What causes leaf color?

[JYb.shopteacher]

So wondering if anyone knows? I’m out doing fall chores here in Michigan and wondering does the change in color of the leaves have anything to do with sugar content. Any scientific proof out there. I was thinking that this could help one figure which to tap and which to not. Thoughts?

[maple flats]

The color was always there from the nutrients taken up by the trees thru the roots. They are over powered by the chlorophyll in the leaves. As the leaves stop producing energy for the tree the chlorophyll is depleted and you then see those amazing colors.

[Revi]

I think it's like a ripe fruit. The sugars turn red, orange and yellow once they are left. Athocyanin is the technical term for the color producing chemical.

[DrTimPerkins]

Very briefly...which is hard for me to do...Dr. Abby van den Berg here at UVM PMRC did her Ph.D. (with me as her advisor) on sugar maple pigmentation. A lot of my early research was on light capture and release (I specialized in foliar fluorescence..the glow -- which humans can't see -- when photons drop from a high energy state to a lower energy state...one way in which plants release captured solar energy) in spruce needles. My Ph.D. advisor, Dr. Richard Klein, was a photobiologist.

Chlorophyll (green), the photosynthetic light-capturing apparatus of leaves, is there in leaves all summer, but degrades in the fall. The color isn't actually green...that is the light that is NOT used but is reflected, so it is what we see.

Anthocyanin (red) is a pigment that is actively produced by leaves in the fall, perhaps as a light screen for leaves to prevent excess solar energy captured at that time of year (and also in the spring) from burning out the photosynthetic system (although there is some evidence that this is not the case, but it is a simple way to explain it). This happens because the first step of photosynthesis after energy is "captured" is to split water. When temps are below freezing this can't happen (water is ice at that point), but the light harvesting system can't be shut off -- so what can you do with the excess energy?

Carotenoids (yellow/orange) are ancillary pigments (they grab photons of light and pass them on to chlorophyll) that are also there all summer, but are masked by the intensity of the green color. When the chlorophylls degrade in the fall, the carotenoids show through.

Light photons, when they hit a plant leaf, are either passed through, reflected, or absorbed by the pigment complex. Those that are absorbed bounce around kind of like a pinball in the carotenoid/chlorophyll complex (which is sort of like an antenna with a funnel, gathering and shunting photons down into the right place) with a until they hit a photosynthetic reaction center, which passes the energy through a series of reactions resulting in the splitting of water and the driving of membrane pumps which pushes protons (H+, derived from splitting water...the O2 is released) across a membrane. This pumping produces a gradient of electrical charge between the inside and outside of the cell membrane. By opening pores in the membrane, the cell can utilize the energy of the gradient (electrically the cell wants to be balanced across the membrane, so protons actively are drawn into the cell). The energy of this drives other chemical reactions to "fix" the physical energy (the electrical charge) into chemical energy, which in turn is used to make sugar.

Sounds complex (and it is), but really it is just a physio-biochemical machine...just at a cellular and subcellular level.

[maple flats]

What he said? Way over my head, must be you need a PhD to understand it.

[DrTimPerkins]

What he said? Way over my head, must be you need a PhD to understand it.

OK...my apologies. I'll try to simplify it.

Think of the sun as a gentle rain. The equivalent to a drop of rain is a photon (a packet of light energy). Not much use there if you want to get energy from it, but if you harness it properly the energy is useable.

So the pigments in leaves (chlorophyll and carotenoids) are like a watershed with a dam. The rain falls on the ground, runs into a stream and the stream is dammed up.

The pigments are the ground, catching the rain (photons of light), and they funnel the light to the stream and into the dam. Inside the dam is a special door where the water (photons) can escape. The water runs through the opening, turning a shaft. In a leaf, the photons are funneled through the opening (a special chlorophyll molecule called a reaction center), which then allows the photon to travel down a pathway. That energy flowing through the leaf turns the shaft which is used to break water (H2O) into H+ and O2, kind of like how flowing water through a dam turns a grindstone. The oxygen is released (thankfully for those organisms that breathe oxygen). The H+ (ground corn) builds up on the inside of the membrane. When there is enough, a door opens and the H+ (corn) flows out. The energy in the H+ (corn) is used to feed pigs (CO2, from the air) and fattens up the pig (crams some carbons together) to make more and fatter pigs (sugar).

So the falling rain (photons of light, physical energy), running through a dam and some machinery eventually is converted into a bunch of fat pigs (sugar, chemical energy). Leaves are just the molecular machinery that makes that happen.

So to go back to why (some) leaves turn red....what happens when it gets cold and starts to freeze, then it warms up a bit and rains hard. Unfortunately you have no way to shut off the dam/shaft/grindstone machine? If you let it run, ice chunks build up and break the machine, but you've still got corn to grind the next few weeks to finish out the crop. Maybe you can build a small diversion channel which would let the water run around the machine long enough for you to squeeze out grinding a bit more corn. That is what some people think the anthocyanin pigment (red) is for. It is a way for plants to divert the light energy hitting the leaves from breaking the machine. Plants have no way to shut off the collection of light by the hillside and dam, but they can perhaps screen some of it out by catching the rain before it hits the ground and makes it to the dam. The red pigments are kind of like an umbrella for the photosynthetic pigments -- they catch some of the photons and keep them from getting to the dam and breaking the machine....at least for a few more days or weeks. Having them there all summer would be bad (why catch the rain you need to grind corn when it isn't freezing), so they are formed only when the weather gets cold. No sense having umbrellas around protecting the ground (chlorophyll and carotenoid pigments) all summer when grinding the corn needs to happen full speed. That would just slow things down needlessly. Better to make them only when they are actually needed.

The process is really kind of simple and elegant...just tiny molecular-level machines (walls, pumps, funnels, streams, dams, factories, transport, electrical energy, chemical energy).

Not sure if that helped or made things worse. :D

[Ed R]

Great job explaining it both ways Dr. Perkins. I was always amazed when I taught as a grad assistant the photosynthesis lab to incoming college freshman. When they finally got that the carbon for this reaction comes from the air it was like a balloon burst in their brains. In the past at least, this has been taught poorly at the high school level, which is a shame since almost all life on earth is due to this reaction.

[Revi]

Great explanation Dr. Tim! Thanks!