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SUMMARY

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eturn to table of contents Corn Roots Grow Throughout the Soil Profile Towards Soil Nutrients. Corn roots grow throughout pore spaces created by the irregular packing of soil colloids (Figure 3). The majority of soil nutrients are extracted by young roots, root hairs, and mycorrhizae (Salisbury and Ross, 1978). Fungal mycorrhizae develop a symbiotic relationship with corn roots. Mycorrhizae extract nutrients from soil and provide some of these nutrients to the corn plant. In exchange, the corn plant provides sugar to the mycorrhizae to support new fungal growth. These corn roots and associated mycorrhizae deplete the nutrient content in the soil immediately surrounding these roots, so new corn roots must continue to grow to extract nutrients from different portions of the soil profile. Soil profile As this micro-zone of soil becomes more acidic, cationic nutrients, such as ammonium (NH 4+ ), 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. Epidermal Root Cells and Associated Mycorrhizae Extract Nutrients from Soil Colloids and the Soil Water Matrix and Retain These Nutrients for Plant Uptake. Corn root 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. 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 5). 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. Soil colloid Soil water 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). Soil profile Figure 5a. 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. Soil water Corn root 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). Corn root Figure 5b. Each nutrient also strives to maintain an equilibrium balance between the amount of nutrient in the soil water and the amount of nutrient associated with the corn root. 58

Soil colloid Soil water Endodermis return to table of contents Corn root Figure 5c. Nutrients do not magically jump from the soil to the corn root. Each nutrient molecule is dissolved and carried by soil water. Endodermal cells Casparian strip For all of the nutrients that are not anions, the vast majority of each nutrient is associated with the soil colloid, and only a small portion of this nutrient is dissolved in the soil water. For nutrients that are anions, the vast majority of nutrients are in the soil water and can move as the soil water moves. Each nutrient also strives to maintain an equilibrium balance between the amount of nutrient in the soil water and the amount of nutrient associated with the corn root. Corn roots contain organic molecules and receptive spaces that can bind and retain nutrients within the root epidermal cells. When this equilibrium is in balance, the majority of each nutrient is associated with the corn root, and only a small portion of this nutrient is dissolved in the soil water. Soil nutrients have now moved from soil to the corn root. Within the corn plant, how is nutrient transfer from the corn root to the rest of the plant regulated for proper growth? To answer this question, it is necessary to understand basic corn root anatomy (Figure 6). Stele: contains central parenchyma tissue Figure 7. The Casparian strip is a layer of plant material inserted between the endodermal cells in the corn root that blocks the transfer of nutrients from the cortical cells to the central portion of the plant root. Nutrients must enter directly into the endodermal cells for eventual transport throughout the corn plant. Corn root Phloem Xylem Endodermis: contains Casparian strip Figure 8. Initial location of nutrients (depicted as orange dots) as they move from soil into cortical cells of the corn root. Epidermis Phloem Xylem Stele Endodermis Cortex Epidermis Cortex: contains several layers of cells 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). Figure 6. Diagrams of horizontal and vertical cross sections of a corn root. 59

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