Exam revision 2 Flashcards
What are phytosiderophores?
Fe-mobilising compounds released in the rhizosphere under Fe and Zn deficient conditions. Phytosiderophores (PS) are chelators for Fe acquisition in the rhizosphere (localised in apical root shoots) and crucial for chlorosis resitance (high-PS release rates in chlorosis-reistant plants, e.g. barley, wheat, and rye).
What is the rhizosphere?
Soil around the root that is influenced by the biology and chemistry of the root. Extent isn’t clearly defined and varies with plant and soil properties (e.g. dry soil - rhizosphere smaller than in moist soil).
Depends on:
Root exudate amount and composition
Sorption capacity of the soil (soil type/OM %/pH)
Water content (diffusion rates away from roots)
Root hair length
Decomp rates of exudates
What is the zone of depletion?
Where the uptake from the root surface is greater than the rate at which nutrient moves to root surface, there becomes a region of [low] developing immediately around the root system which becomes a concentration gradient. This localised area around the root surface (where concentration is lower than found in the bulk soil) is the zone of depletion.
What is nutrient balance?
The difference between the nutrient inputs entering a system (e.g. fertiliser) an the nutrient outputs leaving the system (the UPTAKE of of nutrients for crop or pasture production).
What is the Phosphorus Buffering Index?
xxxx
Describe root hairs and nutrient uptake.
See proteiod roots? XXXX
What are ‘critical values’ for soil tests?
XXXX
What are proteoid roots?
Proteiod roots (cluster or bottle-brush roots) are closely-spaced lateral roots which perform a similar function to mycorrhiza in other plants, and are found worldwide in low-nutrient soils (though are prolific in Australian Proteaceae). Cluster roots have a distinct morphology --> Intense proliferation of closely spaced, lateral ‘rootlets’ occurs along part of a root axis. Root hairs develop along each rootlet and result in a further increase in surface area compared to regions where cluster roots have not developed. They greatly enhances exudates, solubilisation of mineral and organic nutrients and uptake of inorganic nutrients, amino acids and water per unit root mass. Root cluster production peaks at soil nutrient levels suboptimal for growth of the rest of the root system, and may cease when shoot mass peaks. They are often located in surface soil horizons
Describe low- and high-affinity transporters?
XXXX
Describe the antagonistic relationship between nutrients.
XXXX
The formation of AM can help to improve plant nutrient acquisition. Discuss with reference to the formation and functioning of the association. Hint: a diagram may help in answering this Q.
AM are soil fungi that form symbiotic associations with plants. In exchange for carbon/sugars (energy) from the plant, organic nutrients are mobilised in the soil and transferred to plants (inorganic).
- Bi-directional nutrient transfer of soil-derived nutrients, as fungal hyphae can extend cm beyond roots (to combat depletion zones) have high surface area to volume ratios and are VERY efficient at nutrient uptake (N, P, Zn)
- 4-20% of plant’s C (from photosynthate) can be allocated to fungi. If plant weren’t mycorrhizal that C would be stored in roots.
- Influencing C supply to fungi can influence colonisation. If low P, plant sends carbohydrate to rhizosphere for mycorrhiza to use. If enough P, no carbon/carbohydrates redirected so no benefit for mycorrhiza to colonise.
- Plant genotype and species influence how much mycorrhizal colonisation will occur (e.g. brassicas inherently UN-colonised, proteaceae inherently colonised). BUT, within a genotype, other factors (e.g. root length and structure) will influence mycorrhial colonisation rates and dependence.
Arbuscules create a large interface, with a massive surface area between plants and fungi in th cortical cells (highly branched structures).
*These fungi cannot complete life-cycle without these stages (have to form association with plant –> obligate symbiosis).
Nutrients in soil can move by mass flow and diffusion. Describe the main features between the two.
Mass Flow: how water moves (in bulk flow) from soil to the root system (sucked up as the H2O is moved to the root system as the crop is transpiring (transpiration-cohesion-tension mechanism).
- Plants grow by transpiration (evaporating H2O from leaves), this sets up a water potential gradient which draws water from the soil to the roots and up through the plant. As this H2O is being drawn to the roots from the soil it takes with it nutrients dissolved in that transpiration stream.
* Depending on solubility of nutrients, there can be greater or smaller amounts of those nutrients moving to the root system via MF.
* nutrients that are quite soluble are prone to leaching because of this. —> leaching is another form of mass flow: H2O is draining through the soil profile taking with it dissolved nutrients (in solution).
Diffusion: Spontaneous movement of ions due to thermal agitation , from a region of [high] to [low] (e.g. a fertiliser granule in soil, with movement from that granule out into bulk soil, as long as that nutrient is soluble –> driving force is concentration gradient in soil.
As nutrient moves in solution into root cells, a concentration gradient is established. The uptake from root surface is greater than the rate @ which the nutrient is moving to the root surface. There will be a region of [low] developing immediately around the root system which establishes a concentration gradient (zone of depletion).
*Uptake > soil supply : zone of depletion
*Uptake < soil supply : zone of accumulation
Nutrient deficiencies can develop under certain environmental conditions rather than by a low [nutrient] in the soil. For each of the cases below, explain why the nutrient deficiency has developed based on the way the nutrient mainly moves to the root in soil.
- Mn deficiency in lupin after period of dry weather during flowering and early pod growth.
- Zn deficiency develops in grapevines in the McClaren Vale during an unusual period of wet weather in early spring
- Poor pollen viability from B deficiency occur in tropical regions under periods of high humidity
- K deficiency develops in grapevines in spring after a period of cold and wet weather
- N and S deficiency develop in barley on a sand dune but not on heavier soil at the base of the dune
- N deficiency develops after applying high rates of a C-rich compost
- Mn
- Zn: moves through MF 30% and DF 40% - low mobility in soil but high rainfall leads to leaching in soil profile.
- B: B uptake is reduced in high humidity (high humidity=low transpiration + reduced B uptake by mass flow
- K
- N and S: high mobility - MF 98 & 95% - sand has high infiltration of nutrient in solution
- N: e.g. cereal straw/winter prunings - N used to synthesise amino acids & proteins/C used for energy = if low N, microbes utilise N & tie up N in soil to maintain growth (immobilisation @ C:N>25) - if high N there is mineralisation (C:N<25)
Discuss why soil testing for N availability and crop response to N fert. are less successful than those for P.
XXXX
N fert is highly mobile in soils while P is immobile
Provide examples to describe examples of new soil tests and how to validate them to determine if they outperform standard soil testing procedures.
IR spectroscopy (bounce beam of dry soil surface (in situ) to get absorbtion wavelengths
- measure soil matrix elements e.g. CaCO3, clays & types, OM etc.
- chemical properties: C pool, total N, exchangeable cations (Ca, Mg, K, Na, CEC), PBI, pH, EC, ESP
- physical properties: Pbulk, particle sizes (clay, silt, sand), Ov, elements (Si, Al, Fe, Ca, Mg)
These results need to be calibrated against best practice soil tests in order to be validated.
PBI- when used to predict PBI in soil @ Karoonda, SA, laboratory measurements had r2 of 0.895, validating measurements made with the spectral method.
obvious benefit of ease of use - in situ results are also lower footprint, less expensive, quick, etc.
