Lecture 6 Flashcards
(29 cards)
What are the 3 macro-nutrients required by plants?
Nitrogen, Phosphorous, potassium.
What is N used for?
Production of Amino acids, proteins and other cellular constituents.
What results from N deficiency?
Stunted growth and chlorotic leaves (lose chlorophyll).
N is the most limiting nutrient to plant growth.
Why is N difficult to obtain?
N molecules are extremely unreactive, triple bond between N molecules is the 2nd strongest known chemical bond.
Must be fixed into useful ammonia.
How is N fixed industrially?
The Haber-Bosch process.
Nitrogen + Hydrogen -> Ammonia.
Why is the Haber-Bosch process costly?
Requires high temperatures (300-600 degrees) and high pressure (20-80MPa). Uses 1-2% of global energy. 85% of the ammonia produced is used to make fertiliser. 2 tonnes of oil are used to make 1 tonne of ammonia.
There is a correlation between population size and fertiliser usage.
What is the first biological method of nitrogen fixation?
Reduction of nitrogen to form ammonia by microbes that are in a symbiotic relationship with the plant.
20,000 Mt produced annually.
What is the second biological method of nitrogen fixation?
Reduction of nitrate (predominant form of soil N) into ammonia (NH3).
How are these two methods achieved?
Over 250 species of microbes such as bacteria and cyanobacteria fix N using nitrogenase enzyme.
Where do these microbes occur?
Can be free living in the soil or associated with plants (normally in root nodules of legumes).
What are the issues with the nitrogenase enzyme?
Reaction is sensitive to O2 as it inactivates nitrogenase. Also, it requires a large input of energy as ATP and reductase.
How do root nodules form?
Root hairs from an infection thread by invagination of the plasma membrane to allow bacteria in.
Rhizobium bacteria penetrate the infection thread and form bacteroids within vesicles.
Bacteria secret chemicals to induce root growth so root cells grow around the bacteroid forming a nodule which develops vascular tissue to allow nutrient exchange.
Why are nodules costly?
In legumes 1/3 of all carbon goes to the root nodules for growth & maintenance, production of ATP and NADPH and to provide carbon for assimilation of ammonia.
What are 2 adaptation strategies to low nutrient environments.
Parasitism by burrowing into a host plants vascular tissue to obtain N.
Carnivory to gain N from animals/ animal waste.
What effect does ammonia have on plant cells?
It is toxic and causes changes in growth, increased frost sensitivity and nercrosis. Therefore it must be converted to a useful compound.
What is the glutamate synthase cycle?
Produces useful products from ammonia.
Glutamine is used to make AAs and chlorophyll.
Glutamate is used to make AAs and nucleic acids.
What is phosphorous used for in plants?
Production of nucleic acids, metabolites, ATP, sugar phosphatases. Limits photosynthetic rate.
How is phosphorous manufactured?
Grinding up rocks that contain phosphate (lower energy than ammonia production). 98% of this is used as fertiliser.
What are some issues with phosphorus production?
It is non-renewable, wasteful application is widespread and we will reach “peak phosphorous” when it runs out.
How is phosphorous recycled?
Over millions of years, P leaches into the sea from fertilisers and ends up in sediment, then phosphate rocks are produced by tectonic/volcanic activity.
What are some solutions to reaching “peak phosphorous”?
Applying phosphorous more efficiently as 80-90% of fertiliser is bound to soil particles and is biologically unavailable - fertiliser only directly delivers 1-5% of phosphorous demand.
Increase P recycling.
What are the 2 biological mechanisms for increasing P uptake?
Cluster/proteoid roots and mycorrhizal interactions.
What are cluster/proteoid roots?
Proliferation of root growth that increases the root area 25-fold. Allows growth in low P soils.
Most don’t form mycorrhizal symbioses.
How do cluster/proteoid roots work?
Produce high levels of citric acid which is extruded into the soil. Normally P ions bind to metals making them biologically unavailable but citric acid swaps carbohydrate for P ions so plant can take them up.
