Overview – The six elements we know as macronutrients, N, P, K, S, Mg and Ca, are each indispensable for growth. Here we examine how macronutrients contribute to plant growth. Specifically, we look at (1) the availability of nutrients in the soil along with the effects of soil microbes and physical properties on their availability; (2) nutrient uptake from the external environment, across plasma membranes and into plant cells; (3) in some cases, the assimilation of the nutrient into organic molecules;
(4) the distribution and redistribution of nutrients throughout the plant; and (5) regulation of these processes. In parallel, we will examine the genetic basis of a plant’s nutrient-use efficiency (NUE), and evaluate strategies by which to replenish nutrients that growing plants extract from soil.
Study / exam questions ( understanding and comprehension)
· In a plant, what are the functions of each of the macronutrients?
· A farmer’s wheat was growing slowly so she added NH4NO3. When this didn’t stimulate it to grow faster, she concluded that nutritional deficiency was not the problem. Was this a correct assumption? Why or why not?
· Name four organic molecules (each) that incorporate N, P and S.
· Diagram the global cycle of N, P or S. Indicate places where human activities impact the cycle.
· In the TV show Star Trek, Mr Spock observes, “In the strict scientific sense we all feed on death – even vegetarians.” Is this statement also true for plants? Explain your reasoning.
· Compare and contrast the cellular functions of K, Mg and Ca.
· In which chemical form(s) is each macronutrient taken up from the environment? If more than form can be taken up, what factors determine which is the major form?
· Draw the reaction catalyzed by glutamine synthase and discuss why it has an important role in nitrogen use efficiency.
· Diagram three ways that a root system responds to low-P soils. In what ways would you expect the root system to respond differently to soils that are low in total P as opposed to those that are low in available P?
· Soil loses nitrate primarily through leachage, and phosphorus primarily through erosion. Explain.
· Of all the families of nutrient transporters, the best understood are the potassium transporters. Why do you think that is? Hint – In 2003, the Nobel Prize in Chemistry was awarded jointly to Roderick MacKinnon. Why did the Nobel committee choose to honor this research? Is it because of the importance of potassium channels in plant nutrition?
· Explain this paragraph: “Until recently, most of the research into plants and sulfur was focused on the harmful effects of atmospheric SO2, but lately research efforts have switched to ensuring that plants get adequate S for the metabolic needs. However, lichens and bryophytes continue to be adversely affected by atmospheric SO2”.
· In your own words, describe how the cysteine synthase complex controls flux through the sulfur assimilation pathway.
· Why has magnesium been described as the “forgotten element”?
· Where are calcium-carboxylate crystals found and what functions do they have?
· Why is the Casparian strip a greater obstacle to Ca uptake than to the uptake of other macronutrients?
· Calcium and magnesium both form divalent cations. Would it be feasible to suggest that Ca2+ could substitute for Mg2+ in stabilizing ribosomes? Why or why not?
Discussion questions ( engagement and connections)
· What are the advantages to the use of chemically-produced fertilizers? What are three negative impacts / consequences of the use of chemically-produced fertilizers?
· Investigate how nutrients are taken up from trapped insects by insectivorous plants. Is it possible to determine if the molecular processes are co-opted from those in roots, or have evolved separately? How would you go about investigating this question?
· For one nutrient, discuss (1) how soil properties and / or microbes affect its availability, (2) transporters for uptake or reallocation, (3) if appropriate, how it is assimilated into organic molecules, (4) factors involved in its distribution and redistribution throughout the plant; and (5) what is known about the regulation of one or more of these processes.
· Why is it important to have low and high affinity nutrient transporters in the root? Why is it important to have transporters for both influx and efflux?
· What is meant by the term “peak phosphate”? Is there a consensus for when P reserves will run out?
· Why is it important to remove phosphorus from wastewater and sewage? Compare and contrast commonly used processes. How widely used are they?