The Function of Plant Roots
Investigating Water Uptake, Anchorage, and Food Storage in Roots
Aug 7, 2009
Branching, twining, overlapping, and even connecting, the roots of plants grow with as much vigor and riotous enthusiasm below ground as the stems, leaves, and flowers that soar above ground.
Dehydration is a Constant Threat
Unlike animals, plants are not watertight. Plants need to draw carbon dioxide into their leaves for photosynthesis. This requires openings in the leaves so as one gas enters – carbon dioxide – another, in the form of water vapor, can exit and evaporate through those same openings.
When a plant loses water in the form of vapor to the open air, the turgor pressure (internal cell pressure) of its cells decreases. This in turn results in a decrease in the overall stiffness of the plant. If water continues to be lost and is not replaced, the leaves and stem of an herbaceous (nonwoody) plant become increasingly limp and the plant wilts.
Roots as Organs of Uptake
Water and minerals move from soil spaces into the root through the root hair cells that cover some sections of the root. Water and minerals move into root hair cells through one of two processes:
Osmosis. molecules of gases and liquids move of their own accord (no energy input required) from areas of high concentration to areas of low concentration through a process known as diffusion Once balance is reached, no further diffusion occurs. The diffusion of water only is known specifically as osmosis.
The concentration of water molecules is usually higher in the soil spaces around the root and lower inside the root hair cells. Because of this concentration difference and because the thin cell walls of root hairs are permeable to water, water moves by osmosis from the soil spaces into the cytoplasm of root hair cells covering sections of the roots.
Active Transport.
If plants needed to take in water only and soil moisture was always plentiful, osmosis alone would suffice. However, there are situations in which the plant must move molecules against the concentration gradient and expend energy to do so.
The process by which plants use cellular energy to move materials from an area of low concentration to an area of high concentration is known as active transport. Active transport is necessary when roots must absorb dissolved nutrients such as minerals whose concentration in soil spaces is lower than that in root cells. Active transport is also required to keep what little water remains in soil spaces moving into the roots during dry spells and droughts.
The energy for active transport is derived from a molecule of adenosine triphosphate (ATP) and oxygen. Thus roots need a constant supply of oxygen to survive. Normally, roots obtain oxygen from the air in soil spaces but if these spaces are totally filled with water, the plant dies. This is one reason why floods destroy farm crops so quickly and also the reason why houseplants can be killed by over-watering.
Roots as Anchors
The extent of a root system – the depth to which it penetrates the soil and the distance it spreads laterally – is dependent upon several factors, including the moisture, temperature, and composition of the soil. The bulk of most “feeder roots” (roots actively engaged in the uptake of water and minerals) occurs in the upper meter of soil, the part of the soil normally richest in organic matter.
In an intensive study of the fibrous roots of winter rye, H.J. Dittmer found that one plant 20 inches high, consisting of a clump of 80 shoots, had a total of 14 million roots – main roots, secondaries, tertiaries, and quaternaries. The total surface area of the root system was 4,000 square feet – equal to the floor space of two or three good-sized houses – or 130 times the surface area of the stems and leaves. If laid end to end, these roots would have had a length greater than the distance from New York City to Washington, D.C. or around 600 km (360 miles).
Amazingly, all these roots occupied only about 6 liters (2 cubic feet) of soil. Such extensive root systems so thoroughly permeate the soil around them that they not only serve to secure the plant into ground but they also become ecologically important in holding topsoil so that it does not get washed away by rains.
Roots as Food Storage Organs
Most roots are important storage organs, and some, such as those of the carrot, beet, radish, sweet potato, and sugar beet, are specifically adapted for the storage of food.
Food molecules manufactured above ground, in photosynthesizing portions of the plant body, move down the stem and into the storage tissues of the root. Some of this food may eventually be used by the root itself but the majority of the food stored in roots is “digested” and the products transported back up to feed the stem and leaves.
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