Chapter Summary

• Plants are photosynthetic autotrophs that can produce all their organic molecules from carbon dioxide, water, and minerals, including a nitrogen source.

• Mineral nutrients are obtained from the soil solution.

• Plants require 14 essential elements, 6 of which are macronutrients and 8 of which are micronutrients. Deficiency symptoms suggest what essential element(s) a plant lacks. Review Figure 35.1, Table 35.1, Animation 35.1

• The essential elements were discovered by growing plants hydroponically, with their roots suspended in mineral solutions instead of soil.

• Root growth allows plants, which are sessile, to search for mineral resources.

• Plants can regulate the uptake of nutrients by increasing the number or activity of active transport proteins in root epidermal cells. Review Figure 35.2

• Soils contain water, air, and inorganic and organic substances. Soils have living (biotic) and nonliving (abiotic) components. Review Figure 35.3

• A soil typically consists of two or three horizontal zones called horizons. Topsoil forms the uppermost or A horizon. Topsoil tends to lose mineral nutrients through leaching. Loams are excellent agricultural topsoils, with a good balance of sand, silt, clay, and organic matter. Review Figure 35.4

• Soils form by mechanical and chemical weathering of rock. Chemical weathering imparts mineral nutrients to clay particles. Plant litter and other organic matter decompose to form humus. Plants obtain some mineral nutrients through cation exchange between the soil solution and the surface of clay particles. Review Figure 35.5

• Farmers use fertilizers to make up for deficiencies in soil mineral nutrient content.

• In the earliest stages of mycorrhiza formation, the hyphae of arbuscular fungi grow toward strigolactones, compounds that are produced by the plant roots. Review Figure 35.6A

• Some nitrogen fixers live free in soil or water; others live symbiotically as bacteroids within plant roots. The formation of a root nodule requires interaction between the root system of a legume and rhizobia. Review Figure 35.6B

• Several steps in the formation of root nodules and arbuscules are similar and probably involve some of the same plant genes. Review Figure 35.7

• Mycorrhizae are symbiotic root–fungus associations that greatly increase a plant’s absorption of water and minerals, especially phosphorus. They occur in more than 90 percent of terrestrial plant species. Review Investigating Life: Mycorrhizal Fungi Can Replace Fertilizer in Cassava Cultivation

• The arbuscules are the sites of nutrient exchange between the fungus and plant.

• In nitrogen fixation, nitrogen gas (N₂) is reduced to ammonia (NH₃) or ammonium ions (NH₄⁺) in a reaction catalyzed by nitrogenase. Review Figure 35.8, Activity 35.1

• Carnivorous plants are autotrophs that supplement a low nitrogen supply by feeding on insects or other small animals.

• Parasitic plants draw on other plants to meet their needs, which may include minerals, water, or the products of photosynthesis.

• Hemiparasites, such as mistletoes, can still photosynthesize. Holoparasites cannot function as autotrophs because they have lost chloroplast genes that code for components of the photosynthetic apparatus (which they no longer need).

• A strigolactone—a compound in the same category of compounds plants use to attract mycorrhizal fungi—also induces the germination of some parasitic plants, including Striga. Scientists hypothesize that a mechanism evolved in the ancestors of modern Striga to recognize a compound that was already produced by plants to attract arbuscular fungi.