Understanding How Plant Roots Take In Water and Nutrients
Roots play an integral part in plant survival, whether it’s as an anchor or providing life-sustaining water and nutrients. Chris Bond explores just how roots do their job to keep plants alive.
Most folks know what plant roots are and have a vague notion of some of the basic functions they perform. Of course, roots offer an anchorage system to keep plants in place, they store food for the plant in the forms of sugars, starches, and proteins, and roots send water and nutrients up into the plant. Root systems also play a role in vegetative reproduction in some plants. But not everyone knows just how the root system does these amazing things—all without anyone ever really seeing them in action.
There are two primary types of root systems—taproot systems and fibrous root systems. Both serve nearly identical purposes but in different ways. Plants with their respective root systems likely evolved based on the conditions of the native soil where they thrived, or as an adaptation to the types of roots that supported competing plants. Through site-specific evolution and human intervention, it is quite probable that in any typical landscape you will find plants with both kinds of roots growing in proximity to each other.
In taproot plants, the first root or proto-root that develops from the germinating seed is called the radicle. This type of root system is commonly found on most conifers, dicots, and seed-bearing plants. It consists of a predominant root that forms from the radicle and grows downward. Other less dominant root branches will form off of this taproot. Monocots primarily develop fibrous root systems, though there are exceptions. Fibrous roots are a dense mat of similarly-sized roots that do not venture as far away from the shoot of the plant. The radicle in monocots does not stick around for long; the first root that develops and holds the seed in place is aborted and replaced by others shortly thereafter, starting the process of developing the robust stitchwork of the fibrous root system.
To peel back a further layer of roots and how they work, let’s start at the bottom. The end or tip of the root (on a taproot) is protected by a structure called the root cap. The root cap is a mass of hardened cells that are the tip of the spear, driving the root downward, protecting the layers behind it. Think of it as the nose of a space shuttle that can take the brunt of the resistance upon re-entry into the atmosphere.
Behind the root cap is the cortex, which makes up the bulk of the rest of the root. The cortex is loosely packed with cells and empty space, both of which store water and allow it to flow into the xylem vessels, sending water and dissolved nutrients to the above-ground shoot systems. Roots lack openings such as the stomata found in leaves. Rather, roots are covered in thin-walled cells, known as parenchyma cells, which act as a water-absorbing membrane. These are primarily found in the cortex of the root. This entire outer wall of the cortex is known as the epiblema and the main conduit for this transport at the center of the cortex is known as the endodermis. Unlike the rest of the cortex, the cells in the endodermis are more tightly arranged so water does not escape back into the cortex but can be sent on its merry way upwards through passage cells. However, this does not happen until the water and nutrients have passed the root system’s smell test. To that end, the endodermis is a plant guardian. If the plant has inadvertently absorbed any toxic material, the endodermis filters it out and rejects it. A waxy barrier known as the Casparian strip is the gatekeeper at play here. Proteins within the cells allow for the good compounds and molecules to pass, while toxins are weeded out and removed.
Root hairs are an extension of the endodermis. These fine, long, and narrow projections grow out from mature roots, helping increase a root system’s ability to absorb moisture and nutrients by increasing the root system’s surface area, which increases contact between roots and soil.
The stele, pericycle, conjunctive tissue, and vascular bundles round out some of the lineup that make up the typical root. If you really, really want to know more about the intricate functions of roots or have a severe case of insomnia, look up “histogen theory” and/or “Quiescent Center.” For our purposes here, these go way beyond the realm of useful knowledge for the layman horticulturist, this author included.
(For more on how roots work, check out A Root Primer)
The Secret World of Roots
Root systems are a network of connected botanical appendages whose entire mass can sometimes dwarf the above-ground portion of the plant it is supporting. It takes a lot of work behind the scenes to feed and care for all the flowers, fruits, and leaves getting most of the accolades. They don’t, however, work alone. The root (mostly below ground) and shoot (mostly above ground) systems work in unison in a positive feedback loop. Though an oversimplification of the process, it’s safe to say the photosynthesis work the leaves and greens do sends nutrients below to the root system, which is then able to expand and grow, sending more food and water upward so more leaves can be produced. Then more photosynthesis can occur, and more roots can develop and so on.
Roots are constantly attracting and transporting water and dissolved nutrients from the medium they are in. Deep down in the root cells, a pressure builds. This root pressure creates a siphon-like action which forces water and nutrients up into the above-ground portions of the plant while water and nutrients from the surrounding soil are drawn into the root. This is due to the higher concentration of nutrients and minerals inside the root cells than in the soil environment around the root system. In addition to this force, moisture from the soil is continually being absorbed into the roots by the negative water potential within the root cells.
Symplastic vs Apoplastic Movement of Water and Nutrients in Roots
Once inside the roots or root cells, water and dissolved nutrients travel through different root pathways in one of two ways—either symplastic movement or apoplastic movement. Most water and nutrients move through the roots via apoplastic movement. When this happens, water does not actually enter the cells but travels through the passages between cells. Think of this as a hallway connecting bedrooms. This is easier than traveling by symplasticity. With symplastic movement, water and nutrients must cross over a cell membrane to enter into the cytoplasm, a gelatinous material inside the cell. Water and nutrients must then travel upwards by moving through the cell walls. Think of this as gaining access to bedrooms through the adjoining walls instead of taking the hallway.
To recap the process of water and nutrient uptake, we can reduce it to four basic steps:
- Step 1 Water and dissolved nutrients enter the roots through the root hairs by the process of osmosis or root force.
- Step 2 Water and dissolved nutrients then cross over the root cortex either through symplastic or apoplastic movements.
- Step 3 Water and dissolved nutrients enter the xylem.
- Step 4 Water and dissolved nutrients are sent up into the plant stem in the transpiration stream to deliver needed nutrition to all the shoots of the plant.
These processes will go on so long as the plant is alive, and water and nutrients can be found in the soil. Periods of drought will slow down these actions and, in some cases, encourage root growth as plants seek deeper sources of water and nutrition.
Mycorrhizae: Roots’ Fungal Friend
Worthy of inclusion, though not a component of root anatomy or function proper, is mycorrhizae. This lauded beneficial fungus attaches itself to plant roots in a symbiotic embrace that can exponentially increase the reach of any particular root system. As many as 85 percent of plants may owe all or some of their nutrient uptake capacity to mycorrhizae. The fungus not only gets fed while attaching itself to root systems but acts as a force multiplier by increasing the reach of plant roots and the surface area of the root system. The hyphae, or tentacles of the fungus, are much smaller than even the root hairs and can, therefore, mine into much smaller crevices in search of nutrients.
In addition to the basic and more complex functions described earlier, roots also serve to aerate the soil they are anchored in and can pull the plant closer to the dirt for protection from the elements. All of these and more are functions of roots in the care and feeding of the shoots.
(Get more in-depth about mycorrhizae read, Mycorrhizae: The Straight Story.)