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!!