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eturn to table of contents Casparian strip Cell wall Cell cytoplasm Plasmodesmata Plasma membrane Regulatory protein in plasmodesmatum Figure 9. Critical components of an endodermal cell where regulation of nutrient uptake occurs. Selected plasmodesmata contain proteins that allow a particular nutrient to transfer from the external cellular region of the root, through the plasma membrane, and into the cytoplasm of the inner cells for eventual transport throughout the corn plant. 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, the 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. Nutrient Concentrations in These Internal Cells Regulate How Much Additional Nutrient is Transported Through the Plasma Membrane. When a particular nutrient attains a sufficient concentration inside the cell, the nutrient acts as a feedback inhibitor and stops the transfer of additional nutrient across the plasma membrane (Figure 10). Nutrients move to xylem for transport X Nutrients initially in cortical cells Regulatory proteins transfer nutrients Transfer process stops when internal concentration is adequate Nutrients move to xylem for transport Process continues Figure 10. The regulatory process for nutrient uptake occurs at the plasma membrane of endodermal cells. (Different colored dots represent different nutrients.) 60

eturn to table of contents 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). Phloem Xylem Stele: contains central parenchyma tissue Epidermis Nutrients Endodermis: contains Casparian strip Cortex: contains several layers of cells 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. 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. AGRONOMIC PRACTICES TO IMPROVE NUTRIENT UPTAKE It is most important to remember that corn roots must continually grow to extract soil nutrients. Any agronomic practice that supports new root growth improves corn yield. Practices like eliminating compaction or draining wet soils to allow for better root growth improve opportunities for greater yields. Soil is the primary source for 13 of the 16 nutrients necessary for corn growth. If any 1 of these 13 nutrients is limited, corn yield also is limited. Farmers are well aware that maximum corn yields require adequate levels of nitrogen, potassium, and phosphorus and routinely fertilize the soil to supply these nutrients. Today’s high-yielding hybrids remove substantial amounts of all 13 nutrients from the soil as grain leaves the field for transport to market. Nutrient deficiencies other than nitrogen, phosphorus, or potassium deficiency may also appear – especially in sandier and lower organic matter soils. Agronomic practices that monitor for all types of nutrient deficiency and that adequately replace these nutrients in a balanced fertility program improve opportunities for maximum yield. Finally, nutrient transport from soil to corn roots requires adequate water. For farmers who irrigate, the water applied supports nutrient transport as well as overall plant growth. For dry-land farmers, reducing tillage and leaving more mulch or residue on the soil surface reduces water loss via evaporation from the soil surface, thus allowing more water for plant uptake. Whether under irrigated or dry-land production, proper water management is essential for maximum nutrient uptake and subsequent grain yield. This particular article pertains to nutrient uptake in corn; however, this same physiology and these same agronomic practices apply to soybeans, cotton, sorghum, and other crops. 61