EFB530 Plant Physiology

Stress physiology

Plants are sessile, so they either withstand the environmental conditions to which they are subjected, or they die.

Biotic

• Pathogens-bacterial, fungal, viral, other (plant, protists, nematodes)
• Herbivores

Abiotic

• Temperature-cold and heat
• Salinity
• Anaerobiosis (flooding)
• Air pollution
• Wind (mechanical)
• Light-excessive or limiting; UV

Plants have become adapted to environmental conditions through evolution and may become acclimated to a condition by preliminary, short-term, non-lethal exposure

Plants can tolerate a stress condition and survive, or use some mechanism to avoid a stress (annual plants make seeds to overwinter - thus the vegetative foliage avoids freezing stress)

Chilling & freezing stress

Chilling

Chilling injury occurs at temperatures lower than normal growth temperatures, but not freezing temperatures

• temps around 10C
• usually occurs in plants adapted to warm climates

The main damage caused by chilling is to the membranes

• leakage of ions out of the cell
• inefficient functioning of photosynthetic or respiratory ATP synthesis
• leaf lesions, wilting

Chill-sensitive plants have a higher proportion of saturated fatty acids

• saturated lipids solidify at higher temperatures
• membranes change from fluid to gel as temps drop, lose function

Plants can become acclimated by slow exposure to colder temps

• increase the proportion of unsaturated fats in the membranes

Freezing

Freezing damage occurs primarily due to the formation of ice crystals, which damage cell structure

• ice usually forms first in the cell walls and intercellular spaces
• damage occurs when ice crystals grow and puncture into the cytoplasm

Physics of ice formation

• in plants, the temperature of water will drop below its freezing temperature and will still remain liquid = supercooling
• for the transition to solid phase to take place, need ice nucleation points
• when becoming solid, ice gives off heat, so the temperature rises
• when all of the water in the cell wall has frozen, then the temperature begins to drop again

Many plants can avoid freezing injury, because they allow deep supercooling

• the liquid in the intercellular space never makes the transition to solid phase, so ice crystals don't form
• some can supercool down to -35C
• at -40C, ice crystal formation begins spontaneously
• occurs in some hardwoods and some fruit trees (apple, pear, peach, oak, elm, maple, beech, ash, walnut, hickory)

Plants that live in environments colder than -40C usually don't supercool (white birch, quaking aspen, pin cherry, lodgepole pine)

• ice crystal formation begins at -3 or -5C in the cell walls
• over time, water is pulled from the cytoplasm and accumulates on the growing ice crystals (if temps drop slowly)
• these plants must have sufficient intercellular space for ice crystals to grow
• this results in greater concentration of solutes in the cytoplasm (dehydration)
• these plants must also tolerate dehydration, similar effect during drought

Some bacteria can promote ice crystal formation at warm temps (-3 or -5C)

• formation of ice crystals at warmer temps can be damaging to the plants (but is great for making snow at ski areas)
• can isolate mutant bacteria that do not promote ice formation, these can be sprayed on sensitive plants to protect them from frost damage (strawberry)

Acclimation to cold involves the expression of some different genes

• isoforms of enzymes that are active at lower temps
• antifreeze protein

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