This chapter focuses on the steady state activities of plants. It explains how plant nourishes itself; how it exchange gases with the environment; how it transports water and minerals, how it translocates food; how it gets rid of metabolic waste materials and how it regulates growth and development. 125
STEADY STATE ACTIVITIES OF PLANTS Learning Objectives This chapter aims that the students be able 1. to explain how plants maintain homeostasis; 2. to list the different nutritional requirements of plants and their major biological functions; 3. to observe the practices of sustainable agriculture in the community and comment on them through an essay; 4. to demonstrate how stoma works for gas exchange; 5. to trace the pathway and mechanism of transport of food and water in the plant; 6. to discuss how does a plant control and regulate its activities; and 7. to cite practical applications of plant hormones. HOMEOSTASIS is the physiological equilibrium that exists in organisms allowing the necessary metabolic activities to occur. It is brought about by several steady state activities that are well coordinated and regulated. These steady activities are nutrition, exchange of gases, transport of 126
materials, excretion and regulation and coordination. Plants undergo steady-state activities. They absorbed nutrients by the roots from the environment and produce their own food. They exchange gases with the external environment through their stomata. They transport materials in the xylem and phloem. They excrete metabolic waste products in form of crystals. They produce growth regulators and respond to the changes in the environment. Plant Nutrition Plants are capable of producing their own organic compounds through photosynthesis. However, there are some essential elements that are needed to be absorbed from the environment. Complete plant foods contain minerals, inorganic substances. Plants need sixteen essential elements for growth and reproduction. Oxygen, carbon and hydrogen are building blocks of all organic compounds like carbohydrates, lipids, proteins and nucleic acids. These three elements account for about 96 percent of the plant s dry weight. Oxygen comes from water and gaseous oxygen; carbon from the carbons dioxide in the air; and hydrogen from water molecules. The other thirteen essential elements are minerals that occur in nature. These inorganic substances can only be absorbed by plants in mineral ions. Six of these are macronutrients, each makes up at least a tenth of a percent of 127
the total dry weight. The rest are micronutrients or trace elements. If a plant is deficient in any one of these mineral nutrients, it will exhibit specific symptoms. A plant deficient in nitrogen for example, has small-yellowed leaves, while one deficient in phosphorus has stunted purple leaves. Table 1 shows the functions of these elements and the symptoms plants exhibit when they are deficient in them. These minerals are absorbed by plants in forms that plants can use. They are dissolved in water and are absorbed by the roots through active transport. They are transported via the xylem until it reaches the cells of use. The minerals required for plant growth originate from the earth s rocks, eroded over time into small pieces or dissolved in soil water. Although the water enters the roots by simple diffusion in response to a water potential gradient, mineral uptake may be passive or active. Active transport is a more reliable means of accumulating minerals and ions than passive, since it can import materials against a concentration gradient, moving even trace quantities of minerals from the soil into the cytoplasm. 128
Table 1. Macronutrients requirements of plants, their functions and deficiency symptoms exhibited by plants ELEMENT SOME KNOWN FUNCTIONS DEFICIENCY SYMPTOMS MACRONUTRIENTS NITROGEN component of amino acids, proteins, chlorophyll, nucleic acids, coenzymes Stunted growth, delayed maturity, light green older leaves, lower leaves turn yellow and POTASSIUM CALCIUM MAGNESIUM PHOSPHORUS SULFUR Activates enzymes used in protein, sugar, starch syntheses, helps maintain turgor pressure Part of middle lamella (helps cement cell walls together) necessary for spindle formation in mitosis, meiosis Component of chlorophyll, activates many enzymes used in photosynthesis, respiration, protein synthesis Component of some amino acids, proteins, nucleic acid ADP and ATP, phospholipids Component of some amino acids, two vitamins, most proteins die Reduced yields, mottled spotted of curled older leaves, marginal burning of leaves, weak root system, weak stalk Deformed terminal leaves, reduces root growth, dead spots in dicot leaves. Terminal buds die Plants usually chlorotic, (interveinal yellowing of older leaves), leaves may droop Purplish veins in older leaves, stems and branches often turn dark green, reduced yields of seeds and fruits, stunted growth Light green or yellow leaves including veins, reduce growth, weak stems similar to nitrogen deficiency 129
Table 5.1b Micronutrients requirements of plants, their functions and deficiency symptoms exhibited by plants ELEMENT Some Known Functions Deficiency Symptoms CHLROINE IRON BORON MANGANESE ZINC COPPER MOLYBDENUM Aids in root and shoot growth, aids in photolysis and noncyclic pathway of photosynthesis helps synthesize chlorophyll, component of electron transport systems of photosynthesis and respiration Affects flowering, pollen germination, fruiting, cell division, nitrogen metabolism, water relations, hormone movement Chlorophyll synthesis, acts as coenzyme for many enzymes Used in the formation of auxin, chloroplast and stare, component of several enzymes Component of enzymes used n carbohydrate and protein metabolism Essential in enzymemediated reaction that reduces nitrate, component of enzyme used in nitrogen fixation Plants wilt, chlorotic leaves, some leaf necrosis, bronzing in leaves Paling or yellowing of leaves, (chlorosis) between veins at first, gasses develop alternate rows of yellowing and green strips (veins in leaves Terminal buds die, lateral branches begin to grow then die. Leaves thicken curl and become brittle Network of major green ins on light green background. Leaves later become white and fall of Abnormal roots, molted bronzed and rosetted leaves, inter-veinal chlorosis Terminal leaf buds die. Leaves have chlorotic or dead spots. Stunted growth terminal leaves die. Plants may become nitrogen deficient, pale green, rolled or cupped leaves with yellow pots. 130
The xylem caries the dissolved minerals passively upward, presumably because of the transpiration-pull effect, in this way the nutrients are distributed throughout the plant. Leaf and stem cells then remove ion form the xylem solution using active transport pump enzymes similar to those used by the root cells to remove ions from soil water. The rate at which plants can take up various minerals depends on concentration of the minerals in the soil and on the soil s ph. Most plants grow best in a slightly acidic soil, in which mineral nutrients are most soluble. Soil composition affects how plants take up minerals. Sandy soils have mainly large particles and large air spaces, but retain little water and therefore provide few dissolved minerals. Clay soils have very fine particles, little space and hold much more water and dissolved mineral ions. In soils with little organic material, decaying vegetation or compost, for example- calcium is often tied up in an insoluble salt, calcium carbonate In soils with more organic material, calcium ions form complexes with organic acids and remain available to plant. Farmers use both organic compost and mineral containing fertilizers to recondition and replenish soils. Nitrogen Fixation. Nitrogen is abundant in the air but plants cannot use N 2 molecules. They have to be to be processed into absorbable forms. Some soil microbes can break the bonds to change nitrogen atom to ammonium (NH 4 + ). Some of these live in root nodules, which are localized swellings on roots of legumes, and other plants 131
where symbiotic, nitrogen-fixing bacteria live. The nodule residents use up some of the plant s organic molecule, However, the ammonium they produce is used in assembling their amino acids and nucleic acids and most of the ammonium and amino acids remain in plants. Sustainable Agriculture We must conserve our natural resources, including our soil for the plants. One way of doing this is through sustainable agriculture. Sustainable agriculture is capable of maintaining the soil s productivity and utility indefinitely. For a farm to be sustainable, it should be able to produce adequate amounts of quality food while conserving its resources and protecting the environment upon which it depends. recognizes nature and its limitations. It calls for diversified cropping systems, mixed crop or multi-cropping that promote biodiversity which helps keep away serious pests. Questions and Tasks 1. Look around your community and list activities that you consider sustainable agriculture. 2. Write a short essay on the importance of sustainable agriculture. TRANSPORT OF MATERIALS IN PLANTS Water Transport. Roots are the major organs for water absorption in plants. Water form the soil diffuse inward through the epidermal cells 132
or root hairs and is conducted along the walls of cortical cells until it reaches the endodermis surrounding the vascular tissues, xylem and phloem. The casparian strip of the endodermis prevents water from engorging the xylem but water molecules pass through the passage cells to the xylem within. Water moves from roots to stems and into the leaves. Only a small amount of water absorbed is used for growth and metabolism and large percentage is lost through evaporation or transpiration. This strong pull of air causes transpiration creates a continuous negative pressure or tension extending downward form the leaf to the roots this tension pulls water upward I the xylem. As water molecules are lost form the mesophyll cells of the leaf, they are replaced by those from the veins of the leaf and from the veins are replenished by molecules of water from the stem and then from the roots. This creates a tension all throughout the water conducting system of the plants. As long as water molecules continue to vacate transpiration site, water molecules are pulled upward and under tension, likewise, the cohesive power of the water molecules confined in a small tubular xylem cells allowing them to be pulled continuously in a column. Mineral and ions are actively transported into the root cells. This is important to create 133
the osmotic pressure gradient for the water molecule to diffuse inside the root cells. FOOD TRANSPORT. Glucose is produced in the leaves of plants through photosynthesis. Glucose is synthesized into other organic molecules, which are transported to other parts where they maybe metabolized or stored in the root stems, flowers and fruits. Translocation is the process of food conduction in the plants, sieve tubes play a passive role in the bulk transport of organic molecules the molecules are loaded into the sieve tubes by active transport and the energy required is supplied by the neighboring companion cells. Food is transported by bulk for in the phloem and the increase among the food in the leaves creates a potential where water concentration is decreased greatly and solute concentration increased. The food movers in mass in the regions of lower concentration or pressure throughout the phloem. RESPIRATION IN PLANTS Gas exchange is evidently an important function in plants. They require oxygen for cellular respiration and carbon dioxide for photosynthesis. They exchange gases with the environment through the structures of the leaf called stomata and in the stems called lenticels. Stoma consists of two kidney-shape cells, the guard cells and a small pore in between, the 134
stomatal pore. Entry and exit of gases is controlled by the guard cells as they open and close the stomatal pores. Unlike other epidermal cells, guard cells contain chloroplasts so that they can photosynthesize. Their cell walls are of unequal thickness, wherein those next to the pore is considerably thicker than those at the sides away from the pore. When the guard cells are exposed to sunlight they produce sugar, making them hypertonic to nearby cells. Water diffuses into them making them turgid with the thin outer walls pulling the rest of the cell causing the opening of the stomatal ore. At dark, the reverse process takes place, the guard cells loss water and become flaccid, closing the stomatal pore. Open stoma is a path for water loss though evaporation, a process called transpiration. However, loss of water is lower compared to the carbon dioxide intake, thus gas exchange benefit outweighs the disadvantage of water loss. Stems of older plants exchange gases with environment through their lenticels. Lenticels are groups of loosely arranged cells with many intercellular spaces between them. They are efficient structures for gas exchange in stems as most cells in the inner layers are already dead. Roots have specialized structures for gas exchange but gases can readily diffuse through the moist membranes of the root hairs and other epidermal cells. The air must be absorbed form 135
the air spaces between the soil particles, There is no need for special gas-transporting mechanism on plants as most intercellular spaces are filled with spaces. Stomata and lenticels are continuous with the air filled spaces so that gases can penetrate through inner parts of the plants' body. EXCRETION IN PLANTS. Plants excrete little nitrogenous waste materials compared to animals. The nitrogenous waste materials such as ammonia, uric acid and urea are also produced in plants but in small quantity. This is because of minimum protein metabolism taking place in plants. The little metabolic waste produced is diffused in the form of ammonia in the stomata of leave diffuse as nitrogen and released as salts in the soil by the roots. In some plants, the materials remain in the form of crystals in the leaves. When the leaves are shed off, the crystals are likewise eliminated. Chemically the crystals are made up of calcium oxalate or calcium carbonate. The rosette, needle-like raphide and the prismatic crystals are made up of calcium oxalate while the cystolith crystal is example of calcium carbonate. 136
Questions and Task 1. Given a rice plant, explain a. how it gets water from the soil and transports it to the grains; b. how carbon dioxide is exchanged; c. how the food produce in the leaves reach the roots; and d. how the waste materials are excreted. REGULATION AND CONTROL IN PLANTS Plant hormones are a group of naturally occurring organic substances, which in small quantities promote, inhibit or in some other way modify a physiological process. Many of these hormones have been classified on the basis of their chemical structure or the effects they produce. There are five recognized groups of natural plant hormones, auxins, gibberellins, cytokinins, ethylene and abscissic acid. Similar to animal hormones, the synthesis of plant hormone maybe localized but they may occur in a wide range of tissue, or cell site tissues. Nowadays there are man hormones that are synthesized chemically and can initiate responses similar to those caused by the natural hormones. Hence, the term plant growth regulators have been used to refer to both natural and synthetic plant hormones. Auxins, gibberellins and cytokinins are growth-promoting hormones. Auxin produced by the 137
apical meristem moves downward and stimulates and promotes cell elongation in stem. They are responsible for photoperiodism, growth and apical dominance and plants. Auxin. Auxin activates cambium and elicits differentiation of xylem and triggers cell elongation. When IAA contact responsive cells, which are prepared for growth, the cells begin to transport proteins actively out across the plasma membrane. This has the effect of acidifying the cell wall. The protein breaks some of the chemical bonds that hold one cellulose to another and active enzymes that weaken other bonds so that wall becomes weaker. If the protoplast is turgid and pressing against the wall, it extent enough pressure to stretch the weakened wall and the growth results. Immature cells do not grow if auxin is lacking. Root elongation is particularly sensitive to auxin. It will promote growth of excised root section and intact roots, but only at very low concentration. Removal of the root tip or application of auxin antagonist often promotes growth of roots, which suggests that endogenous auxin production roots top is normally high enough to be inhibitory. The response of leaf axil to auxin is not cell elongation but rather inhibition of growth. Apical induced dormancy of axillary buds, the result of each shoot tip having only one active apical meristem, is the phenomenon called apical dominance. Auxin inhibits lateral growth. 138
Abscission occurs as a result of the development of special layer of cells called the abscission layer near the base of the petiole. As the organ ages, the cells wall in the abscission layer weakened eventually. Abscission appears to be dependent on the relative concentration of auxin on either side of the abscission layer. The auxin content of young, rapidly growing organ is relatively higher, decreasing as the organ ages and approaches senescence. A young leaf produce large amount of auxin, but production falls to low steady level in a mature leaf. As long as auxin flows out through the petiole, activity in the abscission zone is inhibited. If the leaf is damaged auxin production drops to such low level that its petiole does not keep the abscission zone a quiescent, perception and transduction, in this case plant will be damaged it making it impossible for the impaired cells to produce auxin to inhibit abscission. Gibberellins. Gibberellins stimulate rapid stem elongation in dwarf plants and are important in breaking dormancy and inducing seed formation. They work on cell elongation and division at the internodes that bring about stem elongation. Cytokinin. Cytokinin promotes cell elongation as well as prevents aging and senescence. Cytokinins are noted primarily for the ability to induce cell division in plant 139
tissues. Cytokinins stimulate cell enlargement in limited number of systems, in particular the cotyledons. They also influence morphogenesis in cultured tissues. Senescence or aging is characterized by breakdown of protein, nucleic acid and other macromolecules, loss of chlorophyll and accumulation of soluble nitrogen production. Senescence is normal consequence of aging process and will occur eve when water and minerals are maintained. Application of cytokinins delays the natural senescence of leaves. Ethylene Gas. Ethylene is a gas that induces ripening of fruits. This gas promotes maturation, stimulates lateral expansion of elongating cell. Promotes fruit ripening and leaf and fruit drop. The presence of ethylene in air causes increased respiration, which in turn leads to the changes in fruit composition that transform hard, acidic inedible fruit into a sugary ripe one. Abscissic Acid. Abscissic acid is produced by cells under stress. It is a growth inhibitor and maintains dormancy in plants. It stimulates buds to form and set of outer leaves that become tough protective bud scales in preparation for winter dormancy. In general it is a growth inhibitor in response to stress. It is also active in seed dormancy. It build up in maturing seeds and suppresses root and shoot elongation in the embryo. It accelerates senescence and in 140
some plants promotes abscission or falling off of leaves or flowers. Some plant activities respond to the changes in the environment. Some plants respond to gravity as in geotropism, to light as in phototropism, to touch in thigmotropism and length of daylight as in photoperiodism. Questions and Task 1. What are plant hormones? 2. How do you keep plants bushy? 3. Explain why plants of same species indoor are taller than those found outdoor. 4. Cite some examples of how plant hormones and regulators are commercially used? 141