SENC Exam 2 Flashcards
Describe how carbohydrate and oxygen supply influence nutrient uptake by roots and discuss the difficulties in accurately estimating root biomass.
Answer
Describe the effect of four unfavourable soil conditions on root growth and/or function
TEMPERATURE
> A study by Russel (1977) showed that barley growth was negatively affected by unfavourable soil temperatures (20ºC vs 10ºC) and that the plants would preferentially grow roots in the higher temperature, more favourable soil soil.
PH
> Different plants have different tolerances (levels of pH dependance) to variations in pH
- Usually related to the environmental conditions that the species evolved in.
> pH dictates nutrient availability, so the plant’s response is related to their nutrient requirements and ability to extract and/or take them up.
> Cotton, for example, is sensitive to acidic soil, and will suffer decreased root length as a result, whereas lupin is the opposite and prefers acidic soil (though has a different response curve to changing pH).
> Lime is used to increase pH
- However, too much CaCO3 can also reduce root growth as the Ca can form bridges between cell walls and reduce the ability of cells to elongate.
SOIL COMPACTION/BULK DENSITY
> Roots can’t create pores, only widen them.
- This ability differs between species/cultivars and depends on root strength
> Roots will preferentially seek out zones of lower density and exploit those regions
> Restriction to root growth (i.e. increased PR) results in decreased shoot growth (e.g. leaf area of tomato)
= decreased root growth
= decreased water and nutrient uptake
= increased drought sensitivity
= decreased shoot growth
= increased root diameter
- There’s still carbohydrate supply from the shoots, so if the roots can’t penetrate further, they grow bigger to exert more force.
MICROORGANISMS
> Can have both a positive and negative effect on plants and root growth
> Microbes compete with plants for nutrients and water
= increased root growth and exploration of the soil in order to find more resources
> Microbes mineralise nutrients
= increased availability for plants
> Production of toxic compounds
> Direct feeding on/damaging of root cells
WATER AVAILABILITY
> increasing drought stress:
= incr. root growth to find/utilise more nutrients
= incr. root : shoot growth
= incr. root density at depth to search for more water
> Hydraulic lift
- Happens at night
- Assumes that plants have a carbohydrate store
- Means that they still have the capacity to take up and store nutrients despite the stomates being closed.
- The ability of some plants to redistribute water throughout the soil profile at night for increased water availability during the day, thus increasing drought tolerance.
- At night, when the stomates are closed, the roots take up water from their deeper roots that have access to deep soil moisture, redistribute it to the shallower roots, then exude it into the soil.
- Neighbouring plants also benefit
> Flooded/waterlogged soils
> Roots require O2 to convert carbohydrate into energy
> O2 diffusion is roughly 1000x slower through water than air
> Generally speaking, plants have similar responses to waterlogged or flooded soil:
- Shallower root systems
- Increased root porosity
> However, some plants, such as lowland rice, have adaptions for surviving in waterlogged/flooded soil. One such adaption is the presence of AERENCHYMA - straw-like pores in the cortex that extend all the way from the shoots, transporting oxygen to the roots. This increases the amount of oxygen available for root function, and because some oxygen is released from the roots, creates a more favourable environment for microorganisms.
SALINITY
SODICITY
LOW [NUTRIENT]
> The initial response to nutrient deficiency is to increase root growth
Discuss how water availability influences root growth
DECREASING WATER AVAILABILITY (drought stress)
= incr. root growth to find/exploit additional resources
= incr. root : shoot ratio
= incr. root density at depth to access more (protected) water
ADAPTION = Hydraulic lift
- During the night, deep roots that have access to higher soil moisture levels take up and translocate water to the shallower roots, which exude the water into the surrounding soil.
- Roots are carbohydrate syncs, hence why they’re able to take up and translocate water despite the stomates being closed.
INCREASED WATER AVAILABILITY (waterlogged/flooded soils)
> Roots require O2 to convert carbohydrate into energy
> O2 diffusion is roughly 1000x slower through water than air
- Not fast enough to replenish O2 utilised by plants and microbes
> Generally speaking, plants have similar responses to waterlogged or flooded soil:
- Shallower root systems
- Increased root porosity
> However, some plants, such as lowland rice, have adaptions for surviving in waterlogged/flooded soil. One such adaption is the presence of AERENCHYMA - straw-like pores in the cortex that extend all the way from the shoots, transporting oxygen to the roots. This increases the amount of oxygen available for root function, and because some oxygen is released from the roots, creates a more favourable environment for microorganisms.
Explain hydraulic lift and how it can improve drought tolerance of plants
> Upper soil strata are usually depleted of water first due to higher rates of evaporation and plant uptake (most roots exist within the top 30cm)
Roots are carbohydrate syncs, and can therefore actively transport water at night despite the stomates being closed.
> At night, deep roots take up water, translocate it to the shallower roots which exude it, thus moistening the surrounding soil.
During the day, there is a greater pool of water available, and the plant takes up the water it exuded (as well as continuing to take up deep soil moisture).
This means that the plants are better able to withstand drought
Helps mobilise nutrients through diffusion and mass flow
Stimulates soil microbes
Describe how rhizosphere soil differs from bulk soil and explain the underlying mechanisms (explain 3 or 4 mechanisms)
NUTRIENTS
MICROBES
PH
Explain how the form in which N is taken up by plants (NH4+, NO3-, and N2) influences the pH in the rhizosphere and how this could influence element availability
Answer
Discuss how organic acid exudation influences P availability and Al toxicity
Answer
Organic acids mobilise P but they are also rapidly decomposed. Explain by which mechanisms plants increase the effectiveness of organic acids mobilising P.
Answer
Explain the differences between AM and ectomycorrhiza
Answer
Discuss the influence of mycorrhizal colonisation on nutrient and heavy metal uptake
Answer
Explain the table (effect of fallow on AM colonisation and plant growth)
Long fallow disorder
Mycorrhizal dependency
Describe how mycorrhiza influence plant community composition in natural ecosystems
Answer
Explain why microbial diversity in soils is high and what this means for the relationship between function and community composition
Answer
Discuss how drought and flooding influences microorganisms in soil
Answer
Explain the interactions between soil animals and microbes and how this influences nutrient cycling in soils
Answer
