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Plant Nutrition and Soils Chapter 39 Soils Soil is the highly weathered outer layer of the Earth’s crust -A mixture of sand, rocks, clay, silt, minerals and microorganisms The Earth’s crust includes about 92 naturally occurring elements -Most are found in the form of inorganic compounds called minerals 2 Soils Most roots are found in topsoil -A mixture of mineral particles of varying sizes, living organisms and humus -Humus consists of partly decayed organic material 3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Partially decomposed organic matter Well decomposed organic matter Minerals leaching from rocks and accumulating from above Weathered bedrock material Leaf litter and plant life Topsoil Subsoil Bedrock 4 Water and Mineral Availability Only minerals dissolved in water in spaces among soil particles are available -Organic soil particles tend to have negative charges, and so attract positive ions -Therefore, active transport is needed to move positive ions into root hairs 5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1. Soil particles tend to + – have a negative charge. + – + – + – + – – Soil particle + – – – + + – – + – ATP + 2. Positive ions are attracted to soil particles. + + 3. Negative ions stay in – – + – + – + – – – – Root hair + + solution surrounding roots, creating a charge gradient that tends to “pull” positive ions out off the root cells. + 4. Active transport is Water required to acquire and maintain K+ and other positive ions in the root. 6 Water and Mineral Availability About half of the soil volume is occupied by pores, which may be filled with air or water -Some of this water is unavailable because it drains immediately due to gravity -However, water that is held in small pores is readily available to plants 7 Water and Mineral Availability 8 Soil Loss If topsoil is lost, soil’s water-holding capacity and nutrient content are adversely affected -Drought and poor farming lead to wind erosion of farmland in the 1930s -The southwestern Great Plains of the US became known as the “Dust Bowl” 9 Soil Loss Measures to prevent erosion include: -Intercropping = Mixing of crops in field -Conservation tillage = Minimal or even no-till approaches to farming Measures to prevent fertilizer runoff include: -Site-specific farming -Integrated nutrient management 10 Acidic and Saline Soils Acidic soils release minerals, such as aluminum, that are toxic to plants Saline soils alter water potential, leading to a loss of water and turgor in plants -Draining marshland in southern Iraq resulted in a salty desert 11 Plant Nutrients The major source of plant nutrition is photosynthesis -Fixation of atmospheric CO2 into simple sugars, using the energy of the sun -However, this is not enough for the synthesis of all the molecules needed by the plant 12 Plant Nutrients Plants require a number of inorganic nutrients -Macronutrients: Used in relatively large amounts -Nine = C, O, H, N, K, Ca, Mg, P & S -Micronutrients: Used in minute amounts -Seven = Cl, Fe, Mn, Zn, B, Cu & Mo A deficiency of any one can have severe effects on plant growth 13 Plant Nutrients 14 Healthy wheat plant Chlorine-deficient plant Copper-deficient plant Zinc-deficient plant 15 Plant Nutrients Assessment of nutritional requirements -Plant seedling is first grown in a complete nutrient solution -Seedling is then transplanted to a solution lacking one suspected essential nutrient -Growth of the seedling is monitored for presence of abnormal symptoms 16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Suspected nutrient is not essential Transplant Normal growth Monitor growth Complete nutrient solution Solution lacking one suspected essential nutrient Suspected nutrient is essential 17 Abnormal growth Plant Nutrients Hydroponic cultures -Plant is suspended in air, with the root rotating through a nutrient bath 18 Plant Nutrients Food security, avoiding starvation, is a global issue -Food fortification is an active research area -Focuses on ways to increase a plant’s uptake and storage of minerals -Using genetically-modified plants 19 Special Nutritional Strategies Plants need ammonia (NH3) to build proteins -However, they lack the biochemical pathways necessary to convert N2 to NH3 Symbiotic relationships have evolved between plants and nitrogen-fixing bacteria -Legumes form nodules that house the bacterium Rhizobium 20 Special Nutritional Strategies 21 Special Nutritional Strategies Nitrogen fixation is the most energetically expensive reaction known to occur in cells -Nitrogenase requires 16 ATPs to break the triple bonds in N2 to form two NH3 molecules Rhizobium fixes nitrogen in exchange for carbohydrates -Nodule formation requires extensive signaling between bacterium and legume 22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Epidermal cell1. Pea roots produce flavonoids (a group of molecules used for Cortex cells plant defense and for making Flavonoids reddish pigment, among numerous other functions). Root hair The flavonoids are transported Rhizobium into the rhizobial cells. 2. Flavonoids signal rhizobia to produce sugar-containing compounds called Nod (nodulation) factors. Nod factors Nod factors 3. Nod factors bind to the surface of root hairs and signal the root hair to grow so that it curls around the rhizobia. 4. Rhizobia make an infection Infection thread thread that grows in the root hair and moves into the cortex of the root. The rhizobia take control of cell division in the cortex and pericycle cells of the root (see chapter 35). 5. Rhizobia change shape and Vesicles with differentiating bacteroids Cell division starts making nodule are now called bacteroids. Bacteroids produce an O2-binding heme group that combines with a globin group from the pea to make leghemoglobin. Leghemoglobin gives a pinkish tinge to the nodule, and its function is much like that of hemoglobin, bringing O2 to the rapidly respiring bacteroids, but isolating O2 from nitrogenase. Differentiated bacteroids 6. Bacteroids produce fixing nitrogen nitrogenase and begin fixing atmospheric nitrogen for the plant’s use. In return, the plant provides organic compounds. 23 Mature nodule Special Nutritional Strategies Symbiotic associations with mycorrhizal fungi are found in about 90% of vascular plants -Substantially expand the surface area available for nutrient uptake -Enhance uptake of phosphorus and micronutrients 24 Carnivorous Plants Often grow in acidic soils that lack nitrogen -Trap and digest small animals, primarily insects, to extract additional nutrients -Have modified leaves adapted for luring and trapping prey -Prey is digested with enzymes secreted from specialized glands 25 Carnivorous Plants Pitcher plants = Have pitcher-shaped leaves with cavity filled with digestive fluid Venus flytrap = When hairs are touched, the two halves of the leaf snap together Sundews = Glandular trichomes secrete both sticky mucilage and digestive enzymes Waterwheel = Uses trigger hairs and snaps to capture and digest small aquatic animals 26 27 Carnivorous Plants The phylogenetic relationship is: Pitcher plants are not related to this clade 28 Parasitic Plants May be photosynthetic or non-photosynthetic -At least 3,000 types of plants Dodder (non-photosynthetic) -Wraps around its host -Relies on it for its nutritional needs Indian pipe (non-photosynthetic) -Hooks into host trees through mycorrhizae 29 Parasitic Plants 30 Carbon-Nitrogen Balance The Intergovernmental Panel on Climate Change (IPCC) has concluded that CO2 is maybe at its highest level in 20 million years -Associated with increased temperatures The C-N ratio in a plant is important for the health of the plant and the herbivore 31 Carbon-Nitrogen Balance Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) catalyzes the first step of the Calvin cycle -Can bind CO2 or O2 -If CO2 binds, a 3-C sugar is made, that can is used to make glucose and sucrose -If O2 binds, photorespiration occurs -Neither nutrient nor energy storage 32 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Photorespiration (no sugars) O2 CO2 CO2 Rubisco Rubisco CO2 Ribulose 1,5-bisphosphate Calvin Cycle 33 Glucose and other sugars Carbon-Nitrogen Balance C3 photosynthesis occurs in mesophyll cells C4 photosynthesis uses an extra pathway to shuttle carbon deep within the leaf -This reduces photorespiration by limiting the Calvin cycle to cells surrounding the vascular tissue where O2 levels are low 34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vascular tissue CO2 Mesophyll cell RuBP Calvin Cycle Glucose 3PG (C3) Sucrose (in phloem) a. C3 leaf CO2 Mesophyll cell C4 pathway Bundle sheath Vascular tissue Bundle CO2 sheath Calvin cell Cycle Glucose Sucrose (in phloem) b. C4 leaf (Kranz anatomy) 35 Carbon-Nitrogen Balance In C3 plants, as CO2 increases, the Calvin cycle becomes more efficient -Increased photosynthesis and plant growth -However, the plants have less nitrogen and minerals per unit mass -Resulting in lower nutritional value for herbivores -More plant matter must be eaten to obtain same amount of nutrients 36 Carbon-Nitrogen Balance One possible outcome of increased CO2 in atmosphere: Photosynthesis CO2 Plant growth Relative levels of protein and minerals in plant tissue Herbivory Human nutrition As ambient temperature increases, respiration rate will also increase -Result is more changes in plant nutrient balance 37 Phytoremediation Phytoremediation is the use of plants to concentrate or breakdown pollutants -Phytodegradation: Contaminant is taken up from soil and broken down -Phytovolatilization: Contaminant is taken up from soil and released through stomata -Phytoaccumulation: Contaminant is taken up from soil and concentrated in shoots -These are later harvested 38 Phytoremediation Trichloroethylene (TCE) may be removed from the soil by poplar trees -Degraded into CO2 and chlorine -A fraction moves rapidly through the xylem and is released through stomata Trinitrotoluene (TNT) may be removed from soil and degraded by poplar and bean plants -But at high concentrations, it is toxic to these plants 39 Phytoremediation Heavy metals, including cadmium and lead, are toxic to animals in even small quantities -400 plant species have the ability to hyperaccumulate toxic metals from soil -However, a concern is that animals eating these plants will be exposed to high concentrations of toxic compounds 40 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phytovolatilization Phytodegradation Phytoaccumulation a. TCE CO2 b. Lead Not degraded 41 Phytoremediation Phytoremediation is a promising technique -Costs 50-80% lower than cleanup methods involving mechanical removal of contaminated soil An illustrative example comes from the 1998 accident at the Aznalcóllar mine in Spain 42 Dike of a holding lagoon for mine waste broke Large amounts of sludge were removed mechanically Hyperaccumulating native plant species are now populating the area 43