Chlorophyll does not reflect green light – how to correct a misconception

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ABSTRACT
Plant leaves are green because they contain the green photosynthetic pigments, chlorophylls a and b. Popular science literature, and sometimes even textbooks, state that the greenness is caused by reflection of green light by chlorophyll. In the present study, we compared the reflectance spectra of green leaves to yellow or white leaves of the same species. Chlorophyll-deficient leaves reflected green light more efficiently than green leaves of the same species, which conclusively refutes the misconception. The data show that the green colour of leaves is caused by preferential absorption of blue and red light by chlorophyll, not by reflection of green light by chlorophyll. The data suggest that the cellulose of the cell walls is the main component that diffusely reflects visible light within plant leaves.

Introduction
The colours of illuminated items are – with few exceptions like the blue colour of the sky – caused by wavelength-selective absorption of light. Wavelengths that are neither absorbed nor pass through, are (diffusively) reflected from the item, and the spectral distribution of the reflected light determines the colour. An opaque object either absorbs or reflects all incident light, and if the object is homogenous like a Lego brick, then the reflection spectrum of the material is essentially a mirror image of its absorption spectrum. In heterogeneous systems, one constituent may mostly reflect and another may absorb.

Chlorophylls a and b show strong absorption in the blue and red spectral regions but absorb poorly green light (500–560 nm) (Lichtenthaler and Buschmann 2001). Due to inhomogeneous broadening, the absorption spectra of both pigments are however wider in vivo than in organic solvents (Van Amerongen, Valkunas, and van Grondelle 2000), enabling wider absorption of photons throughout the illumination spectrum. Plants also contain carotenoids absorbing blue-green light (Lichtenthaler and Buschmann 2001). Nonetheless, due to the sheer number of pigment molecules, green light is overall absorbed only 20–30% less efficiently by leaves of land plants than red or blue light, and green light is also utilised in photosynthesis (Hershey 1995). Ability to utilise green light has been suggested to provide the leaves in lower layers of the canopy and chloroplasts in lower mesophyll layers with excitation energy when the topmost layers efficiently absorb blue and red light (Terashima et al. 2009).

The reflectance of light is widely used in remote sensing to estimate the chlorophyll content per surface area of the terrain. The estimation requires simultaneous recording of reflectance at two wavelengths, from which one is strongly and the other poorly absorbed by chlorophyll. The ratio of reflectance in the red region (strongly absorbed) to reflectance in near-infrared (poorly absorbed) has been found to estimate chlorophyll content better than the red to green reflectance ratio (Le Maire, Francois, and Dufrêne 2004; Datt 1999).

Popularised science texts (see, e.g. Ortega 2020) and even biology textbooks (see, e.g. Raven, Evert, and Eichorn 2005) sometimes explain the green colour of plant leaves by stating that chlorophyll absorbs blue and red but reflects green light. The origins of this hypothesis are unclear, but it appears to be widespread. A Google search requiring the exact wording ‘chlorophyll reflects green light’ gave 4670 hits when tested on 4 October 2020, and Wikipedia repeats the misconception (Wikipedia article titled Chlorophyll; https://en.wikipedia.org/wiki/Chlorophyll, cited 27 April 2020).

Light and colour belong to elementary school science classes and to high-school physics. Studies of conceptions about light, colour and vision have revealed that upper elementary school children (grades 3–6) often have no idea about the roles of incident and reflected light in vision and that children tend to get confused when they find out that a brightly coloured item cannot be seen at all in complete darkness (Ward, Sadler, and Shapiro 2008). Various incorrect (‘alternative’) conceptions of colour, including the idea that colour is a permanent property of an object and independent of the colour of incident light, are common among upper elementary school students (Eaton et al. 1984; Valanides and Angeli 2008), and the misconceptions appear to be recalcitrant against traditional science teaching (Eaton et al. 1984). In Finnish high-school biology textbooks, the colour of plants is simply attributed to the presence of chlorophyll, without a further physical explanation (Happonen et al. 2018a, 2018b).

The misconception that chlorophyll reflects light may not belong to elementary school students’ misconceptions because this misconception requires correct basic understanding on how the colour of an object is formed. Therefore, the misconception is expected to be one of the teachers and their educators. In the present study, we show that the reflectance of green light by plant leaves is not caused by chlorophyll, and plant leaves devoid of chlorophyll show higher, not lower, reflectance in the green region than green leaves. With these data, we seek out to falsify and correct the common misconception about chlorophyll reflecting green light. Furthermore, we provide simple tools for demonstrating the reflectance of light by leaves in a classroom.

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Discussion
White and yellow leaves with very low chlorophyll concentrations (Table 1) show significantly higher reflectance of green light than the green leaves of the same species (Figure 2 –Figure 4), thus immediately falsifying the hypothesis that chlorophyll reflects green light. Our results are in agreement with results of Gitelson and Merzlyak (1996) who showed that increasing chlorophyll a concentration is inversely, rather than directly proportional to the reflectance of green light. Specular reflectance appears to depend on the surface quality and on the chlorophyll content of the leaf. The glossy birch and barley leaves reflect light more strongly than the matt, high- chlorophyll Euphorbia leaves throughout the measured spectrum. The differences in chlorophyll content (Table 1) partially explain the differences in reflectance below 700 nm but above that chlorophylls a and b absorb light very poorly. The present data also show that transmission of light from the background through the leaf can be significant. This was demonstrated by the Euphorbia spectra that changed dramatically when the background was changed, indicating strong dependence of reflectance on the reflectivity of the background. Dependence of leaf reflectance on the background may be important for the interpretation of remote sensing data. While chlorophylls a and b absorb throughout the visible spectrum, carotenoids only absorb blue-green light (Zur et al. 2000). In green leaves, the contribution of carotenoid absorption to the reflectance is difficult to distinguish from the contribution of chlorophyll b, as both absorb roughly at the same range of wavelengths. However, the spectra of the chlorophyll-deficient leaves show that the decrease in reflectance in all pigmented leaves when approaching 500 nm from the long- wavelength side is largely caused by carotenoids. This conclusion is based on the finding that the decrease in reflectance around 500 nm is much larger in the yellow, more carotenoid-rich birch leaves (Figure 2, Table 1) than in the white leaves of variegated barley and Euphorbia with very low carotenoid content (Figures 3 and Figures 4).

If chlorophyll does not reflect green and near-infrared light, what then causes the reflectivity of leaves in these wavelength regions? Our data do not pinpoint a specific biomolecule among the constituents of the leaves, but cellulose of the cell walls is an obvious candidate, as white paper, composed of cellulose, is highly reflective in the whole visible range (see Figure 4). The large amount of cell walls in a plant leaf supports the suggestion that they are mainly responsible for the reflectance of leaves. The sensitivity curve of the human eye reflected in the 1931 CIE luminous efficiency function (see Solomon and Lennie 2007 for a review of the human visual system) peaks in green, and therefore green light has a higher visible impact than other visible wavelength ranges. The spectral sensitivity of the human eye deepens our perception that plant leaves are definitely green, even if green light might be only slightly enriched in the light reflected from a leaf (Figure 4). [snip] In conclusion, plant leaves are green because green light is less efficiently absorbed by chlorophylls a and b than red or blue light, and therefore green light has a higher probability to become diffusely reflected from cell walls than red or blue light.

Chlorophylls do not reflect light.

Just seems to me like further evidence plants benefit from a full RGB spectrum.

As soon as i read chlorophyll Billy Madison popped into my head. "Chlorophyll, more like borophyll". Has nothing to do with the actual post FYI. Thanks for posting this, great write-up.

Pretty cool.

Gotta love the misconceptions from childhood education.

Thanks for the thread, great read.

Milky Weed said:

Just seems to me like further evidence plants benefit from a full RGB spectrum.

Click to expand...

and FR! ))