As largely immobile, rooted-to-one-spot, organisms, plants are adept at sensing their surroundings and responding appropriately, e.g. growing in a way that maximises interception of sunlight for photosynthesis. This is particularly well demonstrated by the shade avoidance mechanism of plants. As light passes through leaves, not only is the intensity of what is transmitted markedly reduced (e.g. this) , but there are changes in the ratio of red to far red wavelengths in that light, which are detected by neighbouring plants. The photomorphogenetic response that is triggered includes extension growth of the shaded plant. This should have the effect of elevating the plant, either above the sunlight-stealing competition or at least into a more favourable light environment. That works well for individual plants, but this shade-avoidance behaviour appears to have been taken to a new level (no pun intended…) by Mónica López Pereira et al..
Investigating sunflower (Helianthus annuus) grown in fields at high-density, they show that a given plant’s stem is inclined towards one side of the space between the rows, and the immediate neighbouring plants are inclined in the opposite direction*. In this way each individual plant’s lean can be interpreted as optimizing its own light environment for maximum personal photosynthetic benefit. Investigations involving manipulation of the spectral composition of light indicate that this stem-inclination response is mediated by changes in the red/far-red ratio of the light, which is perceived by the photoreceptor pigment phytochrome. This individual benefit has a collective benefit from a human ‘appropriation of photosynthate’ point of view in that the overall oil yield from such a mutually-inclined crop of sunflower can be 19–47 % higher than for plants whose stems remain vertical. Whether this behaviour can be ‘encouraged’ in other single-stem crops remains to be seen. But what a great potential to boost crop yield, seemingly ‘overnight’, and in a way that doesn’t appear to require additional inputs of fertilisers, with their attendant problems of eutrophication, etc.
From a plant where individuals’ stems do their own thing, to a species whose stems all incline in one direction, the amazing Cook pine (Araucaria columnaris). Jason Johns et al. report that stems of these trees are inclined towards the north in the southern hemisphere, and southwards when growing in the northern hemisphere. Furthermore, the degree of lean is greater at higher latitudes (i.e. further towards the Poles) in both hemispheres. The cause of – or reason for (they are different!) – this dramatic “hemisphere-dependent directional leaning” remains unknown. Another of Nature’s little mysteries to get your teeth into – if you feel so inclined.
*The story of the discovery of this phenomenon seems to be another of those Pasteurian ‘chance favours the mind that is prepared’ moments. The alternate leaning was only revealed when the study’s senior author – Antonio Hall, a crop eco-physiologist at the University of Buenos Aires, Argentina> – was visiting an unusually dense field of sunflowers, at the end of the growing season. It was only at that time – after the leaves had fallen – that the inclined stems became noticeable.
[Ed. – this fixed stem orientation is in addition to the more familiar sun-tracking behaviour of the growing stems of sunflower, which latter phenomenon was investigated by Hagop Atamian et al. and also discovered to have an advantage in terms of promoting growth.]
FRANKLIN, K. A., & WHITELAM, G. C. (2005). Phytochromes and Shade-avoidance Responses in Plants. Annals of Botany, 96(2), 169–175. https://doi.org/10.1093/aob/mci165
MATHEWS, S. (2006). Phytochrome-mediated development in land plants: red light sensing evolves to meet the challenges of changing light environments. Molecular Ecology, 15(12), 3483–3503. https://doi.org/10.1111/j.1365-294X.2006.03051.x
Messier, C., Parent, S., & Bergeron, Y. (1998). Effects of Overstory and Understory Vegetation on the Understory Light Environment in Mixed Boreal Forests. Journal of Vegetation Science, 9(4), 511. https://doi.org/10.2307/3237266
López Pereira, M., Sadras, V. O., Batista, W., Casal, J. J., & Hall, A. J. (2017). Light-mediated self-organization of sunflower stands increases oil yield in the field. Proceedings of the National Academy of Sciences, 114(30), 7975–7980. https://doi.org/10.1073/pnas.1618990114
Casal, J. J. (2012). Shade Avoidance. The Arabidopsis Book, 10, e0157. https://doi.org/10.1199/tab.0157
Johns, J. W., Yost, J. M., Nicolle, D., Igic, B., & Ritter, M. K. (2017). Worldwide hemisphere-dependent lean in Cook pines. Ecology, 98(9), 2482–2484. https://doi.org/10.1002/ecy.1850
Atamian, H. S., Creux, N. M., Brown, E. A., Garner, A. G., Blackman, B. K., & Harmer, S. L. (2016). Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits. Science, 353(6299), 587–590. https://doi.org/10.1126/science.aaf9793