witnessing the amazing fall foliage that comes with this time of year. The trees are dropping

their leaves in preparation for winter, turning amazing shades of red, orange and yellow in

the process. But why exactly does this happen, and what's going on inside of the plants to

make it happen? Well, I'm here to tell you!

This process of plants dropping their leaves in autumn is called seasonal leaf senescence.

It's thought that this senescence is triggered by the shortening of the days and the drop

in temperature that happens in fall. A series of chemical reactions triggers the mechanisms

of senescence, and the first thing that happens in the destruction of chloroplasts in the

leaves' plant cells.

The mitochondria may be the powerhouse of the cell, but in plant cells it's the chloroplasts

that rule. Chloroplasts contain chlorophyll, the molecules necessary for photosynthesis.

It's through photosynthesis that plant cells use sunlight to create chemical energy. Chlorophyll

is what gives plants their token green color, and as the chloroplasts are destroyed, the

plant's leaves lose their color.

That doesn't explain, however, why leaves turn shades of yellow, orange and red. Well,

chlorophyll isn't the only pigment that helps with photosynthesis. Chloroplasts also contain

carotenoids, accessory pigments that have an orange color. Β-carotene is a carotenoid,

and it's what makes carrots orange.

Carotenoids actually help with photoprotection in plant cells. Basically, if too much energy

builds up in chlorophyll, it can trigger chemical reactions that damage the cell. Carotenoids

quench this excess energy and protect the cell.

When it comes to vibrant fall foliage, it's maples that take center stage for color. Red

maples are known for their red leaves, a color that can't be explained by carotenoids alone.

Maple leaves also contain lots of anthocyanins, Anthocyanins are found in a lot of plants

and can produce colors like pink, purple and blue. In trees like maples, however, they

create a startling red.

Seasonal leaf senescence, of course, is more than a color change. The leaves shut down,

leading to leaf abscission. This is when the leaves to actually fall off the plant. The

petiole, the short stem that connects the leaf to the tree, is walled off by the plant,

causing the leaf to fall.

In summary, as the days shorten and the temperature drops, deciduous trees like maples begin the

process of senescence. They shut down the plant cells in the leaves, starting with the

chloroplasts, which causes them to change color. Eventually, as senescence continues,

the leaves are cut off from the plant and fall to the ground.

Hopefully now you know a little more about fall foliage! If you have any questions, please

let me know in the comments. Thank you for watching; for daily posts about plant science

you can check out www.brilliantbotany.com or follow me over on twitter. I'll see you

all next time.