25 1 Plant Acquire Mineral Nutrients from the Soil Plants are autotrophs that obtain mineral nutrients as ions from soil Collect carbon from atmospheric carbon dioxide Oxygen from water Essential element plant nutrient that if absent causes disruption of normal plant growth and reproduction Categories of essential elements Macronutrients concentrations of at least 1 gram per 1 kg of plant s dry matter Six types Nitrogen Phosphorus Potassium Sulfur Calcium and Magnesium Micronutrients concentrations of 0 1g kg Iron Chlorine Manganese Zinc Copper Nickel and Molybdenum Missing nutrients can be provided by fertilizer Benefits from soil Anchorage for mechanical support for shoot Mineral nutrients and water from soil solution O2 for root respiration from air spaces between soil particles Services of other soil organisms including bacteria fungi protists and animals such as earthworms and arthropods Soil consists of three major horizons layers of soil Topsoil Subsoil Supports plant s mineral nutrients needs Accumulates materials from topsoil and parent rock Parent rock bedrock Where soil arises Humus material used as a food source for microbes that break down complex organic molecules and release simpler molecules into soil solution improves texture of soil and provides air spaces increasing O2 availability Three ways to restore nutrient content of soil after leaching washing away or harvesting crops Shifting Agriculture People move to another location after soil no longer supported plant growth Came back after weathering and accumulation and breakdown of organic matter Today human population too great and people do not want to move Organic Fertilizers Compost partially decomposed plant matter Manure Add nutrients to soil much more rapidly than natural weathering but still allow for slow release of ions with little leaching as materials decompose Used to supply mineral nutrients directly in forms that can be immediately taken up by Leaf proteins NH4 NO3 Bacteria cause the changes in substance Inorganic Fertilizers plants Chemical NH4 NO3 bacteria changes NH4 to NO3 25 2 Soil Organisms Contribute to Plant Nutrition Formation of Mycorrhizae Plant roots produce compounds called strigolactones stimulate rapid growth of fungal hyphae toward root Sites of nutrient exchange between fungus and plants are arbuscules Formation of Nitrogen Fixing nodules Group of plants called legumes form symbioses with several species of soil bacteria known as rhizobia roots of these plants release flavonoids and other chemical signals that attract rhizobia flavonoids also trigger transcription of bacterial nod genes Nod genes products that synthesize Nod nodulation factors cause root cortex to divide and form primary nodule meristem Bacteroids form of bacteria that fix nitrogen bacteria enter root via infecion thread Mycorrhizae expand root surface area 10 fold to 1 000 fold can get into pores in soil that are inaccesible to root hairs Primary nutrients plants obtains from mycorrhizae is phosphorus Bacteria have enzyme nitrogenase that converts N2 to NH3 process called nitrogen fixation Carnivorous plants Obtain some nutrients by digesting arthropods Typically grow in boggy soils where little nitrogen or phosphorus is available Digestion hydrolosis provies those missing nutrients Hemiparasites photosynthesize but derive water and mineral nutrients from living bodies Parasitic plants of plants Holoparasites completely parasitic and do not perform photosynthesis 25 3 Water and Solutes are Transported in the Xylem by Transpiration Cohesion Tension Osmosis movement of water trough selectively permeable membrane toward region of higher solute concentration lower water concentration Water potential psi tendency of solution water solutes to take up water from pure water across membrane water potential of pure water is 0 Solution with water potential less than zero has tendency to take up water from pure water Lower more negative water potential greater driving force for water movement across membrane Water potential has two components Solute potential As solutes are added concentration of free water is reduced More solues lower water potential Solute potential is usualy negative Pressure potential More pressure decreases the tendency of cell to take up more water Solute potential is sum of solute potential and pressure potential measure all three potentials in megapascals Mpa Physical structure of many plants is maintained by positive pressure potential of their cells if pressure potential drops i e plants does not have enough water the plant wilts Two major challenges when soil solution containing mineral ions passes through root cell plasma membrane Membrane is hydrophobic whereas water and mineral ions are polar Some mineral ions must be moved against their concentration gradient These challenges overcome by membrane proteins Aquaporins membrane channels which water can diffuse can change the rate of osmosis but not direction water always moves to region of more negative water potential Ion channels and proton pumps Transport proteins can move ion with greater concentration than root into plant by facilitated diffusion and secondary active transport Proton pump uses energy from ATP to move protons out of cell Protons cause an electrical gradient the region just outside of the cell becomes more positively charged than the inside of the cell Proton concentration gradient develops with more protons just outside the cell than Water and ions from soil solution move through roots to xylem by one of two pathways consists of cells walls which lie outside the plasma membranes and intracellular spaces apoplast is continuous meshwork through which water and solutes can flow without ever inside the cell Apoplast having to cross membrane Symplast passes through continuous cytoplasm of living cells connected by plasmodesmata movement of water and solutes into symplast is tightly regulated End result is the water and minerals end up in the xylem which constitute the xylem sap Caspian strip blocks movement of water and ions through apoplast Leaves must be alive for water to move in the xylem Almost a century ago the transpiration cohesion tension theory was proposed to explain water transport in xylem Transpiration evaporation of water from cells within the leaves Cohesion tensiion pulls column of water held together by cohesion through xylem from root Tension Evaporation from leaf produces tension in mesophyll Water vapor