Education Zone

Mineral nutrient flux through the apoplasm cannot be allowed to reach all of the plant as otherwise any toxic substances in the external solution would permeate throughout the whole plant. For this reason a waterphobic layer of suberin protects the endodermis and prevents solutes passing through in the apoplasm into the stele, forcing them into the symplasm (a network of interconnected cytoplasms joined by structures called plasmodesmata). This layer of suberin is called the Casparian strip. This produces a situation whereby solutes can travel quite freely through the rhizodermis and cortex, via the apoplasm or any intercellular space, but once they reach the Casparian strip must enter the cell before any further progress can be made. This ensures that only required nutrients enter the main part of the plant and only in controlled quantities. Exceptions to this include areas of the apical zone where the Casparian strip is yet to fully form and gaps caused by emerging lateral roots. Despite this initial freedom of movement some studies have shown that most uptake into cells occurs in the rhizosphere, particularly when low concentrations are present (Vakhmistrov, 1967). This is shown by the predominance of ATPase-H+ pumps present in the cells of this layer (Felle, 1982; Parets-Soler et al., 1990). This may be due to another layer of suberized cells at the exodermis but this is hotly contested.

Ions are free to enter the symplasm rather than the apoplasm at any point providing they can get into the cell. From here they are transported along this pathway from cell to cell by diffusion and mass flow. The gateways between cells are regulated by calcium ions. During this passage some ions may be taken into the cells vacuoles rather than be passed to the xylem. This is particularly the case for nutrients that are deficient. This withholding of nutrients in times of deficiency can lead to higher root to shoot ratios, which then hopefully should aid correction of the deficiency due to increased ability to obtain nutrients.

Ions that do make it to the stele are loaded into the xylem from parenchyma cells. An ATPase-H+ pump is used in this loading. It pumps H+ ions into the xylem and then uses the gradient to transport cations by an antiport process, and anions either by a symport process or along the electrochemical gradient. The ions are pumped into the apoplasm of the xylem as it is dead tissue. Some young xylem cells however are not dead and so accumulate nutrients in their symplasm. It is believed these nutrients then leak into the apoplasm when these cells die. This method is believed to account for approximately 10% of shoot demand in maize (McCully and Canny, 1988).

The loading of solute into the xylem together with the one- way movement of water in the symplasm gives the xylem a very low water potential, causing water to rush in. This provides the hydrostatic pressure required for the xylem to function, forcing the sap up the plant. The rate of flow of the sap is affected by similar factors as transport of ions into cells due to the loading mechanism that produces it. Studies using xylem sap have tried to determine nutrient contents in a variety of conditions. They are hampered however, by (a) the lack of transpirational pull, (b) nutrient cycling, and (c) movement of nutrients directly from phloem to xylem.

• Effects nutrients have on the development of plants

• Effects nutrients have on yield

• Factors that effect the rate of mineral uptake

• Explanation of the mineral uptake into the cells of plants

• Uptake of mineral nutrients in the apoplasm

• Transport of nutrients throughout the plant

• Nitrogen nutrition