The Importance of Water

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BISC 367 - Plant Physiology Lab Spring 2009
Plant Biology Fall 2006

• Notices
• Photosynthesis lab report due Feb. 09
• Lecture test Feb 10
• Please email water relations data to Doug
  Wilson myself
• Reading material (Taiz Zeiger)
• Chapter 12 assimilation of mineral nutrients

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Water relations data
Plant Treatment Pressure Bomb (Individual readings) Pressure Bomb (MPa) (average) Leaf Press (psi) Individual readings Leaf Press (MPa) Average Osmometer Reading (mmol/Kg) Osomometer Reading (mmol/Kg) Average
Poplar dry 1.80314 and 3.039, 2.3 35,39,55 0.2965 804, 1007 905.5
wet 1.26625/1.16495/0.9117, 1.25 1.1143 60,85,52, 29, 30 0.4528 632, 589, 588 603
Geranium dry 0.75975/0.82053/0.72936 0.76988 73,85, 60 0.5447 294, 293, 274, 298 289.75
wet .38494/.51663/.48624. 0.462603333 90,58,62, 45 0.4826 280, 263, 299, 272 278.5
Bean dry 0.86105 0.86105 42 0.2896 405  
wet 0.35455 0.35455 37 0.2551 353  
Corn dry - - - -    
wet - - - -    
1psi0.00689476MPa
35 0.2413 Convert to Ys using vant Hoffs eqtn
39 0.2689
55 0.3792
60 0.4137 Poplar, bean Ys is lower for dry but not for gernanium why?
The units are MPa 85 0.5861
The sign is NEGATIVE 52 0.3585
73 0.5033
85 0.5861
These readings look good 90 0.6205
Combine with data from other group 58 0.3999
62 0.4275
42 0.2896
37 0.2551
???This data doesn't jive with the pressure bomb
I agree!
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Measuring Yw
Relative water content

• Assesses the water content of plant tissues as a
  fraction of the fully turgid water content
• relevant when considering metabolic /
  physiological aspects of water deficit stress
• Considered to be a better indicator of water
  status and physiological activity
• Captures effects of osmotic adjustment
• Osmotic adjustment lowers the Yw at which a
  given RWC is reached
• Simple technique
• Leaf disks are excised, weighed (W) then
  allowed to reach full turgidity and re-weighed
  (TW). Leaf disks are dried to obtain their dry
  weight (DW).
• RWC () (W DW) / (TW DW) X 100

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Water uptake by roots

• Water crosses the roots using 3 possible pathways
• Apoplastic pathway
• Water moves via cell walls
• Symplastic pathway
• Water moves through the cells passing through the
  plasmodesmata
• Transmembrane pathway
• Water moves through cells but independently
  enters and exits each cell

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Water uptake by roots

• Casparian strip forces water to enter
  endodermal cells
• must cross plasma membrane
• Allows plant to select what can pass on to the
  xylem
• Important for discrimination against toxic ions
  etc.
• Usually consider a single hydraulic conductance
  for entire root

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Water movement - an overview
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Inorganic ions in the soil

• Soil particles carry a negative charge
• Bind cations

• Anions are not readily bound
• NO3- is soluble
• PO42- binds to Al3 or Fe3 and can be
  unavailable
• SO42- reacts with Ca2 to form gypsum (CaSO4)

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Ion transport across the root

• Ions can cross the root in the apoplast or
  symplast
• All ions enter the symplast at the endodermis
  before entering the stele (vascular tissue)
• To enter the cells of the xylem ions must move
  back to the apoplast
• Note the casparian strip
• prevents outward movement of ions
• Can allow a higher level of ions to build in the
  xylem relative to the soil

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Ion uptake into a cell

• Driving force for ion uptake is the
  electrochemical gradient
• Conc. gradient across membrane
• Electrical gradient across membrane
• At eqm the conc. difference across the membrane
  is balanced by the electrical difference
• Calculate electric potential for given ion using
  Nernst equation
• All living cells have an electrical difference
  across the membrane - membrane potential

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Ion uptake into a cell

• Membrane potential is established by several ions
  coming to eqm
• Ability to come to eqm (or steady state) is
  influenced by membrane transport processes

• Only K is close to eqm.
• Anions have a higher than predicted conc
• Cations have a lower than predicted conc

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Ion uptake into a cell

• Membrane potential is set by
• Passive diffusion
• Electrogenic pumping (primarily H)
• H-ATPases
• Located on PM (plasma membrane) - P-ATPases
• Pump H into cell wall
• and tonoplast (membrane surrounding vacuole) -
  V-ATPases
• Pump H into vacuole

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Ion uptake into a cell

• H gradients drive 2o transport across PM and
  tonoplast
• In vacuole H is high
• Anions move in to balance charge
• Ys falls
• Water moves in - turgor increases
• H-pyrophosphatase also moves H into vacuole
• Utilize energy of PPi hydrolysis

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Ion Composition

• K acquired passively
• Na actively pumped out to apoplast and vacuole
• H actively pumped out to apoplast and vacuole
• Acidic apoplast and vacuole, neutral cytoplasm
  (regulates cell pH)
• Anions are actively acquired
• Ca2 is actively pumped out

Passive transport Active transport
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Nitrogen assimilation

• Only C, H, and O are more abundant in plants
  than N
• N is abundant in the atmosphere as N2
• not readily available
• Triple N-N bond needs lots of NRG to break

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Nutrient assimilation

• Energetically costly!
• NO3- reduction to NH4 utilizes 25 of a plants
  NRG requirements
• Requires large amounts of reductant
• Most occurs in stroma of chloroplast (cp)
• Dependent on photosynthetic e- transport
• photoassimilation

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Nitrogen assimilation

• Nitrate uptake is inducible
• Low and high affinity carriers exist
• Carriers are synthesized in response to
  external NO3 and is influenced by
• plant N status
• form of N available in the soil
• Sustained protein synthesis is necessary
• NO3 that enters root cells has 3 fates
• Storage in the vacuole
• Assimilation in root cells
• Translocation in the xylem for assim. in leaf
  cells

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Nitrogen assimilation

• Assimilation of N via reduction of NO3
• NO3 NO2 NH4 NH2
  group of amino acid NRG cost 12 ATP

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Nitrate assimilation

• Nitrate absorbed by the soil is reduced in the
  cytosol by nitrate reductase (NR)
• NO3- NAD(P)H 2H NO2-
  NAD(P) H2O
• NR is the major Molybdenum containing enzyme in
  plants

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Nitrate assimilation

• NR is tightly regulated
• Gene transcription and enzyme activation are
  stimulated by
• NO3
• Light
• enhances activation by NO3
• links NO3 assimilation with NRG
• CHO
• Inactivation of NR is stimulated by
• Dark
• Mg2
• NR is regulated by a NR kinase
• phosphorylated and non-phosphorylated states
  are active
• if the phosphorylated form is transferred to
  darkness an inhibitor switches NR off
• activity is restored in the light by
• inhibitor release
• phosphatase

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Model for the post-translational modulation of NR
Kaiser, W. M. et al. J. Exp. Bot. 2001
521981-1989 doi10.1093/jexbot/52.363.1981
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Nitrate assimilation

• NO2- is toxic and must be utilized immediately
• Transported to cp (leaf) or plastid (root)
• Reduced by nitrite reductase (NiR)
• NO2- 6 Ferredoxinred 8 H
  NH4 6 Fdox 2 H2O
• NiR is regulated by light/NO3 (inducers), and by
  amino acids (repressors)
• NiR levels are higher than NR

Reduction of NO2- Relies on e- produced by
photosynthesis
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Ammonium assimilation

• NH4 is toxic and must be utilized rapidly
• Dissipates pH gradients

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Ammonium assimilation

• Glutamine synthetase (GS) combines NH4 and
  glutamate
• Glu NH4 ATP Glutamine
  ADP Pi
• Glutamate synthase (GOGAT) transfers the amide of
  glutamine to 2-oxoglutarate
• Glutamine 2-oxoglutarate Fdred/NADH
  2 glutamate Fdox
• Transamination rxns transfer amide N to other
  amino acids
• Glu oxaloacetate
  aspartate and 2-oxoglutarate