Lecture 7 - roots, stems, structure and transport Flashcards
What are the features of the root system of Horsetails (Equisetum)?
Rhizomes: underground stems for anchorage
Leaves and roots form whorls around each node.
once LARGE DOMINANTs, now small weeds. But extremely persistent ones!
Very simple structure, similar above and below ground.
Very primitive.
But the vascular tissue allowed the organism to EXPAND because of transport
Describe the Angiosperm root system.
primary root (tap root)
branching lateral roots
branching frequency and pattern dictate form
What are root hair cells?
single epidermal cells
can account for >50% of root surface area
increase capacity for water absorption - nutrients are dissolved in the water
The marker of a fully developed root: a single cell with the projection into the soil/water matrix.
SURFACE AREA is the key here – diffusion and uptake is correlated with surface area.
What is the apoplast pathway?
Between cells
Gradient drive
Cannot extend beyond Casparian strip (waxy substance that blocks gap between cells)
What is the symplast pathway?
Within cells
Controlled by transporters
Aquaporins the cell “water channels
Controls entry to vascular tissues
What are tracheids and vessels?
Dead, lignified cells
Tracheids: elongated
communicate with adjacent cells via pits.
Vessel elements: shorter, wider
communicate end-to-end via perforation plates to form vessels. Having multicellular vessels leads to much more rapid water transport
How is water moved by evapotranspiration?
Evaporation of water from leaves results in suction
negative pressure in the xylem vessels
This suction force or tension can pull water from the roots providing that
The walls of the xylem vessels can withstand the pressure
The water molecules stick to the walls and to each other, forming a continuous column
To get water up all trees there must be 3MPa pressure in the vessel
What are the causes of cavitation (water column breakage)?
Cavitation
Water is replaced by water vapour e.g.
Drought: tensions in xylem large; lower pressure in the water than the air may result in an air bubble pulled through a pit.
Freeze/thaw: gases less soluble in ice than water so dissolved gas results in small bubbles which cause problems when conduit thaws.
Or damage - Air leaks
can lead to Collapse:
Forces so great, walls pulled in & collapse – blocking flow
How do plants get round cavitation?
water will find other way through - Ways round this, are through the bordered pits in the vessels – water WILL FIND path of least resistance if the pressure remains high.
Gas bubble may re-dissolve when forces through xylem are reduced (e.g. at night)
Xylem cells that are narrower (i.e. Tracheids) are better able to resist bubble formation or rupture. Those that are wider (Vessels) achieve greater rates.
What limits tree height?
One factor that limits tree height is the maximum tension a water column can withstand
If the column breaks tension is lost and air bubbles get in (embolism)
The water tension at the top of the tallest
trees is close to the maximum
this needs A LOT of force in a TALL TREE – water transport is a LIMITING FACTOR ON TREE HEIGHT
How does the phloem transport nutrients?
Phloem part of the vascular tissue
Transport through living cells.
Moves carbohydrates from where synthesized or stored to where needed to support growth and respiration.
i.e. Moves C (mainly in the form of sucrose) from SOURCES to SINKS
Sources: Both leaves (P/s) and potato tubers (storage organ) are examples of sources.
Sinks: Young leaves, roots and developing fruits are examples of sinks.
What is the structure of the phloem?
LIVING CELLS
In the root – at the centre so they are within the casparian strip – prevents (or limits ) nutrient loss by diffusion
In the stem: in bundles between the xylem bundles.
Structure – living cell with nuclei, and cytoplasmic connections between sells (PLASMODESMATA)\ via sieve plates.
With parenchyma packing and companion cells
Why are structural tissues a cost?
Carbon spent on constructing structural tissue is at the expense of growth - because it is not photosynthetic
Plants must trade off growth rate with survival.
Most plant morphology and physiology reflects these trade offs.
What is carbon gain dependent on?
Carbon gain dependent on:
Physiology of photosynthesis:
1) the light intensity and the ability to absorb light energy (amount, type and organisation of chlorophylls)
2) ability of chloroplast to use light energy to produce ATP and NADPH in the thylakoid membranes
3) ability to fix atmospheric CO2: the rate of CO2 fixation is dependent on the availability of CO2, and amount of CO2 fixing enzyme (RUBISCO) present.
4) The ratio of photosynthesis to photorespiration. RUBISCO is capable of fixing both CO2 (photosynthesis) and O2 (photorespiration) - O2 competes with CO2 for binding sites on Rubisco. High rates of photorespiration come at the expense of photosynthesis.
Carbon gain also dependent on:
Plant Morphology: the proportion of plant mass allocated to photosynthetically active leaves - LMR
Leaf area: the amount of leaf area produced by a given amount of leaf mass. Influences the ability of a plant to intercept light energy - SLA
Leaf structure: of the cells in a leaf, what proportion are present as photosynthetically active tissue, such as mesophyll, versus non photosynthetically active tissue such as structurally important tissues that make leaves strong, such as sclerenchyma)
What is carbon uptake?
Carbon uptake:
- rate of photosynthesis per unit leaf area
- which depends on chlorophyll allocation and efficiency
- amount of leaf area
Amount of leaf area controlled by:
- Leaf mass ratio (LMR, % plant mass in leaves)
- Specific leaf area (SLA, amount of leaf area per unit leaf mass)
