Plant Nutritional Requirements Lecture Presentation PDF

Introduction to Biology II Chapter 36: Plant Nutritional Requirements Chapter 36 Opening Roadmap Chapter 36 Learning Objectives At the completion of this section, you should be able to ….. Define micronutrients and macronutrients, and identify some of the nutrients required for plant growth,...

Introduction to Biology II Chapter 36: Plant Nutritional Requirements Chapter 36 Opening Roadmap Chapter 36 Learning Objectives At the completion of this section, you should be able to ….. Define micronutrients and macronutrients, and identify some of the nutrients required for plant growth, Explain how the composition of soil affects nutrient availability. Specifically, how anions and cations differ in their availability to plant roots and how roots can influence soil pH to enhance the absorption of ions. Describe how plants absorb useful nutrients but prevent toxins from entering the shoot system using passive and active control mechanisms. Explain how fungi and symbiotic bacteria enhance the absorption of nutrients by plants, and how nitrogen-fixing bacteria infect the roots of legumes. Identify examples of specialized methods some plants use to obtain nutrients, such as how parasitic plants absorb nutrients from other plants, how epiphytes obtain nutrients from their surroundings, and how carnivorous plants obtain nutrients from digesting animals. 36.1 Nutritional Requirements of Plants Essential Nutrient Definition: – Element or compound necessary for normal growth and reproduction. – Most vascular plants have 17 essential elements. Primary Constituents of Dry Mass: – Carbon, Hydrogen, and Oxygen constitute 96% of the dry mass of plants. Macronutrients Macronutrients: Plants require significant quantities of certain elements from the soil. Major components of nucleic acids, proteins, and phospholipids. Key Macronutrients: Nitrogen, Phosphorus, and Potassium. Act as limiting nutrients, influencing plant growth. Micronutrients Micronutrients: – Plants require micronutrients in small quantities – Usually act as cofactors of specific enzymes Micronutrients may be vital to plant health, even though they are required in tiny amounts What Happens When Key Nutrients Are in Short Supply? Mobile Nutrients (e.g., N, K, P, Mg): – Transferred from older leaves to newer leaves – Scarcity reflected in the deterioration of older leaves. Immobile Nutrients (e.g., Ca, Fe): – Remain tied up in older leaves – Scarcity reflected in deficiency symptoms in newer leaves.  36.2 Soil: A Dynamic Mixture of Living and Nonliving Components Soil formation starts with the weathering of solid rock through rain, running water, and wind forces. As organisms inhabit the soil, they contribute decaying organic matter known as humus. Humus: decomposed organic matter; reserve of nutrients Soil: A Dynamic Mixture of Living and Nonliving Components Soil texture is crucial because it influences – Root penetration for water and nutrient uptake – Provides structural support to plants – Regulates water retention – Affects oxygen availability for cellular respiration Loam, the optimal soil for plants, consists of balanced proportions of sand, silt, clay, and abundant humus. Mature Soils Are a Complex Mixture of Organic and Inorganic Components Plants absorb water and minerals from upper layers of soil Contains a wide range of organisms Complex, fragile ecosystem Conservation Soil erosion: soil is carried away from a site by wind or water Sustainable agriculture: farming techniques for preventing soil erosion: − Planting rows of trees as windbreaks − Minimizing the amount of plowing and tilling needed to control weeds − Planting crops in strips that follow the contour of hillsides Role of Ionic Charge and Soil Texture Anions: negative ions, interact with water molecules via hydrogen bonding: – Readily available to plants for absorption – But easily washed out of soil by rain (called leaching) Cations: positive ions, dissolve in soil water but are not immediately available, like anions. – Electrostatically attached to negatively charged organic material and clay Role of Soil pH Soil pH can influence availability of essential elements pH scale indicates relative concentrations of hydrogen ions in a solution: – Acidic soils are found in regions with conifer forests – Alkaline soils are found in areas rich in limestone (CaCO3) Role of Soil pH Cation exchange: – Protons bind to negatively charged soil particles causing release of bound cations, such as magnesium or calcium – Nutrients released by cation exchange are available for uptake by plant roots 36.3 Nutrient Uptake Most nutrient uptake occurs just above the growing root tip – the zone of maturation: – Epidermal cells in this region have root hairs: ▪ Increase surface area available for nutrient and water absorption – Root hairs create zone of nutrient depletion in soil surrounding them – Depleted nutrients promote growth of root tip – the root tip uses up soil nutients in a local area and then move on to find more nutrients - vital to plant’s health Mechanisms of Nutrient Uptake Nutrients pass through plant cell walls freely; but plasma membranes are selectively permeable – Root hairs have large surface area; contain large numbers of membrane proteins - bring nutrients to cytosol of root cells Anions Root hairs absorb anions (- charge) against an electrochemical gradient using cotransporters, which transport two solutes simultaneously. Proton pumps establish an electrochemical gradient, enabling plant roots to absorb essential cations (channels) and anions (channels and symporters) Nutrient Transfer Via Mycorrhizal Fungi (1 of 2) Fungi and plant roots that live in association are called mycorrhizae: – Mycorrhizae fungi and plants are symbiotic: live in physical contact with each other Symbiotic relationship between plant and fungi is mutualistic: – Fungal symbionts obtain sugars from plant – Plant symbionts receive soil nutrients such as nitrogen from the mycorrhizal fungi (from decomposing soil) Nutrient Transfer Via Mycorrhizal Fungi (2 of 2) Fungi are very efficient acquiring nutrients required by plants, for two reasons: 1. Networks of filamentous hyphae increase the surface area available for absorbing nutrients by up to 700% 2. Fungi can acquire nutrients from soil that are unavailable Mechanisms of Ion Exclusion Not all ion uptake is beneficial to plants Natural soils and soil contaminated by waste products contain enough cadmium, zinc, nickel, lead, sodium or other metals: poison enzymes in most plants Sodium poisoning key issue in environments: – Ocean coastlines – Habitats near roads treated with salt to melt snow – Irrigated farmlands Exclusion of Plant Toxins Plants may exclude detrimental ions by two methods: 1. Passive exclusion, root cells lack transporters to bring in toxic ions 2. Active exclusion, plants have mechanisms for coping with toxins that enter their cells Passive Exclusion in Roots Some ions cannot enter root system if: – Root-hair cells lack membrane protein required for ion to enter cell – Ions that enter root and move through root cortex via apoplastic pathway is blocked from entering xylem by the Casparian strip Active Exclusion by Metallothioneins and Phytochelatins Plants also have mechanisms for coping with toxins after they enter cells Metallothioneins and phytochelatins: – Synthesized by special enzymes – Bind to metal ions and prevent them from acting as poison Active Exclusion by Antiporters Tonoplast: – Membrane surrounding large central vacuole – Second mechanism for actively neutralizing specific toxins involves transport proteins located in tonoplast Proteins in tonoplast allow plant cells to remove toxic substances from cytosol and store them in vacuole, where they cannot poison enzymes 36.4 Nitrogen Fixation Nitrogen gas (N2): – Makes up ~80% of the atmosphere – Plants and other eukaryotes cannot use nitrogen in this form: N2 is unreactive; great deal of energy to break triple bond Plants absorb nitrogen such as ammonium (NH4+) or nitrate ions (NO3−) Nitrogen fixation: – Few species of bacteria and archaea able to absorb N2 and convert it to NH3, nitrites (NO2), or nitrates (NO3) Role of Symbiotic Bacteria Nitrogen-fixing bacteria can take up residence inside plant root cells Infected root cells of legumes form nodules, where nitrogen-fixing rhizobia are found: 36.5 Nutritional Adaptations of Plants Autotrophs: 99% of living plants; Synthesize their own sugar through photosynthesis: ▪ 95% of vascular plants take up nutrients from soil ▪ 80% of plants obtain soil nutrients from mycorrhizal fungi However, a few plants don’t follow these rules; some plants: – Parasitize other plants – Appear to live on air – Catch and digest insects Parasitic Plants Parasites: – Live on or in a host, obtaining water or nutrients from host and reducing host’s fitness Most parasitic plants are photosynthetic and use haustoria to extract water and ions from the xylem of host plant Epiphytic Plants Epiphytes: non-parasitic: grow in absence of soil, often on leaves or branches of trees Plants absorb water and nutrients from rainwater, dust, and particles that collect in their tissues:  Carnivorous Plants Carnivores trap insects and other animals, kill their prey and absorb the prey’s nutrients Can make their own carbohydrates via photosynthesis and use carnivory to supplement nitrogen available in the environment Sundews: modified leaves containing sticky hairs that trap insects Also: Venus flyraps and Pitcher plants