Following an interminable winter, spring finally arrives. The soil warms up to about 55°F and you carefully apply a handful of balanced garden fertilizer around the base of each of your rosebushes. You scratch it into the soil with a rake, apply water, and retreat to your warm kitchen for a cup of coffee.

Then what happens? How do the nitrogen, potassium, phosphorus and other nutrient elements in the fertilizer get inside your plant to perform all their required functions? The answer(s) to this question get complicated, so much so that it will require three columns to address them properly. And even then, we'll be looking at only the tip of an iceberg — the subject of plant nutrient uptake fills books.

In this first of three installments we will discuss some of the anatomical features of your rosebush that are involved in nutrient uptake. In the second, we'll review some of the basic chemistry that must happen before nutrient uptake can occur. And in the final column we'll talk about the three main mechanisms of plant nutrient uptake and how they work.

Let's start with some basic plant anatomy. As you know, anatomy deals with structure (as opposed to physiology, which deals with function). The main structures involved in nutrient uptake are the roots, the xylem, and the stomata. Let's begin with roots. The illustration of a young root shown at top left (scanned from Plant Anatomy by Katherine Esau) shows the various types of cells that can be found in the root when it is examined in microscopic cross section. At the root surface are epidermal cells, and many of these are modified into what are called 'root hairs.' These have a large surface area and are major players in water and nutrient uptake. Inside the epidermis are the thin‑walled cells of the cortex and endodermis. Contained within the endodermis is a structure known as the 'Casparian strip,' whose function is still under investigation. Inside this strip lies the pericycle, which encloses the vascular tissues in the xylem and phloem. These are pipes through which water and sugars move around inside the plant. The darkened cells and arrows in the drawing show the pathway of water as it makes its way from the soil into the xylem.

The xylem elements are continuous from the roots to the leaves. You can visualize them as bundles of pipes running up the plant stem. The individual xylem pipes are connected to one another at their sides and ends. Water moves from pipe to pipe through tiny pores called pits that line the sides and ends of the pipes. These bundles make their way up into the leaf where they lie adjacent to other types of leaf cells.

The second illustration (scanned from Physiology of Woody Plants by Kramer and Kozlowski) represents a cross section of a leaf. You can see that the xylem bundles are adjacent to thin‑walled cells called spongy mesophyll. These are loosely arranged within the leaf leaving large air gaps between them. Air can escape from the leaves through openings called stomata that are bound by what are known as guard cells. In the illustration, these are shown on the underside of the leaf. The August/September 2007 issue of American Rose included an article about stomata. You may want to review it.

Stomata are nothing short of miraculous. They open and close as the guard cells gain or loose turgidity from absorbing or loosing water. Gasses, such as carbon dioxide, oxygen and water vapor, that enter and leave the leaf do so mainly through the stomata. Stomata are very sensitive to environmental conditions such as light, temperature and relative humidity of the atmosphere. In general, light, low CO₂, high humidity, and high leaf water content cause them to open. Through control of the stomata, the plant can regulate the rate and direction in which these gasses are exchanged with the atmosphere, as well as the rate of water loss from the leaf. As we will see in the next column, the rate of water loss, or transpiration, has a direct bearing on the rate of nutrient uptake.

Nutrient Uptake – Part II

In the last column we reviewed the key anatomical structures in roses, indeed in all woody plants, that are involved in nutrient uptake. These were: the fine roots and root hairs, the xylem (a system of interconnected pipes that leads from the roots to the leaves), and the stomata (tiny pores in the leaves that enable gases such as water vapor to enter and/or leave the leaf). In this installment, we'll review some of the chemistry involved in nutrient uptake. Fortunately, this chemistry is relatively straight forward and quite well understood.

The main chemical process involved is called ionization. We've all witnessed ionization many times, but perhaps were unaware of what we were looking at. Here's an example. Take a spoonful of table salt crystals, then stir them into a glass of warm water and watch what happens. Before long the crystals will 'disappear.' They will not settle out. They will become, essentially, part of the water. You are witnessing ionization in action.

The table salt is made up of two elements: sodium (Na+) and chlorine (Cl‑). When they are combined as sodium chloride (NaCl), table salt, the plus charge and the minus charge act like little magnets, hooking the two elements together. But when they enter the water, the sodium and chlorine come apart or ionize. They are still there but are no longer visible and no longer connected. They are suspended in the water.

Before a plant can extract a nutrient from the soil solution, the nutrient must be in ionic form. This is a very important point to remember. Plants don't take up, say, potassium nitrate (KNO₃). They take up a potassium ion and a nitrate ion that have been dissolved (ionized) in the soil solution. All of the plant macro‑ and micro‑nutrients must first be converted to their ionic form in the soil before they are available to the plant. A bit of this is accomplished by soil microorganisms, but most ionization occurs as the elements are dissolved in water. That's why we must water generously after we fertilize.

As noted above, some ions have a positive charge. These are called cations. While others, the anions, carry a negative charge. Whether an ion has a positive or a negative charge depends on the number and arrangement of electrons around it, a subject beyond the scope of this article. As far as the plant is concerned, the key cations are potassium (K+), calcium (Ca+), magnesium (Mg+), and iron (Fe+). The most important anions are nitrate (NO₃‑), phosphate (H₂PO₄‑), sulfate (SO‑) and chloride (Cl‑).

So, now that you have fertilized your roses and watered the fertilizer deeply into the soil, the soil solution contains a broth of both positively charged and negatively charged nutrient ions floating freely about, much like the glass of water described above that contained sodium and chloride ions.

Now, let's focus on the soil itself. Soil consists of particles of varying sizes. The surfaces of the particles have regions called exchange sites that hold mainly negative charges. The number of these exchange sites is related to the sizes of the particles. Soils that contain lots of clay (i.e. very small particle size) have vastly more exchange sites than soils containing larger particles, such as sand. Organic materials like humus also contain large numbers of exchange sites. Soils such as this are said to have a high 'ion exchange capacity.'

You can probably see where this is going. The charged particles in the soil moisture are the ions we talked about and will tend to attach themselves to the exchange sites on the soil particles and the positive cations to the negative sites and the negative anions to the positive sites. The next step is for the plant to pick up these ions, a process called ion exchange, bring them into the plant and transport them to the places where they are needed for growth. We'll discuss how they do this in the next column. Stay tuned.

Nutrient Uptake – Part III

Spring is here. You have applied fertilizer to the soil around your rose bushes and watered it in deeply. The mineral nutrients in the fertilizer have dissolved (ionized) in the soil water. Some of these ions have attached themselves to charges on the surfaces of the soil particles, while most remain dissolved in the soil solution. The soil is moist, but pockets of air exist between and among the particles. Okay, so now what? How do those nutrient ions get inside your plant? And how do they move to the flowers, leaves, buds and other areas where they are needed for growth and metabolism?

In very simple terms, there are three ways this can happen, or three mechanisms of nutrient uptake and transport:
 (1) ion exchange,
 (2) diffusion, and
 (3) mass flow.
Let's take them one at a time.

Ion exchange or contact exchange. Nearly all of the ion exchange sites in the soil, whether they reside on soil particles or on the surfaces of organic material, carry negative charges. This means that they attract and hold positively charged ions or cations. Remember from the last column which nutrients have positive charges? (Hint: potassium, calcium, magnesium, iron, zinc, etc.). In the exchange process, the plant root surfaces secrete positively charges ions that the plant doesn't need. The plant basically 'swaps' these for the positively charged ions it wants and leaves the unneeded ions on the soil exchange sites. As roots proliferate and plow forward into the soil they exploit new regions, exchanging and taking up ions that they need.

This process is important but not as important as you may think. For one thing, it works for cations only – but many key nutrients are anionic, or negatively charged (nitrate, phosphate and sulfate for example). Furthermore, the total nutrient uptake that occurs via ion exchange is minor because most of the exchangeable ions are in the soil solution and not stuck to the exchange sites.

Diffusion. This is simply the migration of ions from a region where they are highly concentrated to a region of lower concentration. So when roots deplete ions from an area in the rhizosphere (soil near the roots), if these ions are present in higher concentrations outside the rhizosphere they can move in to replace the depleted ions. This mechanism is particularly important for uptake of phosphorus and potassium.

Mass flow. This is the most important uptake process. Remember in Part I of this series we talked about a system of pipes, called xylem, that runs up the stem from root hairs to leaves and terminates at pores called stomata. During daylight, when the stomata are open, water vapor from inside the leaf evaporates out through the pores in a process called transpiration. (I plan to write more about transpiration in a future column).This causes the tissues inside the leaf to dry slightly, which puts a tension on the water column in the xylem, pulling water up through the plant in a process somewhat analogous to sipping water up through a drinking straw. This lost water is replaced by soil moisture taken up by the roots.

In mass flow, many of the dissolved ions in the soil moisture are simply pulled into the plant and up the xylem. This is called a 'passive' process because the plant doesn't need to expend energy for it to happen. The source of the energy for transpiration is the heat that causes evaporation (i.e., the sun).

A final important point concerns the movement of ions from outside the root cells to inside the root cells into the xylem. Some ions move through the cell walls while others move across the cell membranes. Cell membranes are comprised of a lipid sheet with protein molecules embedded in and on it. Some of these molecules act as 'channels' through which certain ions pass. Others serve as 'carriers' that ferry specific nutrient ions across the membrane into the cell. This process requires that the plant expend energy.

Implications for rose growers? When leaves lose water faster than the roots can replenish it, they go into a condition called water stress and the stomata close. When stomata close two things happen:
 (1) CO₂ uptake ceases, so photosynthesis stops, and
 (2) transpiration ceases, so nutrient uptake through mass flow stops. This leads to a key point, one that every serious rosarian already knows: water is the most important summer fertilizer.

This article was provided to the TVRS as a courtesy by the American Rose Society.