Plants Tropisms - correcting some common misconceptions before we start

Gravitropism

The ability of a plant organ to orient itself with respect to the gravity vector has impressed and fascinated people for hundreds of years. Indeed gravitropism is the most visible example of the ability of plants to respond with precision to their environment. Every time you look at a plant you are looking at gravitropism in action. The orientation of the main stem, the branches, the leaves, the flowers and the fruits are usually determined predominantly by gravitropism.

Gravitropism examples

Because a gravitropic response can be seen within 30 minutes in young, easily grown seedlings, it was one of the first plant physiological processes subject to experimental study. Nearly 200 years of study followed but our understanding of gravitropism is still very incomplete. Why? I would argue that the apparent simplicity of the response has been partly to blame. Gravitropism is a very complex process that only looks simple. It is even possible that there is more than one type of gravitropism. Nearly all teaching of gravitropism, especially at an elementary level, ignores these two issues. Worse still many simple misconceptions about gravitropism remain uncorrected and are propagated year after year in research articles. So before I discuss gravitropism in detail, lets start by trying to correct some of these misconceptions.

Misconception 1: Plant organs respond to gravity by growing up or by growing down. Not true. There are examples of roots and shoots being maintained at all specific angles between 0o (vertically down) and 180o (vertically up).

There are 79,000 Google hits for the phrase “shoots grow up” and 392,000 for the phrase “roots grow down”. Such statements can be found in some textbooks and sadly even in recent scientific papers. But look at this picture of a poppy flower stalk as it develops.

poppy gsa

The red arrow heads indicate a group of cells that “flow” down the stalk as the stalks elongates. The youngest flower stalk is on the left. The gravitropic response of that group of cells changes as the cells elongate. When the cells are very small, the region has an upward orientation, so the flower bud is raised. A little later the region is growing down, so the flower bud is below the hook of the stalk. Subsequently that region is horizontalas it forms part of the hook. Finally the same zone is growing up as the flower opens. This change in gravitropic behavior is happening within one set of cells as they extend. So the angle at an organ (or part of an organ) grows can be variable.

Misconception 2: Shoots are negatively gravitropic and roots are positively gravitropic. The terms positive and negative are mechanistically meaningless as they simply describe whether an organ moves up or down.

Think of a root that grows horizontally. If one displaces that root so that its tip is now pointing downwards, a gravitropic response will cause the root to grow upwards - so this must be negatively gravitropism? But if the root is displaced so the tip is upwards, the gravitropic response which restores the horizontal orientation is downwards - so this is positive gravitropism? No. Mechanistically the gravitropic responses that caused the root to grow up or to grow down were equivalent so the terms “positive” or “negative” are meaningless. But you will find nearly 7000 Google hits for the phrase “positive gravitropism” or “negative gravitropism”.

Misconception 3: Root and shoot gravitropism are very similar. No. Root gravitropism and shoot gravitropism might share some processes but they differ fundamentally

Another example of the danger of misleading simplification is the way in which root gravitropism and shoot gravitropism are often regarded as analogous, sharing the same fundamental processes but controlled in some way to give different outcomes. The reason that this is so odd is that the way in which roots and young shoots grow is totally different. The gravitropic response of a young cress stem (the hypocotyl) involves the movement of the existing structure - no cell division is needed to re-orient the organ.

cress gravi composite

But the gravitropic response of young cress primary root involves redirecting future axis of elongation - the new cells being fed into the elongation zone of the root elongate on a different axis. The gravitropic response of a root involves very little movement of an existing root structure. Not surprisingly because an existing root could not move laterally in soil!

maize root gravi composite

These web pages do not aim to provide a summary of all that is known about gravitropism. Instead I want to help those interested in gravitropism to think about the subject in a more productive way.

Phototropism

As in the case of gravitropism, phototropism is much more complicated than it seems and misconceptions are all too common.

Phototropism misconception 1. Phototropism shows that plants can detect the direction of light. No. Phototropism results from the differential illumination of the flanks of an organ.

It is commonly and erroneously stated that phototropism is the movement of a plant organ in response to the direction of light. Although a plant exposed to unilateral light usually moves towards (often termed positive phototropism) or away from the direction of the light (negative phototropism), that result is simply a consequence of the way the experiment is conducted. Using a single light source as the phototropic stimulus, imposes on the plant a very unnatural condition and the resulting organ movement tells on nothing about the ability of the plant to respond to the directionality of the light . Plants have evolved to grow in environments where light hits the plant from all directions. Consequently if plants relied on detecting the direction of light, plants would be unable to show a phototropic response if more than one flank were exposed to light. Some very simple experiments where phototropic curvature was induced by two or more light sources showed that the direction of a phototropic stimulus was not the cause of phototropic induction (Gleed, D., Firn, R.D. and Digby, J. (1994) How is a phototropic stimulus perceived by hypocotyls. J. Exp. Bot. 45, 409-412). For example, if cress seedlings were placed between two lights, exposing each of two opposing flanks to phototropic stimulation, phototropic curvature would be induced towards the brightest light if the two light sources differed by > 10%. Plants also showed an impressive phototropic response if two flanks, 90 degrees apart, were illuminated (see photograph below). However, in this case the plant bends, not towards either light, but to a position dependent on the relative fluence rate of the two lights.

cress photo 2 flanks 2
Studies of the distribution of light within plant organs shows that the light gradients are complex because of light scattering within the organ (Parsons, MacLeod, Firn & Digby, 1983. PCE. 7, 325.Light gradients in shoots subjected to unilateral illumination — implications for phototropism. This alone would make it very difficult for the plant to detect the directionality of light.

One explanation as to why too many people erroneously believe that plants showing a phototropic response must be detecting the direction of light, is the widely held view that gravitropism and phototropism must share common mechanistic elements. If you believe that gravitropism is clearly induced by a directional stimulus (gravity), and you believe that gravitropism and phototropism must share common mechanistic elements, then it follows that you might conveniently believe that phototropism must also involve the detection of a directionality. However, one might question whether plants detect the direction of gravity in any meaningful way - this will be discussed elsewhere on this site.

Phototropism misconception 2: there is one kind of plant phototropism. No, there are possibly more than two.

Plants respond to light in many ways, most of which were only discovered long after people had begun to study phototropism. There are at least two families of photoreceptors in plants, one responding maximally to blue wavelengths and the other responding most to red wavelengths. Both red light and blue light can influence the rate of cell extension in young seedlings. Hence if one imposes a light gradient across a young extending organ, it is predicable that differential elongation patterns will be imposed on the growing organ and curvature would be predicted. This simple idea was advanced by Blaauw in the early part of the 20th C as the explanation for phototropism. However, several experiments have caste doubt on the Blaauw model (for example Macleod, K., Firn, R. D. and Digby, J. 1986. The phototropic responses of Avena coleoptiles.—J. exp. Bot. 37: 542–548) as a universal explanation of phototropism but such experiments do not, indeed they cannot, show that a Blaauw effect might not be responsible to some forms of phototropism. Thus just because the phototropic response of oat coleoptiles to very low fluence blue cannot be explained by a Blaauw model, it does not follow that root phototropism in response to red light could not involve some contribution of a Blaauw effect. So care is needed when reading about phototropism to ensure that one is comparing like with like.

Phototropism misconception 3: phototropism is easily studied without gravity interfering. No. Phototropism nearly always involves gravitropic stimulation

Once a seedling starts to bend towards a light, it is inevitably subjected to gravitropic stimulation. This can be clearly seen by turning off the light causing the phototropic response and watching the plant rapidly bend upwards due to the gravitropic response (possibly helped also by autotropism). One can “titrate” the two types of tropic stimulation by placing a seedling horizontally then placing a light directly beneath it. The plant will bend towards the strongest stimulation - if the light is bright enough the plant will bend downwards but if the phototropic response is weaker then the plant will bend upwards. So watching a plant subjected to prolonged phototropic stimulation one will see the net result of the two tropistic responses.

Phototropism misconception 4: phototropism studies in the laboratory mimics what happens in the field. No. phototropism or heliotropism - which is the party trick?

Is phototropism as studied in a dark room in any way meaningful or is it a party trick? It is radical even to raise this issue but without a recognition that physiological processes have evolved to give an organisms greater fitness then it is easy to start thinking that the traditional experimental approach is meaningful and probes the mechanisms in the most imaginative way. But why is the phototropic response of very young seedlings, growing in total darkness, towards a single extremely low energy light source, meaningful?

Richard D Firn

December 2009

 

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