Today your hip, non-hipsta’ gardener is chillin’ inside, staying warm. In the midst of the mild to extreme swings in temperature experienced this week, I deliver a second post in a blog trilogy regarding plant cold hardiness adaptations.
Last time, I discussed how leaf buds might survive the howling winds of Indiana winter, and by the time you have read the last word of this post, it will be clear different plant tissues respond differently to thermometric changes in our winter wonderland.
Why is ice a big deal to members of the plant kingdom? To sum it up, two words: lysis and cytorrhysis. Lysis is the rupture of cells, which in this case occurs when the razor sharp ice crystal edges slice through the cell membrane. Cytorrhysis involves the irreversible inward collapse, rather than outward going explosion, of the cell membrane and wall. Plants, shockingly enough, do not appreciate such incidents.
Temperate tree and shrubs species undergo large changes in metabolic activity, known to botany nerds as acclimation. Plants capable of hanging with a rugged eskimo use one of two basic strategies: supercooling or freeze tolerance. Plants undergoing normal, gradual, temperature changes are able to acclimate to environmental conditions except when…
- Temperatures drop below a plant’s maximum tolerance even after aforementioned internal changes,
- Sudden, extreme drops in early autumnal temperatures prior to the aforementioned internal changes,
- Sudden late vernal freezes after the plant has de-acclimated, or
- Moody winter temperatures swing wildly, stimulating premature de-acclimation.
Supercooling is one adaptive strategy used by cold hardy species such as oak, beech, ash, apple, rhododendron and roses. Normally water below its freezing point, 0°C, crystallizes around some “foreign” or insoluble particle (ice nucleation). There is often something biological present, such as bacteria, in the plant cell. Soluble substances such as glucose, glycerol and ethylene glycol drop the temperature at which impure water will freeze. Pure water can be supercooled, remaining a liquid down to -40°C. Cellular water freezes instantly below this temperature killing cell, thus the reason for altitude and arctic timberlines. Even more wild is the fact that there are antifreeze proteins somehow involved with ice prevention in cold hardy plants!
Plants such as red twig dogwood, willow and paperbark birch use freeze tolerance to protect cell tissue through forming ice outside the cell wall, dehydrating the interior, so there is a concentration of dissolved particles in the cell’s cytoplasm. The same suppression of water’s freezing point occurs with salting roads or adding antifreeze to your car.
FYI- A couple of wild facts I discovered along the way: there are antifreeze proteins somehow involved with ice prevention in cold hardy plants! Also, ectothermic plants develop glass-like substances to withstand temperatures down to -196°C. Glass, people! Inner passage cruise anyone?
Josh Steffen, Horticulture and Facilities Manager