Karl Foord, UMN Extension Educator
Well, here we are in Minnesota where most parts of the state are sitting under several feet of snow. This led me to think about what was the state of my apple and maple trees.
The plants in our state are adapted to northern climates with harsh winters that are unfavorable to plant growth. One mechanism that these plants have adopted to survive involves a suspension of growth termed dormancy.
The bud that overwinters in an apple tree is a miniature shoot with apical meristem, leaf and perhaps flower primordial, and axillary buds enclosed by modified leaves termed bud scales. Bud scales protect the bud from mechanical injury, restrict gas exchange and prevent desiccation.
Preparation for winter and true dormancy
The buds are the photoperiod receptors and in preparation for winter undergo a series of physical and physiological changes triggered primarily by the shorter days of late summer. These short days (actually long nights) trigger the production of abscisic acid (ABA) which acts as a growth inhibitor. ABA has been found to build up to high levels in the fall. Although cool temperatures are not the primary trigger they facilitate dormancy of the buds. There is a point at which the bud cannot be induced to grow even given under optimum environmental conditions. At this point the bud is said to be in true dormancy. The only way the bud can be induced to grow is by experiencing a chilling period. Temperatures need to be below 45° F (7.2° C) and last for between 800 and 1,000 hours for northern adapted apples (Table 1). It may be the presence of ABA that inhibits growth and only after this inhibitor decays over time that the plant has the ability to respond to favorable environmental conditions. This removes the internal block to growth, but external factors such as low temperature can also inhibit growth in the early spring.
Dormancy can be distinguished from quiescence where the bud is in a resting state in response to adverse environmental conditions, but will resume when the environmental conditions become favorable again. Fascinatingly enough, roots overwinter in a in a quiescent state.
When the soil begins to warm, promoters of growth such as gibberellin and cytokinins build up, signaling the bud to resume growth.
Intracellular water management for plants in cold climates
Another aspect to surviving harsh winters other than the dormancy state is the management of cellular water either through deep supercooling or intracellular dehydration. Temperate woody plants utilize one of these two mechanisms.
Supercooled water is water below 32° F (0° C) that remains in a liquid state. Supercooled water can remain in the liquid state down to -36.6° F (- 38.1° C) and in the presence of dissolved solutes to -43.6° F (-42° C). This temperature is called the Homogeneous Nucleation Point. Without nucleating points no ice crystals will form above this temperature, and plants avoid cold damage by not allowing nucleating points. At temperatures below -43.6° F (-42° C) ice will form and the plant cells will be damaged or killed. Most temperate plants in North America utilize this mechanism.
Plants growing in parts of the world where temperatures fall below -43.6° F (-42° C) utilize a different mechanism. These plants avoid injury by preventing intracellular (within the cell) ice formation. Water freezes in the extracellular spaces which pulls liquid water out of the living cells leaving them dehydrated. These plants avoid damage by freezing but can be injured by dehydration. This mechanism permits plants to survive in areas where the temperatures drop below -43.6° F (-42° C).