When gardeners typically think of color in their garden, flowers are undisputedly the first ‘thing’ that comes to mind! However, come autumn, the changing color of leaves on deciduous trees and shrubs suddenly usurps the mighty flower for the center of attention. Autumn color is clearly the reason for many an afternoon ride through the countryside, but it is also one of the perspectives about plants that we fail to consider when designing a garden. Indeed, fall color can often provide more intense and longer lasting color than flowers of certain plants! It is also fun to pare together late blooming perennials with trees and shrubs that have harmonizing or complimentary autumn coloration. Fall color is the result of the interplay of the three groups of pigments found within a leaf: anthocyanins, carotenoids and chlorophyll.
The aspect of color that many fail to understand is the color of a leaf – or any object for that matter – is not the spectrum of light that a pigment absorbs, but rather the spectrum of light that is reflected! Anthocyanins are a group of water soluble pigments that provide blue to purple to red coloration that, depending upon the plant, can reveal themselves in leaves, flowers, fruits, stems and/or roots! For the Gardener, they are responsible for the blue or purple foliage of the various blue foliaged Hosta (such as Hosta sieboldiana ‘Elegans’), the European Copper Beach (Fagus sylvatica ‘Atropunicea’), Diablo Ninebark (Physocarpus opulifolius ‘Diablo’) and the Purple Leaved Smokebush (Cotinus coggygria ‘Atropurpurea’). For leaves, anthocyanins serve as ‘sun block’ and protect cells within the leaf by absorbing light within the blue-green and ultraviolet spectrum. Obviously, in order for Anthocyanins to act as a ‘sun block’, they have to be located within the cells of the outer covering or the epidermal layer of the leaf. For those plants without purple or blue foliage, pigment production can also be triggered by rising sugar contents; when the sugar concentration reaches the proper threshold, anthocyanin production is initiated, resulting in the deep blue coloration of ripened Blueberries, or the purple color of Eggplant Skin and Concord Grapes! Higher sugar content is also responsible for triggering their production in autumn foliage too!
The cooler temperatures and shorter days trigger the development of a corky abscission layer at the point where a leaf is attached to the stem. This layer allows the leaf to drop from the branch in autumn. This layer also creates a blockage, preventing the sugars produced through photosynthesis from exiting the leaf. It is this rise in sugar levels which stimulates the production of Anthocyanins and the resulting red and purple fall colors! In Red Maples (Acer rubrum, pictured above), Balckgum (Nyssa sylvatica) and Sumac (Rhus species), Anthocyanin production pathways predominate!
The Carotenoids are another group of over 600 individual pigments whose ‘job’ it is to protect the inner cells of a leaf. Most importantly, Carotenoids act as a ‘body guard’ for the protection of chlorophyll molecules within the chloroplasts. Unlike Anthocyanins, Carotenoids are not water soluble, but rather are fat soluble. Thus, they cannot simply mix with the liquids within a cell, but need to be attached to the cell membrane. The most well-known members are Beta-carotene, a precursor to Vitamin A that provides the orange color to carrots and Lycopene, which is responsible for the bright red coloration of a ripe tomato! Carotenoids absorb energy in the blue and green light spectrum, and reflect or appear to the eye as yellow, orange or red colors. Also unlike Anthocyanins, Carotenoids are found in nearly all leaves throughout the growing season at levels or concentrations roughly 1/3 that of chlorophyll. In golden foliaged or golden variegated plants, the colors of the Carotenoids are most strongly evident, although many chartreuse or golden foliaged plants of spring will fade to a light green come summer as the chlorophyll concentration slowly builds and becomes the predominant pigment. Carotenoids also differ from Anthocyanins as they are an active participant in the process of photosynthesis. Carotenoids do not actually conduct photosynthesis, but rather pass along the energy that is absorbed to the adjacent chlorophyll molecules, giving them the title of an accessory pigment! Chlorophyll molecules can become damaged at any given instant during strong light situations by essentially becoming ‘overworked’, since the quantum mechanics of photosynthesis is certainly taxing upon the chlorophyll molecule. Carotenoids serve to protect chlorophyll by bleeding off or absorbing some of the excess energy. Like Anthocyanins, Carotenoids also serve to protect by providing a layer of ‘sunscreen’ to block various damaging wavelengths of light.
Chlorophyll is of course the workhorse of photosynthesis. Chlorophyll absorbs energy in the red and blue spectrum of visible light and reflects the unused green spectrum. Hence, the green color of a leaf! Chlorophyll is contained within disc-like cells called chloroplast that are predominantly located within the central mesophyll of a leaf and more specifically, in the upper palisade layer of cells. Chlorophyll is the pigment responsible for photosynthesis – the process by which Carbon Dioxide and Water are converted to Oxygen and Carbohydrates. Chlorophyll is a huge molecule (C55H70MgN4O6) that is insoluble in water and like Carotenoids, is attached to the cell membranes of the chloroplast cell. Despite its size, chlorophyll is not a longed lived and stable molecule, but breaks down rather quickly in bright sunlight during the stressful processes of photosynthesis. Consequently, during warm temperatures and sunlight – especially on bright sunny days – plants are required to constantly synthesize chlorophyll to replace those molecules that are breaking down. Come autumn, as the corky Abscission Layer develops and reduces the transfer of nutrients traveling from the roots to the leaves, the plants ability to produce chlorophyll is also compromised. The reduction in chlorophyll production and resulting concentrations allows the Carotenoids and Anthocyanins to become more apparent to the eye and fall color begins! In some trees, such as Birches, Hickories and Bottlebrush Buckeyes (as pictured above), the Carotenoids prevail and the fall color is a striking yellow!
The best fall color appears during cool, dry and sunny late summer and autumn days. The cool temperatures slow the production of chlorophyll, while the bright sunshine promotes the breakdown of chlorophyll and promotes the production of anthocyanins. The dry weather also reduces the water available for the plant, resulting in an increased concentration of sugars within the leaf. These increased sugar levels promotes the production of anthocyanins. Hence, our dry August through early October in New Jersey of this past year, along with the prolonged cool temperatures and bright sunny days has allowed our trees to produce a saturnalia of bright, autumn colors!