Plant Adaptations to Chilling and Freezing

So could we…?  Could we ever have banana plantations or groves of oranges here in Wisconsin?  Probably not  So why is it that we do have oodles of apple orchards, cherry trees, and even peaches?  Where are these crops located in Wisconsin?  Why is Door County most famous for these crops?  Why is it that an apple tree can survive cold Midwestern winters but a banana can’t survive in the fridge? Acclimation vs. Deacclimation  Through natural selection, plants in high latitudes and developed cellular responses to cold weather that enable them to tolerate or avoid cold weather  Acclimation: a plant’s transition from sensitive to hardy (usually in fall, or when moved from a greenhouse)  Deacclimation: a plant’s transition from hardy to sensitive (usually in summer)  Deacclimation is faster than acclimation, which is why an extended spring thaw is very bad – takes longer to become hardy again Strategy 1: High osm  High osmolarity (high conc) lowers the freezing and boiling temperatures  e.g. if you want your pasta to boil quicker, add salt to it  e. g. if you want to melt ice on your sidewalk, you add salt  A high solute concentration increases the atmospheric pressure, and this makes it so that a lower temperature is needed for ice to form. Strategy 2: Super Cooling     Ice forms when water gets below freezing, right? WRONG! Ice forms when impure water gets below freezing! Pure water can be cooled to -40 F/C before it will freeze!  -40 is the same in both F and C  Ice needs something to ‘hold onto’ in order for crystals to form.  This ‘something’ for ice to form is called a nucleator.  Without any impurities, ice will not form at temperatures above -40. Strategies 1 and 2 together  Inside the cell, high osm lowers the freezing point at which ice would form because of increased atms  Outside the cell, pure water can remain ice-free up to a temperature as low as -40 because of an absence of nucleators  For example, the newly formed leaf buds in fall are kept ice-free by the pure water around them within the bud structure  Both hypertonic water and pure water have lowered freezing points, but for different reasons  I know, weird! Strategy 3: Osmotic Alterations  Some plants survive the cold by tolerating the formation of ice inside the cell, but limit the loss of water from the cell  By pumping up the osmolaric concentration of the cells ‘innards’, it can reduce the loss of water caused by freeze-induced dehydration  This can be via salts, sugars, or other solutes  For example, some frost-tolerant crops become sweeter with cold weather because of Strategy 3 Strategy 4: Membrane Changes  We mentioned earlier that protein pumps can shut down if the lipid cell membrane freezes (solidifies)  Quick Biology Review: the membrane of living cells is called a ‘phospholipid bilayer’  This simply means that the cell membrane is double-layered with phosphate heads on the outside and lipid tails on the inside. Unsaturated vs. Saturated  When we talk about a ‘saturated fat’, we mean that the fat molecules have the maximum amount of hydrogen  They are saturated with hydrogen molecules  If we replace some of the hydrogen in the lipid (fat or oil) with double bonds in the carbon, the molecules form a “kink”  More kinks = more fluidity Kinked Unsaturated Lipids  This should be familiar to you  Animal fats tend to be solid at room temp because they are usually found at body temperature (e.g. butter)  This is also true for warm weather plants  Cold weather plants tend to have unsaturated fat because they need to be kinked to prevent solidification  E.g. Vegetable oil is a liquid at room temperature  Solidification basically ‘freezes’ the cell and stops the cell metabolism  The kinked unsaturated lipids prevent this from happening by maintaining a liquid state at low temps Strategy 5: Cutin Wax  You probably remember that the cuticle is the outer layer of the epidermis that produces cutin.  Cutin is a waxy layer that prevents the plant from dehydration and disease  An increased cutin layer will reduce the damage caused to a plant by chilling and freezing by preventing water loss  Again, frost damage is dehydration damage  Plants exposed to cool weather have more waxy cutin Summary  Strategy 1: High intercellular osmolarity = high atm = reduced freezing temperature  Strategy 2: Absence of nucleators (pure water) outside the cells = reduced freezing temperature (to -40 degrees)  Strategy 3: Osmotic alterations = reduced loss of water to ice (e.g. Snow peas sweeten after each frost)  Strategy 4: Unsaturated lipids in the phospholipid bilayer of the cell’s membrane  Stays liquid at lower temps  Strategy 5: Increased cutin production