Gravitropism is the ability of a plant to turn in response to gravity. Roots have gravitropism, bending to turn down and stems negative gravitropism to turn up. But what happens if you remove a plant’s ability to sense where down is?
In roots, plants feel where down is in the root cap. If you remove the root cap carefully, and then tip the plant on its side, the root will continue to grow without changing direction until the root cap regenerates. Once the root cap can signal to the root, cells on one side of the root elongate to bend the root downwards.
The flight of STS-3 posed a challenge to the plants on board. Once in orbit they would be in perpetual freefall and there’d be no sense of ‘up’. What effect would this have on the root cap? Slocum, Gaynor and Galston compared the responses of the oat and mung bean seedlings on board in their paper Cytological and Ultrastructural Studies on Root Tissues. Seedlings for both plants germinated either a few hours before launch or in orbit.
The oats were fine. Both the flight and ground-based oat seedlings had normal root structure. The same was almost true for the mung beans too. Most of the roots were normal, except for the root-cap in the flight sample. The root cap cells on the mung beans in space had had a very bad time. Most of the cells were degenerated. If you compare the control sample below (left) with the flight root (right) you can see one of them is not well.
It seems that the ability of plants to adapt to microgravity varies on the plant, so it’s not enough to extrapolate from one to all.
This is another paper that continues to get cited today. Most recently Simple sequence repeat markers reveal multiple loci governing grain-size variations in a japonica rice (Oryza sativa L.) mutant induced by cosmic radiation during space flight by Wang et al in Euphytica 2014. There’s also research on peas citing it like Ultrastructure and metabolic activity of pea mitochondria under clinorotation in Cytology and Genetics 2012.
If gravity is essential, then it might become something we have to fake in space. The usual idea is to gently rotate a space station to give a sense of centripetal force. Spin faster and it is possible to subject plants to hypergravity, as noted by Nigel Chaffey earlier this year. Perennial favourite Arabidopisis is the subject of a 300g (yes, three hundred times the force of gravity) in this paper from AnnBot. Subjecting humans to this level of gravity would be a Very Bad Idea.
You can read more posts on papers from our spaceflight supplement by clicking the STS-3 tag.
Slocum R.D., Gaynor J.J. & Galston A.W. (1984). Cytological and Ultrastructural Studies on Root Tissues, Annals of Botany, 54 (supp3) 65-76.
Brykov V.O. & I. P. Generozova (2012). Ultrastructure and metabolic activity of pea mitochondria under clinorotation, Cytology and Genetics, 46 (3) 144-149. DOI: http://dx.doi.org/10.3103/s0095452712030036
NAKABAYASHI I. (2006). Hypergravity Stimulus Enhances Primary Xylem Development and Decreases Mechanical Properties of Secondary Cell Walls in Inflorescence Stems of Arabidopsis thaliana, Annals of Botany, 97 (6) 1083-1090. DOI: http://dx.doi.org/10.1093/aob/mcl055
Wang J., Tianqing Zheng, Xiuqin Zhao, Jauhar Ali, Jianlong Xu & Zhikang Li (2013). Simple sequence repeat markers reveal multiple loci governing grain-size variations in a japonica rice (Oryza sativa L.) mutant induced by cosmic radiation during space flight, Euphytica, 196 (2) 225-236. DOI: http://dx.doi.org/10.1007/s10681-013-1026-8