The color most associated with plants is the color they are turning away. Photosynthesis works by absorbing light and using the energy from that light to make sugars the plant can use. Think about the colors of the rainbow red, orange, yellow, green, blue, indigo and violet, in that order.
Chlorophyll a picks up some red and orange light while it reflects yellow, green and blue. Its favorite colors, though, are indigo and violet, as it absorbs those colors at nearly double the rate it picks up red and blue. To perform photosynthesis, violet light is the most important color, and it's from these wavelengths that plants get most of their energy.
The reason for this is because out of the visible spectrum, red light is the longest wavelength light that the photosynthesis process can use, but it has the least energy. As we move through the spectrum from red, wavelengths become shorter and shorter while energy increases. Violet light is the shortest wavelength, and it has the highest energy. Plants are typically green because they reflect green and yellow light, but if they absorbed all colors equally, they would appear black.
While some plants do, in fact, have black leaves , this is definitely not the norm. If photosynthesis is all about harvesting energy from light, then why not use all of it?
Wavelengths absorbed by chlorophyll and other photosynthetic pigments generate electrons to power photosynthesis. All photosynthetic organisms have chlorophyll a which absorbs violet-blue and reddish orange-red wavelengths. Chlorophyll a reflects green and yellow-green wavelengths.
Accessory photosynthetic pigments, including chlorophyll b and beta-carotene, absorb energy that chlorophyll a does not absorb. Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane, such as in thylakoid membranes of the chloroplast or membrane infoldings found in photosynthetic prokaryotes. This pattern could result from light-dependent regulation of respiration Croce et al.
In our calculations, we assumed that the leaf respiration in the light was the same as R d. Such processes include photorespiration Krall and Edwards, , nitrate assimilation Nunes-Nesi et al. QY inc decreased slower under green than under red or blue light Figure 6A. The lower QY inc under blue light than under green and red light at high PPFD can be explained by disparities in the light distribution within leaves. Blue and red light were strongly absorbed by lettuce leaves Similar low green absorptance was found in sunflower Helianthus annuus L.
Brodersen and Vogelmann, , and spinach Vogelmann and Han, Absorption of red light decreased slower with increasing depth than that of blue light Vogelmann and Han, ; Brodersen and Vogelmann, Green light absorption peaked deeper into leaves, and was more evenly distributed throughout leaves, because of low absorption of green light by chlorophyll Vogelmann and Han, ; Brodersen and Vogelmann, The more even distribution of green light within leaves, as compared to red and blue light, can explain the interactive effects between PPFD and light spectrum on leaf photosynthesis.
The more uniform green light distribution within leaves may be a key contributor to higher leaf level QY inc under high PPFD because less heat dissipation of excess light energy is needed Nishio, ; Terashima et al. Reaction centers near the adaxial leaf surface receive more excitation energy under blue, and to a lesser extent under red light, than under green light, because of the differences in absorptance. Consequently, under high intensity blue light, NPQ is up-regulated in the chloroplasts near the adaxial leaf surface to dissipate some of the excitation energy Sun et al.
Since less green light is absorbed near the adaxial surface, less heat dissipation is required. This indicates more upregulation of heat dissipation in the top of the leaves under blue, than under green light Evans and Vogelmann, On the other hand, the bottom half of the leaves can still utilize the available light with relatively high QY inc , since the amount of light reaching the bottom half is relatively low, even under high PPFD Nishio, By channeling more light to the under-utilized bottom part of leaves, leaves could achieve higher QY inc even under high intensity green light.
In our study, high QY inc under green light and low QY inc under blue light at high PPFD Figure 6 can be thus explained by the large disparities in the light environment in chloroplasts from the adaxial to the abaxial side of leaves due to differences in leaf absorptance. Green light also resulted in a higher A g,max As discussed before, the high A g,max under green light resulted from the more uniform light distribution under green light, allowing deeper cell layers to photosynthesize more.
Overall, under high PPFD , the differences in light distribution throughout a leaf are important to quantum yield and assimilation rate, since it affects NPQ up-regulation Sun et al. We examined the effect of light quality and intensity on J and V c,max Figure 8. For the light-dependent reactions, the interactive effect between light spectra and PPFD found for CO 2 assimilation and quantum yield was also observed for J Figure 8A.
This similarly can be attributed to a more uniform energy distribution of green light among reaction centers throughout a leaf and weaker upregulation of non-photochemical quenching with increasing green light intensity Sun et al.
Unlike J , V c,max was largely unaffected by light spectra Figure 8B and was not correlated with A g data not shown. Similarly, Wullschleger noted a strong linear relationship between J and V c,max across C 3 species. The ratio between J and V c,max in our study 1. These results suggest that the interactive effect of light spectra and PPFD resulted from effects on J , which is associated with light energy harvesting by reaction centers, rather than from V c,max. Figure 9. The color scheme representing the nine spectra is the same as Figure 8.
The Emerson enhancement effect describes a synergistic effect between lights of different wavebands red and far-red on photosynthesis Emerson, McCree attempted to account for interactions between light with different spectra when developing photosynthetic action spectra and applied low intensity monochromatic lights from to nm with white background light to plants.
His results showed no interactive effect between those monochromatic lights and white light McCree, We tested different ratios of blue, green, and red light and different PPFD s, and similarly did not find any synergistic or antagonistic effect of different wavebands on any physiological parameters measured or calculated.
The interactive effect between PPFD and light quality demonstrates a remarkable adaptation of plants to different light intensities. Many early photosynthesis studies investigated the absorptance and action spectrum of photosynthesis of green algae, e. Extrapolating light absorptance of green algae and suspension of chlorophyll or chloroplast to whole leaves from can lead to an underestimation of absorptance of green light by whole leaves and the belief that green light has little photosynthetic activity Moss and Loomis, ; Smith et al.
Photosynthetic action spectra developed on whole leaves of higher plants, however, have long shown that green light effectively contributes to CO 2 assimilation, although with lower QY inc than red light Hoover, ; McCree, ; Inada, ; Evans, The importance of green light for photosynthesis was clearly established in more recent studies, emphasizing its role in more uniformly exciting all chloroplasts, which especially important under high PPFD Sun et al.
The idea that red and blue light are more efficient at driving photosynthesis, unfortunately, still lingers, e. Light-emitting diodes LEDs have received wide attention in recent years for use in controlled environment agriculture, as they now have superior efficacy over traditional lighting technologies Pattison et al. LEDs can have a narrow spectrum and great controllability.
This provides unprecedented opportunities to fine tune light spectra and PPFD to manipulate crop growth and development. Therefore, red and blue LEDs are sometimes considered optimal for driving photosynthesis. This claim holds true only under low PPFD. Green light plays an important role in photosynthesis, as it helps plants to adapt to different light intensities.
The wavelength-dependent absorptance of chlorophylls channels green light deeper into leaves, resulting in more uniform light absorption throughout leaves and providing excitation energy to cells further from the adaxial surface. Under high PPFD , this can increase leaf photosynthesis. Plant evolved under sunlight for hundreds of millions of years, and it seems likely that the relatively low absorptance of green light contributes to the overall photosynthetic efficiency of plants Nishio, There was an interactive effect of light spectrum and PPFD on leaf photosynthesis.
The strong absorption of blue light by chlorophyll creates a large light gradient from the top to the bottom of leaves. The large amount of excitation energy near the adaxial side of a leaf results in upregulation of nonphotochemical quenching, while chloroplasts near the bottom of a leaf receive little excitation energy under blue light. The more uniform distribution of green light absorption within leaves reduces the need for nonphotochemical quenching near the top of the leaf, while providing more excitation energy to cells near the bottom of the leaf.
We also found that the interactive effect of light spectrum and PPFD on photosynthesis was a result of the light-dependent reactions; gross assimilation and J were strongly correlated. We detected no synergistic or antagonistic interactions between blue, green, and red light. The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
JL and MI designed the experiment, discussed the data, and revised the manuscript. JL performed the experiment, analyzed data, and prepared the first draft.
Both authors contributed to the article and approved the submitted version. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Note that the initial increase in QY inc became more pronounced after correction of light suppressed respiration. Note that the pattern of QY inc after correcting of alternative electron sink B is similar to quantum yield of PSII measured by chlorophyll fluorescence by Weaver and van Iersel The light response curves are shown in Figure 3.
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