Bonsai, being a living art form, undergo seasonal changes
making it an ever-changing canvas - especially in deciduous or broad-leafed trees. Autumn is one of my favorite seasons as my Japanese Maples put on a spectacular
dazzling show of fall color. Summers green turns to brilliant reds, fiery
oranges, dazzling yellows and rich purples.
As daylight hours diminish and
temperatures drop, hormones cause a transformation in leaf pigmentation. Chlorophyll, the
green pigment in leaves, absorbs both red and blue light from the
sun thus reflecting back the color green. Chlorophyll molecules are attached to
the cell membrane in disc like structures called chloroplasts. Photosynthesis
occurs here and the endothermic process transfers the light energy absorbed by
the chlorophyll into carbohydrates - starches and sugars - that drive the bio
chemical reactions that cause trees to grow, flower and produce seed.
Chlorophyll decomposes rapidly in bright
sunlight, and therefore during the growing season the tree is constantly synthesizing
it. Other pigments present in leaves are carotenoids and anthocyanins as well
as tannins in some species. Carotene
absorbs blue blue/green light and is a much more stable molecule than
chlorophyll. Thus, when chlorophyll decreases in the leaf, the remaining
carotene causes the leaf to appear yellow. Anthocyanins, unlike chlorophyll and carotenes, are actually dissolved in
the fluid sap of the inner cell and the color exhibited is reliant on the
sensitivity of the pH levels. If the sap
is very acidic the pigment will be a bright red color; the lower the acidity the
color is more purple. Anthocyanin pigments are formed by reactions between
sugars and certain proteins in the cell sap.
The range and intensity of
autumn colors is greatly influenced by the weather. Low temperatures destroy
chlorophyll, and if they stay above freezing, promote the formation of
anthocyanins. Bright sunshine also destroys chlorophyll and enhances
anthocyanin production. Dry weather, by increasing sugar concentration in sap,
also increases the amount of anthocyanin. Therefore, the brightest autumn colors are
produced when dry, sunny days are followed by cool, dry nights.
Here is an excellent
website devoted to the chemistry involved in fall color change: http://www.esf.edu/pubprog/brochure/leaves/leaves.htm
