Stress PhysiologyChapter 25
Water stress – drought toleranceHeat stress and heat shockChilling and freezingSalinityO2 deficiency
Responses to water stress
Osmotic adjustment
Stomatal closure•hydropassive - guard cell dehydration•hydroactive - guard cell metabolism; ABA, solutes, etc.
Leaf abscision and reduced leaf growth•reduces surface area for water loss•Smaller leaves lose more heat via convective heat loss
Increased root growth•with reduced leaf expansion, more C transported to roots•increases water supply
Increased wax deposition on leaf surface•reduces cuticular transpiration, increases reflection
Induction of CAM in facultative CAM plants•in response to water or osmotic stress
Also many responses at the cellular level:
Proteins increase and decrease in response to water stress
One special group of proteins: LEA-proteins (late embryogenesis abundant)
Accumulate in dehydrating leaves, and during seed ripening
Function: protection of membranes (hydrophylic proteins)prevention of destructive crystallization of proteins
Table 25.3
2. Heat StressAnd Thermotolerance
Energy loss: ReradiationConvectionConductionTranspiration
Energy storage
Ion leakage isa sign of membranedamage dueto high temps.(or freezing.)
Fig. 25.10
Photosynthesisdeclines beforerespiration
AtriplexTidestromia
What happens when plant tissues reach harmful temperatures?
•Membranes lose function because they become too fluid.•Soluble proteins may denature, degrading function•Membrane-bound proteins may become dysfunctional because of denaturation or excessive membrane fluidity.
These effects can be seen in the changes in photosynthesis, respiration, and ion leakage of membranes.
Fig. 1.5
Adaptive or acclimation responses to high temperatures
1. Vertical leaf orientation2. Leaf pubescence3. Altered membrane fatty acids
more saturated fatty acids that don’t melt as readily
4. Production of heat shock proteins (HSPs) in response
to rapid heat stress“molecular chaperones”, increase enzymes resistance to denaturation; help maintain proper protein folding
3. Chilling and freezing stress
Symptoms Slower growthLeaf necrosis or damage“Soggy” looking leaves
Inhibition of photosynthesis, translocation, increased degradation of proteins
The central problem - loss of membrane function.Chilling can cause membranes to lose fluidity.
Freezing can rupture membranes (ice crystals)Extracellular ice can dehydrate protoplastFreezing induced xylem embolisms can result from air bubbles released from ice as it thaws.
Chilling sensitiveCornPhaseolus beanRiceTomato Cotton etc
Young root sectionsIncubate at 25°C for 24hMeasure conductivityKill roots at high temperatureMeasure conductivity
Nayyar et al. 2005 - chickpea
Membrane fatty acid composition determines fluidityat different temperatures.
Saturated f.a. have nodouble bonds; all carbonsare saturated with -H.
Ratio of chain length:dbl. bondsdetermines melting point
Longer chains and fewer double bonds meanhigher melting temperature
Palmitic 16:0(major constituent of palm oil)
Acclimation and adaptation response to low temperatures include an increase in membrane unsaturated fatty acids.Chilling-resistant species have higher unsat’d/sat’d ratio.
Oleic acid is 18:1, Pea shoot is 17.8 not 12.8
summary of fatty acids: High % unsaturated – melts early, good for coldLow % unsaturated – melts late, good for hot
I’ve got cold membranes and I think I’m gonna freeze
Those unsaturated fatty acids, help me deal with it please
Take a trip to the tropics, it’s melting that’s got me beat
Need my saturated fatty acids so I can cope with the heat.
Membrane rap
Pinus aristata, S.F. Peaks
What makes arctic and alpine species tolerantof freezing?
How is that overwinteringbuds (e.g. winter deciduoustrees) tolerate temperaturesthat would kill summer leavesand buds?
Frost tolerance increases as buds “harden” for winter.
ABA is thought to induce hardening
Dealing with chilling and freezing stress
1.Altered membrane fatty acids
2. Solute accumulation can lower freezing point.“antifreeze” compounds
3. Limiting ice nucleation using “antifreeze” proteins that slow ice formation.
4. “deep supercooling” mechanisms that preventice formation down to -400C!
5. ABA seems to induce freezing tolerance.
Hardening in hardwood forest species1- short days, low temperature: induction of chilling tolerance - stops growth, remove water from xylem2- freezing: tolerant to -50 to -100°C
Deep super cooling: no ice formation above -40°C. Oak, elm, maple, beech, pear, apple, Engelmann spruce, subalpine fir.
Cross tolerance: They are also extremely dehydration resistant. Again LEA and HSP proteins may be involved. Some of the anti-freeze proteins are also involved in pathogen attacks!!
Spring: freeze tolerance is lost quickly – spring damage to flower buds!!