Every autumn we enjoy the annual harvest of our vineyards, as well as the spectacular beauty of the fall colors. My recent trip to Aspen punctuated this season for me. The mixture of reds, umbers, and yellow result from chemical processes that take place in the vines as the seasons change from summer to winter. It is similarly reflected in the Aspen trees and this connection marked me this year.
Three leaf pigments in grape vines responsible for color and its changes in the autumn are: chlorophylls, carotenoids, and tannins.
Chlorophyll absorbs the sun's radiant energy and is necessary for photosynthesis, the chemical reaction in which carbon dioxide and water are transformed to sugars, used for food by vines and trees. During the growing season, chlorophyll is continually being produced and leaves appear green.
As days get shorter and temperatures cooler in the autumn, the leaves stop their food-making process. This is called senescense. Nitrogen and phosphorus are slowly withdrawn from the leaves to be stored in twigs and branches during the dormant winter period. The loss of these nutrients, due to reduced exposure to sunlight with shorter days, gradually stops the production of chlorophyll. The green leaf color fades and other pigments are unmasked to show their colors. The timing of chlorophyll loss varies among different species, thus some leaves will remain green longer than others.
It seems that aspen trees at 10,000 feet in Colorado and Pinot Noir grown at 1000 feet in west Sonoma County hold a common thread. Did you know that enormous groves of aspen trees are all one organism, sharing a common root system?!
Carotenoid pigments are responsible for the yellow and orange colors in leaves. They are also located in the chloroplasts and assist chlorophyll in the capture of sunlight for photosynthesis. Caratenoids are always present in the leaves, but are not visible for most of the year because of greater amounts of chlorophyll. The yellowish colors are unmasked as chlorophyll degrades. Carotenoids are also responsible for the yellowing of leaves at any time during the year if there is a deficiency in nutrients or disease that reduces normal chlorophyll production.
Tannins are responsible for brown hues in the leaves. The golden yellow in some leaves are a result of tannins along with the yellow carotenoid pigments. These compounds are always present in the leaves, but only become visible as chlorophyll and carotenoids disappear. I did not realize that tannins are present in both the leaves and the fruit. Tannins are bitter substances responsible for the color and flavor of tea. They are waste products of plant metabolism, deposited in the cell walls. They often accumulate in dead tissue.
Leaves that fall decompose and restock the soil with nutrients that make up part of the spongy humus layer of the vineyard and forest floors. These absorb and hold rainfall. Fallen leaves also become food for numerous soil organisms that are critical to the health of the ecosystem.
Other then that, very good blog!