Raven BIOLOGY OF PLANTs chapter 30
The Uptake of Inorganic Nutrients by Roots Is an Energy
Dependent Process
The uptake, or absorption, of inorganic ions takes place through the epidermis, largely the root hairs, of younger roots. Current evidence suggests that the major pathway followed by ions from the epidermis to the endodermis of the root is symplastic—that is, from protoplast to protoplast via plasmodesmata. Ion uptake by the symplastic route begins at the plasma membrane of the epidermal cells. The ions then move from the epidermal cell protoplasts to the first layer of cortical cells (probably an exodermis) through plasmodesmata in the epidermal-cortical cell walls (Figure 30–16). Movement of ions into the root continues in the cortical symplast—again, from protoplast to protoplast via plasmodesmata—through the endodermis, and into the parenchyma cells of the vascular cylinder by diffusion.
Nutrient uptake from soil by most seed plants is greatly enhanced by naturally occurring mycorrhizal fungi associated with the root systems (page 312). Mycorrhizas are especially important in the absorption and transfer of phosphorus, but the increased absorption of zinc, manganese, and copper has also been demonstrated. These nutrients are relatively immobile in soil, and depletion zones for these minerals quickly develop around the root and root hairs. The hyphal network of mycorrhizas extends several centimeters out from colonized roots, thus exploiting a large volume of soil more efficiently.
The mineral composition of root cells is far different from that of the medium in which the plant grows. For example, in one study, cells of pea (Pisum sativum) roots had a K+ ion concentration 75 times greater than that of the nutrient solution. Another study showed that the vacuoles of rutabaga (Brassica napus var. napobrassica) cells contained 10,000 times more K+ ions than the external solution.
Because substances do not diffuse against a concentration gradient, it is clear that minerals are absorbed by active transport (page 85). Indeed, the uptake of minerals is known to be an energy-dependent process. For instance, if roots are deprived of oxygen, or poisoned so that respiration is curtailed, mineral uptake is drastically decreased. Also, if a plant is deprived of light, it will cease to absorb salts after its carbohydrate reserves have been depleted (Figure 30–21) and will finally release minerals back into the soil solution. How the ions enter the mature vessels (or tracheids) of the xylem from the parenchyma cells of the vascular cylinder has been the subject of considerable debate. At one time it was suggested that the ions leak passively from the parenchyma cells into the vessels, but substantial evidence now indicates that the loading, or secretion, of ions into the vessels from the parenchyma cells is a highly regulated, energydependent process. Hence, ion transport from the soil to the vessels of the xylem requires two active, or energy-dependent, events: (1) uptake at the plasma membrane of the epidermal cells and (2) secretion into the vessels at the plasma membrane of the parenchyma cells bordering the vessels.
Inorganic Nutrients Are Exchanged between Transpiration and Assimilate Streams Once secreted into the xylem vessels (or tracheids), inorganic ions are rapidly transported upward and throughout the plant in the transpiration stream. Some ions move laterally from the xylem into surrounding tissues of the roots and stems, while others are transported into the leaves
Much less is known about the pathways followed by ions in the leaves than about those in the roots. Within the leaf, the ions are transported along with the water in the leaf apoplast, that is, in the cell walls. Some ions may remain in the transpiration stream and reach the main regions of water loss—the stomata and other epidermal cells. Most ions eventually enter the protoplasts of the leaf cells, probably by carrier-mediated transport
mechanisms similar to those in roots. The ions may then move symplastically (through protoplasts via plasmodesmata) to other parts of the leaf, including the phloem. Inorganic ions may also be absorbed in small amounts through the surfaces of leaves; consequently, fertilization by direct application of micronutrients to the foliage has become standard agricultural practice for some crop plants.
Substantial amounts of the inorganic ions imported into leaves through the xylem are exchanged with the phloem of the leaf veins and exported from the leaf together with sucrose in the assimilate stream (Figure 30–22; see also the discussion of assimilate transport that follows). For instance, in a study of the annual white lupine (Lupinus albus), transport in the phloem accounted for more than 80 percent of the fruit’s vascular intake of nitrogen and sulfur, and 70 to 80 percent of its phosphorus, potassium, magnesium, and zinc intake. The uptake of such inorganic ions by developing fruits is undoubtedly coupled to the flow of sucrose in the phloem.
Recycling may occur in the plant as nutrients reaching the roots in the descending assimilate stream of the phloem are transferred to the ascending transpiration stream of the xylem (Figure 30–22). Only those ions that can move in the phloem—phloemmobile ions—can be exported from the leaves to any great extent. For example, K+, Cl–, and phosphate (HPO4 2–) are readily exported from leaves, whereas Ca2+ is relatively immobile. Solutes such as calcium, as well as boron and iron, are characterized as phloem-immobile.