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Stress Physiology Chapter 25 Water stress – drought tolerance Heat stress and heat shock Chilling and freezing Salinity O 2 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
2. Heat Stress And Thermotolerance Table 25. 3
Energy loss: Reradiation Convection Conduction Transpiration Energy storage
Atriplex Tidestromia Photosynthesis declines before respiration Fig. 25. 10 Ion leakage is a sign of membrane damage due to high temps. (or freezing. )
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 orientation 2. Leaf pubescence 3. Altered membrane fatty acids 4. more saturated fatty acids that don’t melt as readily 5. 4. Production of heat shock proteins (HSPs) in response 6. to rapid heat stress 7. “molecular chaperones”, increase enzymes resistance to denaturation; help maintain proper protein folding
3. Chilling and freezing stress Symptoms Slower growth Leaf 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 protoplast Freezing induced xylem embolisms can result from air bubbles released from ice as it thaws.
Chilling sensitive Corn Phaseolus bean Rice Tomato Cotton etc
Young root sections Incubate at 25°C for 24 h Measure conductivity Kill roots at high temperature Measure conductivity Nayyar et al. 2005 - chickpea
Membrane fatty acid composition determines fluidity at different temperatures. Saturated f. a. have no double bonds; all carbons are saturated with -H. Ratio of chain length: dbl. bonds determines melting point
Longer chains and fewer double bonds mean higher 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 cold Low % unsaturated – melts late, good for hot Membrane rap 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.
Pinus aristata, S. F. Peaks What makes arctic and alpine species tolerant of freezing? How is that overwintering buds (e. g. winter deciduous trees) tolerate temperatures that would kill summer leaves and 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 prevent ice formation down to -400 C! 5. ABA seems to induce freezing tolerance.
Hardening in hardwood forest species 1 - short days, low temperature: induction of chilling tolerance - stops growth, remove water from xylem 2 - 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!!