Saturday, October 16, 2004

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A Day in the Leaf

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Saguaro The sun rises over arid Arizona. Among the diverse desert flora is the tall Saguaro, standing at a height that took many decades or even centuries to achieve. For yet another day it must fight the difficult battle to survive water scarcity and the unpredictable nature of the desert environment as well as human intervention. East northeast, way across the country, the Tomato vines in my own backyard in Maryland soaked up the sunrise, hoping for the full daily seven to eight hours of sunlight required so they can bear their juicy red fruit.Tomato

These two organisms have something in common. That's right. They're both plants (and flowering plants at that). At first sight, they seem like very different plants: a cactus and a nightshade. When you look a little closer, their differences don't seem so obvious. Then again, look even closer than that, and suddenly they're different as night and day! Well, let's stop where they have fewer differences, and we'll take a glimpse at what it is that these chosen two have in common as members of the Kingdom Plantae and Phylum Magnoliophyta, and how they utilize these features for their respective lifestyles.

Plants, like any organism, need to evolve into forms that allow them to survive in the habitat that was dished out for them. In general, leaves act as sites for photosynthesis, respiration, and transpiration (and flowers, which are modified leaves, are for reproduction), roots are where they take up nutrients and water from the soil, and the stem is their leaf-root liaison. There are exceptions!

Keeping the Guard Cells Busy

Now the sun has risen, it will send its light onto the plants of the world. For another day now, leaves (or other organs evolved to serve the function of leaves) will photosynthesize, transpire, and respire. A lot of exchange is involved here, which takes place in the stomata (that's plural for stoma), the tiny pores scattered over the leaf. The guard cells are two cells located on each stoma that open and close the pore as needed. For photosynthesis, carbon dioxide enters the stomata and with water in reaction with the solar light and chlorophyll that where absorbed into the chloroplasts of plant cells and into their thylakoid membranes, producing glucose and oxygen:

6 CO2 + 6 H2O + Light + Chlorophyll -----> C6H12O6 (glucose) + 6 O2

More specifically, solar energy and chlorophyll react, water donates an electron to this reaction, and the resulting oxygen gets released out the stomata. Other energy from these reactions phosphorylate ADP (adenosine diphosphate) into ATP (adenosine triphosphate). The co-enzyme NADP+(nicotinamide adenine dinucleotide phosphate) is reduced to NADPH. ATP and NADPH are products of this light-dependent reaction, which is followed by carbon fixation (where the carbon dioxide comes in), including the Calvin Cycle, which is known as light-independent reactions or dark reactions, and this yields the sugars that feed the plant.

Plant respiration is similar to animal respiration in that oxygen is taken in and carbon dioxide is released. This occurs through the stomata of course. Transpiration is also a very common leaf process in which the plant loses water through the stomata. More than 50% of a plant's water is lost this way. The reason for it? Well, sometimes the plant has more water than it needs or can hold, so it releases it. This also cools the plant, so it's just like when we animals sweat. Cooling also allows for less evaporation, so that's why the stomata are usually more numerous on the underside of leaves, where it's cooler because of less sun exposure.

And at the Root of the Matter...

Roots keep the plant anchored into the ground. There's more to it, though. They take up water and nutrients from the soil. Soil conditions and climate dictate how these roots grow for a given plant. Some roots go very deep into the ground to get underground water. Other plants have a very shallow but far reaching root system to catch more rainwater. Along with water, they absorb nitrates, phosphates, sulfates, and other nutrient mineral salts. Roots may be adapted for storage purposes as well, but this is also dictated by soil and climate conditions. The presence or absence of a root cap at a given time determines the root's growth: when there is a root cap, generally the root will grow downwards because of gravity. The cap wears away during growth, and when it is gone and hasn't been replenished yet, the root pretty much goes wherever! Root hairs are single-celled epidermal growths that help absorb water. There are two basic kinds of root systems: the taproot and the fibrous root system.
Clay uses its negative charge to hang on to cations for the plant roots.
Clay's donations to the roots.
Cations (positively charged ions) such as potassium and sodium are exchanged a lot between roots and soil. Soil is full of these nutrients because of its organic matter content from a lot of sources, such as bodily waste and dead organism decomposition. Solubility of these nutrients relies on soil pH. Also, the nutrient content of soil probably relies on the particle content, what ratios there are of clay, silt, and sand.

Now let's see how this all works with the Saguaro and the Tomato!

Carnegiea gigantea

Tall, in the Desert, and Pointy

The Saguaro cactus (Carnegiea gigantea) lives in the Sonoran Desert and grows only about an inch a year. Considering the height of these things, often over 30 feet, it takes a long time to get there! Furthermore, this is your run-of-the-mill stereotypical cactus, the kind you usually see in cartoons or otherwise depicting the Southwest USA somehow (although Arizona is the only state in the US where Saguaros are found). It grows arms, often up to five and sometimes more than that. It is such a tall and rather fragile organism, usually residing in rocky bajadas, the plains at the foot of desert mountains. Therefore, it must have the leaf, stem, and root system that fits it just right in such an environment!

The creamy white Saguaro flowers, the state flower of Arizona, open only once for a full day and never open again! They produce many thousands of seeds, but sadly a wide majority of them never live past about six weeks after germination. And the Saguaro struggle goes on from there. They tend to grow well under a "nurse" plant that provides it some shade and moisture to nurture its growth. The Saguaro cactus is an endangered species.

Cactus spines are modified leaves, although they really don't serve a leaf's purpose very much. Mostly what they do is drip what little rainwater they get back down to the ground so the roots can pick it up. Appropriately enough, spines also serve as protection. It is a good thing the Saguaro, like most cacti, doesn't have any full leaves, since a higher leaf area, therefore more stomata, means more water loss via transpiration, a process a desert dweller just cannot allow to happen.

So how does the Saguaro photosynthesize? They must do it with their stems. Amid all of the protective spines, waxy coatings (cuticles), and air pockets (to keep the plant cool to allow less evaporation of water), there is still an autotrophic organism that must produce its sugars and emit its gases. It is hard, though. An organism in a dry environment must do everything it can to keep from losing water, so there are significantly fewer stomata per surface area on the Saguaro than on flowering plants of other families. So photosynthesis is limited, and this limits their growth, which is why they only grow about an inch a year.

The Saguaro's root system must also be set just right for the organism's dry desert lifestyle. Along with reduced leaves, most desert plants adapt by having a shallow root system. Why is that? They want to catch as much as they can from the little rain they get. So these plants, even the extremely big and heavy Saguaro, must have roots that don't grow quite so much downwards as they do radially (to the side, basically). The Saguaro has a three-foot-deep taproot, and from it grow lots of littler roots that spread out as far as they can close to the surface of the ground. Saguaros grow in areas with a lot of high winds, and these little roots also help to keep it from falling over by reaching around rocks and whatever else they can grab to hold this great cactus in place. Also, they reach very far out, often the same distance as the height of the Saguaro, and this way they can pick up more rain water. The roots also have terminating mechanisms in their extremities that help prevent water loss.

Not So Tall, Wetter Climate, and Not So Pointy

Tomatoes are great! Solanum lycopersicum or Lycopersicon esculentum, they're the same thing. People used to think they were poisonous because of being so closely related to tobacco and nightshade! Glad we don't think that anymore. Tomatoes originated in the Andes but can be grown in any nice temperate climate and work well with well-drained soil. They don't need the excruciatingly strict water retention qualities the aforementioned desert succulents need. Therefore, they have the full leaves we're all familiar with, but they still have to orient themselves just right to take in plenty of light for photosynthesis.

Lycopersicon esculentum OR Solanum lycopersicum
Tomatoes can be grown easily enough in a well-lit home garden, but they require a lot of sunlight. Like the Saguaro, they too have a shallow root system, although really their roots depend a lot on how they were planted. If they grew from a seed, they have a taproot. If they grew from a clipping, they'll have a fibrous root system.

Tomatoes are prone to many parasites and other pests that are detrimental to its leaves and roots, so this causes some agricultural stress. Much of the research being done on Tomatoes is on these microorganisms (as well as a lot of genomic studies). Luckily, Tomatoes have developed their own methods to ward off these pests naturally.

There are a lot of leaves on the Tomato, and whether a big fully grown leaf or a new leaflet, they are all well positioned for capturing solar energy. The Tomato leaves are all compound, which sort of means there is another leaf growing on the other side of the stem as one leaf. Depending on gardening modifications, the Tomato plant can exhibit determinate or indeterminate growth. Determinate grows into into a bush. Indeterminate makes it an ever-growing vine.

To the Point

Is "pointy" my favorite word for this issue? Anyway, I highly recommend learning more about the Saguaro and the Tomato, as from what I've gathered while putting together information for this Article, they are very neat organisms. I'm already hoping to get out to Southern Arizona one of these days to see a Saguaro in person, and I have always been into cacti! The Tomato has shown itself to be more than just a fruit you mash up into spaghetti sauce, and about a million other food items. Check out the links below!

Sources and Information


If there is any information in this Article you know to be false or falsely implied, let me know.

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© 2004-2007 Katrina G. Moncure unless otherwise stated. Do not copy anything on this site without my permission. All rights reserved.