The Science Behind Fall Leaf Color

Pigments of Fall Color
David C. Zlesak, Regional Extension Educator, Horticulture
University of Minnesota

Brilliantly colored leaves and fruit transform the fall landscape into enchanting scenes and is a hallmark of Minnesota and other Northern-tiered states. Heightened interest in plants and nature in the fall is evident as many take advantage of the opportunity to enjoy autumn camping, drives, and hikes and participate in seasonal artistic crafts and activities. As the window of peak fall color moves across the state, the details of when and for how long it will last each year is largely uncertain. Factors such as wind, temperature, and precipitation all influence how long we have to enjoy the fall display and motivates many of us to rearrange our schedules to make sure we do not miss the magnificent show. Changing colors signal timely biochemical responses as plants move their nutrient resources to prepare for the upcoming winter and make their fruit more appealing to hungry critters that will inadvertently transport seeds to other locations.

The colors we see in the foliage and fruit of plants are primarily influenced by three pigments- chlorophyll, carotenoids, and anthocyanins. Chlorophyll gives plants their green color and traps light energy so it can be stored in a chemical form through the process of photosynthesis. Chlorophyll is fat soluble and found within specialized lipid-containing structures called chloroplasts. Chlorophyll primarily absorbs blue and red light (grow lights help fuel photosynthesis and appear purple because they are rich in red and blue light). Carotenoids are accessory pigments to chlorophyll and help channel light energy to chlorophyll for photosynthesis and are typically yellow to orange in color. Carotenoids are found in chloroplasts with chlorophyll and in similar structures lacking chlorophyll called chromoplasts. Carotenoids help make more of the light spectrum (besides red and blue light) useful for photosynthesis and also help to protect chlorophyll molecules from being damaged by intense light. Chlorophyll is a relatively short-lived molecule, and throughout the growing season it continually degrades and new chlorophyll is synthesized to replace it. Carotenoids are critical to help extend the life of chlorophyll molecules.

Perennial plants adapted to our climate respond to primarily decreasing temperatures and shortening days (detected by plants as lengthening nights) to trigger the onset of dormancy. Nutrients within foliage of deciduous plants can be retained by the plant by reabsorbing and transporting it to overwintering structures (i.e. stems and roots). Synthesis of new chlorophyll to replace what has been degraded slows and less and less chlorophyll is found within the leaf. The colors of other pigments, once masked by the green color of chlorophyll, become evident. Carotenoid pigments provide the brilliant yellow and gold colors as typically found in foliage of aspen, ginko, and some maples.

Anthocyanins are another group of plant pigments and are made up of a molecule called anthocyanidin joined with one or more sugar molecules. Unlike chlorophyll and carotenoids, anthocyanins are water soluble and found within the water containing vacuole of plant cells. Anthocyanins typically appear as pink, red, scarlet, or purple. Anthocyanins share a common 15 carbon chemical structure. Many different anthocyanins with slightly different colors and other properties can be generated according to which carbon position sugars and hydroxyl groups are attached. Some anthocyanins such as delphinin typically produce colors more in shades of purple and blue, while other anthocyanins tend to be pink or red. In addition to sugar availability within the plant, other factors such as pH, metal ions, and temperature can influence how light interacts with anthocyanins and the final color we see. For instance, flower color of some plants like bigleaf hydrangea can be manipulated by changing soil pH and manipulating metal ions such as aluminum.

Anthocyanins absorb strongly in the ultra violet (UV) spectrum and act as antioxidants protecting cells from direct UV damage and damage from free radicals. DNA, in particular, can be damaged by the high energy in UV light and anthocyanins help plants by serving as a natural sunscreen. Some plants have red-tinted young growth rich in anthocyanins to especially protect young, sensitive tissue. In addition, leaves growing under higher light levels often have more anthocyanin pigment than leaves in more shaded locations. This is commonly seen in fall where leaves deeper within the canopy appear lighter red than those at the top of the canopy. Another example is burning bush (Euonymus alata), a shrub well-known for its brilliant red fall color. Plants growing in more shaded locations tend to have less anthocyanin and appear florescent pink, while those in full sun have more and are burning red.

Some plant species naturally produce more anthocyanins than others and variants within some species have been selected which produce elevated levels of anthocyanin all season. Such plants typically appear deep purple during the growing season from a combination of anthocyanin and green chlorophyll. ‘Purple Palace’ coral bells, ‘Chocolate’ white snakeroot, ‘Crimson Frost’ birch, ‘Diablo’ ninebark, and ‘Crimson King’ Norway maple are examples of plants with anthocyanin-rich, purple foliage throughout the growing season.

Anthocyanin production tends to increase in the fall to help protect leaves as degradation of cellular components and reabsorption of nutrients occurs. As cellular components are degraded, free radicals can be generated and anthocyanins help protect cells from damage. In addition, anthocyanins are beneficial to human health as an antioxidant protecting cells from free radicals as well and helping to prevent cancer. Many doctors advocate diets including antioxidant supplements and antioxidant-rich foods such as red cabbage and deeply pigmented fruits like cranberries, blueberries, and blackberries.

Fall anthocyanin accumulation can be unpredictable and varies from year to year. This is due to variation in such things as sugar levels in plants, amount of cloudy versus sunny weather, temperatures (cooler temps often favor anthocyanin production), and general plant health. Anthocyanin production can also be stimulated when plants are under stress. For instance in years where there has been drought going into fall, plants often color sooner and their color can be more intense. Carotenoid pigments on the other hand, are present at more consistent levels throughout the growing season and provide more consistency in fall color than anthocyanins.

Additional Notes: Many ornamental plant cultivars possess variegated foliage and origin of the variegation is frequently just a chance mutation within a particular cell layer of the growing point leading to reduced chlorophyll. With such cell layer mutations, at least one cell layer produces normal green leaf regions and at least one cell layer produces white regions in the leaf which is low in or lacking chlorophyll. Often the white region of the leaf is caused by cells not producing enough carotenoids rather than lacking the ability to synthesize chlorophyll. Without sufficient carotenoids to protect chlorophyll, chlorophyll is degraded quickly and such tissue can appear white or cream colored. In addition, plants which have uniformly colored, golden-green foliage may also be due to reduced carotenoid presence or function.

Hardening off tender plants grown under low light indoors before planting outside is important. By exposing plants gradually to increased light levels, wind, and temperature extremes, damage from sun scorch and dehydration can be minimized. During the hardening-off period carotenoid and anthocyanin pigments can be synthesized to help protect chlorophyll and other light sensitive components in plant cells.

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