Phototropism - what is it for?

Richard D Firn and John Digby

Department of Biology
University of York
York
YO1 5DD

Correspondence should be addressed to: Richard D Firn, PhD.
Email: drf1@york.ac.uk

Submitted for publication: July 18th 2001


Keywords: phototropism, suntracking, heliotropism



ABSTRACT

The ability of young cress seedlings to track an artificial sun has been recorded using a simple time-lapse system. The video revealed that the young seedlings have a well developed ability to "sun-track".  The seedlings use hypocotyl differential elongation to orient their photosynthetic cotyledons such that the angle of incidence of the light is more favourable than could be achieved in static vertical seedling. It is proposed that the phototropic ability of seedlings evolved to enhance the efficiency of light capture of young seedlings. Consequently, studies of phototropism, which have nearly universally been conducted by studying the growth of a seedling towards a fixed light at right angles to the initial orientation of the seedling, have been highly artificial. Some of the interesting features of phototropism may be more readily appreciated if the physiology is considered in relation to the ecophysiological purpose of the process.


INTRODUCTION

The ability of many young seedlings to grow towards the light has been studied intensively since the mid 19th century and it was studies of this piece of physiology that was the starting point of a wave of research that led to the discovery of the plant hormone auxin. As a piece of physiology it was one of the easiest to study in a simple laboratory, indeed Darwin's classic studies were conducted in his study at Down House. The movement of an organ towards a light source was at that time termed heliotropism, recognising that the response must have evolved in response to sunlight (1). However by the beginning of the 20th century studies of this physiology had been taken into the laboratory where it was renamed phototropism and where unilateral light exposure to artificial light became the normal stimulus. The sensitivity and precision of the phototropic response towards dim unilateral light sources in laboratory studies fascinated physiologists and they began a century of work that focused on the mechanisms of light perception and the mechanisms for causing the differential elongation that brought about curvature. Such work has culminated with the discovery of photoreceptors responsible for phototropic perception (2) but there is still disagreement as to how differential elongation is controlled (3). However, what is phototropism for? Reading advanced texts and reviews one would have to assume that the role of phototropism must be self evident because it is rarely considered.  More elementary texts sometimes make unconvincing suggestions such moving the plant nearer the sun or aiding seedlings when growing under a rock face or on a bank. The inadequacy of these explanations of the selective forces that must have driven the evolution of the exquisite sensitivity of phototropism have been noted and it was suggested that seedlings may have evolved a phototropic response in order to provide a capacity to track the sun (4). Using a simple time lapse digital camera and a simple "artificial sun", it has now been possible to demonstrate conclusively that young dicot seedlings can indeed suntrack effectively.
In other words, the 19th century term (heliotropism) for the physiological response now called phototropism was most appropriate. The implications of considering phototropism in this new light are discussed


MATERIALS AND METHODS

Cress (Lepidium sativum) seeds were sown in 50mm plastic petri dish bottoms in Levingtons Compost (a peat-based multi-purpose compost) and watered with tap water. The dishes were placed for 4 days in a plant growth room (25oC 16h day/20oC 8h night) illuminated with white fluorescent light. The dished were placed in a tranparent plastic container to reduce water loss.
After 4 days the seedlings were removed and placed in a 22-25o C dark room and then exposed to a small unilateral light source (30mm  diameter  6v 5W MR11 quartz halogen lamp operating at 4v DC giving 14 micromoles m-2 s-1 ) that was attached to a 20cm arm. For phototropism experiments the arm was kept stationary such that the light was at right angles to the flank of the seedling. For suntracking experiments, a small AC motor drove the arm such that it arced over the seedlings during the following 12 h to simulate the movement of the sun.
At intervals, images of the seedlings were captured (PictureWorks Live) using a Kodak DVC-300 USB webcam attached to Pentium PC running Windows 98. The experiment has been repeated in excess of 20 times with similar results.


RESULTS

Young cress seedlings show a vigorous phototropic response towards a fixed unilateral light (Video 1). When the light source is moved during a 12h period to mimic the movement of the sun the seedlings are seen to initially move towards the "rising sun" then they reverse their direction of movement and follow the direction of the light's movement throughout the remainder of the day. From mid-day onwards they are quite efficient at suntracking (Video 2). Similar results have been obtained with mustard and Arabidopsis seedlings.


DISCUSSION

Young seedlings clearly have an ability to suntrack and the similarity of the initial bending responses to either a fixed or to a moving unilateral light strongly suggests that the suntracking response is essentially the net results of multiple phototropic responses throughout the day. It is clear from the time-lapse video and from the analysis of individual frames that such an ability to track the sun enables the seedling to orient its photosynthetic organs more appropriately throughout the day to enhance light capture. A more complete analysis of this potential photosynthetic gain is now being undertaken for simulated sunpaths as would be experienced by plants growing at various latitudes.
The fact that light grown seedlings show such a well developed ability to track the sun suggests that this ability could have been the driving force in the evolution of the physiological processes that we have traditionally associated with phototropism. The appreciation of the ecophysiological importance of phototropism may help us appreciate more fully why plants posses such remarkable "phototropic" sensitivity. The extreme sensitivity of plants to small light doses (light doses that could not easily be considered to be of ecophysiological significance) may be related more to an ability to respond to small light gradients than an ability to respond to low absolute light doses. It has been shown that seedlings show a good phototropic response to multiple light sources (5) and can respond to very small inequalities of exposure when given multiple exposures (4,6). This ability to respond to small gradients can be seen to be especially important once the plant "locks-on" to the sun because at that point the light gradient across the hypcotyl will be much smaller than it is at "dawn". The unilateral light at right angles to the hypocotyl in "early morning" is an exception and throughout most of the day much smaller angles of incidence are inducing differential elongation.
Further studies of very young seedlings growing in their natural environment are now needed to confirm the ecological significance of the demonstrated ability to suntrack.


CONCLUSIONS

It has been clearly shown that yound seedlings possess a competence to suntrack which appears to be caused by multiple phototropic responses in response to the varying position of the sun. The abandonment of the term heliotropism to describe the bending of young seedlings when grown in a light gradient, yet its retention by some of those studying the movement of leaves or flower heads in response to the movement of the sun, is now seen to be inconsistent. However, there are several features which suggest that the suntracking of leaves and the suntracking of young dicot seedlings are distinct physiological processes hence it might be sensible to retain the term phototropism for the latter but to recognise that its purpose is to facilitate suntracking (7). The increasingly complex interactions between plant photoreceptors (2) and their ecophysiological significance may be understood more easily if greater emphasis is paid to studying plants under conditions that simulate the conditions under which they evolved.


ACKNOWLEDGEMENTS

Our thanks to students past and present for keeping us interested in suntracking.


REFERENCES

  1. Pfeffer, W. (1905) The Physiology of Plants (Translated by Ewart A.J.) Oxford University Press
  2. Briggs, W.R. and Huala, E. (1999) Annu. Rev. Cell Dev. Biol. 15, 33-62.
  3. Firn, R.D. (1994) Phototropism. Chapter 9.2 in Photomorphogenesi in Plants. Eds Kendrick, R.E. and Kronenberg, G.H.M.. Kluwer.
  4. Firn, R.D. (1988) Phototropism. Biol. J. Linn. Soc. 34, 219-228.
  5. Gleed, D., Firn, R.D. and Digby, J. (1994) How is a phototropic stimulus perceived by hypocotyls. J. Exp. Bot. 45, 409-412.
  6. Arisz, W.H. (1915) Untersuchungen uber den Phototropismus. Rec. Trav.bot. neerl., 12, 44-216.
  7. Koller, D (2000) Plants in search of sunlight. Adv. Bot. res. 33, 35-131.

© 2001 Epress Inc.