spectrometers reveal how plants detect their neighbours
by Dr Michael Forster, Edaphic Scientific
Plants use light signals, or cues, from their environment to maximise their ability to grow, survive and reproduce. Light is not constant, however, and varies markedly throughout the day, week and year. Day lengths change from summer to winter, the amount of light varies between morning and afternoon, and even over the course of several minutes the light environment can change as clouds move in front of the sun.
From a plant’s perspective, perhaps the most important light cue occurs when a neighbouring plant starts to grow near or over the top of another plant. Crowding a plant, or growing a canopy over a plant, can be detrimental as the energy resources in sunlight are now diverted to the neighbouring plant. In the short term, this is a nuisance and can be tolerated. Over the long term, however, it can be fatal to the plant being shaded. The shaded plant needs to either adapt its physiology so it can tolerate the new, low light environment or it can grow up and away from the competing neighbour.
an experiment to bend plants towards the light
Plants commonly move towards the light. You can do an experiment yourself by germinating seeds on your windowsill. The seedlings will slightly bend towards the window as they grow. If you move the plate or tray on which the seedlings are growing so they are facing away from the light, over the next few hours or days the seedlings will bend back towards the light. Another famous example is sunflowers tracking the movement of the sun throughout the day.
Shade, or a decrease in light, can be caused by more than just a new canopy growing near or above a plant. A false light cue could lead to potentially damaging physiological or growth changes in plants. Therefore, plants need to be certain that they are indeed being shaded by a competing plant and the light environment has not change for some other, potentially temporary, reason. How plants go about this can be revealed by optical spectrometer measurements.
Spectrometers are scientific instruments for the measurement of electromagnetic wavebands or the light spectrum. There are many different types of spectrometers including mass, time-of-flight and magnetic spectrometers. Another type of spectrometer, optical spectrometers, are designed to measure the intensity of light as a function of wavelength. Edaphic Scientific has a range of optical spectrometers designed for this purpose.
Edaphic Scientific’s MR-16 portable optical spectrometer.
quantity versus quality of light
Plants can perceive both the quantity and the quality of light. The quantity of light refers to how much energy is available in light. At night there is very little light so there is virtually no energy available to the plants. Whereas around midday the quantity of light is at a maximum.
There is not a lot a plant can do when the quantity of light changes. This can occur throughout the day as there is less quantity in the morning versus midday, or in full sun versus under cloud cover. Plants have adapted over an evolutionary time scale to cope with consistently low quantity or high quantity light environments. Plants can be sun-adapted or shade-adapted. That is, their physiology is geared towards a life living in full sun or in deep shade – think of herbs in the understorey of a rainforest versus a grass growing in the field. Plants are often categorised into shade tolerant or shade intolerant species although this simple categorisation is not so black and white. Some shade intolerant species, for example, can tolerate a little bit of shade. Even on the same individual tree, leaves on the outside of the canopy maybe shade intolerant and those leaves on the inside of the canopy shade tolerant. This scientific paper discusses the complexities involved with shade tolerant and intolerant species.
The quality of light is a little harder to explain and is related to electromagnetic spectrum.
The image above shows the electromagnetic spectrum from very short gamma rays to very long radio and audio waves. Plants are primarily interested in the spectrum covering ultraviolet (UV), visible (VIS) and infrared (IR). Within this part of the electromagnetic spectrum, good quality light is found in the blue, red and near infrared regions. For a plant, this is good quality light as more light in these wavebands improves photosynthesis, growth and everything else relevant to a plant.
spectrometers reveal quantity versus quality of light
The image below was taken with Edaphic Scientific’s HSM-01 spectrometer and shows the spectra between 380nm and 780nm in full sunlight and cloudy weather. Both data were collected around midday. Note the absolute count on the y-axis of both data graphs. These numbers represent how much energy was measured by the HSM-01 spectrometer at 1nm resolution. In full sunlight, the maximum value is approximately 27,000 uW whereas on the cloudy day it is approximately 1,400 uW. There is a large difference in the quantity of light between the sunny versus cloudy day. However, note that the shape of the distribution, or the shape of the curve, is practically identical. If we removed the values on the y-axis there is little chance to distinguish which graph is the sunny or the cloudy day. The quality of light on both the sunny and cloudy day is the same.
Now consider the next image below which was also taken with Edaphic Scientific’s HSM-01 spectrometer. The images on the left hand side of the graph are the same as the two images above of the sunny and cloud days. The images on the right hand side are spectrometer measurements taken beneath the canopy of a plant. Note the vastly different shapes between the curves, but also note clouds absorb far more quantity of light than this particular leaf canopy.
Under a plant canopy, the quality of light has changed markedly. Note the large amount of absorbance in the blue region (400 to 499 nm wavelengths) and in the red region around 670nm wavelength. These are the light signals that plants can detect that provide information on whether they are beneath or near a canopy.
Out of interest, the two images below is the full spectrum between 300 and 1100nm that can be generated by the HSM-01 spectrometer. The first spectrum is full sunlight and the second is light beneath a plant canopy.
red to far red ratio and phytochromes
The specific cue or signal which plants use to detect neighbours is the ratio of red to far red light (R:Fr). Or, more specifically, the ratio of red light at 660nm to far red light at 730nm. From the images above, on the full sunlight day and the cloudy day without a plant canopy the R:Fr ratio was approximately 1.2. With the plant canopy, and on both full sunlight and cloudy days, the R:Fr ratio was approximately 0.1. The quantity of light does not affect the R:Fr ratio but the quality of light absolutely does. This is an unequivocal signal to plants that there is a canopy near or above them.
The receptor inside the plant that detects red and far red light is known as phytochrome. This is an extremely important growth regulating protein controlling all sorts of function from stem elongation to seed germination.
Optical spectrometers can determine both the quantity and the quality of the light environment for plant growth. Such knowledge is not only useful for scientific research into the physiology of plants, but it is also of extreme practical use. For example, greenhouse or glasshouse managers can ensure their plants are growing under the right type of housing materials and important wavebands are not being absorbed. In incubators and growth cabinets, spectrometers can be used to ensure artificial LED growth lights are providing sufficient quality of light. These are just some of the many uses for spectrometers.
references and further reading
Ballare, C. L. (1999) Keeping up with the neighbours: Phytochrome sensing and other signaling mechanisms. Trends Plant Sci. 4, 97–102.
Casal, J. J. and H. Smith (1989) The function, action and adaptive significance of phytochrome in light-grown plants. Plant Cell Environ. 12, 855–862.
Franklin, K. A. (2008) Shade avoidance. New Phytol. 179, 930–944.
Holmes, M. G. and H. Smith (1975) The function of phytochrome in plants growing in the natural environment. Nature 254, 512–514.
Holmes, M. G. and H. Smith (1977) The function of phytochrome in the natural environment. II. The influence of vegetation canopies on the spectral energy distribution of natural daylight. Photochem. Photobiol. 25, 539–545.
Smith, H. (1982) Light quality, photoperception and plant strategy. Annu. Rev. Plant Physiol. 33, 481–518.