The Physiology of Flowering Plants - KHAM PHA MOI

ION UPTAKE AND TRANSPORT IN THE PLANT 107plants in limited amounts if at all. The ions which serve as sources ofthe essential elements for flowering plants are listed in Table 4.1. Forterrestrial flowering plants the chief source of mineral ions is the soil.The mineral rock particles of the soil yield ions by weathering whichgradually brings them into solution; ions are also released by theaction of microorganisms on dead organic material. The ion concentrationof the soil solution rises as the water content of the soil falls,but except under very dry conditions the solution is very dilute. It hasbeen shown that a solution corresponding in ionic composition tothat of a soil solution will support good growth of crop plants providedit is frequently renewed or applied as a flowing solution so thatit is not depleted. In a natural soil the ions in the solution areconstantly being replenished. The laws of physicochemical equilibriumensure that, as ions derived from soil solids are removed fromthe solution, more ions dissolve from the rock particles. Soil Piconcentration is always low, 1 mg L –1 or less; it has been calculatedthat the Pi of the soil solution may need to be renewed 10 or moretimes per day to meet the P demands of a growing crop. Nitrate alsoneeds rapid replenishment; it is absorbed rapidly by plants, N beingneeded in larger quantities than most other elements, and beingextremely soluble, nitrate is easily washed downwards in rainwater.Not all the ions in the soil are totally free in the soil solution. Thecolloidal matter in the soil, both inorganic clay particles and organicparticles, ‘humus’, which help to retain water in the soil (Chapter 3)also serve to retain ions by adsorption. The colloidal constituents ofthe soil usually carry a net negative charge; cations, being positivelycharged, are adsorbed to the negatively charged groups on the clayand on the organic particles. These ions are held at the surface of thesoil particles by electrostatic attraction only loosely and can beexchanged for other cations; by washing a soil with a concentratedsolution of a salt such as NH 4 NO 3 the soil cations can be displacedinto the solution in exchange for the ammonium ions (NH 4 + ). Formost of the anions, there is little adsorption because of the lack ofpositive charges on the soil colloids. There may be some adsorption ofphosphate ions, especially of the trivalent PO 4 3– which has a highelectric charge density, and phosphate ions can also replace hydroxyland silicate anions in clays.4.3.2 Ion uptake by the rootAdsorption, absorption and accumulationThe region of most active ion uptake by roots is the same as for wateruptake, i.e, the young region of the root behind the apical meristem,the root hair region. The uptake of cations begins with the adsorptionof the cations to the cell walls where polysaccharides carrynegative charges, which attract H + ions moving out of cells by theaction of proton pumps (Section 4.3.4, p. 117). These H + are thenaccessible to be exchanged for soil solution cations, which thusbecome electrostatically bound to the cell walls. The binding sites