The Ups and Downs of Plant Growth
What environmental factors have the most important effects on plant development?
When I posed this question to students in class (back in the day when I was boring college undergraduates), it typically took quite a while before someone finally said “Gravity!”. (I usually had to jump up and down like I was on a pogo stick before they would even think about the force of gravity.)
Yes, this mysterious unseen force (gravitation) has profound effects on both root and shoot development in plants.
Indeed, growing plants are constantly monitoring and responding to the force of Earth’s gravity in a physiological process called gravitropism (a.k.a., geotropism).
Those tall, straight pine tress in the forest and those rows of field corn all growing away from the center of the Earth are manifestations of plant negative gravitropism.
And, conversely, roots growing downward in the soil toward the center of the Earth are manifestations of positive gravitropism.
Most people, I presume, don’t consider the effects of gravity on plants until, for example, they see a potted plant that’s been tipped on its side, it’s young shoots bending up, away from gravity.
Or, they may see a field of “lazy corn” that have their shoots growing along the ground, which are examples of gravitropic mutants. That is, due to a genetic mutation, these plants are agravitropic or ageotropic, not displaying a normal response to gravity. (Much has been learned about the nature of plant gravitropism from studying such mutants, e.g., see Ref 1 below.)
Once students recognize the effects of gravity on plants, a good follow-up question is:
What would a plant seedling look like if there was no gravity?
Plants in Space
Of course, before people were able to travel to space, away from Earth’s gravity, there was no way to conclusively answer this question.
About the best that scientists could do to try to answer this question was to use a device called a clinostat, which uses rotation to negate the effects of gravity on plant growth.
Briefly, the results of early experiments re. plant growth in space can be summarized as follows: “Spaceflight research conducted prior to 1987 clearly showed that orientation of roots and shoots differs from that normally observed on Earth. Roots were frequently disoriented, while shoots more typically were able to be directed upwards when grown in the presence of light.” (from: Ref 2 below)
Since the early years of plant research in space, literally hundreds, if not thousands, of papers have been published regarding various aspects of plants in space, from purely basic research to actually growing plants on spacecraft as a source of food for astronauts, for example super dwarf varieties of wheat (see Ref 3 below).
But, I’m not going to try to delve into this vast literature here.
What prompted this post was a report published in the 7 December 2012 issue of the open access journal BMC Plant Biology (see Refs 4 & 5 below).
This report, briefly described here, has to do with “root waving”.
According to Prof. Stan Roux (U. Texas): “Less studied effects of gravity (on plants) are undulations due to the regular periodic change in the direction root tips grow, called waving, and the slanted angle of growth roots exhibit when they are growing along a nearly-vertical surface, called skewing. Although diverse studies have led to the conclusion that a gravity stimulus is needed for plant roots to show waving and skewing, the novel results just published by Paul et al. (2012) – Ref 5 below – reveal that this conclusion is not correct.” (from Ref 4 below)
“The elongation zone in roots of ground control and flight plants. The blue frame defines a region of about 225μm along the length of each root from the transition zone into the elongation zone. The cross bars in the rectangle define cell boarders in this area for each root.” (Figure 3F and legend from Ref 5 below)
Their conclusion: Generally, the seedlings grown on the International Space Station were smaller but exhibited the same degree of waving seen in similar plants grown on Earth.
Just for fun: Video tour of the International Space Station
More plants in space: Botany experiment will try out zero gravity aboard space station.
1. Chen, R., E. Rosen and P. H. Masson (1999) “Gravitropism in Higher Plants.” Plant Physiology, Vol. 120, pp. 343-350. (Full Text)
2. Dutcher, F. R., E. L. Hess and T. W. Halstead (1994) “Progress in plant research in space.” Advances in Space Research, Vol. 14, pp. 159–171. (Abstract)
3. Coyne, C. (1996) “Space Wheat – To Boldly Grow Where No One has Grown Before.” The Salt Lake City Tribune, April 15, 1996. (PDF)
4. Roux, S. J. (2012) “Root waving and skewing – unexpectedly in micro-g.” BMC Plant Biology, Vol. 12, p. 231. (Abstract)
5. Paul, A.-L., C. E. Amalfitano and R. J. Ferl (2012) “Plant growth strategies are remodeled by spaceflight.” BMC Plant Biology, Vol 12, p. 232. (Abstract)
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