Unit 5 - Minerals Nutrients and Absorption.doc.pdf

AG123 CROP PHYSIOLOGY UNIT 5 MINERAL NUTRITIONS AND ABSORPTION OF MINERAL SALTS INTRODUCTION Knowledge on mineral nutrition in plants is very important in agriculture. Much of the nutrients required by plants would...

AG123 CROP PHYSIOLOGY UNIT 5 MINERAL NUTRITIONS AND ABSORPTION OF MINERAL SALTS INTRODUCTION Knowledge on mineral nutrition in plants is very important in agriculture. Much of the nutrients required by plants would already have been taught in the subject “Introduction to Soils” (please refer to these notes). All our crops depend on the availability of mineral nutrients in the soil and sustained crop yields depend on our ability to manage soils in such a way that the essential elements are available in adequate quantities in the soil. This has been achieved in developed countries where very high crop yields are now obtained. We will learn how the plant obtains these nutrients from the soil and how they move within the plant. We will later see how CO2 and O2 are exchanged with the atmosphere through the leaves. We have already learnt how water moves into and within the plants. The elements essential for plant growth are absorbed by plant roots from the soil at the same time as water. Minerals are however selectively absorbed by plant roots and they do not move as freely into and within the plant as water does. ELEMENTS FOUND IN PLANTS One way of finding out what element plants require is to find out what element it contains. We know that C, H and O are required by the plants (we will hear more about these when we talk about photosynthesis and respiration). We can find out what other elements the plant requires by drying the plant (expelling all the water in it and ending up with what is called dry matter) and then determining what elements are contained in the dry matter. An example on this process is shown in Table 1. It is obvious from Table 1 that the mineral composition of the plant (i.e. corn) is quite different from that of the soil, which contains and supplies the mineral nutrients. Soils are largely composed of A1, Si, and Fe. This is because not all the nutrients in the soil are easily soluble in water (and therefore available to the plant) and also plant roots are selective in their absorption of nutrients. There are three main ways in which we can find out which nutrients the plant requires: 1  (a) Nutrient culture - hydroponics. (b) Sand culture - also called slop culture. (c) Nutrient film techniques. Table 1: Analysis of corn tops and corn leaf ------------------------------------------------------------------------------------------------------------ Element Maize shoot (% dry wt.) Maize leaf (% dry wt.) ------------------------------------------------------------------------------------------------------------ Oxygen 44.4 - Carbon 43.6 - Hydrogen 6.2 - Nitrogen 1.5 3.2 Potassium 0.92 2.1 Phosphorus 0.20 0.31 Sulfur 0.17 0.17 Calcium 0.23 0.52 Magnesium 0.18 0.32 Chlorine 0.14 - Silicon 1.2 - Sodium - - Iron 0.08 0.012 Manganese 0.04 0.009 Copper - 0.0009 Boron - 0.0016 Molybdenum - - Zinc - 0.003 Aluminium 0.89 - Undetermined 7.8 - ------------------------------------------------------------------------------------------------------------ Source: Modified from Salisbury and Ross (1992, p. 117) ESSENTIAL ELEMENTS Besides C, H and O, additional 16 elements are considered essential for plants. Two main criteria have been set up to judge if an element is essential: 1. An element is essential if the plant cannot complete its life cycle (i.e. form viable seeds) in the absence of that element. 2. An element is essential if it forms part of any molecule or constituent of the plant that is itself essential in the plant. Either criterion is sufficient to demonstrate essentiality. Most of the 16 essential elements satisfy both. Other less recognized criteria are sometimes used: 2  3. An element is essential if it acts directly inside the plant and does not cause some other element to be more readily available or antagonizing the effect of another element. 4. An element is essential if deficiency symptoms appear on plants when they are grown without addition of the element to the nutrient solution, even though such plants form viable seeds. It is usually easier to show that an element is essential than that it is not; this is because the elements we can detect with our analytical instruments are limited by the sensitivity of our instruments. There is therefore a possibility that other elements will be added to our essential list as the sensitivity of our instruments improves. Table 2 lists the 16 elements presently believed essential to plants. The Table shows the forms in which the elements are available; their atomic weights; an approximate adequate concentration in the plant, and the approximate number of atoms needed compared to molybdenum. Table 2: Essential elements for higher plants ------------------------------------------------------------------------------------------------------------ Element Form Atomic wt. Concentration Relative Rel.# of atoms Available in dry tissue compared to to plants (mg/kg) (%) Molybdenum ------------------------------------------------------------------------------------------------------------ Mo MO- 95. 95 0.1 0.00001 1 + ++ Cu Cu4 ; Cu 63.54 6 0.0006 100 Zn Zn++ 65.38 20 0.0020 300 ++ Mn Mn 54.94 50 0.0050 1,000 B H3BO- 10.82 20 0.002 2,000 Fe Fe3++; Fe++ 55.85 100 0.010 2,000 - Cl Cl 35.46 100 0.010 3,000 S SO-- 32.07 1,000 0.1 30,000 - - P H2P4 ; HPO4 30.98 2,000 0.2 60,000 Mg Mg++ 24.32 2,000 0.2 80,000 Ca Ca 40.08 5,000 0.5 125,000 K K+ 39.10 10,000 1.0 250,000 - + N NO3 , NH 14.01 15,000 1.5 1,000,000 O O2, H2O 16.00 450,000 45 30,000,000 C CO2 12.01 450,000 45 35,000,000 H H2O 1.01 60,000 6 60,000,000 ------------------------------------------------------------------------------------------------------------ Source: Modified from Salisbury and Ross (1992, p. 120). These essential elements can be divided into two groups: 3  1. Micronutrients or trace elements: the first 7 elements listed are needed in tissue concentrations equal to or less than 100 µg g-1 of dry matter. 2. Macronutrients: the remaining elements in the Table are needed in concentrations of 100 µg g-1 dry matter, or more. For information on the symptoms exhibited per respective elements when there are deficiencies, refer to your notes on “Introduction to Soils”. At this of this subject, we will concentrate on how minerals are absorbed and then trans-located through plants. MINERAL ABSORPTION Roots as absorption organs Mineral nutrients are found in very dilute form in the soils solution and a very extensive root system is required to enable the plant to absorb enough nutrients. In fact all plants do have fairly extensive root systems; the big trees have bigger and deeper roots and they can absorb nutrients from greater depth in the soil. Most of our annual food crops have fairly shallow root systems and they absorb from the top meter or so of the soil. They thus exhaust nutrients from the top layer of the soil and leave those further down untouched. The ability of trees to absorb nutrients from great depths explains the way in which long fallows under forest restore fertility to the surface soil. Our knowledge about roots is limited because roots are underground and hence difficult to observe. We however know that there is large turnover of the smaller roots in the soil. The bigger tree roots are mainly structural, while the smaller absorbing roots are replaced annually. The cylindrical and filamentous (long and narrow) shape of the roots is important for the absorption of water and nutrients from the soil. The sectional area and together with the root cap enables roots to force their way into the soil. The filamentous shape enables roots to explore much more soil volume per unit root volume than would spherical or disc shaped roots. Root hairs also contribute to the efficiency of ion absorption; these are modified epidermal cells found near the root tip just behind the region of root elongation. The presence of mycorrhizae also helps in nutrient absorption. Mycorrhizae Mycorrhizae are non-pathogenic or weakly pathogenic symbiotic fungi living in root cells, mostly in epidermal and cortical in cells. The fungi obtain organic nutrients from the host plant and in turn improve the nutrient and water absorption properties of roots. The association is therefore mutually beneficial. The presence of mycorrhizae could 4 make a difference between the successful growths of a plant in some soils – say those very poor in nutrients or otherwise. Foresters are more conscious of this fact than agriculturalists and they always make sure that the nursery soil always contains some soil from a well-established stand of the forest species in which mycorrhizae are bound to be found. The presence of mycorrhizae is particularly beneficial in soils, which fix phosphorus, like in many PNG soils. They have the ability to extract the fixed phosphorus. They also hasten the movement of other essential ions like NH+, K+ and NO3-. In fertile soils mycorrhizae are not well developed. There are two main groups of mycorrhizae – the ectomycorrhizae and the endomycorrhizae. The fungal hyphae of the ectomycorrhizae form a mantle outside the root and also inside the root in the intercellular spaces of the epidermis and cortex. They do not penetrate the cells but an extensive network called the Hartig net is formed. Ectomycorrhizae are more common in tree species. Annual species commonly have endomycorrhizae; although tree species also have them. There are 3 sub-groups but by far the most common one is the vesicular arbuscular mycorrhizae (VAM). VAM fungus produces an internal network of hyphae between cortical cells that extends out into the soil where they absorb nutrients and water; they also penetrate directly into the cytosol of cortical cells where they form vesicles and arbuscules (and hence their name). Mineral elements in soil All mineral ions occur in very dilute solutions in the soil. The cations are adsorbed to the negative charged sites in clays and organic matter, so are less easily leached from the soil. The phosphate anion is also found in very dilute solutions but it is often reversibly precipitated as salts of aluminum, calcium or iron. The ions NO3-, SO4++ and Cl- are more soluble and are repelled by negatively charged sites of clay and organic matter. Because of this they remain in soil solution and are therefore easily leached from the soil. NH4+ is adsorbed to soil colloids, but there is little NH4+ in the soil because most of it is rapidly oxidized to NO3-, and absorbed in this form by plants. Movement of ions into the root There are three mechanisms by which dissolved mineral nutrients reach the root surfaces, and then get absorbed. These mechanisms are mass flow, diffusion and root interception. For details explanations with accompanying illustrations, refer to the “Supplementary Notes”. 5  General principles of solute absorption Four principles of solute absorption were observed even before the advent of electron microscopes and the discovery of the detailed structure of biological membranes: 1. If cells are dead and not metabolizing, their membranes become much more permeable to solutes – they in fact leak out. 2. Water molecules and dissolved gasses like N2, O2, and CO2 penetrate all membranes rapidly. The mechanism for this is not clearly understood (see Salisbury and Ross, 1992 for further explanations). 3. Hydrophobic solutes penetrate at rates positively related to their lipid solubility (see Salisbury and Ross, 1992 for examples). 4. Hydrophilic molecules and ions with similar lipid solubility penetrate at rates inversely related to their size (see for Salisbury and Ross, 1992 examples). Characteristics of solute absorption Accumulation cells can absorb certain essential solutes much faster and for a longer time than others. The concentration of such solutes with the cells becomes much higher than in the external solution. This is called accumulation. The accumulation of certain solutes is common in all living cells. The accumulation of a particular ion depends on the ion and the species of plant concerned, thus ions are selectively absorbed. For instance, when roots are fed with a mixture of ions like K+ and Na+, plant roots will absorb a lot of K+ but little Na+ or in a mixture of K+ and Cl+, plant roots will absorb a lot more K+ than Cl-. This is sometimes referred to as selective absorption. Retention of ions: Once ions have been absorbed by the cytoplasm or vacuoles of cells they do not readily leak out. Rapid leakage takes place when membranes are damaged by heat, poisons, lack of O2 and the removal of Ca++. There is however very slow leakage, absorption being unidirectional. Effect of solute concentration on solute absorption In natural vegetation (or crops, if not manured) the concentration of the major plant nutrients is so low that diffusion of these nutrients to the roots is much more limiting to the rate of uptake of the root itself; in well fertilized or manured crops absorption through membranes limits the rate of uptake. An increase in the external solute concentration 6 around roots speeds solute absorption, but rates at both low and high concentrations across the membrane. Actual absorption does not rise linearly as concentration increases but approaches a maximum at concentrations near those normally existing in soils. This reinforces the observation that roots become increasingly efficient in absorbing certain ions as they become depleted in the soil. Furthermore, a high internal concentration of ions somehow “signals” the cell to slow down in absorption. When the ions are required in large quantities, say when there is active shoot growth, absorption is much faster. Plants can therefore regulate and optimize their internal solute concentration in different environments. Diffusion and facilitated diffusion If the chemical potential of a solute is lower inside a membrane than outside, the solute will diffuse inward until equal chemical potential inside and outside is attained. The concentration of neutral molecules like sugars is kept low in the cytoplasm because they are converted to sugar phosphates, or starch, or cell wall polysaccharides, or are broken down to CO2 and H2O. The concentration of many ions is also kept low; e.g. NO3- – is reduced to NH4+ or SO4- is incorporated into amino acids, and HPO4- converted to sugar phosphates and nucleotides. The inward movement of such ions can continue by diffusion alone. When ions penetrate membranes by diffusion they do not move through the lipid bilayer as water does. They are absorbed through channels in proteins by a process called facilitated diffusion. They (ions) combine with membrane proteins called ionophore, which speed up their movement across membranes. The antibiotics, such as gramicidin, nigericin and valinomycin are examples of ionophores. This is one way in which ions can be transported across membranes but by no means the only way because ions are transported even if the outside solution is less concentrated than the inside one (against a concentration gradient). In such cases more active mechanisms must operate. Active transport It is now known that metabolic energy is required to move ions against a concentration gradient. A number of possible mechanisms, all involving ATPase to generate ion pumps, carriers and electrical potentials have been invoked to explain how this takes place. The explanations are however not universally accepted and they are still at the theory or educated guess stage and we will not go into them here. It is enough to say that metabolic energy is used through ATP. 7 FURTHER READING Salisbury, F.B. and Ross, C.W. (1992). Plant Physiology (4th edition). Wadsworth Publishing Company, Belmont, California. 8