By Charles Bryant, Huerfano County Weed Manager/CWMA Board Member
A Wintertime Miracle
During the frigid winter months I often take notice (and am actually in awe) of the few signs of green growth that can miraculously withstand subzero temperatures. When “green” growth is thought of during the winter months the image of a stately pine tree or some other coniferous species generally comes to mind. While our evergreen species certainly steal the limelight when it comes to plants that are universally known to withstand cold temperatures, our winter annual plant species tend to take a backseat or are outright ignored. These winter annual species lead lives that are counter to what most individuals understanding of what plant lifecycles are. Rather than emerging with the onset of longer days and favorable weather in the spring as most other annual and perennial species do, our hardy winter annuals prefer to begin their lives during the shortest, coldest and darkest days of winter.
Death by Osmosis?
Germinating in the fall and spending the bulk of the winter months as a seedling, it is a curious fact that these small underdeveloped plants can withstand temperatures that would prove deadly to other plants. We are all familiar with the symptoms associated with frost\freeze damage of tender vegetation. The curled over, withered and discolored appearance of frost stricken plants are actually the result of the cell membranes within the plant tissue rupturing due to the growth of ice crystals. As these ice crystals form and enlarge at the extracellular level, they not only work as a means of perforation, but they also desiccate the plants cells. The entrained water within the plant tissue cell is basically stolen through osmosis to provide the building material for the formation of ice crystals on the outer walls of the cell. This causes a collapse of the cell membrane resulting in cell lysis (death).
Built to Survive
Our winter annuals have developed a unique ability to combat the destructive effects of ice crystal formation at the cellular level. Their ability to withstand subzero temperatures is made possible by what are known as antifreeze proteins, also known as ice binding proteins (IBPs). Plant IBPs were only discovered about thirty years ago (Griffith et al., 1992), so we are somewhat at the beginning of our understanding. Rather than simply lowering the freezing point, these IBPs actually manipulate the shape and size of ice crystal formations. The IBPs adsorb to the extracellular ice crystals, exclusively on the basal and prism planes of the ice crystal. The presence of the IBP on these two planes inhibit the growth of sharp ice crystal formations that normally damage\perforate the apoplast of plant cells. Basically, the formation of sharp crystalline structures is prevented at the contact points between the cell and ice crystal.
The cell membrane of cold tolerant plant species is also specially adapted to withstand cold temperatures by having a higher composition of unsaturated lipids. In plants that contain cells with higher levels of saturated lipids (like warm weather species) the membrane formation is much more organized and rigid, causing greater fragility of the cell membrane. Cold tolerant plant species (whose cells predominately contain unsaturated lipids) form a membrane that has a more branched out, net like structure that freezes together at odd angles. These net-like unsaturated lipid formations in cold tolerant plant species allow much more flexibility of the cell membrane and compliment the protection provided by the ice binding proteins. A fitting visual comparison for the two differing frozen membrane structures would be that of a neatly stacked pile of frozen lumber (saturated lipids) vs. a pile of frozen tree branches (unsaturated lipids). The flexibility of an unsaturated lipid dominate membrane provides a much more resilient barrier than that of the rigid saturated lipid membrane.
Common Invasive Winter Annuals
So where can these impressive little organic antifreeze factories be found in Colorado? EVERYWHERE! Cheat grass is perhaps the winter annual species of greatest concern. It can be found this time of year growing everywhere from pastures to cracks in the sidewalk. That strange little tuft of reddish green grass observed in January or February is almost certainly cheat grass. The short-lived invasion of the either loved or hated blue mustard plant that fills the air with its pungent fragrance each spring is another widespread non-native. Redstem filaree (aka Stork’s Beak) is yet another common non-native plant species that bears pinkish purple flowers and that is common among a variety of sites in our state.
Nature’s Antifreeze in Ice Cream
The next time you see that strange little patch of green growth in the winter, take a moment to appreciate the tenacious character of our often overlooked winter annuals and the science behind their survival. Perhaps pay homage by enjoying your favorite frozen dairy treat because believe it or not, there is a relation. The Food and Drug Administration has actually approved the use of IBPs in yogurt and ice cream products. Referred to as “ice structuring proteins” on food product labels, you have already likely consumed IBPs in one form or another. These IBPs found in dairy products are not derived from a plant source though, they are actually proteins isolated from coldwater fish that possess similar IBPs. Rather than harvesting these IBPs from fish, they are replicated on a larger scale using genetically modified yeast. As our understanding of IBPs develops we are likely to encounter their use in what can seem like unlikely areas. Who thought antifreeze (protein) could taste so good? 😉