Figure 3. Corn roots grow through pore spaces created by the irregular packing of soil colloids. Roots can extract only those nutrients (represented by the orange dots) from soil immediately surrounding the root tissue (represented by the green box). New roots must grow through different parts of the soil profile to extract additional nutrients.
Step 2. Roots solubilize nutrients associated with soil colloids into the soil water matrix.
Plant roots exude different chemical compounds to draw nutrients from the soil into the plant (Kochian, 2000). For most of the nutrients, plant roots exude hydrogen (H+) ions into the soil matrix and acidify the soil immediately surrounding the root (Figure 4).
Figure 4. Corn roots exude hydrogen (represented by blue arrows) into the soil profile to acidify soil immediately surrounding the root (represented within the green box).
As this micro-zone of soil becomes more acidic, cationic nutrients, such as ammonium (NH4+), calcium (Ca++), magnesium (Mg++), and potassium (K+), are easier to extract from soil cation exchange sites and are more available for plant uptake. Nutrients like phosphorus exist in a form in soil that becomes more water-soluble at lower pH and are, therefore, more available for plant uptake. The ability of corn roots to acidify the adjacent soil allows corn plants to extract nutrients from higher pH soils as well as from acidic soils. The bulk of the soil profile that is not adjacent to corn roots continues to maintain its pH as long as the buffer capacity of the soil is greater than the acidifying properties of nutrient uptake and metabolism in the soil.
Step 3. Epidermal root cells and associated mycorrhizae extract nutrients from soil colloids and the soil water matrix and retain these nutrients for plant uptake.
Nutrients do not magically transfer from the soil to the plant. Nutrient transfer is a two-part process that consists of two sets of dynamic equilibria that strive to maintain a balance in the soil profile (Figure 5a, href="#fig5b">Figure 5b, href="#fig5c">Figure 5c). Each nutrient strives to maintain an equilibrium balance between the amount of nutrient associated with the soil colloid and the amount of nutrient dissolved in the soil water.
The central core of the root contains parenchyma cells and vascular tissues (xylem and phloem) that serve as conduits to transport nutrients to all portions of the corn plant. The outermost portion of the corn root, which consists of the cortex and epidermis, is separated from the central core of the root by a ring of endodermal cells called the endodermis. Between each group of endodermal cells lies the Casparian strip, plant material that is impermeable to nutrients (Figure 7). Initially, as plant roots extract nutrients from soil, these nutrients reside in the root cortex (Figure 8).
Step 4. The corn plant expends energy to actively transport nutrients from epidermal root cells, through the plasma membrane, and into cells within the central portion of the root.
For nutrients to pass from the cortex to the stele or central portion of the root, these nutrients must pass directly through the plasma membrane located within these endodermal cells. Regulation of nutrient uptake occurs at this plasma membrane (Figure 9). The plasma membrane is impermeable to nutrients. However, the plasma membrane contains thousands of plasmodesmata filled with proteins capable of selecting and transporting specific nutrients from the external portion of the root, through the plasma membrane, and into cells located within the central portion of the root. This nutrient selection and transfer process requires biochemical energy to function properly.
Step 6. The corn plant transports nutrients from these internal root cells via the xylem to other portions of the plant to support growth.
Xylem and phloem vascular tissues are closely associated with parenchyma cells that now contain higher concentrations of essential nutrients. Nutrients are loaded into the vascular tissue for long-distance transport to whatever part of the corn plant is in need of this nutrient (Figure 11).
Figure 11. After nutrients pass through the plasma membrane and enter parenchyma cells in the central portion of the root, they are available for xylem transport to other portions of the corn plant.
Step 7. As nutrient concentrations within the internal root cells decrease, these cells then allow additional nutrients to move from epidermal cells into internal root cells to continue the process of nutrient uptake.
As nutrients are transferred to vascular tissue, concentrations of these nutrients in endodermal cells decrease sufficiently to allow the transfer protein to transport additional nutrients across the plasma membrane. Continuous regulation of this transfer protein allows the corn plant to maintain a proper balance of all nutrients for plant growth.
The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary.
Product performance is variable and subject to any number of environmental, disease and pest pressures. Individual results may vary. Some of the information set forth may be based on statements by the manufacturers.