BIOLOGY Volume 2 - A Guide to General Biology - 2004

19. HOMEOSTASIS

19.3. Thermoregulation

19.3.2. Plant Adaptations to Low Temperatures

Plants native to northern temperate regions and the tundra exhibit numerous adaptations that allow them to make the most efficient use of the short warm summer. Species growing in polar regions of both the northern and southern hemispheres must endure long periods of severely adverse conditions, notably low light levels combined with surface cold and permafrost in the soil. Unsurprisingly, mosses and low-growing herbs with shallow ROOT systems, which become completely buried under snow in winter, predominate here. Among vascular plants, There is a high proportion of annuals that rapidly complete their life cycle and survive freezing temperatures as dormant seeds. Trees of the temperate latitudes display many structural and physiological adaptations to winter. For instance, deciduous trees shed their leaves in autumn (triggered by a specific plant hormone, Abscisic acid), thereby drastically reducing Transpiration rates during the period when soil Water is frozen (Section 16.2.8). Leaf fall also helps prevent damage from snow and wind at a time when Photosynthesis would be severely limited anyway due to low light intensity, low temperatures, water deficit, and nutrient scarcity. During these periods, future growth zones—buds protected by modified scale leaves—enter a dormant state, and their metabolic activity is suppressed by abscisic acid. Northern and alpine forests are dominated by conifers with needle-like leaves covered by a thick cuticle. This Structure reduces summer transpiration rates as well as the risk of wind and snow damage in winter, while allowing photosynthesis to resume immediately upon the arrival of the warm season, giving these species an advantage over deciduous trees. Various annual plant species have short growing seasons and survive the cold period by producing frost-resistant seeds and underground Organs.

Many temperate plants require exposure to low temperatures in order to break dormancy. For example, lilac buds develop faster in spring after a cold winter than when the shrub is grown under continuously warm conditions. Other Examples of the effects of low Temperature on plant growth are discussed in Chapter 16 and Section 22.4.