Chapter 37 Plant Nutrition

Overview: 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 Figure 37.1

Plants derive most of their organic mass from the CO2 of air Concept 37.1: 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

Macronutrients and Micronutrients More than 50 chemical elements Have been identified among the inorganic substances in plants, but not all of these are essential A chemical element is considered essential If it is required for a plant to complete a life cycle

Researchers use hydroponic culture To determine which chemicals elements are essential Figure 37.3 TECHNIQUE Plant roots are bathed in aerated solutions of known mineral composition. Aerating the water provides the roots with oxygen for cellular respiration. A particular mineral, such as potassium, can be omitted to test whether it is essential. RESULTS If the omitted mineral is essential, mineral deficiency symptoms occur, such as stunted growth and discolored leaves. Deficiencies of different elements may have different symptoms, which can aid in diagnosing mineral deficiencies in soil. Control: Solution containing all minerals Experimental: Solution without potassium APPLICATION In hydroponic culture, plants are grown in mineral solutions without soil. One use of hydroponic culture is to identify essential elements in plants.

Essential elements in plants Table 37.1

Nine of the essential elements are called macronutrients Because plants require them in relatively large amounts The remaining eight essential elements are known as micronutrients Because plants need them in very small amounts

Symptoms of Mineral Deficiency The symptoms of mineral deficiency Depend partly on the nutrient’s function Depend 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

The most common deficiencies Are those of nitrogen, potassium, and phosphorus Figure 37.4 Phosphate-deficient Healthy Potassium-deficient Nitrogen-deficient

Concept 37.2: 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 The eventual result of this activity is topsoil A mixture of particles of rock and organic material

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

The film of loosely bound water 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

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

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

Irrigation 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 Erosion 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 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 Soil Reclamation 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

Plants require nitrogen as a component of Concept 37.3: 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

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

Concept 37.4: 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)

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

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

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

Development of a soybean root nodule Infection thread Rhizobium bacteria Dividing cells in root cortex Bacteroid 2 The bacteria penetrate the cortex within the Infection thread. Cells of the cortex and pericycle begin dividing, and vesicles containing the bacteria bud into cortical cells from the branching infection thread. This process results in the formation of bacteroids. Developing root nodule Dividing cells in pericycle Infected root hair 1 2 3 Nodule vascular tissue 4 3 Growth continues in the affected regions of the cortex and pericycle, and these two masses of dividing cells fuse, forming the nodule. Roots emit chemical signals that attract Rhizobium bacteria. The bacteria then emit signals that stimulate root hairs to elongate and to form an infection thread by an invagination of the plasma membrane. 1 The nodule develops vascular tissue that supplies nutrients to the nodule and carries nitrogenous compounds into the vascular cylinder for distribution throughout the plant. Figure 37.11

The Molecular Biology of Root Nodule Formation The development of a nitrogen-fixing root nodule Depends on chemical dialogue between Rhizobium bacteria and root cells of their specific plant hosts

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

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

Epiphytes, Parasitic Plants, and Carnivorous Plants Some plants Have nutritional adaptations that use other organisms in nonmutualistic ways

Staghorn fern, an epiphyte Mistletoe, a photosynthetic parasite Exploring unusual nutritional adaptations in plants 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