Shuttle Season

Today's newsletter started as a simple question: What's going on when leaves turn colors in the fall?

The overall story is pretty simple—as leaves turn color in the fall, plants move nutrients out of dying leaves so they can safely store these scarce nutrients for the winter—but the piece I've never understood is how plants actually "move" nutrients.

I've written about autumn leaves several times in previous newsletters, including writing about why leaves turn red, how fall colors send messages to aphids, and why autumn leaves are vital to stream ecology, but I've never done a deep dive into the ways that plants actually move nutrients out of dying leaves.

First of all, leaves are critical to plants because this is how plants photosynthesize and feed themselves, but each year's crop of leaves also represents a tremendous energy investment. Plants cannot afford to simply drop their leaves in the fall and lose this investment, so they undertake an extremely complex and highly choreographed series of steps to ensure that as many essential nutrients as possible are broken down into their component pieces and shuttled to storage locations for the winter.

What you may not realize is that the turning of colors and the dying of leaves in the fall is not a passive process; in fact, it is one of the most important and active phases of a plant's life. Ultimately, this process of carefully capturing and moving nutrients is so effective that up to 98% of the nutrients that might have been lost are recycled, and that matters because elements like nitrogen, phosphorus, sulfur, and metallic ions are very hard to acquire and are critical for new growth the following spring.

What's tricky for plants is that they can't simply turn off a switch at the end of summer. Breaking down and moving nutrients requires that the fundamental machinery of a leaf remains intact until the last possible moment, and powering all this activity means that a leaf still has to produce a substantial amount of energy.

This dance is so complex that six different classes of thousands of genes need to be activated and coordinated in a precise series of steps. Macromolecules like proteins, nucleic acids, and chlorophyll first need to be carefully broken down into their component pieces with specialized enzymes. Then, as photosynthesis grinds to a halt and leaves stop producing sugars, stored lipids must be mobilized to power the relocation of nutrients to winter storage locations in a plant's branches, stems, and roots.

My question at the top of the page was how these nutrients are moved, and it turns out that they are moved via something called the symplastic transport pathway, utilizing microchannels called plasmodesmata.

This pathway links the fluid contents of cells via a connected network of microchannels to allow a wide range of materials to move through plant tissues without having to repeatedly cross cell membranes. Once they've been moved into the transport pathway, nutrients can be shuttled to a plant's phloem and easily transported to a variety of storage locations.

The arrival of fall colors feels serene and peaceful, but under the surface there's a furious scramble that continues until the last possible moment. By the time a plant severs the connection, and a leaf falls to the ground, it's little more than an empty shell of its vibrant summer self.

Further Reading:

There's a voluminous scientific literature on this topic, but this technical paper provides a decent starting point, along with an extensive bibliography.