Horticultural Responses to Temperature

Your Mother’s Roses  “As a kid, I can distinctly remember that when September came, my mother would begin covering her rosebushes at night with plastic sheeting. While I appreciated her commitment to her rosebushes, I couldn’t help but shake my head at the stupidity of my mother. After all, how could a thin, plastic sheet protect those flowers from 30-degree weather? I guess I just realized that sometimes, you just have to let parents learn from their mistakes. The problem was, this always worked!” - the teenage Mr. Kohn So, wait a minute…  So we clearly have some inconsistencies here…here’s a couple questions to consider:  Why is it that sometimes a plant will suffer frost damage at temperatures above freezing (e.g. 36o)  How is it that ice can melt at sub-freezing temperatures?  E.g. on the roof of your house or on a salted-sidewalk  Why are some plants better at surviving frost than others?  If plants aren’t ‘warm-blooded’, how can they survive in winter when cold-blooded animals have to hiberate?  Do plants hibernate? The Big Question  The BIG question here is…  How do temperature changes affect the ability of a plant to grow and produce a crop (food, flowers, etc.)?  The ‘slightly less big but still pretty good sized’ question is…  Knowing how temperature affects a plant, how can we respond as horticulturalists to maximize plant productivity in a given environment? The Plant Cell  To answer these questions, we have to know a little bit about the plant cell before we can continue.  Things to keep in mind…  Plant cells are very different from animal cells, but face the same kind of challenges  For example, both animal cells and plant cells have to eliminate cellular waste and toxins; if animals have a bloodstream and kidneys to filter out waste, how to plants do it?  Plants don’t ‘feel’ things – i.e. they don’t feel cold or pain or hunger but they do still respond to it. Fact 1: Selective Cell Membranes  Plants have cell membranes that are selectively permeable.  Selectively permeable: the membranes of plant cells let some things in but not others; they can choose what solutes come in and which solutes don’t  To make this possible, plant cells use specialized proteins that block or allow solutes.  Some proteins are just like gates that passively let some solutes flow in  Some proteins are like pumps that actively move these solutes into the cell Fact 2: Osmolarity & Osmosis  Osmolarity is simply how concentrated a solution is  It is based on the number of particles in the volume of fluid (not by weight or mass).  Osm = number of particles in a solution  Osmosis is the movement of water from a solution with low osmolarity to a solution with high osmolarity  i.e. water tries to dilute things, so it wants to go from a watered down solution to a highly concentrated solution High Solutes Low Solutes Selectively Permeable Membrane Osmosis: Water will move from an area with low solutes (low osmolarity) to an area with high solutes (high osmolarity) Fact 3: Turgor Pressure  Because of osmolarity-induced osmosis, living plant cells have a property called Turgor Pressure.  Turgor Pressure: the pressure within a cell due to movement of water into that cell  The turgor pressure of a plant cell is about 11 atmospheres  1 atm (atmosphere) of pressure is equivalent to the air pressure at the earth’s surface at sea level  The pressure inside of a plant cell is 11 x the air pressure at sea level  This enables a plant to send shoots up and roots down Turgor Pressure and Cell atm  Turgor pressure can change within the cell as a result of the osmolaric conditions around that cell High osm (cell H2O out) Low osm (cell H2O in) Turgor Pressure & Cells  For example, imagine you have packed a salad for lunch.  If you pour salty salad dressing on your salad before you leave, what will happen to the salad?  Well, when lunch comes, you lettuce will be a disgusting, limpy mush  If you pour your salad dressing on right before you eat it, your salad will still be crisp and crunchy  The salt in the salad dressing sucks the water out of the lettuce leaf cells, causing them to collapse  Turgor pressure makes lettuce crisp and juicy Fact 4: Plasmolysis  Plasmolysis: if a cell is placed in a highly concentrated solution (hypertonic), water moves out of the cell, causing it to shrink and lyse (burst).  Lysis: bursting a cell; Lysol kills bacteria by bursting it  Deplasmolysis: if a cell is placed in a diluted solution (hypotonic), water moves into the cell, causing it to swell.  Salty Drink: cells shrink - plasmolysis  Pure & Clear: cells swell - deplasmolysis Hyper, Hypo, and Iso  Hypertonic solution = hyper concentrated outside  Isotonic solution = same concentration inside and out  Hypotonic solution = low concentration outside Fact 5: Vacuoles  Plants do not have kidneys or livers to process their cellular waste  To eliminate this waste and prevent it from reaching levels of toxicity, each plant cell has a large central vacuole.  The plant vacuole comprises roughly 80% of the cell volume A little more about vacuoles  The central vacuole is enclosed by a membrane called the tonoplast  The fluid inside the vacuole is called the cell sap.  The color of some flowers, the smell and flavor of fruits, veggies, and berries, and the taste of many plants comes from the cell sap Roles of the Vacuole  Storage – not just for waste, but also for salts, minerals,     nutrients, pigments, proteins, and others. Growth – the vacuole is a catch-all and materials can be retrieved if needed Structure – osmolarity and osmosis cause the vacuole to swell, preventing wilting Regulation – the vacuole can assist in regulating the rigidity of the cell and change the osmotic pressure Defense – toxins and bitter-tasting compounds that protect a plant from predation can be found here Summary  Fact 1: plant membranes are lined with protein channels     that are selective Fact 2: plant cells are affected by osmolarity (how concentrated a solution is) and osmolarity (water moves from low conc. to high conc.) Fact 3: the movement of water into cells due to osmolarity and osmosis creates a turgor pressure inside the cell of 11 atm Fact 4: when a cell is immersed in highly concentrated water, it shrinks as water is pulled out – plasmolysis Fact 5: the plant’s storage organ, the vacuole, can regulate turgor pressure to keep it at 11 atm So wait, isn’t this about temp?  You might wonder when temperature will make its way into our discussion about plants and temperature  Well, here it is… Plant Responses to Freezing  On a chilly night, cooling temperatures will cause the following changes to a plant cell…  Protein pumps require energy; plant cells will be less able to build     up the osmolarity of plant cells, reducing turgor pressure The vacuoles will begin to shrink as they are less able to increase the osmolarity inside the cell As ice crystals form inside the cell, water will be drawn out of the vacuole and into the ice causing plasmolysis Ice also acts like a monkey wrench in the delicate machinery of the inside of a plant cell Finally, ice formation will cause the cell to lyse, interrupting the flow of water through the plant via the xylem tubes  The water column inside the plant will be interrupted, causing irreparable damage Irreparable Frost Damage  While plants can tolerate chilling or freezing to varying extents, at low enough temperatures all face the following outcomes as water is pulled out of the cell  Protein Pump Shutdown  Lost Turgor Pressure (as water moves out of the cell to form ice –      frost damage is dehydration damage) Buildup of toxicity – as water is lost, concentration of waste increases inside the cell Interrupted cell function and stopped cell metabolism Cell damage from the cell membrane pulling away from the cell wall during plasmoylsis Cell lysis (rupture) and cell death Disrupted water flow via the xylem Frost Damage