Transcript The Importance of Water

BISC 367 - Plant Physiology Lab
Spring 2009
Plant Biology
Fall 2006
BISC 367
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
BISC 367
Water relations data
Plant
Treatment
Pressure Bomb (Individual readings)
Poplar
dry
wet
1.80314 and 3.039, 2.3
1.26625/1.16495/0.9117, 1.25
Geranium
dry
wet
dry
wet
dry
wet
0.75975/0.82053/0.72936
.38494/.51663/.48624.
Bean
Corn
Leaf Press (MPa) Average
35,39,55
1.1143 60,85,52, 29, 30
0.86105
0.35455
-
Osomometer
Osmometer
Reading (mmol/Kg)
Reading (mmol/Kg)
Average
0.2965 804, 1007
905.5
0.4528 632, 589, 588
603
Pressure Bomb (MPa) Leaf Press (psi)
(average)
Individual readings
0.76988 73,85, 60
0.462603333 90,58,62, 45
0.86105
42
0.35455
37
1psi=0.00689476MPa
35
39
55
The units are MPa
The sign is NEGATIVE
These readings look
good
Combine with data
from other group
0.5447 294, 293, 274, 298
0.4826 280, 263, 299, 272
0.2896
405
0.2551
353
289.75
278.5
-
0.2413
0.2689
0.3792
60
85
52
73
85
0.4137
0.5861
0.3585
0.5033
0.5861
90
0.6205
58
62
42
37
0.3999
0.4275
0.2896
0.2551
???This data doesn't jive
with the pressure bomb
I agree!
Convert to Ys using
van’t Hoffs eqtn
Poplar, bean Ys is
lower for dry but not
for gernanium –
why?
Measuring Yw
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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).
disks are dried to obtain their dry weight (DW) .
RWC (%) = [(W – DW) / (TW – DW)] X 100
Leaf
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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
BISC 367
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
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)
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
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
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
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
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
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
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
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
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
Nitrogen assimilation
• Assimilation of N via reduction of NO3
NO3
NO2
NH4+
NRG cost = 12 ATP
NH2 group of amino acid
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
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 52:1981-1989; doi:10.1093/jexbot/52.363.1981
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 NO2Relies on eproduced by
photosynthesis
Ammonium assimilation
• NH4+ is toxic and must be utilized rapidly
– Dissipates pH gradients
Ammonium assimilation
• Glutamine synthetase (GS) combines NH4+ and glutamate
Glu + NH4+ + ATP
Glutamine + ADP + Pi
• Glutamate synthase (GOGAT) transfers the amide of glutamine to 2oxoglutarate
Glutamine + 2-oxoglutarate + Fdred/NADH
2 glutamate + Fdox
• Transamination rxns transfer amide N to other amino acids
Glu + oxaloacetate
aspartate and 2-oxoglutarate