Chapter 37: Plant Nutrition

Where does mass come from?  Mineral nutrients-- essential elements  minimal contribution to mass  Water – 80-85% of herbaceous plants  over 90% lost via transpiration  Retained H 2 O = solvent, maintain turgidity  Assimilated CO 2 makes up most of organic molecules mass.

How do plants get nutrients?

Dry Weight  95% organic (carbs, CHO, cellulose).  5% inorganics  nitrogen, sulfur, and phosphorus  Found in many organics

Plant Nutrients  Essential nutrients-- required for a plant to grow from a seed and complete the life cycle.  Plants require 9 macronutrients and 8 microsnutrients  Hydroponic cultures identify essential nutrients

Hydroponics

Macronutrients  Needed in large quantities  9 macronutrients  carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus (found in organics)  potassium, calcium, and magnesium (ions)

Micronutrients  Needed in very small amounts  8 micronutrients  iron, chlorine, copper, zinc, magnanese, molybdenum, boron, and nickel.  Cofactors of enzymatic reactions.  Lacking micronutrients can kill/sicken a plant.

Mineral Deficiencies (ex: Chlorosis)  Since Mg and Fe are involved in the production of chlorophyll, their deficiencies are marked by yellow leaves (chlorosis)

Mineral Deficiencies  Mobile Nutrients  symptoms of the deficiency will appear first in older organs  Ex: chlorosis affects older leaves first  Immobile Nutrients  affect young parts of the plant first  Older tissue = adequate supplies for shortages

Soil  The type of soil (composition/texture) determines the plants that can survive  Originates from weathered rock (minerals)  Topsoil (product of weathering)  Rich in decaying organic material (humus), bacteria and rock  Horizons  Horizontal layers of soil

Soil

Soil Texture  Depends on the size of its particles (coarse sand to microscopic clay)  Loams  Most fertile soils (equal silt/sand/clay)  fine particles = large surface area for retaining minerals and water  course particles = air spaces that supply oxygen to the root (respiration)

Waterlogged Soil  Too much H 2 O  Water in air spaces can suffocate a plant  Unfavorable molds can grow and attack plant.  Mangroves have long, hollow, tube roots that allow them to survive in marshland.

Topsoil Organisms  1 teaspoon = approx. 5 billion bacteria.  Organisms’ alter the physical and chemical properties of the soil.  Earthworms aerate soil (burrowing) add mucus holds fine particles together.  Bacteria (ie: nitrogen fixation)  Plants extract nutrients, affect soil pH, and reinforce the soil

Humus  Decomposing organic material  Formed by the action of bacteria and fungi on dead organisms, feces, leaves, etc.  Crumbly soil -- retains water, porous enough for aeration of roots  Rich in minerals from decomp.

How does Soil Hold Water? Electrostatic attraction between soil particles and water molecules. Plants extract the more loosely bound water molecules Surface of soil is charged (hydrophilic)

How does Soil Hold Minerals  Positively charged minerals (K +, Ca 2+, Mg 2+ are not leached out by rain because they adhere to negative surface of clay (remember origins!)  Negatively charged mineral ions (NO 3 -, H 2 PO 4 -, SO 4 2- ) leach out more easily.

Cation Exchange  (+) mineral ions are made available to the plant when H + ions in the soil displace the mineral ions from the clay particles.  cation exchange  roots secrete H + and compounds that form acids in the soil solution

How does Soil Hold Water?

Soil Mismanagement  The Dust Bowl (1930’s)  Farmers removed grasses that held soil in place during droughts  Topsoil was blown away and soil useless

Fertilizers  Agriculture requires a lot of minerals  Fertilizers replace the minerals in the soil  “ ” Fertilizer = 10% nitrogen, 12% phosphorus and 8% potassium  Used to increase crop yield  Organic Fertilizers (ie: manure)  Too much = pollution of groundwater

Water Availability and Irrigation  Top limiting factor for plant growth  Irrigation = bring water to dry areas (ie desert)  Irrigated water that evaporates leaves salt behind.  Lowers soil H 2 O potential  plasmolysis

Wind and Erosion  Wind destroys topsoil of many crops.  Planting trees or other crops (alfalfa) in rows between fields.  Acts as a windbreak for soil

Wind and Erosion

Sustainable Agriculture  The goal is to eventually develop farming techniques that are maximally conservation minded, environmentally friendly, and profitable.

Human Influence  Humans have made many areas uninhabitable for both flora and fauna  contamination and/or overdevelopment.  Phytoremediation – Use of plants that utilize toxic contaminants to clean up our mess  Ex: alpine pennycress (Thlaspi caerulescens) accumulates zinc in its shoots

Nitrogen Fixation  Atmosphere = 80% nitrogen, yet plants suffer from deficiencies  Plants cant use N 2 (g) directly  Must be taken as NH 4 + or NO 3 -  Decomp. of humus by microbes releases usable nitrogen into the soil

Denitrifying Bacteria  Convert NO 3 - (usable) into N 2 (not usable) which diffuses into atmosphere  Denitrifying bacteria remove usable nitrogen from the soil

Nitrogen-Fixing Bacteria  Nitrogen Fixation  Soil microbes convert N 2 (not usable) into NH 3 (will be usable)  Nitrogen fixation restocks the soil with usable forms of nitrogen (specifically ammonia)  Nitrogenase complex of enzymes catalyze rxns

Nitrogen Cycle in the Soil

Nitrogen Fixation  Once bacteria release NH 3, it picks up extra H + to become NH 4 + which plants can assimilate directly  Much NH 4 + oxidized to NO 3 - (also absorbed)  Once absorbed, plants incorporate nitrogen into amino acids and organics

Symbiosis  When a species participates in a direct relationship with an individual of another species.  Often involve microorganisms.  Can be good, bad, or neutral for one or both of the organisms.

Nitrogen-Fixing Root Nodules  Rhizobium bacteria exist in specialized projections in roots of legumes  The bacteria fix nitrogen into usable forms.  The plant supplies nutrients to the nodule through xylem and phloem.  Relationship is good for both.  Plant gets nitrogen, the bacteria obtains nutrients.

Root Nodules

Mycorrhizae  Symbiosis b/t fungi and root of nearly all plants.  Early evolutionary adaptation??  Fungi gets a good (protected) environment with nutrients from plant  Plant Benefits:  Fungus surface area and absorbs minerals in the soil and supplies them to the plant.  Fungi secretes growth factors that stimulate root growth.  Fungi produce antibiotics

Plant Modifications  Parasites – Plants that steal nutrients from other plants (ie: tap into vascular tubes)  Carnivores – Plants that eat meat (ex: venus fly trap)