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David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum La nuova biologia.blu Anatomia e fisiologia dei viventi S Transport in Plants How Do Plants Take Up Water and Solutes? Terrestrial plants obtain water and mineral nutrients from the soil. Water is needed for photosynthesis, for transporting solutes, for cooling the plant, and for internal pressure for support. Plants lose large quantities of water to evaporation, which must be replaced. How Do Stomata Control the Loss of Water and the Uptake of CO2? Leaf and stem epidermis has a waxy cuticle to minimize water loss, but it also prevents gas exchange. Stomata, or pores in the leaf epidermis, allow CO2 to enter by diffusion. Guard cells control the opening and closing. How Do Stomata Control the Loss of Water and the Uptake of CO2? Stomata open and close rapidly in response to changes in water. Changes in pressure arise from changes in K+ concentrations in the guard cells. How Are Water and Minerals Transported in the Xylem? Transpiration: water vapor diffuses from the intercellular spaces of the leaf to the outside air. As water evaporates from walls of mesophyll cells, surface tension of remaining water increases. The increased tension (negative pressure potential) draws more water into the walls from the cells. Cohesion between water molecules in the column prevents the column from breaking. The Transpiration–Cohesion–Tension Mechanism There is a continuous column of water in the xylem, caused by cohesion. How Are Water and Minerals Transported in the Xylem? The transpiration–cohesion–tension mechanism accounts for water movement through the xylem. Xylem vessels are dead and have no cell contents; fused end to end, they form a long tubular “straw” of lignified cell walls. They provide both structural support and the rigidity needed to maintain a gradient of pressure. How Are Substances Translocated in the Phloem? Movement of carbohydrates and other solutes through the phloem is called translocation. • Products of photosynthesis are called photosynthates • Content of the phloem is called phloem sap How Are Substances Translocated in the Phloem? Structure of phloem: Sieve tube elements meet end-to-end; plasmodesmata in end walls enlarge to form sieve plates, and most of the cell contents are lost. Companion cells are produced as daughter cells along with the sieve tube element when a parent cell divides. How Are Substances Translocated in the Phloem? Photosynthates are translocated from sources to sinks: • Sources: organs that produce more sugars than they require (e.g., leaf or storage root) • Sinks: consume sugars for growth or storage A given organ can be a sink at some times and a source at others. How Are Substances Translocated in the Phloem? Phloem sap flows under positive pressure; bulk flow— from regions of higher pressure potential to lower. Two steps require energy: •Transport of solutes from source into the sieve tube— loading •Removal of solutes at sinks—unloading How Are Substances Translocated in the Phloem? The pressure flow model of translocation: What Nutrients Do Plants Require? Mineral nutrients: inorganic elements, obtained mostly from the soil. Includes sulfur (S), phosphorus (P), magnesium (Mg), iron (Fe), and calcium (Ca). An essential element is one required for the plant to complete its life cycle, and no other element can substitute. • Macronutrients—at least 1 g per kg of dry plant matter is needed • Micronutrients—less than 100 mg per kg is needed Table 36.1 How Do Plants Acquire Nutrients? Plants cannot change location to find nutrients (they are sessile). Except for CO2, plants’ nutrient supply is localized— how can they find scarce nutrients? Roots mine the soil for minerals and water as they grow. Plants must also deal with variation in micro environments around them. How Do Plants Acquire Nutrients? Plants regulate nutrient uptake. They have specialized transporters for specific ions. Genes for ion transporters are regulated at the transcriptional level by the amount of nutrient in cells. How Does Soil Structure Affect Plants? Soils contain both living and nonliving components. The air spaces are important sources of oxygen for plant roots. How Does Soil Structure Affect Plants? Most soils have recognizable layers or horizons, called the soil profile. • A horizon or topsoil: layer from which plants get nutrients; • B horizon or subsoil: accumulates materials from the topsoil above and parent rock below; • C horizon or bedrock: is in the process of breaking down to form soil. How Does Soil Structure Affect Plants? Soil fertility: ability to support plant growth. Partly determined by proportions of sand, silt, and clay. Sandy soils—mineral nutrients tend to leach out of the topsoil and are carried to deeper horizons where they are unavailable to plant roots. Clay binds water and mineral ions and there is less leaching; but the particles are tiny and pack tightly—no air spaces. Loam has an optimal mix of sand, silt, and clay, and also organic matter. Best for agriculture. How Does Soil Structure Affect Plants? Soil organic matter or humus is used as a food source by microbes that break down complex organic molecules and release simpler molecules into the soil solution. Humus also provides air spaces that increase O2 availability to plant roots. How Does Soil Structure Affect Plants? Cation exchange. Cation exchange capacity affects soil fertility. How Do Fungi and Bacteria Increase Nutrient Uptake by Plant Roots? Mycorrhizae are associations of fungi with plant roots; occur in more than 90% of terrestrial plants. Some plants form associations with nitrogen-fixing bacteria. Legume plants form symbioses with bacteria Rhizobium. The nutrient most often in short supply is nitrogen while the main nutrient the plant gets is phosphorus. How Do Carnivorous and Parasitic Plants Obtain a Balanced Diet? Carnivorous plants augment their nutrient supply by capturing and digesting insects. Some plants are parasitic— getting water, mineral nutrients, or photosynthate from the bodies of other plants. What Determines the Transition from the Vegetative to the Flowering State? Plants fall into three categories in terms of maturation and flowering: • Annuals complete life cycle in one year and have little or no secondary growth • Biennials take two years to complete the life cycle Produce vegetative growth the first year and store carbohydrates in underground roots (carrots) or stems (onion). • Perennials live three or more years How Do Angiosperms Reproduce Sexually? Most angiosperms reproduce sexually—producing the genetic diversity that is the raw material for evolution. It involves mitosis, meiosis, and alternation of haploid and diploid generations. How Do Angiosperms Reproduce Sexually? The haploid gametophytes develop from haploid spores in the flower: Megagametophytes (female) are called embryo sacs; develop in the ovules. Microgametophytes (male) are called pollen grains; develop in anthers on the stamens. How Do Angiosperms Reproduce Sexually? Microsporocytes undergo meiosis to produce 4 haploid microspores. Each develops a cell wall and divides mitotically to form 2 haploid cells in each pollen grain: the tube cell forms the pollen tube that delivers the sperm to the embryo sac. After pollination the generative cell divides by mitosis to form 2 sperm cells. How Do Angiosperms Reproduce Sexually? A megasporocyte undergoes meiosis to produce 4 haploid megaspores; 3 undergo apoptosis. The surviving megaspore undergoes 3 mitotic divisions with no cytokinesis to produce 8 haploid nuclei. Cell wall formation leads to a gametophyte (embryo sac) with 7 cells and 8 nuclei. How Do Angiosperms Reproduce Sexually? At one end of the gametophyte are 3 cells—egg and 2 synergids. Synergids attract the pollen tube and receive the sperm nuclei. Three antipodal cells at the opposite end usually degenerate. The central cell has 2 polar nuclei. Sexual Reproduction in Angiosperms (Part 1) Sexual Reproduction in Angiosperms (Part 2) How Do Angiosperms Reproduce Sexually? Pollination: transfer of pollen from anther to stigma. The evolution of pollen made it possible for male gametes to reach the female gametophyte without water. This selective advantage allowed pollen-bearing plants to colonize land. In many species, wind transports pollen. Most angiosperms rely on animals (insects, birds, and bats) to transfer pollen. How Do Angiosperms Reproduce Sexually? Germination of the pollen grain involves uptake of water from the stigma and growth of the pollen tube. The pollen tube grows through the style to reach the ovule. Downward growth is guided by a chemical signal released by the synergids in the ovule. How Do Angiosperms Reproduce Sexually? Double fertilization: • One sperm cell fuses with the egg cell, forming a diploid zygote • The other sperm cell fuses with the two polar nuclei in the central cell, forming a triploid (3n) cell What Determines the Transition from the Vegetative to the Flowering State? Plants initiate flowering based on cues such as photoperiod (day length) and temperature. Short-day plants (SDPs) flower when the day is shorter than a critical maximum. Long-day plants (LDPs) flower when the day is longer than a critical maximum. Day Length and Flowering What Determines the Transition from the Vegetative to the Flowering State? Photoperiodic plants actually measure length of night, not day. Experiments with cocklebur, (SDP): Day length was varied in one group, night length in another. The critical night length was 9 hours. Adapted from Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014 Inc. All rights reserved