Chapter 35

1. Solutions that were thought to be pure were not, as they contained trace amounts of elements that could not be detected at the time. Recently developed chemical methods made it possible to detect essential elements that were not previously recognized.

2. Yellow leaves in a young plant may be the result of an iron or a sulfur deficiency; yellowness in older leaves suggests a nitrogen deficiency.

1. Plants express specialized transporter molecules to move nutrients in soil water across root cell membranes. The more of a certain transporter that is made, the more its nutrient moves into the plant.

2. Plants can grow toward nutrients in the soil and orient stems and leaves to maximize exposure to light and air for photosynthesis.

1. Heavy irrigation after a prolonged dry period may produce runoff of topsoil (the A horizon) and leaching of ions (especially anions) into the subsoil, making fewer nutrients available to plant roots. Converting land use from virgin deciduous forest to crops will change the composition of living organisms in the soil, as many organisms that live in association with tree roots will disappear. The soil structure and texture will also change, because roots will no longer be present to hold the soil together and make air spaces. The soil chemistry will change, because crops take up nutrients from the soils and the nutrients are removed from the system when the crops are harvested.

2. Cation exchange frees ions bound to soil particles into the soil solution, where they can enter plant roots.

3. There are no differences between organic and inorganic nitrogen fertilizers in terms of plant nutrition; both enter the plant root as nitrate NO₃^–.

1. In both cases, the plants supply the other organism with photosynthate (e.g., sugars). Mycorrhizae supply phosphorus; bacteria in nodules supply fixed nitrogen.

2. Numerous species fix nitrogen. Loss of one species might allow populations of other species to expand and take on additional nitrogen fixation. Loss of all nitrogen-fixing species would mean that only abiotic methods (e.g., industrial methods) could be used for nitrogen fixation. Loss of nitrogen-fixing species would likely reduce overall nitrogen in the soil, meaning less would be available for plant growth.

3. The corn crop depletes the soil of nitrate. The soybeans do not require nitrate, as they have nitrogen-fixing nodules. When corn is rotated with soybeans, the soil becomes replenished with fixed nitrogen via free-living bacteria.

1. Carnivorous plants capture animals, digest their proteins, and absorb the amino acids. The primary nutrient they acquire is nitrogen.

2. The experiment with mutant Arabidopsis suggests that Arabidopsis uses either its own or exogenous strigolactones for growth regulation and has the appropriate receptor and response mechanisms. This reinforces the idea that an ancient mechanism to attract beneficial microbes is also used for modern plant growth regulation. Or the reverse might be true: the original function of strigolactone might have been as a plant hormone, and its role in plant–microbe interactions might have evolved later.

3. Holoparasitic plants can gain reduced carbon through association with hosts, so the genes encoding photosynthesis functions are not under selection pressure, because having them would not confer any survival and reproductive advantage for the parasites. So any mutation that renders such a photosynthesis gene nonfunctional will not be deleterious.

1. Root colonization was maximal 315 days after spores were added. The lag time was probably due to the need for the spores to germinate and grow into hyphae that could colonize the cassava roots.

   

2. 1. Cassava root crop production was just as good with fungal spores alone (38 g) as with fertilizer alone (35 g). Adding spores to fertilizer had a positive effect (43 g).

   2. A t-test for paired samples could be used to test for significance in differences for spores-treated and untreated experiments.

1. Yes, some of the data support the researchers’ conclusion that nickel is an essential micronutrient. Specifically, the germination data presented in the graph show that seed germination rate is related to the nickel quantity present in the seed. Seeds produced from plants grown under extremely low nickel concentrations have significantly lower germination rates than seeds produced from plants grown in higher concentrations of nickel. Because barley plants have high rates of germination failure without nickel, they cannot complete their life cycle in the absence of this nutrient. Over three generations, the plants were gradually depleted of nickel. This makes nickel an essential nutrient.

2. The data show that the mass of seeds produced by a plant is not affected by lack of nickel availability to the plant. This means that production of seeds within a parent plant occurs through processes that are not dependent on nickel. This conclusion does not contradict the conclusion from Question 1, which states that germination of seeds is dependent on nickel. Seed production within a parent plant and germination of the seed once it leaves the parent plant occur by two separate processes, only one of which is dependent on nickel.

3. Because soil is a complex mixture of organic and inorganic matter, it is more difficult to control the nutritional content of soil than it is to make a liquid growth medium with a defined and well-controlled content. The researchers would not have been able to easily remove nickel from soil and verify its nickel content. The results from using hydroponic growth conditions can be extrapolated to growth in soil because barley is able to adequately obtain the nutrients it needs from both hydroponic solutions and soil. Therefore there is no reason to think that different results would be observed using soil.