by Leslie Cox; Saturday; October 6, 2018
October has definitely started off with some clout this week…first frost hit on the 4th. Same date as in 2012…both years taking credit for being the earliest frost dates we have had here in our garden. Next earliest frost date for the season was on October 11th in 2016. The latest date I have ever recorded for the first frost of the season was November 11th in 2014. We had quite a long and beautiful Indian summer that year.
Feeling quite gypped this year at the less than stellar autumn display of burgundy, red, orange, rust, and yellow. With not more than a week of mixed sun and cloud at the end of September, the plants barely had enough time to get revved up for colouring. And even the ones who started changing hues a bit early did not hang onto their leaves long after the wind gusts blew through here on the 29th.
Sooo…have you ever wondered how those wonderful leaf colours develop from summer-green leaves? The answers lie buried in science.
Chlorophyll. Everyone knows this biological material…the green pigment contained in the chloroplasts of leaves and responsible for photosynthesis. The driving force behind a plant’s ability to manufacture food using sunlight to convert carbon dioxide taken in through the leaves and water containing necessary nutrients pulled up from the soil.
However, what we do not see reflected in leaf colour over the summer are the other pigments also found in the chloroplasts…anthocyanin, carotene and xanthophyll.
Anthocyanin. The pigment responsible for red, purple, and blue colours because it absorbs blue, blue-green, and green from the light spectrum. Extreme sensitivity to acid/alkaline levels (pH) in the sap of the plant where this pigment is found determines the colour displayed. Low pH (acid) produces red; high pH (alkaline) produces purple.
Carotene. The pigment responsible for orange, orange-red, and yellow colours because it absorbs the ultraviolet, violet, and blue from the light spectrum. But when both carotene and chlorophyll occur in the same leaf, they combine to remove the red, blue, and blue-green when sunlight falls on the leaf. The light then reflected by the leaf appears green. In this case, the carotene pigment is an accessory absorber, transferring the energy it garners from the sun to chlorophyll. And because carotene is a more stable compound than chlorophyll, it remains in the leaf after the chlorophyll disappears…causing the leaf to turn to shades of orange and yellow.
Xanthophyll. This is another accessory pigment which helps to absorb chlorophyll in certain plants in low light situations. This pigment is part of the carotene group but contains oxygen where the other carotenes do not. Hence, it produces yellow tones rather than the orange and orange-red colours exuded from the oxygen-starved carotene pigment.
Now, this is just a rough overdraft of what goes on inside plant leaves as light levels shorten through autumn and temperatures dip towards zero. But it is amazing how these environmental triggers spark the plant into producing a cork-like membrane across the end of the leaf stem where it attaches to the branch.
This separation layer forms a tight seal between leaf and plant. It effectively stops all water from entering the leaf from the plant and all manufactured food leaving the leaf and entering the plant. The plant either begins to die back if it is an annual or herbaceous perennial…or begins to enter dormancy if it is a deciduous plant.
With no water coming from the plant to renew the chlorophyll in the leaves, it begins to disappear. At this point, the plants whose leaves contain the sugar-saturated anthocyanin pigment turn brilliant shades of red and purple as autumn progresses.
Plants whose leaves contain the oxygen-starved carotene pigment are lit up through the fiery orange tones…from apricot through to deep orange-red.
And the brown-coloured leaves, such as the oak? These are produced from a build-up of waste products in the leaves and a polyphenol compound called tannin. Found in many plant species, tannin compounds assist in regulating plant growth. They are also valuable in protecting the plant from predation…even triggering the production of a pesticide to deal with a specific pest.
Sooo…anyone else feeling a little gypped we have not had much of an Indian summer in which to enjoy the flashing shades of reds, oranges, yellows, and golds…dancing on an autumn breeze…gently waving goodbye to summer?