#016 - A Pigment of Truth

The Science Behind Foliage Color: Hue Knew?

My coursework for landscape architecture included a few classes in Plant Biology, which opened my eyes to the science behind photosynthesis. That’s the process by which plants, algae, and some bacteria convert light energy into chemical energy, creating sugars (their version of food) from carbon dioxide and water. Plants absorb light primarily through the pigment chlorophyll. The green part of the light spectrum is not absorbed but is reflected, which is the reason that most plants have a green color.

I wanted to write this Hot Plant Tip ever since I read an article that suggested if we ever find a planet similar to earth, it might look purple instead of green. Scientists theorize that an Earth analog orbiting a different kind of star could be dominated by bacteria that don’t rely on visible light or oxygen, like some do here on Earth. These bacteria use simpler forms of photosynthesis powered by infrared light, which doesn’t produce oxygen and relies on pigments that are often purple rather than green. That article made me reflect on the fact that even here on Earth, chlorophyll is not the only pigment plants rely on. There’s more going on in every leaf than most people realize.

Besides chlorophyll, many plants also produce pigments like carotenoids and anthocyanins. These pigments help plants expand their light capture, manage energy, and respond to environmental stress. They also influence how a plant appears over time as it deals with changing seasons, light conditions, and resource availability. This Hot Plant Tip might feel a bit different than previous ones. It is less about planting design and more about understanding the processes that shape how plants look and function, which is just as important!

Chlorophyll

Chlorophyll is essential for photosynthesis. It allows plants to convert sunlight into the sugars that fuel growth. There are two primary forms found in green plants, chlorophyll a and chlorophyll b. Their slightly different structures allow them to absorb different parts of the light spectrum. Chlorophyll a captures red and violet-blue light, while chlorophyll b helps cover the blue and red-orange range.

Both types of chlorophyll reflect green light, which is why most healthy foliage appears green. Each molecule of chlorophyll contains nitrogen and magnesium, which the plant reabsorbs as the leaf begins to senesce. In the lead-up to dormancy, chlorophyll production slows, and the pigment begins to degrade. As it fades, the visual role of other pigments becomes more prominent. This breakdown process is orderly and deliberate, and it is the reason fall color becomes visible in many deciduous species.

Carotenoids

Carotenoids are a group of pigments that absorb in the violet and blue-green regions of the light spectrum and reflect yellow, orange, and red. They are present in most green tissues during the growing season, although they are often overshadowed by chlorophyll. In photosynthesis, they serve as accessory pigments, helping to broaden the range of usable light. Carotenoids act as internal safety valves. When sunlight exceeds what a plant can use, they absorb the excess energy and release it as heat. This prevents the buildup of harmful reactive oxygen molecules, which can damage cells while plants are experiencing extra harsh sun or drought.

Unlike chlorophyll, carotenoids degrade slowly. When chlorophyll begins to break down in autumn, carotenoids remain, often resulting in yellow or orange foliage. This is what creates the golden fall color seen in species like ginkgo, birch, hickory, and some maples. These pigments are not produced in response to autumn cues. They are simply unmasked as the green fades.

Other cultivated plants have reduced chlorophyll from the start. Golden-leafed forms of Ceanothus (Ceanothus griseus var. horizontalis 'Diamond Heights'), Honey Locust (Gleditsia triacanthos ‘Sunburst’), or Smoke Bush (Cotinus coggygria ‘Golden Spirit’) show chartreuse foliage throughout the growing season. These plants rely more heavily on carotenoids rather than chlorophyll, and as a result, often grow more slowly due to reduced light capture.

Often the intensity of the yellow coloration is influenced by sunlight, with warmer, sunnier conditions producing a more vibrant yellow, while cooler or shaded conditions result in greener foliage.

Anthocyanins

Anthocyanins are water-soluble pigments stored in the vacuoles of plant cells. Their color varies depending on cell sap pH, appearing red in acidic environments, purple in neutral ones, and blue in more alkaline conditions. Unlike chlorophyll and carotenoids, anthocyanins are not directly involved in photosynthesis. They are often produced in response to environmental stress, including bright light, cold temperatures, nutrient limitations, or excess sugar.

In fall, some plants begin to accumulate sugars in their leaves as phloem transport slows. This sugar buildup can trigger anthocyanin production, resulting in red or purple foliage. Unlike carotenoids, which are unmasked as chlorophyll fades, anthocyanins are created when needed. The intensity of red fall color often depends on sunny days, cool nights, and the plant’s genetic ability to produce anthocyanins in the first place.

In Southern California, we don’t get the dramatic autumn palettes of the Northeast. But we do see anthocyanins at work. Plants like Red-Flowering Currant (Ribes sanguineum), Californa Buckwheat (Eriogonum fasciculatum), and Manzanita (Arctostaphylos species) often display red or purplish pigments in new growth or drought-stressed foliage. These shifts are especially common at leaf margins or in sun-exposed areas during dry periods.

Anthocyanins also appear at other times of year. Many plants produce red or bronze new growth in spring. This is a common trait in oaks, Mahonia, Berberis and even some tropical species. In these cases, anthocyanins help shield young leaves from UV damage before their photosynthetic machinery is fully functional. In winter, some broadleaf evergreens and conifers accumulate anthocyanins in exposed foliage, leading to bronzing or purpling during periods of cold and bright light. And just like golden or chartreuse cultivars express carotenoids more prominently, many cultivars emphasize anthocyanins to achieve red, burgundy, or purple foliage like Japanese Maple (Acer palmatum ‘Bloodgood’), Smoke Bush (Cotinus coggygria ‘Royal Purple’), Chinese Fringe Flower (Loropetalum chinense ‘Purple Diamond’) and Redbud (Cercis canadensis ‘Forest Pansy’).

Beyond foliage, anthocyanins are also responsible for the red, blue, and purple tones in many fruits and vegetables, including blueberries, blackberries, and red cabbage. They act as antioxidants, protecting plant cells from free radical damage. The same qualities that make these pigments beneficial to plants have also made them prized in human nutrition.

Why Multiple Pigments Are Important

Chlorophyll alone cannot capture all of the sunlight available for photosynthesis. It absorbs strongly in the red and blue-violet portions of the spectrum but leaves a significant portion of light, especially green and yellow wavelengths, underutilized. Carotenoids expand the range of light that a plant can use. They absorb light in the blue-green part of the spectrum and pass that energy along to chlorophyll molecules. This makes light capture more efficient.

Carotenoids also play a critical photoprotective role. Under high light conditions, especially when carbon dioxide is limited by drought or cold, excess energy can build up in the photosystems and generate harmful reactive oxygen species. Carotenoids help dissipate this energy as heat, protecting the photosynthetic machinery from damage. Chlorophyll cannot perform this function on its own.

Anthocyanins offer a different kind of protection. Although they do not contribute directly to photosynthesis, they act as optical filters and antioxidants. By absorbing excess blue and ultraviolet light, they help reduce stress in sensitive tissues, especially in young leaves or senescing foliage. Their presence can also signal chemical defenses to herbivores, acting as a visual deterrent. The ability to produce anthocyanins gives plants more flexibility in dealing with stress and adds a visual dimension to seasonal change.

Together, these pigment systems give plants the ability to adapt to a wide range of environments, manage light efficiently, and display a dynamic range of color through the year.

One Last Tangent

Carotenoids also show up in animals, at least indirectly. Female house finches, for example, tend to prefer males with brighter red plumage, a trait that comes from eating carotenoid-rich foods. Some males may appear more yellow than red, likely because their diet includes fruits with different carotenoid compositions or lower overall pigment content. In this case, plumage color can signal how effective a bird is at finding high-quality resources. Plants produce these pigments for their own purposes, but their influence on ecological signaling extends far beyond the leaf.

Final Thought

Our interest in plant pigments often lies on the surface with vibrant and unusual cultivars or seasonal changes that stand out. But beneath these familiar moments lies the intricate world of plant biology. These visual shifts are often the result of pigments that are doing much more than coloring a leaf. From light capture to stress response, these microscopic molecules play a critical role in how life on Earth is powered and sustained. For designers, gardeners, and scientists alike, understanding plant pigments is not just about noticing surface details. It is about recognizing the layered complexity of living systems we often take for granted. The science of plants is anything but simple, and that complexity is exactly what makes it so compelling.