Chapter 37 Plant Nutrition

The branching root system and shoot system of a vascular plant A nutritional Network Every organism Continually exchanges energy and materials with its environment For a typical plant Water and minerals come from the soil, while carbon dioxide comes from the air The branching root system and shoot system of a vascular plant Ensure extensive networking with both reservoirs of inorganic nutrients

Nutritional Requirements of Plants Plants require certain chemical elements to complete their life cycle Plants derive most of their organic mass from the CO2 of air but they also depend on soil nutrients such as water and minerals Figure 37.2 CO2, the source of carbon for Photosynthesis, diffuses into leaves from the air through stomata. Through stomata, leaves expel H2O and O2. H2O O2 CO2 Roots take in O2 and expel CO2. The plant uses O2 for cellular respiration but is a net O2 producer. Roots absorb H2O and minerals from the soil. Minerals

Symptoms of Mineral Deficiency The symptoms of mineral deficiency depend partly on the nutrient’s function and partly on the mobility of a nutrient within the plant Deficiency of a mobile nutrient Usually affects older organs more than young ones Deficiency of a less mobile nutrient Usually affects younger organs more than older ones

Mineral Deficiencies in Plants The most common deficiencies are those of nitrogen, potassium, and phosphorus Figure 37.4 Phosphate-deficient Healthy Potassium-deficient Nitrogen-deficient

Soil Quality Soil quality is a major determinant of plant distribution and growth Along with climate The major factors determining whether particular plants can grow well in a certain location are the texture and composition of the soil Texture Is the soil’s general structure Composition Refers to the soil’s organic and inorganic chemical components

Texture and Composition of Soils Various sizes of particles derived from the breakdown of rock are found in soil along with organic material (humus) in various stages of decomposition Humus important component of topsoil consists of decomposing organic material formed by the action of bacteria & fungi on dead organisms, feces, fallen leaves, etc. It prevents clay from packing tightly together and builds a crumbly soil that retains water but is still porous enough to aerate roots. It serves as a reservoir of mineral nutrients. The eventual result of this activity is topsoil (a mixture of particles of rock and organic material) The most fertile soils are usually loams (made up of roughly equal amounts of sand, silt, and clay. Loamy soils have enough fine particles to provide a large surface area for retaining minerals and water, but also have enough coarse particles to provide air spaces containing oxygen that can be used by roots for cellular respiration.

Soil Layers The topsoil and other distinct soil layers, or horizons Are often visible in vertical profile where there is a road cut or deep hole Figure 37.5 The A horizon is the topsoil, a mixture of broken-down rock of various textures, living organisms, and decaying organic matter. The B horizon contains much less organic matter than the A horizon and is less weathered. The C horizon, composed mainly of partially broken-down rock, serves as the “parent” material for the upper layers of soil. A B C

Extracting Water from Soil After a heavy rainfall, water drains away from the larger spaces of soil But smaller spaces retain water because of its attraction to surfaces of clay and other particles The film of loosely bound water is usually available to plants Figure 37.6a (a) Soil water. A plant cannot extract all the water in the soil because some of it is tightly held by hydrophilic soil particles. Water bound less tightly to soil particles can be absorbed by the root. Soil particle surrounded by film of water Root hair Water available to plant Air space

Cation Exchange in Soil Acids derived from roots contribute to a plant’s uptake of minerals When H+ displaces mineral cations from clay particles Figure 37.6b (b) Cation exchange in soil. Hydrogen ions (H+) help make nutrients available by displacing positively charged minerals (cations such as Ca2+) that were bound tightly to the surface of negatively charged soil particles. Plants contribute H+ by secreting it from root hairs and also by cellular respiration, which releases CO2 into the soil solution, where it reacts with H2O to form carbonic acid (H2CO3). Dissociation of this acid adds H+ to the soil solution. H2O + CO2 H2CO3 HCO3– + Root hair K+ Cu2+ Ca2+ Mg2+ H+ Soil particle –

Soil Conservation and Sustainable Agriculture In contrast to natural ecosystems Agriculture depletes the mineral content of the soil, taxes water reserves, and encourages erosion The goal of soil conservation strategies Is to minimize this damage

Commercially produced fertilizers Contain minerals that are either mined or prepared by industrial processes “Organic” fertilizers Are composed of manure, fishmeal, or compost

Maintaining Crops while Reducing Fertilizer Use Agricultural researchers Are developing ways to maintain crop yields while reducing fertilizer use Genetically engineered “smart” plants Inform the grower when a nutrient deficiency is imminent Figure 37.7 No phosphorus deficiency Beginning phosphorus deficiency Well-developed phosphorus

Topsoil from thousands of acres of farmland Irrigation & Erosion Irrigation, which is a huge drain on water resources when used for farming in arid regions Can change the chemical makeup of soil Topsoil from thousands of acres of farmland Is lost to water and wind erosion each year in the United States

Certain precautions can prevent the loss of topsoil Contour Tillage Certain precautions can prevent the loss of topsoil Figure 37.8

The goal of soil management Is sustainable agriculture, a commitment embracing a variety of farming methods that are conservation-minded Some areas are unfit for agriculture Because of contamination of soil or groundwater with toxic pollutants A new method known as phytoremediation Is a biological, nondestructive technology that seeks to reclaim contaminated areas

Nitrogen and Plant Growth Nitrogen is often the mineral that has the greatest effect on plant growth Plants require nitrogen as a component of Proteins, nucleic acids, chlorophyll, and other important organic molecules

Plants and Nitrogen Availability Nitrogen contributes most to plant growth Nitrogen is required for proteins, nucleic acids, chlorophyll, etc. Nitrogen cannot be obtained from atmospheric nitrogen (N2) N2 must be converted to ammonium (NH4+) or nitrate (NO3–) for plants to absorb nitrogen The process of nitrogen fixation involves the conversion of N2 to NH3 (ammonia) by nitrogen-fixing bacteria NH3 can be converted to NH4+ by picking up an H+ ion from the soil solution or to NO3– by nitrifying bacteria that oxidize NH3

Soil Bacteria and Nitrogen Availability Nitrogen-fixing bacteria convert atmospheric N2 To nitrogenous minerals that plants can absorb as a nitrogen source for organic synthesis Figure 37.9 Atmosphere N2 Soil Nitrogen-fixing bacteria Organic material (humus) NH3 (ammonia) NH4+ (ammonium) H+ (From soil) NO3– (nitrate) Nitrifying bacteria Denitrifying bacteria Root NH4+ Nitrate and nitrogenous organic compounds exported in xylem to shoot system Ammonifying bacteria

Improving the Protein Yield of Crops Agriculture research in plant breeding Has resulted in new varieties of maize, wheat, and rice that are enriched in protein Such research Addresses the most widespread form of human malnutrition: protein deficiency

Symbiosis in Plants Plant nutritional adaptations often involve relationships with other organisms Two types of relationships plants have with other organisms are mutualistic Symbiotic nitrogen fixation Mycorrhizae

The Role of Bacteria in Symbiotic Nitrogen Fixation Symbiotic relationships with nitrogen-fixing bacteria Provide some plant species with a built-in source of fixed nitrogen From an agricultural standpoint The most important and efficient symbioses between plants and nitrogen-fixing bacteria occur in the legume family (peas, beans, and other similar plants)

Nitrogen Fixing in Plants Along a legumes possessive roots are swellings called nodules Composed of plant cells that have been “infected” by nitrogen-fixing Rhizobium bacteria Figure 37.10a (a) Pea plant root. The bumps on this pea plant root are nodules containing Rhizobium bacteria. The bacteria fix nitrogen and obtain photosynthetic products supplied by the plant. Nodules Roots

Nitrogen Fixing in Plants Inside the nodule Rhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cell 5 m Bacteroids within vesicle (b) Bacteroids in a soybean root nodule. In this TEM, a cell from a root nodule of soybean is filled with bacteroids in vesicles. The cells on the left are uninfected. Figure 37.10b

Nitrogen Fixing in Plants The bacteria of a nodule Obtain sugar from the plant and supply the plant with fixed nitrogen Each legume Is associated with a particular strain of Rhizobium

Symbiotic Nitrogen Fixation and Agriculture The agriculture benefits of symbiotic nitrogen fixation Underlie crop rotation In this practice A non-legume such as maize is planted one year, and the following year a legume is planted to restore the concentration of nitrogen in the soil

Mycorrhizae and Plant Nutrition Are modified roots consisting of mutualistic associations of fungi and roots The fungus Benefits from a steady supply of sugar donated by the host plant In return, the fungus Increases the surface area of water uptake and mineral absorption and supplies water and minerals to the host plant

The Two Main Types of Mycorrhizae In ectomycorrhizae The mycelium of the fungus forms a dense sheath over the surface of the root a Ectomycorrhizae. The mantle of the fungal mycelium ensheathes the root. Fungal hyphae extend from the mantle into the soil, absorbing water and minerals, especially phosphate. Hyphae also extend into the extracellular spaces of the root cortex, providing extensive surface area for nutrient exchange between the fungus and its host plant. Mantle (fungal sheath) Epidermis Cortex Endodermis Fungal hyphae between cortical cells (colorized SEM) 100 m (a) Figure 37.12a

The Two Main Types of Mycorrhizae In endomycorrhizae Microscopic fungal hyphae extend into the root Figure 37.12b Epidermis Cortex Fungal hyphae Root hair 10 m (LM, stained specimen) Cortical cells Endodermis Vesicle Casparian strip Arbuscules 2 Endomycorrhizae. No mantle forms around the root, but microscopic fungal hyphae extend into the root. Within the root cortex, the fungus makes extensive contact with the plant through branching of hyphae that form arbuscules, providing an enormous surface area for nutrient swapping. The hyphae penetrate the cell walls, but not the plasma membranes, of cells within the cortex. (b)

Agricultural Importance of Mycorrhizae Farmers and foresters Often inoculate seeds with spores of mycorrhizal fungi to promote the formation of mycorrhizae Seedlings so inoculated grow more vigorously than those without the fungal association

Epiphytes, Parasitic Plants, and Carnivorous Plants Some plants have nutritional adaptations that use other organisms in nonmutualistic ways Epiphytes are plants that grow on other plants but nourish themselves by absorbing minerals & water from the rain using their own leaves Parasitic plants absorb sugars & minerals from their living hosts via specialized roots Carnivorous plants are photosynthetic but obtain some nitrogen and minerals by killing and digesting insects and other small animals

Exploring Unusual Nutritional Plant Adaptations Staghorn fern, an epiphyte EPIPHYTES PARASITIC PLANTS CARNIVOROUS PLANTS Mistletoe, a photosynthetic parasite Dodder, a nonphotosynthetic parasite Host’s phloem Haustoria Indian pipe, a nonphotosynthetic parasite Venus’ flytrap Pitcher plants Sundews Dodder Figure 37.13