Winter, Spring, Summer, Abscission

It's happening everywhere right now!  Plants are chopping off their own organs, and they are piling up in yards all over town!  How come no one is worried about this epidemic of leaf death??!!  Well, it happens every year, so I'm pretty sure the plants are going to recover.  Still, why on earth would plants get rid of their most important organs?  That's what we'll address in today's post.

Closeup of leaf abscission zone on sourwood.

In the picture above, you can see the color difference between the pale pink of a leaf petiole (technical term for a leaf stem), and the bright red of a sourwood twig.  The line between those two differently-colored plant parts is called the abscission (ab-SIZH-uhn) zone. 

Fresh leaf scar where the abscission zone dissolved and the leaf fell off.

This time of year, the layers of abscission zones are changing.  One layer is hardening and filling up with a corky substance called suberin.  Suberin is waterproof and heals what would otherwise be a wound where the leaf falls off.  The leaf scar in the picture above is dry and not losing sap because suberin has sealed the wound.  The second layer in the abscission zone is made of thin-walled, weak cells that self-dissolve when the plant is ready to shed its leaves.  Abscission zones are usually quite noticeable this time of year on any plant that is in the process of losing its leaves.  Take a look at the next two pictures and find the abscission zones.

Sourwood leaves and petioles (stems) about to undergo abscission.

The abscission zone is at the base of the leaf petiole where it attaches to the twig.

It is extremely unusual for living organisms to shed any part of themselves except for the production of offspring.  Some lizards have tails that fall off to distract predators, and many plants lose their leaves in the fall - but I can't think of other examples of falling-off body parts.  Of course, most organisms constantly rebuild their outer-coverings and some organisms can replace body parts that are bitten off, but voluntary amputation is strange, indeed! 

The loss of body parts comes at a huge cost.  Plants work all summer to catch enough sunlight to grow more leaves and get bigger, and leaf abscission every fall would seem to waste that energy.   But as with the lizards that lose their tails, there are also benefits.  Lizards' bodies escape to live another day and regrow another tail.  Plants benefit from shedding leaves by not having to maintain those leaves during the winter.  Leaves are tender tissues that would become disfigured and die when frozen.  Try putting some lettuce leaves in the freezer over night and then take them out to thaw.  You will notice they turn to mush when they return to room temperature.  In order for plants' leaves to survive winter, they would have to be tough, like holly, magnolia or spruce leaves, which take much more energy to produce.  Plants with leaves that survive freezing grow more slowly than ones that shed their leaves.

Dogwood with remnants of chlorophyll along veins and lots of anthocycanins (red pigment).

Plants have many ways to minimize the costs of losing their leaves.  They move all available nutrients out of their leaves and down into their roots to save the food for the next growing season.  Leaves fall near the plant that grew them and decompose, releasing their nutrients into the soil and further increasing the amount of nutrients recovered by the plant.  In this way, deciduous plants grow their own mulch.  Some plants, like walnut trees, even deposit compounds in their leaves that suppress the growth of competitor plants as the leaves decompose throughout the winter and spring. 

Rainbow of fall colors.

As leaves senesce (slow down and die) in the fall, they turn the variety of amazing colors we are so familiar with.  Plants' normal color is green, due to the most important compound in the world: chlorophyll.  Chlorophyll is the substance in plants that allows them to absorb sunlight and use the energy from sun to make food, a process called photosynthesis.  In the fall, chlorophyll breaks down, revealing other colorful substances plants use for photosynthesis: xanthophyll (ZAN-tho-fill), a yellow pigment, and carotene (CARE-oh-teen), an orange pigment.  As temperatures drop, some plants make anthocyanin (AN-tho-SIGH-uh-nin), a red pigment that helps the plants store sugars for winter.  Some plants reveal tanins (TAN-ins) in their leaves in the fall.  Tannins are brown in color and are thought to be waste molecules produced by plants.  They have a bitter flavor, though some tannins are pleasant, including the ones found in tea leaves.

Leaf scar on a buckeye showing scars where the leaf veins were sealed off with suberin.

So leaf abscission is a trade-off that works in parts of the world with four seasons.  Plants in the tropics and plants in colder regions keep their leaves.  Tropical plants don't have to deal with cold, so they don't shed their leaves unless there is a yearly dry season.  Plants nearer the poles of the planet don't have a long-enough growing season to start from scratch every year, so they have to grow slowly and produce evergreen leaves and needles.  We lucked out, and we get to see the beautiful fall colors that accompany leaf abscission.

Gettin' Twiggy With It

A trip to the Chicago Botanic Garden this morning provided me with much blog fodder for this and the next few posts.  Spring is early this year, bringing a bounty of beautiful sights to the Botanic Garden. 

Greenish yellow weeping willows and orange willow shrubs on the left side.

Many trees and shrubs have responded to the spring weather, even if they haven't leafed out yet, by becoming quite colorful.  Their twigs have begun to manufacture photosynthetic pigments near the surface of the bark, making for yellow, orange, red and green twigs.  Forget everything you ever learned about plants - they photosynthesize using bark! (OK, don't forget anything, but you can add on.)  The picture above shows a lovely spring scene with willow trees and shrubs revealing their spring pigments.

Crimson tipped shrub willows.

The brilliant colors of the shrub willows drew me in for a closer look.  Up close, they have yellow stems with bright red tips.  The greenish yellow of the lower stems is probably a mix of chlorophylls and xanthophylls (here is an explanation of pigments in this earlier post).  The red is likely due to anthocyanins, but there is almost certainly chlorophyll also present in the twigs masked by the stronger red pigments.

Willow twigs with crimson tips.

Red is a common pigment 'choice' for plants that are active in cold weather.  The red may act to filter out some excess light and act as a sunscreen for the plant.  Plants can't photosynthesize as quickly when it's cold out, and too much light can overload the slow system.  Red pigments also tend to absorb more heat than other pigments, and even a tiny increase in temperature can increase the rate of photosynthesis.  In this crimson-tipped willow, the narrow tips would be especially likely to freeze, so red pigments there could help them be more active in the cold.  Alternatively, since this plant is growing in a botanic garden, it is likely the product of selective breeding for aesthetically pleasing but physiologically useless traits - so the colorful twigs could just be pretty and not useful at all.

Red dogwood twigs.

Above you can see entirely red twigs of a shrubby type of dogwood.  I can attest that many types of dogwood twigs are often red in the wild as well as in botanic gardens.  People and nature seem to favor red twigs for winter growth.  The overall effect (below) of these red twigs is startlingly beautiful.

Dogwood shrubs.

Many plants opt for green chlorophyll for winter twigs, as seen this variety of rose-related shrub below.  These stems can actively photosynthesize any time the temperature and light are favorable.  The tough, thick stems are able to survive freezing where leaves cannot.  When the temperatures rise to predictably non-freezing levels, these roses will leaf out and photosynthesize in earnest for the growing season.

Rose stems.

When we came to Chicago in October, forecasts said it would be the worst winter ever.  Instead, it's been a record-breakingly warm winter.  Spring seems to be competing to outdo winter's numbers.  It's been 80 degrees for days now.  Plants that use temperature as a trigger to emerge from winter's dormancy are already leafing out.  Those that use day length as the gauge for the start of spring still look like they should for this time of year - leafless and grey.  I suspect the day-length strategy will work better this year, since Chicago has been known to have freezes into April.  Trees that leaf out early stand a good chance of having to grow new leaves after their first ones get frozen off.  Late leaf growth combined with twig pigmentation is a good strategy for climates with unpredictable spring temperatures.  Using twigs to photosynthesize can give a tree a good head-start on the growing season without the risk of having tender plant parts frozen off.

Turnips

Turnips are ridiculously under-appreciated.  They are the easiest vegetable to grow.  They are marvelously delicious.  They produce anti-cancer compounds and their nutritional profile is similar to broccoli even though they look more like potatoes.  Their greens are the richest-tasting greens of all cooking greens.  To top it all off, they are in my favorite vegetable genus, Brassica.

The picture below is of an enormous purple turnip at the Chicago Botanic Garden, a fabulous botanic garden with an extensive fall vegetable section.  This turnip is pure white on the inside (I assume - I didn't cut it open), and it should also be white below the soil.  Sunlight causes the root epidermal cells to become pigmented in this variety of turnip.  The pigment seen here is an anthocyanin, but some turnips have a green suntan from chlorophyll production. 

A gigantic turnip!

The turnip in the portrait above, since it's very large and pigmented, is likely to have some zip to its flavor, much like a radish.  The greens will be piquant as well, like mustard greens.  The root would be delicious cooked in a stew with other vegetables, and the greens could be tamed by throwing out the first round of steaming or boiling water if necessary. 

My favorite turnips are Hakurei turnips, which are pure white regardless of sun exposure, smaller, and not hot.  They are sweet and fruity and even the greens can be eaten raw.  You can find them at farmers' markets in the fall and spring.  Their texture is divine when cooked - smooth and silky, and I like them sauteed or cooked into soups.  It's absurd to think of eating turnips without the greens in my book, so I always get the roots cooking while I prep the greens.  Then I cook the greens with the roots for the last few minutes for a great combination of flavors and textures.  YUM.

Turnip and greens, Brassica rapa.

 To grow turnips, just sow a thick line of seeds and cover them with a little soil.  As the turnips grow, you can thin the young plants by collecting some greens before the roots start to fill out.  In just a few weeks the turnip roots will grow and you can harvest them as you need them for several weeks.

Turnips and butterhead lettuce.

Turnips are members of the genus Brassica, which is a group of unassuming weedy-looking plants with fast growth rates and fantastic variation in growth forms.  Each brassica species modifies a different plant part to store energy, usually in response to humans breeding the plants to make agricultural varieties.  Turnips store energy in their roots (as do rutabagas), and the rest of the plant looks pretty normal.  Other brassicas put lots of energy into leaves (cabbage and kale), leaf stems (bok choi, seen below), flower buds and stems (broccoli and cauliflower), leaf buds (Brussels sprouts, seen below), stems (kohlrabi), and seeds (canola, mustard).  It's fascinating to me that these plants are so closely related with such striking similarities in leaf and flower structures but with such vast differences in other plant parts.

Brussels sprouts, Brassica oleracea var. gemmifera.

Bok choi, Brassica chinensis.

Brassicas generally are quite nutritious and low in calories.  Consuming these vegetables regularly appears to have a protective effect against many cancers, though the mechanism is not well understood.  For optimal amounts of the cancer-fighting compounds, eat these vegetables raw or lightly steamed, not cooked into oblivion.  Some brassicas contain bitter compounds detectable by a subset of the human population.  These people can't enjoy the wonderful flavors of brassicas because they find them to be too bitter.  Also, children are better at tasting bitter compounds than adults, which explains why they more commonly dislike vegetables. 

Pigment Pondering

No pictures today, which is ironic considering it's a post about plant pigments.  Think of it as an exercise for the imagination.  I promise pictures for next time, but today interesting fonts and colors will have to suffice to spice up the text. 

The previous post (which had lots of pictures), described the amazing plant pigment called betalain, found in Celosias and beets (and bougainvillias and cacti).  Very strangely, betalains have never been discovered in the same plant as today's pigment, anthocyanins.  Anthocyanins are the workhorse, common pigments, and betalains are the superhero pigments.  But anthocyanins are not without some amazing characteristics.  Come along and find out.

Anthocyanins are a group of molecules that are usually red, but sometimes they are blue, orange or even yellow.  They share a similar chemical structure and method of production in plants, and they are everywhere.  Every time you look at a plant and see red, unless you're angry or it's one of the plants with betalains mentioned above, you're looking at anthocyanins.  Red fruit, red leaves, red stems, and red flowers are all due to anthocyanins.

In plants, anthocyanins have many functions.  They can act as sunscreen, which is why immature leaves are often reddish.  Some trees' leaves produce anthocyanins in the fall to protect the dying leaves while their nutrients are recovered by the parent plants.  Anthocyanins are major antioxidants in plants, and they protect the plants' DNA from other types of radiation in addition to UV light.  Flowers use anthocyanins to attract pollinators.  Fruits use anthocyanins to attract dispersers.  Plants that grow amongst snow produce anthocyanins because they help the plant stay warmer and grow faster. 

Some anthocyanins change color with a change in pH.  The one in red cabbage turns red in acidic solution, blue at a neutral pH and greenish-yellow at a basic pH.  You can experiment with this at home with a red cabbage, some vinegar, some water and some baking soda.  The anthocyanins in some hydrangeas are red when the soil pH is around 6.5, and they are blue when the pH is a little lower, around 5.5.  The reason for the hydrangea color change has to do with the increased solubility of aluminum in very acidic soil.  The aluminum is used to make anthocyanins when it's present, which accounts for the blue color.  When aluminum is not available at a higher pH, the anthocyanins are made using iron.

All pigments are molecules that selectively reflect light.  Light coming from the sun, or from a light bulb, is white light, which contains light of all wavelengths.  Pigments absorb white light and hold on to most wavelengths, but a few specific wavelengths are bounced off the pigment.  Whatever those reflected wavelengths are are what your eyeballs detect when you look at the pigment.  Something that's red reflects light with only wavelengths of approximately 700 nanometers (very small).  Something that's violet reflects light with wavelengths of about 400 nanometers (even smaller).  Something that's white is not pigmented - it reflects all wavelengths.  Something that's black is also not considered a pigment - it reflects no light.  That's why looking at something that is black is like looking at a dark room - there is no light coming to your eyes from either.  Black clothes are hotter in the sun than white clothes because the black molecules in the clothes absorb all light that hits them.  The black molecules hold on to the light as heat.  And that is why I'll be wearing a white T-shirt tomorrow on the farm (and I'll bring my camera!).