that inner cell structures such as the endoplasmic reticulum, and other cell organelles tends to become more concentrated on the lower side of the cells and thus produces metabolites on that side, resulting in cell growth in that direction. 
 
Plant hormones such as auxins and gibberellic acid are involved in plant tropism, but all of the exact mechanisms are still not understood.  It has been proposed that auxins produced in the root tip are distributed to cells behind the root tip, affecting the elongation of exterior cells and thus direction of growth.  Differences among corn varieties in root growth direction tendencies, some with more deep, narrow growth and others with more lateral growth, is evidence that genetic factors are influencing metabolism that affects root growth direction.
 
Phototropism is also affected by plant hormone production and distribution.  Specific wave lengths in natural sunlight affect the auxins involved in the differential cell elongations, ultimately resulting in growth direction towards the light source.  Exact mechanisms involving photo receptor compounds that allow specific wave lengths of light to turn on the growth cells is not clear, despite many researcher’s attempts to study specifics.
 
Tropism in plants is obvious and complex.  Surely some of those 30,000-40,000 genes in corn are involved, and mostly we can only screen for the effects of the phenomenon with selecting the best performing corn plants and admire the fact that tropism exists.
(Corn Journal 4/19/2018)