Outline and chapter notes to accompany chapter 15 PLANTS AND CROP PRODUCTION Jan., 2001 A. PLANTS CAPTURE THE SUN'S ENERGY AND MAKE MANY USEFUL PRODUCTS Autotrophs are organisms (like plants) that can make their own organic compounds from simpler materials. Autotrophs are also producers in ecological systems because the compounds that they produce are also used by other organisms. Plants are autotrophs that obtain their energy from sunlight. Heterotrophs are organisms (like animals) that must obtain organic compounds from other organisms. Most heterotrophs are either primary consumers (e.g., animals that eat plants) or secondary consumers (e.g., animals that eat primary consumers). A smaller number of heterotrophs are decomposers (like most fungi and bacteria) that break down organic compounds into much simpler forms and return them to the soil or water. PLANT PRODUCTS OF USE TO HUMANS include: Food Juices and fermented beverages Medicines Spices, frangrances, and flavorings Wood (as lumber, as fuel, and as a source of paper) Dyes Rope and other fibers (and baskets, etc.) Flowers and other parts used as decoration and adornment PHOTOSYNTHESIS Overall equation 6 CO2 + 6 H2O ------> C6H12O6 + 6 O2 carbon dioxide + water glucose + oxygen Requires chlorophyll and other pigments to capture energy from light. Chlorophyll looks green because it absorbs most parts of spectrum except for green and yellow wavelengths (Fig. 15.2). Photosynthesis takes place in two major steps (Fig. 15.3): Light reactions take place in chloroplasts and use light energy to split water and release oxygen. Protons (H+ ions) are pumped into the interior of the thylakoids (wafer-like disks inside the chloroplast) and are used to build up the molecules NADPH and ATP. Dark reactions do not require light energy. ATP and NADPH are used to "fix" carbon dioxide and build up glucose. The glucose may be converted into other carbohydrates (or sometimes other molecules) for use at a later time. Stored carbohydrates are used as food sources by other organisms. NITROGEN FOR PLANT PRODUCTS: Plants use nitrogen compounds to make proteins, nucleic acids, and a variety of bitter-tasting compounds called alkaloids. Plants must obtain their nitrogen in soluble form, most often as nitrates. Nitrates are often a limiting nutrient, meaning that the plants would grow more vigorously if extra nitrogen were made available. NITROGEN CYCLE: Nitrogen-fixing organisms fix atmospheric nitrogen, that is, they incorporate the nitrogen into compounds, most usually in the form of ammonium ions. Most of the ammonia is converted first into nitrites and then into nitrates. Most of the nitrates are absorbed by plants, but some is recycled into atmospheric nitrogen gas. The nitrates absorbed by plants are used to build proteins, which are sometimes eaten by animals and used to build animal proteins. Decomposer organisms break down proteins and other nitrogen compounds and release ammonium ions to the soil or water. Specialized types of bacteria convert ammonium ions into nitrites or nitrates, while other bacteria release nitrogen gas into the atmosphere. OBTAINING NITROGEN: Most plants absorb nitrates from the soil. Plants living in nitrogen-poor habitats with abundant sunshine sometimes attract symbiotic nitrogen-fixing bacteria. The plant supplies the bacteria with food and habitat; the bacteria fix nitrogen and produce nitrogen compounds that the plants use. This is an example of mutualism, a type of symbiosis in which two species are closely associated and mutually benefit each other. A few carnivorous plants obtain their nitrogen by digesting insects and other animals. B. PLANTS USE SPECIALIZED TISSUES AND TRANSPORT MECHANISMS. TISSUE SPECIALIZATION IN PLANTS Plant cells are organized into tissues and organs. Leaves, stems, and roots are organs. Most plants are vascular plants in which the stems contain two types of conducting tissues called xylem and phloem. These vascular tissues permit specialized organs in different locations, including leaves (organs of gas exchange and photosynthesis), stems (organs of storage and transport), and roots (anchoring and absorbing organs). The most advanced group of vascular plants are the flowering plants or angiosperms. WATER TRANSPORT IN PLANTS Osmosis is a passive movement of water across a semipermeable membrane in response to concentration gradients. Plant cells take up water by osmosis, which creates turgor, a fluid pressure that makes plant cells stiff. Evaporation of water from the leaves of plants is called transpiration. It is controlled by the opening or closing of pores called stomates on the undersides of leaves. The ascent of water through plant stems is governed by transpiration pull: the water is pulled upwards by the cohesion of hydrogen bonds as water evaporates out of the leaves. A few plants are capable of rapid movement by loss of turgor in selected places. C. CROP YIELDS CAN BE INCREASED BY OVERCOMING LIMITING FACTORS FERTILIZERS Fertilizers add limiting nutrients to the soil, thereby increasing crop yields. The most common fertilizers supply nitrates, phosphates, and potassium. Inorganic chemical fertilizers are frequently obtained over great distances; they often require mechanized agricultural methods, and their overuse can damage crops, deplete soil, and cause runoff problems and eutrophication. Organic fertilizers are more often locally available, require less technology for their use, and are less liable to be overused because they release their nutrients slowly. Crop rotation can increase crop yields by planting species that replenish the soil with certain nutrients that other crops have depleted. Runoff from fertilizers can cause overgrowth of algae in lakes and ponds, accelerating the process of eutrophication, a type of ecological succession in which ponds gradually fill in as different types of plant communities replace one another. SOIL IMPROVEMENTS AND CONSERVATION Soil is weathered rock plus organic material (Fig. 15.9). The organic material in soil is called humus. Good soil takes hundreds of years to form, and should thus be treated as a nonrenewable resource. Sustainable agriculture uses practices that do not deplete the soil. IRRIGATION Water can be supplied to plants by irrigation, either by spraying or by drip irrigation. HYDROPONICS Some plants can be grown without soil, a practice called hydroponics. Most hydroponic systems require large initial investment and abundant energy. Most hydroponic systems are monocultures in which only a single plant species is present. Hydroponic systems must always be monitored for plant diseases, which can spread rapidly in monocultures. CHEMICAL PEST CONTROL Each year, billions of dollars of crops are lost to crop pests (insects, rodents, fungi, etc.) Chemicals can be used to control plant pests, but: Many chemicals may harm nontarget species. Long-lasting chemicals may travel up the food chain, where their concentration increases by biomagnification (Box 15.2). Pest species resistant to these chemicals can become even more devastating. INTEGRATED PEST MANAGEMENT (IPM) Instead of attempting to eradicate pest species entirely, agricultural practices are used to determine an economic impact level (EIL), meaning a pest population level below which an acceptably low level of economic damage is sustained. Pest species are kept below this economic impact level by a variety of measures: maintaining natural enemies of the pest species whenever possible; interrupting the life cycle of the pest species (e.g., by crop rotation); using pheromones to interfere with mating of the pest species; monitoring levels of the pest population; using chemical pesticides only when necessary. Mixed cultures (multiple crop species) are preferred to monocultures (one species only) because pests can spread more easily through monocultures. Monocultures also deplete soil nutrients faster than mixed plantings. ALTERING PLANTS THROUGH ARTIFICIAL SELECTION Artificial selection can be used to improve crop species. This method has been used since the beginning of agriculture. ALTERING STRAINS THROUGH GENETIC ENGINERING Genetic engineering can now be used to introduce genes from one plant species into another. Caution must be used whenever plant strains are introduced to new localities: climate, natural pests, and agricultural practices may differ significantly from place to place, and these differences may prevent one plant strain from being introduced elsewhere. ---------------------------------- Jan., 2001